wood preservatives, dyes typically found in textile mill effluent and leakages or spills of various liquid fuels.96 Dioxins in PCBs can now be destroyed by solar energy and these laborat
Trang 2solutions, while those which are stable in water have too wide
a band gap
12.6.4 Fuels
Environmental considerations are increasingly becoming more
important in the utilization of solar energy to obtain new fuels
at a competitive cost One of the ideal cycles is water splitting
to produce hydrogen Hydrogen can be made by following any
of the paths outlined above e.g thermochemical, photo-
chemical, electrochemical or in a pairwise hybrid process
The main features of the use of hydrogen as an energy
carrier, the ‘Hydrogen Energy’ concept, were first outlined by
Bockris and Triner8’ in 1970 and are shown in Figure 12.11
Hydroglen forms the intermediate link between the primary
energy sources and the energy-consuming sectors ’ It is also
independent of the primary energy sources used for its produc-
tion Even if these change, the intermediary energy systems of
transmission, storage and conversion can remain unaltered
The hydrogen economy system is completely cyclic Water
from lakes, rivers or oceans is converted into hydrogen and
oxygen Its combustion product is water vapour: which is
returned to the biosphere, and it is the least polluting of all the
synthetic fuels The well-known solar furnace at OdeilloX2 was
first used to produce hydrogen and oxygen in the early 1980s,
but the challenge is to make the process cost-effective, some-
thing which could come with the increasing emphasis on the
true costs of fossil-fuel-induced environmental pollution
Hydrogen can also be applied in the traditional reactions of
the chemical industry, or can be reacted with carbonaceous
material, such as carbon dioxide, carbonates, or biomass to
produce o’rganic fuels and chemicals ”
Energy input
water Conversion system
oxygen hydrogen production
water
Figure 12.11 The main features of the use of hydrogen as an energy
Another route for the thermochemical transport of solar energy is through the CO2 reforming of methane The endo- thermic reaction of C 0 2 with CH4 to yield CO and H2 was suggested over ten years ago.83 Methane and carbon dioxide are reacted in a solar receiver (reformer) at high temperature
to produce energy-rich products These can be stored at ambient temperatures and can be transported over distances
of some hundreds of kilometres to the consumer site Here the reverse reaction takes place in a thermal reactor (methan- ator), releasing the chemical energy.s4 The process could take place on one site, with the reformer-storage-methanator linked by pipework
12.6.5 Chromogenic materials
Chromogenic materials offer the possibility of developing advanced glazings which combine variable control of solar gain with efficient thermal insulation.” In a major review of the technical properties and merits of known electrochromic phenomena in 1984, Lamperts6 pointed out that most of the wealth of technical literature and patents dealing with elec- trochromic materials and devices was primarily for electronic information display or other small-scale applications Conse- quently, only minor attention had been paid to electrochromic devices as transmissive devices Since then, transparent aper- tures employing photochromic, thermochromic or elec- trochromic materials have been the focus of intensive world- wide research by many g r o ~ p s ~ ~ ~ ~ concerned with the efficient use of energy in buildings
The electrochromic window is the most advanced example
of these efforts It is basically a multilayer thin-film device which performs as an electric cell, and consists of an electro- chromic layer and a counter-electrode, or ion-storage layer, separated by an ion conductor For window applications these layers are commonly sandwiched between two transparent electronic conductors which are deposited onto transparent substrates (e.g glass or polymeric materials) In operation a d.c electric field is applied across the transparent conductors and ions are driven either into or out of the electrochromic layer, causing reflectance and/or absorptance modulation of visible and near-infrared electromagnetic radiation and hence changes in the optical properties of the device The elec- trochromic layer may be caused to colour or bleach in a reversible way under the influence of the external electric field The principal aim of current research is the development
of a stable, durable, all solid-state electrochromic device -the
‘smart window’ E7
Liquid-crystal-based chromogenic materials have also been successfully used as electrically activated devices s9 Two transparent electrodes provide an electric field to change the orientation of liquid-crystal molecules interspersed between the electrodes The orientation of the liquid crystals alters the optical properties of the device Two main types of liquid crystal systems, the guest-host and polymer-dispersed or encapsulated devices, have been identified for large areas Their disadvantages are that their unpowered state is diffuse, haze remains in the activated (transparent) state and ultravio-
let stability is poor A third approach uses suspended particle
devices: but various technical problems such as long-term stability and cyclic durability have slowed their development Two non-electrically activated devices use photochromic or thermochromic materials Their research histories date back
at least 100 years.87 When photochromic materials are ex- posed to light they change their optical properties, only reverting to their original properties in the dark Photochro- mic plastic has been developed for ophthalmic use and could become useful for regulating solar glazings E7 Thermochromic materials display a large optical property change when a
Trang 31211 8 Alternative energy sources
particular temperature is exceeded Above this critical temp-
erature, transmittance is reduced and if this temperature is
close to a comfort temperature thermochromism could be
used for automatic temperature control in buildings *'
12.6.6 Transparent insulation materials
A new technology is emerging from European initiatives
which will bring about revolutionary changes in the building
i n d ~ s t r y ' ~ Transparent (or translucent) insulation materials
are a relatively new class of materials which combine the uses
of glazing and insulation in the traditional design of any solar
thermal system While the primary use of glazing in buildings
has also been to allow light to enter, its ability to transmit
radiation gives it the subsidiary function of providing solar
heat
Insulation suppresses conduction and convection losses
from buildings, but the traditional opaque materials such as
polystyrene granules or foam which have been developed for
this purpose are equally effective in suppressing solar gain
from the outside of the building For large energy gains both
high irradiation levels and high values for the product of the
solar transmittance and absorptance of the absorber are
essential The influence of insulation (U-value) depends on
the temperature level of the system and the desired heat
storage period For low U-values very good absorption is
needed in the thermal wavelengths, although infrared select-
ive coatings can also be used on the front cover or the absorber
plate to reduce infrared radiation losses 91 Convection losses
can be greatly reduced by the use of structured materials such
as capillaries and honeycombs or low-pressure systems Until
recently no natural or man-made product could offer both
high-transmission, low-conduction and strong convection
suppressant characteristics, but by the mid-1960s it was
possible to conceive the potential benefits of such a material 92
Following rapid advances made in Germany during the 1980s
the term 'transparent insulation' was accepted as best describ-
ing the goal of the t e ~ h n o l o g y ~ ~
The first ex erimental results were presented in 1985 by
Wittwer et a,.' and the following four generic types which
displa different physical properties were first proposed in
Absorber-parallel Multiple glazing, plastic films
Absorber-perpendicular Honeycombs, capillaries
Cavity structure Duct plates, foam
(Quasi)-homogeneous Glass fibres, aerogels
By 1990 several systems had become commercially available
and considerable potential for more scientific research work
had been identified
1986: B
12.6.7 Solar detoxification
The ability of sunlight to detoxify waterborne chemicals is well
known through the cleansing of polluted streams as they flow
through areas open to direct solar radiation Solar detoxifica-
tion uses this natural process of degradation to produce
non-hazardous substances from hazardous organic chemicals
As conventional detoxification methods do not always deal
adequate1 with chemical wastes, the solar route is attracting
attention.'6 Among the advantages are that, by using the sun
as the energy source, there is no added airborne pollution or
use of conventional fossil fuels Solar detoxification breaks
down the hazardous chemicals into an environmentally benign
or easily treated end-product in one step However, conven-
tional processes often remove the wastes from the water and
take them elsewhere for treatment, thus increasing the possi- bility of further contamination
The detoxification of water is a photochemical process which destroys contaminants by the chemical action of light and a semiconductor catalyst 97 When exposed to sunlight, the
catalyst absorbs the high-energy photons and reactive chemi- cals, the hydroxyl radicals, are formed These radicals are powerful oxidizers and break down the contaminant mol- ecules, typically forming carbon dioxide, water and dilute mineral acids (e.g hydrochloric acid), which can be neutra- lized in a post-treatment process before the treated water is discharged
Among the common toxic chemicals which could be solar treated are trichloroethylene and other chlorinated solvents, 96 pesticides wood preservatives, dyes typically found in textile mill effluent and leakages or spills of various liquid fuels.96 Dioxins in PCBs can now be destroyed by solar energy and these laboratory techniques should reach the market by 1995.9x
12.6.8 Chemical heat storage
Thermal energy storage is essential for many solar thermal applications The pro erties of suitable salt hydrates were first
mixed with 3 4 % borax as a nucleating agent if complete crystallization is to be obtained, was the most-tried material, with a transition temperature close to 30°C The problem of a barrier being formed between the liquid and solid phases proved very difficult to solve and numerous polymeric stabi- lizers were tried Many different salt-phase change materials have been tried in the past two decades including calcium chloride hexahydrate and sodium acetate trihydrate, and modified varieties covering a range of transition temperatures from about 8°C to 58"C, several of which were commercially available in 1990.97 For high temperatures, molten nitrate salt receivers have been designed for the 10 MWe Solar One electricity generating pilot plant discussed in Section 12.4 discussed by Telkes' 8 in 1974 Sodium sulphate decahydrate,
12.6.9 Other applications
12.6.9.1 Surface transformation of materials
Highly concentrated solar energy, typically greater than 1 MW m-2, provides a controlled method for delivering large nux
densities of broadband radiation to solid surfaces, thus creat- ing the solar-induced surface transformation of materials Candidate technologies identified by Pitts et al lo' are shown
in Table 12.2
12.6.9.2 Thermochemical heat pump
A thermochemical heat-pump system, consisting of a fixed- focus parabolic solar collector, a stationary thermochemical metal hydride storage unit and various sub-systems has been investigated at the laboratory stage lo' It could provide a small, independent solar-powered home-energy centre for countries with good solar radiation conditions The basic principles are that sunlight is concentrated in the fixed-focus solar concentrator and heats the Stirling engine generator, producing electricity, and dehydrogenates the high- temperature magnesium hydride storage unit The freed hy- drogen is transferred to a second low-temperature hydride storage unit, and may be recycled to the high-temperature unit (hydrogenation) through a valve The chemical systems are as follows:
Storage: M g H + M g + H - 7 5 k J m o l - ' Release of stored heat: Mg + H + MgH + 75 kJ mol-'
Trang 4Hydropower 1211 9
Table 12.2
(1) Chemical vapour deposition
(2) Diffusion coatings
(3) Layered thin films
(4) Melted powdered coatings
( 5 ) Rapid thermal annealing
(6) Self-propagating high temperature
Electronics
As yet, a Stirling engine for remote, maintenance-free
applications has not been developed, but this remains one of
the main development goals of the project team.lo2 Main
parameters for an aperture area of 3 m2 are 4 kWh of
high-temperature energy (heat) for cooking + 3.4 kWh of
electricity + 3 kWh of heat for domestic hot water + 3 kWh
cooling energy for a refrigerator
12.7.1 Introduction
Hydroelectric power is the world's largest commercially avail-
able renewable energy source, accounting for about 6.7% of
the total primary energy consumption lo3 Water has been used
as an energy source for thousands of years, but the various
traditional designs of watermill used until the nineteenth
century could only lead to a technical dead end.'04 None of
them were capable of using a head of water much greater than
their own diameter Further progress followed with the deve-
lopment of the water turbine which was subsequently linked to
an electric generator Although credit for the world's first
hydroelectric plant is often attributed to the US plant which
started in the autumn of 1882 at Appleton, Wisconsin, two
plants were already operating in the UK at that time lo' The
earliest was Sir William Armstrong's small hydroelectric plant,
rated at just under 5 kW, which was constructed in 1880 to
light his picture gallery at Cragside, Northumberland, some
1.5 km away The first public supply of electricity was reported
from Surrey in 1881, when electric current generated from the
waters of the River Wey was used to light the streets of
Godalming The cables had to be laid in the gutters as there
was no legal authority to dig up the streets
The world's first large hydroelectric plant was built in 1895
at the Niagara Falls in the United States, with two turbines
each rated at 4100 kW.'06 The subsequent development of
alternating current by George Westinghouse in 1901 allowed
electric power to be transmitted over long distances lo' By
1903 Canada had a 9.3 MW plant, also at Niagara Falls, and
the era of modern hydropower had commenced The first
reliable survey of water turbines manufactured and installed
throughout the world in the late 1920s108 suggested that about
40% of the world's electricity was generated by hydropower,
with the United States and Canada having a combined oper-
ating potential capacity of over 13 000 MW and five other
countries (France, Japan, Norway, :Sweden and Switzerland)
with operating potential capacities greater than 1000 MW A
few of the earlier hydropower plants, known as run-of-the-
river plants, could not generate any power when the river was
low during the dry season but by the 1930s the use of large dams had been established in the United States The creation
of the Tennessee Valley Authority in 1933 with their compre- hensive approach to the planning and development of river basins set a pattern which has been widely followed in other countries lo' Since then there has been a steady growth in hydropower throughout the world although the percentage share of hydropower in meeting world electricity demand had fallen to about 25% by the early 1990s
During the 1980s the total output from North America and Europe remained unchanged but their share of the total world output dropped from just under 60% to 45% Among the developing countries, Brazil, Ghana, Mozambique, Zaire and Zambia obtained over 85% of their electricity from hydropower lo3.109
The potential for development of hydropower over the next
40 years is so great that it could provide an output equivalent
to the total electricity generated in the world from all sources
in the early 1980s Most of this potential is in the developing countries, some of whom could, in theory, increase their present use of hydropower by a factor of ten or more
12.7.2 The basic hydropower plant
The basic principles of hydroelectric power generation are shown in Figure 12.12 Water at a high level, often stored behind a dam, falls through a head z Its gravitational poten- tial energy is converted to kinetic energy and the flowing water drives a water turbine The rotating turbine shaft drives the electric generator to produce electricity
The theoretical maximum velocity is obtained by equating the gravitational and potential energies as follows:
gz = 112 v z
If the volumetric flowrate is Q (m3 s-l), density p (kg m-'),
then the power output in watts is given by
12.7.3 Types of turbine
Turbines can be classified according to the direction of the water flow through the blades, e.g radial, axial or combined- flow turbines, or as reaction, impulse or mixed-flow turbines
In reaction turbines there is a change of pressure across the turbine rotor, while impulse turbines use a high velocity jet impinging on hemispherical buckets to cause rotation There are three basic types of turbine broadly related to low, medium or high heads
Trang 512/20 Alternative energy sources
Turbo- generator \
j
Figure 12.12 The basic principles of hydroelectric power generation (after McVeigh’)
Propeller or axial flow turbines are used for low heads in the
range from 3 to 30 metres They can have relatively inexpens-
ive fixed blades, which have a high conversion efficiency at the
rated design conditions but a poorer par€-load efficiency,
typically 50%, at one third of full rated output Alternatively,
the more expensive Kaplan turbine has variable-pitch blades
which can be altered to give much better part-load efficiency,
perhaps 90% at one third of full rated output
The Francis turbine is a mixed-flow radial turbine and is
used for medium heads in the range from 5 to 400 m It has
broadly similar performance characteristics to the fixed-blade
propeller type and its speed is controlled by adjusting the
guide vane angle
The best-known impulse turbine is the Pelton wheel Each
bucket on the wheel has a centrally placed divider to deflect
half the flow to each side of the wheel It is normally used for
heads greater than 50 m and has good performance character-
istics over the whole range, very similar to the Kaplan turbine,
reaching 60% efficiency at one-tenth of full rated output The
speed is controlled by a variable inlet nozzle, so that with a
constant head, the delivered torque to the generator is propor-
tional to the flowrate and the turbine speed can be held at that
required for synchronous generation at the particular grid
frequency This type of installation is known as a constant-
speedkonstant-frequency system and optimization of the
power output is relatively easy I1O In smaller installations,
optimum power cannot be obtained at constant speed where
the hydraulic head is both relatively low and variable over a
wide range
A detailed description of methods which can be used for
optimizing electric power from small-scale plant has been
given by Levy.”’ He points out that small hydroelectric
systems will become more financially attractive through deve-
lopments of low-cost power converters (from 100 W upwards),
special variable-speedkonstant-frequency generators and
cheap computing units for on-line power measurement and
optimizing control This means that many run-of-the-river
sites that were considered in the past to be unsuitable for
electricity generation can now be used
12.7.4 Hydropower potential
The Earth’s energy flow diagram (Figure 12.1) shows that just over 4 x W flows in the hydrological cycle of evaporation, rain, other precipitation and storage in water and ice A very small proportion of this hydrological energy flow, probably between 0.01 and 0.015%0, is considered to be theoretically available for conversion into hydropower 109~111 This theor- etical world hydropower potential is calculated as the total energy potential of river discharges relative to a datum of sea level or the base level of erosion for closed basins and is widely quoted as 44.28 x 10” kWh per annum 109*111 How ever, this figure does not seem to include the 3.94 x lo1* kWh for the former USSR, which was separately listed by the 1980 World Energy C ~ n f e r e n c e , “ ~ and there is also some doubt as to whether the 6 X 10” kWh estimated for the People’s Republic
of China has been included.’” A better assessment is the
‘technically usable hydropower potential’, which allows for the unavailability of certain river reaches, mainly those near estuaries This is less than half the theoretical value The
‘economic potential’ includes all hydropower resources which are regarded as economic compared with alternative sources
of electric power at the time of the assessment These can be classified into three categories: operating, under construction and planned, as shown in Table 12.3 The economic operating hydropower potential of 372.1 GW represented just under 16% of the technically usable potential
The world operating potential of some 372.1 GW could, in theory, have provided 372.1 X lo9 X 365 X 24 watt-hours or 3.26 X 10’* kWh The actual energy generated was 1.65 X 10” kWh.7 This represents 50.6% of the potential, a typical figure for most hydroelectric plant Not only are there seasonal fluctuations in water availability, but the demand for electric- ity fluctuates and plants need to close for maintenance In the United States and Canada the figure of (actual energy gener- ated) divided by (theoretically available potential) was 47.7%
in 1979 This ratio is known as the load factor As the electrical
power from a hydroelectric plant can be used directly without the conversion losses and wasted heat associated with conven- tional fossil fuel power plant, the primary energy equivalent of
Trang 6Hydropower 12/21
Table 12.3 Hydropower potential (GW) [after reference 109)
Operating Under construction Planned
53.1 34.1 17.2 128.9 30.3 5.7 96.1 6.7
9.1 40.5 5.4 34.6 21.8 5.9 10.7 2.3
42.0 92.4 22.9 39.0 19.4' Unknown 22.5 3.6
Figures from Asia probably do not include data from the People's Republic of China."'
Figures may not include all small hydropower plant
Estimated
hydroelectricity is usually taken as about three times its actual
output
A common conversion is that 4000 kWh of 'electricity
generated' is considered to have the primary energy equiva-
lent of one tonne of oil '03
By 1980 there had been a steady growth in hydropower for
many years at about 3.5% per annum, representing a doubling
period every 20 years This figure was used by the 1980 World
Energy Conference to estimate that hydropower could be
quadrupled by 2020, reaching a total of over 1600 Mtoe A
more realistic figure was suggested by McVeigh' with a logistic
equation approach giving a growth rate of just under 3% By
the early 1990s, however, it could be seen that growth in the
decade of the 1980s had only averaged 2.0%, and that the
doubling period had stretched to about 35 years Reasons for
this could include the gradual reduction in performance of
some of the older hydro schemes due to silting, and the
changing patterns of rainfall, which, in turn, could be due to
global warming
12.7.5 Pumped storage
Pumped storage systems are used at times of peak demand for
electricity The water can be pumped to an upper storage
reservoir usually at night when the demand is low, and then
allowed to flow down through the turbines, generating elec-
tricity when it is required
Although small pumped storage schemes were first built in
the 1890s the first large system in the UK was built at
Ffestiniog, Wales, in 1963, with four 90 MW generators each
coupled to separate pumps and turbines on the same vertical
shaft This was soon followed by a 4 x 100 MW system at
Cruachan in Scotland
The largest pumped storage system in Europe was com-
pleted iin 1984 at Dinorwig, near Llanberis in North Wales
During its construction 3 million tonnes of rock were exca-
vated from the heart of the mountain between two reservoirs
and 16 km of shafts and tunnels were created."2 The upper
reservoir is 568 m above the underground power station and
the horizontal distance between the upper and lower reser-
voirs is 3200 m There are six turbogenerator units, each rated
at a nominal 300 MW It can generate at full output for about
5 hours The overall efficiency of any pumped storage system
is less than the 'once-through' conventional plant, as the pumping efficiency during the return flow to the upper reser- voir must be included This pumping efficiency, typically about go%, reduces the overall efficiency to about 70-75% However, the economics are quite different Pumping to the upper reservoir only occurs when the electricity tariffs are iow Electricity is supplied to meet peak demands when tariffs are usually at their highest Further, the use of a pumped storage system reduces the need for additional conventional plant which would only be needed for very short periods each year The pumped storage plant at Dinorwig can be generating electricity within 10 seconds of requirement
Other benefits, apart from the reduction in utilization of both high-cost oil-fired and low-efficiency coal-fired plant during the peak demand periods, include a reduction of both the extent and the duration of frequency excursions arising from large losses of generation output or the sudden increase
in consumer demand ex erienced at the end of many popular television programmes This is often up to 2000 MW in a few minutes Dinorwig has given the system the ability to pump prior to the impact of the television 'pick-up', thus
creating an artificial demand As the real demand increases,
the pumps can be reversed to generate within 90 seconds
12.7.6 Small-scale hydropower
One of the needs in many parts of the world is for electrical power in remote regions far from a conventional transmission system Small-scale hydropower is again becoming considered for an increasing number of these applications
Recently, the energy policies adopted by the different Member States of the European Economic Community to reduce their dependence on third countries as suppliers of energy, together with the technical improvements outlined above, have made it possible for small hydropower plants to become competitive in many parts of Europe 'I3
The early history of hydropower up to the 1930s was largely dominated by small plants, less than 1 MW in capacity, but then the economies of scale began to favour large-scale development Until fairly recently it was necessary to match the turbine design very carefully to the particular site This resulted in an expensive special 'one-off' hydropower gen- erator The smaller the application, the greatsr the installed
Trang 712/22 Alternative energy sources
cost per kilowatt of capacity The need for these specially
designed systems has been largely overcome by the use of
standardized turbines and associated equipment, with the
acceptance of some loss in overall plant efficiency and perfor-
mance
As outlined above, one of the major factors which could
favourably influence the economics of small-scale hydropower
is the development of microprocessor-based electronic load
governors These can overcome problems of instability in
matching waterflow to a variable demand and can also reduce
costs as expensive mechanical controls are no longer necess-
ary
Definitions of the size of any hydro scheme into Large,
Small, Mini and Micro appear in the literature, but there
seems to be no agreement on what these sizes represent, as
Table 12.4 shows
Bazaga'I3 points out that the definition of small hydropower
plant is 'not exactly the same in the different Member States of
the EEC' and bases his analyses on a power capacity less than
or equal to 10 MW Among the distinctive features of small
hydropower plants which he identifies are:
1 They are usually run-of-the-river and the energy produced
depends on the available flow
2 They rarely contaminate the environment and do not give
off heat
3 They can be built in a short period of time, with standard
equipment and well-known construction processes
4 Projects can be developed which combine electricity gen-
eration with other uses
5 The power source is reliable, within its hydrological limita-
tions The equipment and facilities involved have a long
life require little maintenance and seldom break down
6 The technology involved is well developed and overall
efficiency is over 80%
7 Operating systems are often automatic, leading to low
operation and maintenance costs
By the 1980s the country with the greatest experience in
small-scale hydropower development was the People's
Republic of China, where near1 100 000 plants have been
have a rated output of some 300 kW and much of their
projected increase in hydropower over the next 20 years will
also be small-scale
constructed in the past 20 years.' 7 ' Most of their recent plants
12.7.7 Economic, social and environmental issues
The costs and benefits of hydropower plant are usually
evaluated by an economic comparison with conventional
thermal or nuclear power stations The main factors which
must be considered, in addition to increases in construction
costs, are changes in the cost of fossil fuels and in environ-
mental protection regulations Although there has been a
steady growth in power station construction costs in all
countries over the past two decades, thermal and nuclear
power station costs have risen at a greater rate than those of
hydropower plants There are two reasons for this The technology and management of the construction of hydro- power plants has improved relative to conventional power station construction and new environmental protection and safety regulations have adverselv affected the cost of nuclear and coal-fired power stations ''' These new regulations have resulted in greatly increased expenditure for the control of air and water pollution with coal-fired stations, and for radiation monitoring and control together with improved safety stan- dards in nuclear installations Some of the adverse effects of hydropower schemes, such as the essential reinforcement of river banks or compensation for moving and resettling whole communities from flooded land, have always been included in the overall construction costs
An economic advantage when considering the later stages in any hydropower scheme is that dams with existing hydropower schemes can be raised to provide both additional storage capacity and a potentially increased output Turbine gen- erators can be added to some existing storage reservoirs to create new generating capacity
The economics of any hydropower system are absolutely site-specific, depending critically on the topology, geology and hydrology of the site.64 These factors influence the power capacity and developments costs, which, in turn, depend on what is required from the system (e.g a high or low load factor) or whether storage is required or not Hydropower, like tidal power is highly capital-intensive and can have a very long life, often over a hundred years for the basic civil engineering work With the low operation and maintenance costs, together with the other advantages outlined by Bazaga above, the main economic problems arise from the financial requirements of hi h interest rates and the demand for short fuel cost, and the UK Watt Committee also commented64 that
it is paradoxical that investment in hydro schemes looks extremely favourable in retrospect
Rivers and streams are regarded in the great majority of countries throughout the world as a public resource Their use
in potential hydropower schemes is subject to government control Hydropower development may be socially acceptable
to some sectors of the community and have quite disastrous effects on others For example, the construction of the Aswan High Dam in Egypt resulted in the destruction of the sardine fishing industry in the Eastern Mediterranean, but this was balanced by the development of a new fishing industry on the newly created Lake Nasser '07 There have been many studies
on the adverse impacts on health which can result from the large dams associated with hydropower projects107 and it would appear that there is still a need for major health- education programmes to be associated with these projects, so
that diseases such as bilharzia and malaria could be elimi- nated Other associated environmental problems include the need for extensive drainage systems on newly irrigated land and the threats to new dams caused by widespread deforesta- 'payback' periods PAgain, as with tidal power, there is a zero '
Table 12.4 Power output ranges
Source UK Watt Committee (1990)64 Hurst and Barnett (1990)'14 Bazaga (1 988) 'I3
Trang 8Wind power tion and soil erosion many kilometres upstream Some existing
aquatic and terrestial ecosystems have been disrupted and
there may have been a loss of visual amenities in scenic areas
On the other hand the United Nations Hydropower
Panel"' has also drawn attention to the positive effects of
hydropower reservoirs on the environment The creation of
regulating reservoirs has been shown to make a substantial
improvement in the water supply for domestic, industrial and
agricultural purposes in many cases The danger of catastro-
phic floods has often been eliminated The overall effects of
hydropower schemes throughout the world have been bene-
ficial, although there have been some largely unanticipated
adverse reactions with the environment These could either be
reduced or eliminated through careful resource planning
12.7.8 Summary
Hydropower is the only renewable energy resource with a fully
developed technological base and a relatively predictable
growth rate over the next few decades Its indlustrial infra-
structure is well established in many countries, and it provides
very subs,tantial proportions of the electricity demand in a
number of countries Although it accounted for only 6.7% of
the worldl's primary energy consumption in 1990, this figure
could easily rise to over 10% by the middle of the next
century It is particularly suitable for the needs of remote
communities in the developing countries
62.8 Wind power
12.8.1 Iintroduction
Energy from the wind is derived from solar energy, as a small
proportiomn of the total solar radiation reaching the Earth
causes movement in the atmosphere which appears as wind on
the Earth's surface.' The wind has been used as a source of
power for thousands of years and the traditional horizontal
axis tower mill for grinding corn, with sails supported by a
large tower; rather than a single post, had been developed by
the beginning of the fourteenth century in several parts of
Europe Its use continued to expand until the middle of the
nineteenth century, when the spread of the steam engine as an
alternative cheaper, source of power started its decline
Nevertheless, before the end of the nineteenth century several
countries used the windmill as one of their main sources of
power In the Netherlands"' there were about 10 000 wind-
mills giving power outputs of up to 50 kW In Denmark
housemills were often mounted on the roofs of barns and,
together with industrial mills, were estimated to be producing
about 200 MW from over 30 000 units.*16 In the United
States'lj an estimated 6 million small multi-bladed windmills
for water pumping were manufactured between 1850 and
1940
Work on the development of wind-generated electricity
started in Denmark in 1890 when Professor P La Cour
obtained substantial support from the Danish government,
which not only enabled him to erect a windmill at Ashov but
provided a fully instrumented wind tunnel and laboratory
Between 1890 and his death in 1908, Professor La Cour
developed a more efficient faster-running windwheel, incor-
porating a simplified means of speed control, and pioneered
the generation of electricity The Ashov windmill had four
blades 2;!.85 m in diameter mounted on a steel tower 24.38 m
high Power was transmitted through a bevel gearing, to a
vertical shaft which extended to a further set of bevels at
ground level, and the drive was connected to two 9 kW
generators - the first recorded instance of wind-generated electricity By 1910 several hundred windmills of up to 25 kW capacity were supplying villages with electricity The use of wind-generated electricity continued to increase in Denmark and a peak of 481 785 kWh was obtained from 88 windmills in January 1944
Large-scale modern windpower dates from the designs of an American engineer, Palmer C Putnam118 in the 1930s He
was responsible for the Smith-Putnam windmill which was erected at Grandpa's Knob in central Vermont in 1941 It had two blades with a diameter of 53.34 m and at that time it was the world's largest ever windmill, a record it was to hold for the next 35 years The synchronous electric generator and rotor blades were mounted on a 33.54 m tower and electricity was fed directly into the Central Vermont Public Service Corporation network The windmill was rated at 1.25 MW and worked well for about 18 months until a main bearing failed in the generator, a failure unconnected with the basic windmill design It proved impossible to replace the bearing for over two years because of the war and during this period the blades were fixed in position and exposed to the full force of the wind Also, in 1942, cracks had been noticed around some rivet holes, but these were considered to be so small that they could be ignored On 26 March 1945, less than a month after the bearing had been replaced, the cracks widened suddenly and a spar failed, causing one of the blades to fly off The S
Morgan Smith Company, who had undertaken the project, decided that they could not justify any further expenditure on
it, apart from a feasibility study on the installation of other units in Vermont This indicated that the capital cost per installed kilowatt would be some 60% greater than conven- tional systems
Although sceptics have tended to regard this experiment as
an expensive failure, it was the most significant advance in the history of windpower For the first time synchronous genera- tion of electricity had taken place and been delivered to a transmission grid Both mechanical failures were due to a lack
of knowledge of the mechanical properties of the materials at that time Bearing design and the problems of fatigue in metals have been studied extensively since then and similar failures are less likely to occur in modern windmills.'19 Their research programme included an extensive series of on-site measurements, which proved that the actual site at Grandpa's Knob had a mean wind velocity of only 70% of the original estimated velocity and that many other sites should have been selected The technical problems of converting wind energy into electricity had been largely overcome and the possibility
of developing wind power as a national energy resource in any country with an appropriate wind climate has been estab- lished However, very few wind turbines were to be built over the next 30 years
12.8.2 Wind-energy potential
Wind has a dependable annual statistical energy distribution but a complete analysis of how much energy is available from the wind in any particular location is rather complicated It depends, for example, on the shape of the local landscape, the height of the windmill above ground level and the climatic cycle Somewhat surprisingly, the British Isles have been studied more extensively than practically any other country in the world'20,'2' and the west coast of Ireland, together with some of the western islands of Scotland, have the best wind conditions with mean average wind speeds approaching 9 ms-I The kinetic energy of a moving air stream per unit mass
is iV2 and the mass flow rate through a given cross-sectional
Trang 912/24 Alternative energy sources
area A is pAV, where p is the density The theoretical power
available in the air stream is the product of these two terms:
IpA V3
If the area A is circular, typically traced by rotor blades of
diameter D , then a/4D2 = A , and the power available be-
where Cis the coefficient of performance or power coefficient
The maximum amount of energy which could be extracted
from a moving airstream was first shown by the German
engineer Betz, in 1927, to be 16/27 or 0.59259 of the theore-
tical available power This efficiency can only he approached
by careful blade design, with blade-tip speeds a factor of six
times the wind velocity, and is known as the Betz limit Modern
designs of windmills for electricity generation operate with
power coefficient values (C) of about 0.4, with the major
losses caused by drag on the blades and the swirl imported to
the air flow by the rotor.'22 Any aerogenerator will only
operate between a certain minimum wind velocity, the starting
velocity Vs, and its rated velocity VR Typically, VR/V/S lies
between 2 and 3 If the pitch of the blades can be altered at
velocities greater than V,, the system should continue to
operate at its rated output, the upper limit depending only on
the design In some systems the whole rotor is turned out of
the wind to avoid damage at high wind speeds An annual
velocity duration curve for a continuously generating windmill
is shown in Figure 12.13
The effect of the height of the windmill tower on the
performance can be significant and empirical power law
indices have been e ~ t a b l i s h e d ' ~ ~ relating the mean wind
velocity V to the height H , in the equation V = H a A value of
a = 0.17 is the accepted value in the UK for open, level
ground, but this rises to 0.25 for an urban site and 0.33 for a
city site An ideal site is a long, gently sloping hill
The mean annual wind velocity is normally used to describe
the wind regime at any particular location, but the output from
a windmill is proportional to V3 Since a transient arithmetic
increase in wind velocity will contribute much more energy to
the rotor than an equal arithmetic decrease will deduct, the
mean of V3, which is always much greater than the cube of the
mean annual wind velocity, should be used For example, if
the mean wind velocity is 8 ms-' the most common variation
Hwrs per arnum
Figure 12.13 Annual velocity duration curve for a continuously
generating windmill (after McVeigh')
in wind velocity occurs at frequent short intervals between 6
ms-' and 10 ms-' and 83 = 512, whereas 1(63 + lo3) = 608
A useful concept is the velocity exceeded for 50% of the year (4380 hours), shown in Figure 12.13 as Vs0 This is quite close
to the mean annual wind speed and has been used to give the annual extractable e n e g E , if the rotor shaft is attached to an electrical generator as
E , = 3.2289 D 2 Vjo3 kWh The Betz limit, outlined above, is purely theoretical, and in practice the power extraction efficiency will be reduced if
either: 1 2 j
1 The blades are so close together or rotating so rapidly that
a following blade moves into the turbulent air created by a preceding blade; or
2 The blades are so far apart or rotating so slowly that much
of the air passes through the cross section of the device without interfering with a blade
The rotational frequency of the wind turbine must be matched
to particular wind speeds to obtain the optimum efficiency The power extraction is therefore a function of the time taken
by a following blade to reach the position occupied by the preceding blade, and the time taken for the normal airflow to become re-established once the disturbed air has left that position This has resulted in a very important parameter - the tip speed ratio - defined as the speed of the turbine blade tip divided by the speed of the normal airstream, or oncoming wind A more detailed analysis can be found in the standard literature
For the great majority of wind-power applications, however, it is more important to know the probability that a minimum site wind velocity will be exceeded Long periods of
no wind or only light winds are obviously unacceptable Matching the wind turbine to the characteristics of any parti- cular site has needed the use of probability functions, the best known being the Weibull function
12.8.3 Small to medium-range windmills
Multi-bladed windmills for water pumping are still being manufactured in several countries and an estimated one million were in use in the early 1 9 8 0 ~ " ~ These windmills have
a high solidity, or area of blade relative to total swept area This gives a high starting torque but a relative low power coefficient, typically about 0.2 Wind energy was considered
to have a significant role in pumping water in the developing countries by the United Nations Technical Panel,'" hut they also identified three problems with existing designs: they were too complicated for local manufacture, too expensive and too difficult to maintain and repair Several new designs appeared
in the late 1970s and early 1980s These could be made locally and were relatively inexpensive, but a wider educational programme was still needed before the technology could be disseminated
Small low-solidity wind turbines for generating electricity in the range up to 10 kW are widely available in many countries Windmills in Sri Lanka, for example, locally developed in the early 1980s, could give an output of up to 400 W and would cost no more than $200 to build.126 Prototypes are used to charge locally manufactured lead-acid batteries which power low-energy consumption fluorescent tubes This provides an electric lighting system at about half the cost of conventional kerosene lamps
Isolated communities in good wind areas, especially in mountain regions, on islands or in coastal areas, can meet their power needs in the 10-1000 kW range by a combination
of wind power and a suitable back-up system
Trang 10Wind power 12/25
By the mid-1980s the combination of wind and diesel
generators was attracting very considerable international res-
earch and development activity The results of much of this
work were summarized by Lipman in 1990, 127 who pointed out
that a wind power system may be fully meeting an autonomous
load at one moment and be in considerable power deficit a few
seconds later Strategies which were being tried included
various types of load control, both long- and short-term
energy storage, hybrid systems using flywheels and multiple
diesels'27 or a pumped hydroelectric system
Among ?he smaller UK companies the Northumbrian Ener-
gy Workshop (NEW) have helped the government of the
Seychelles with wind-resource assessment using data loggers
they have installed under a United Nations Development
Programme (UNDP) NEW is also continuing to support a
UNDP project in the very different climatic conditions of
Mongolia for which they supplied 27 Marlec WG910 50 W
windchargers and four Dyna Technology 200 W windchargers
NEW, together with the National Centre for Alternative
Technology in Wales, and Marlec have also supplied some
small solar-photovoltaic-wind hybrid systems for projects in
Tanzania and Kenya
Most of the Marlec WG510 windchargers are exported to
remote parts of both developing and developed countries A
particularly interesting user is the 'Footsteps of Scott Expedi-
tion', which reported using their Marlec aerogenerator at
temperatures down to -40°C and windspeeds exceeding force
12 and averaging 40 mph over 12 h They re orted 'faultless'
operation under these extreme conditions 128129
12.8.4 The vertical-axis windmill
The mo'dern vertical-axis windmill is a synthesis of two earlier
inventions These are the Darrieus13' windmill with blades of
symmetrical aerofoil cross section bowed outward at their
mid-poiint to form a catenary curve and attached at each end to
a vertical rotational axis perpendicular to the wind direction
and the S a v o n i ~ s ' ~ ' windmill or S-rotor, in which the two arcs
of the 'S' are separated and overlap, allowing air to flow
through the passage The Darrieus windmill is the primary
power-producing device, but, like other fixed-pitch high-
performance systems, is not self-starting The blades rotate as
a result of the high lift from the aerofoil sections, the S-rotor
being used primarily to start the action of the Darrieus blades
The wind-energy conversion efficiency of the Darrieus rotor is
approxiimately the same as any good horizontal system'32 but
its potential advantages are claimed to be lower fabrication
costs and functional simplicity 133 In 1981 the largest Ameri-
can Darrieus machine, with three blades, had developed 500
kW A 4 MW machine jointly funded by the Canadian
National Research Council and the Institut de Recherche
d'Energie du Quebec was completed on a site in the St
Lawrence river valley, Quebec, in 1985 An earlier feasibility
study concluded that Darrieus machines up to 8 MW in size
could be built
In the UK, an analysis of the Darrieus rotor suggested to
Musgrove'34 that straight-bladed H-shaped rotors with the
central horizontal shaft supporting two hinged vertical blades,
could tie a more effective system A variety of designs based
on Musgrove's work in the UK during the 1970s and early
1980s have been studied and a small, 6 m diameter, three-
bladed version was commercially available by 1980
This work was followed by a 25 m diameter 130 kW machine
at Carmarthen Bay, which started a test and monitoring
programme in November 1986 Full details of the develop-
ment of this design are a ~ a i 1 a b l e I ~ ~ Following the highly
successful trials, a larger version, known as VAWT 850, was
inaugurated in August 1940 The '850' refers to the swept
area of the blades Its rated capacity is 500 kW, with a cut-in windspeed of 6 m s-' and a shutdown windspeed of
23 m s-1.136 Musgrove also considered the possibility of siting groups or
clusters of windmills in shallow offshore locations in the UK
such as the Wash Two advantages of this proposal are the higher mean windspeeds and the greatly reduced environ- mental objections
12.8.5 The development of large horizontal-axis wind turbines and some national programmes
Details of the largest horizontal-axis wind turbines built or planned in Europe during the period from the late 1970s to the mid-1980s showed that four countries Denmark, Germany, Sweden and the UK, had major programmes.' In 1979: the Danish machine at Twind, rated at 2 MW with a blade diameter (three blades) of 54 111, became the largest in the world since the Smith-Putnam machine 137 This was a private venture and it never achieved the full rated power
The official Danish programme for large electricity produc- ing wind energy systems started in 1977 with a joint pro- gramme directed by the Energy Ministry and the Electricity Utilities Their major project was the design, construcrion and operation of two machines, Nibe A and B, which were erected
in 1979 These turbines are sited close to each other and are identical, apart from their rotor blades Those for the A machine are supported by stays while the blades of the B
machine are self-supporting
Construction of a 2 MW wind turbine near Esbjerg, in Western Jutland, was completed in 1988, with grants from the
EC.138 The main parameters were a blade diameter (three blades) of 61 m; a hub height of 60 m and a rated windspeed of
15 m s-' The estimated annual output was 3.5 GWh y-' and the estimated capacity of Danish windfarms was approaching
100 MW at the same time 139 The German wind programme, known as the Growian programme, had some 25 projects in operation during the early 1980s, ranging from some small, low-cost units rated at
15 kW for production in developing countries and a medium- sized 25 m diameter twin-bladed 265 kW machine, the Voith- Hutter commissioned in 1981, to the large Growian 1 machine, rated at 3 MW, with the world's largest blade diameter of 100
m The rated capacity of the German Research, Development and Demonstration programme was 8 MW towards the end of the 1980s
The main feature of the Swedish programme was relared to the design, construction and operation of two large-scale prototypes, located at Maglarp in the province of Skane in southern Sweden, and Nasudden on the island of Gotland These projects, with rated capacities of 3 MW and 2 MW, respectively, formed the main basis of Swedish work during the decade
In the United States the first major project in the official wind energy programme was the ERDA Model Zero (MQD- 0) 100 kW windmill which consisted of a two-bladed, 38.10 m diameter, variable-pitch propeller system driving a synchro- nous alternator through a gearbox, mounted on a 30.48 m high steel tower 140 The blades were located downstream from the tower and a powered gear-control system replaced the tradi- tional tail fin of earlier designs This initial test programme was designed to establish a database concerning the fabrica- tion, performance, operating and economic characteristics of propeller-type wind turbine systems for providing electrical power into an existing power grid
The next in the series, the MOD-1 windmill, became the world's largest machine in May 1979, when it was commis- sioned This was also a twin-bladed downwind horizontal axis
Trang 1112/26 Alternative energy sources
machine with a blade diameter of 60.96 m and rated at 2 MW
Problems of interference with television signals were over-
come but a low-level, low-frequency noise could only be
reduced by lowering the speed of rotation and output This
resulted in design changes in the later machines in the series
The MOD-1 machine was dismantled in 1983.'"
Subsequent machines in the programme were planned to
reach the MOD-5, rated at 7.3 MW with a blade diameter of
122 m, but the overall economics of windpower meant that
efforts concentrated on designs in the 300-500 kW range AS
early as 1983 a spokesman for the General Electric Company
said that the future market for large wind turbines was very
doubtful as forecasts of electricity load growths were lower
than expected and subsidies for the use of renewable energy
systems in the United States were planned to end.'''
The UK programme could be regarded as dating from the
early 1950s when two 100 kW machines were built, the John
Brown machine which was erected in the Orkneys, and the
Enfield-Andreau machine, which was eventually built in Al-
geria in 1957,'O A wind database was also established in the
1950s by the Electrical Research Association Preliminary
work with a design feasibility and cost study of large wind
turbine generators suitable for network connection was car-
ried out in 1976 and 1977 by a group comprising British
Aerospace Dynamics Group, Cleveland Bridge and Engin-
eering Co Ltd Electrical Research Association Ltd, North of
Scotland Hydro-Electric Board, South of Scotland Electricity
Board and Taylor Woodrow Construction Ltd A reference
design was evolved for a 60 m diameter turbine in 1977 This
became the WEG (Wind Energy Group) 3 MW design for the
machine which was eventually inaugurated at Burgar Hill,
Orkney, in November 1987 The main design features of the
twin-bladed horizontal axis machine included a rated output of
3 MW at 17 m s-' a blade diameter of 60 m and a hub height
of 46 m A smaller prototype, a 20-m diameter 250 kW
machine, was commissioned in the summer of 1983
A 1 MW wind turbine at Richborough, Kent, began gen-
erating at the end of 1989 It is also a project in the European
Commission's large wind-energy machines programme The
site was selected as typical of an average mainland UK
location, and has a mean annual windspeed of 6.8 m s-l at a
hub height of 45 m, compared with the 10.5 m s-' at Burgar
Hill It has three blades, each 26.5 m long and is an 'extended'
version of the James Howden 300-750 kW horizontal-axis
range
In parallel with the development of the range of prototype
machines outlined above, plans for the first commercial wind-
farm in the UK were being finalized at Delabole, Cornwall, in
1991 145
During 1985 the National Engineering Laboratory (NEL) at
East Kilbride, Glasgow, established a National Wind Turbine
Centre (NWTC) at Myres Hill some 8 km to the south-west of
the NEL, near the village of Eaglesham The 7-ha site is on a
moorland ridge 350 m above sea level with an open outlook
and clear view of the Irish coast.lZ9 It has three 'universal'
concrete foundation pads, each complete with a monitoring
hut and capable of taking a wide range of machines of both the
conventional (horizontal-axis) type and vertical-axis
machines A further test pad of the same design is located at
the NEL A high mast fitted with a wide range of anemometry
and other metereological instrumentation will provide infor-
mation of the wind profile and climate over the 50 m height
1 Providing independent accreditation of machine perfor-
2 Supplying engineering and technical expertise to improve
The NWTC has been created to assist companies by:
mance and quality;
the design and cost-effectiveness of machines;
3 Seeking to promote reliability through assuring high stan- The main data system collects signals at a central building at Myres Hill, and transmits these data by microwave link to the NEL Power Systems Engineering Division for analysis in a real-time facility The microwave link allows two-way commu- nication and some control activities may be undertaken remotely in the future "' Facilities for loading the different types of machine are as follows:
1 Machines with induction generators will supply electricity
to the National Grid;
2 Machines with synchronous generators will be connected to individual resistive load units with programmable con- trollers;
3 Water pumping and direct heat production machines will utilize a 70 000-litre water reservoir on the test site
A major review of renewable energy in the UK carried out in 198g3' concluded that the onshore technical potential
of windpower was 45 TWh yr-', and that, in principle, some
30 TWh yr-' could be provided by the year 2025 Making the assumption that a broadly similar ratio of installed capacity to annual output experienced in California"' could be applied to these projected UK figures, the equivalent UK installed capacity in 2025 would be about 20 GW This methodology based on California data, but remembering that several UK systems have been installed in California, was probably used
as the basis for the 1991 ETSU report to the CEC.'46 which gives a potential of 17 GW for land-based windpower in the
UK This is quoted as being 'technically available' in 1990, with a further 12 GW for offshore wind by the year 2000 There now appears to be no technical barrier to this potential For example, the Watt Committee6' considered that the main strategic reason for introducing wind power or any other renewable energy source into the UK system would be
to increase the security of the system by adding to the diversity
of the plant At least 20% of the system peak load could be accepted from variable supply sources, such as windpower, without significant cost penalties on the operation of existing plant
dards of materials and quality of manufacture
12.8.6 Some environmental issues
The most important effect of wind energy is that it is relatively non-polluting during its working lifetime when it has a zero
fossil fuel input A life-cycle analysis would show that very
minor pollutants were emitted during the construction and site works period
Several negative effects are often quoted in the litera- ture.'@ The first is the visual impact, but visual impact is difficult to quantify and depends on subjective judgements The blades could also cause a rotating shadow pattern which might present visual problems A major area of concern has been the danger of birds colliding with the blades, but this was considered to be a negligible hazard 14'
Electromagnetic interference can be caused by a windmill and the considerable amount of research suggests that where significant TV interference appears possible, remedial action should be taken before the problem arises The area around the Burgar Hill site for the WEG 3 MW machine on mainland Orkney had poor reception prior to 1983 A new repeater station was installed so that better signal reception was established before the wind turbines were operating Noise is also a problem, particularly at low frequencies, and the amount of data is sparse 14' A large amount of low-frequency noise appears to be common to all machines and the only solution is to site them sufficiently far away from objectors
Trang 12Geothermal energy 12/27
In the nineteenth century it was believed that the residual heat in the centre of the Earth was the source of the natural geothermal phenomena such as hot springs with jets of steam that could be seen on the Earth’s surface.’48 It is now widely accepted that there are two heat sources The first arises from radioactive decay and the geological evidence points strongly
to potassium, uranium and thorium contained in the rocks that form the Earth’s crust.149 The second comes from the mantle which lies below the crust and which may also contain small concentrations of radioactive elements The crust is some 30-35 km thick below the continental land masses and the boundary between the crust and the mantle is known as the Mohorovicic seismic discontinuity, or Moho 14R,149 According
to plate tectonic theory the crust is not a solid shell but consists
of rigid segments or plates which can move relative to each other over the mantle Pressure builds up at plate boundaries and the resulting sudden movement results in earthquakes and promotes the movement of large masses of molten rock or magma upwards into the Earth’s crust, causing volcanic activ- ity The thermal effects of the interaction between plates can extend several hundred kilometres from the boundaries Major plate boundaries are well known and indicate the areas where exploitation of Earth’s heat would be most likely to be successful
Measurements of temperatures taken in mines and bore- holes penetrating into the crust show that, with the exception
of a very shallow zone near the Earth’s surface, the tempera- ture rises as the depth increases The rate of increase, or thermal gradient, is between 20 and 30 K per kilometre for non-volcanic regions, with a smaller increase for older and a much higher increase near magma penetration Most of the heat which reaches the Earth’s surface does so by conduc- tion, but some is transferred by convection to the free water in the outer few kilometres of the crust This can occur by the simple process of groundwater sinking through permeable rock such as the sandstones or limestones, or where the rock has been fractured, into the hotter regions and then circulating back towards the surface, or by the heating of the groundwater
by igneous activity 149 Temperatures of these hydrothermal fluids in the range from 100” to 200°C are common and in places have reached 400”C, conditions which can result in some of the water flashing (changing state from liquid to vapour with rapid reduction in pressure) into steam and appearing as hot springs or geysers The average value for terrestrial heat flow on the continental land masses is about 0.06 wm-2,150,151 A slightly higher value 0.063 W I I - ~ , is also widely quoted in the literature, 152,153 but in the exploited geothermal fields the heat flow carried to the surface by the fluids can be from 200 to 1700 times this value 153
This could severely limit potentiai wind sites Safety is an
obvious problem and blades have been known to become
detached and fly off Some towers have suffered structural
failure in high winds
12.8.7 Summary
By the end of the 1980s approximately 100 manufacturers had
been identified who had supplied well over 100 000 wind
turbines and pumps of various sizes throughout the world 139
Of these, some 20 000 were connected to the grid with a total
installed capacity of over 2000 MW The largest single group
of installations is in California, where they are situated mainiy
in the Altamont Pass region, some 60 miles east of San
Francisco near Livermore, the Tehachapi region, about 100
miles from Los Angeles near Mojave and the San Gorgonio
Pass region, a ain about 100 miles from Los Angeles and near
Palm Springs!” In Europe the installed capacity in 1991 was
as shown in Table 12.5
Of the 100 manufacturers identified above, only about 30
were regarded as ‘well established’ by the European Wind
Energy Association ‘All but a handful’ were based within the
European Community and a series of goals for the exploita-
tion of Europe’s wind energy was also established in 1991147 as
The three leading countries (Table 12.5) had plans to
enhance their installed capacities during the 1990s Both
Denmark and the Netherlands were aiming at 1000 MW each,
with 250 MW in the Netherlands by 1995 Germany was
planning to install at least 100 MW over a five-year period 14’
Italy’s projections were to provide an additional 300 MW to
600 MW over the decade
Table 12.5 Installed European capacity, 1991 (in MW)”’
‘I 2.9 Geothermal energy
Geothermal energy is thermal energy stored in the Earth
Although the Earth’s heat can be regarded as an infinite
source of energy prolonged exploitation can exhaust a geo-
thermal field Geothermal energy is, therefore, not strictly a
renewable source of energy compared with, for example, solar
energy or hydro power
12.9.1 Geothermal resources
Compared with the proved reserves and ultimate resources for the fossil fuels, which are published at regular intervals, estimates for both national and world geothermal heat re- sources must be regarded with very considerable caution Armstead1j4 points out that it is of more immediate interest to have an approximate idea of the amount of geothermal energy that could be obtained under existing economic and operating conditions, but that any attempt to estimate this must be highly speculative Many regions in the world have had no geothermal exploration and it is very difficult to place any confidence in much of the published work However, as a starting point the World Energy Conference’” produced an assessment which is given in Table 12.6
In the early 1990s total world primary eilergy consumption was approximately 3.6 X 10” J and the primary energy equivalent of electricity generated was about a quarter of this
Trang 1312/28 Alternative energy sources
Table 12.6 Estimates of geothermal resources for electricity
generation (IOzo J)
Resource base, taking into consideration
the continental land masses to a depth
of 3 km and a datum of 15°C
to be of adequate temperature for electricity
generation
Assume that the overall recovery and
conversion efficiency is about 2.2%
One fifth is convertible by existing technology
410 000
Of this resource base only 2% is assumed 8 200
180
36 Data derived from references 154 and 155
figure Table 12.6 suggests that the convertible geothermal
resource is some forty times greater than the world annual
production of electricity This figure may be of the right order,
but cannot be regarded with confidence
The World Energy Conference also gave a figure of 2.9 X
10'' J for the estimated recoverable thermal energy which was
theoretically available for direct applications at lower temp-
eratures Again, this figure must be fairly speculative as it
amounts to over 7% of the total estimated resource base for
electricity generation When estimates of geothermal re-
sources are made for individual countries the same reserva-
tions must be applied The wide range of estimates which can
be given emphasize the need for caution For example, a series
of estimates for Japan ranged from 40 000 MWe for the next
thousand years, representing some 35% of the total world
potential, to 8650 MWe lS4
12.9.2 Geothermal areas, fields and aquifers
The surface of the Earth can be classified into three main areas
as follows: Is'
1 Non-thermal areas with temperature gradients between 10
and 40 K per kilometre of depth;
2 Semi-thermal areas with temperature gradients approach-
ing 80 K per kilometre of depth;
3 Hyperthermal areas with considerably larger temperature
gradients
An important distinction must be made between a geo-
thermal area and a geothermal field Many thermal areas are
associated with rock of low or zero permeability and cannot be
exploited under existing economic and operating conditions
Geothermal fields contain the hot water or steam in perm-
eable rock formations and a number of these are operating
commercially They can also be classified into three main
types: 15'
1 Semi-thermal fields which can produce hot water at temp-
eratures up to 100°C from depths up to 2 km;
2 Wetfields which produce water under pressure at tempera-
tures greater than 100°C When this water reaches the
surface, its pressure falls and some flashes into steam, the
remainder being boiling water at atmospheric pressure;
3 Dry fields which produce dry saturated or superheated
steam at pressures above atmospheric pressure
Another source of useful hot water is the low-grade aquifer,
which can produce water up to a temperature of about 75°C by
drilling to depths of between 1.5 and 2 km, corresponding to a
temperature gradient of 30 K per kilometre Low-grade aquifers can be found in non-thermal areas but are only worth exploiting if they are located fairly close to an appropriate application, such as space heating in a town or city
12.9.3 Thermal applications
The earliest application of geothermal energy was the use of natural hot springs for bathing or medical purposes One hot spring near Xian, the capital city of ancient China, has been used for over a thousand years and still attracts many bathers The history of the use of geothermal energy for industrial applications probably started in the Larderello area of Tus-
cany, in central Italy, in 1827 Thermal energy from the hot wells was used in the crystallization of boric acid, which was also obtained from the natural pools formed from condensed steam and rainwater*53 and a flourishing chemical industry developed there over the next hundred years
In Europe, one of the richest geothermal sources is in Hungary, where geothermal baths have been used for hundreds of years Geothermal heating was first introduced in the 1960s and is used in greenhouses covering a total area of 2 million m2 Uses in other agricultural applications include corn drying and poultry farms
An increasing number of applications for space heating have also been reported since the 1960s Examples of two different types are the semi-thermal fields in Iceland and the low-grade aquifer found in the Paris basin In Iceland the Reykjavik Municipal District Heating Services were able to sell hot water
at less than one fifth of the cost of heating with oil Is' The capital cost of the geothermal plant installed in the Paris basin per housing unit was US$ 2000 (at 1980 prices) which was said to be comparable with any conventional system 15' District heating using geothermal energy was first introduced in Hungary in 198.5, and by 1990 over 6000
dwellings in six cities were being su plied during a heating
in the UK is in the Wessex basin The first borehole at Marchwood in 1979/1980 produced a flow of 30 1 s-l at 72°C
from a depth of 1700 m This was originally intended to pre-heat feedwater for the power station but this was closed before the project could be taken further 158 The second was the Southampton borehole The pumping rate was too low for
an extensive heating scheme, but a later, smaller, scheme proposed by the Southampton City Council in 1987 now provides about one MW of geothermal heat as part of a larger
12 MW system The maximum pumping rate is limited to 12 1 s-' to ensure an operating life of 20 years.'j8 A very small
warm water scheme (ca 22°C) is operating from a 250 m
experimental borehole in Cornwall for a horticultural applica- tion.(j4
The main features of a typical geothermal district heating system are shown in Figure 12.14, based on information from reference 1.59 The first borehole establishes the chararacter- istics of the aquifer and then becomes the production well, out
of which the hot water is pumped The second borehole is the reinjection well, which is used to dispose of the saline water after the heat has been extracted in the heat exchanger This well is approximately one kilometre from the production well
to delay the return of cold water for 25-30 years The auxiliary boiler can provide additional heat at periods of high demand and the whole system is connected to the housing units and buildings by a pipeline system This must be no further than one kilometre from the geothermal wells, both for reasons of cost and prevention of heat loss Among other thermal applications widely quoted in the literature are greenhouse heating including soil warming, drying of organic products, salt extraction and industrial process heating
period which lasts about six months ! The main development
Trang 14Geothermal energy 12/29
Figure 12.14 A typical geothermal district heating system (after
McVeigh')
12.9.4 Electricity generation
The history of electricity generation also started in the
Lardereilo area, when a simple steam engine coupled to a d.c
generator was driven by steam from the geothermal field This
provided some electric lighting for the town of Larderel-
The steam engine was replaced by a 250 kW turbo-
alternator in 1913 Until 1958 Italy was the only country where
natural steam was used for power generation on an industrial
scale Production commenced in New Zealand in that year,
followed by the Geysers field in the United States in 1960,
when the total installed capacity in the world was 369 MW 'jO
Developiment over the next decade to 697 MW by 1970
represented an annual growth rate of 6.5%, but the 1970s saw
a consid.erable increase in growth Table 12.7 shows the
installed and projected geothermal electrical generating capac-
ity for 1980, 1987, 1989, 1990 and 2000, taken from data
presented at the United Nations Conference on New and
Renewable Sources of Energy in 1981'57 and from references
64 and 160 in 1990
Data published by Shaw and Robinson'", also in 1981, put
installed capacity in 1980 as 2082 MW This gives a growth rate
of 11.6% per annum during the 1970s By examining the
known orders for new plant they concluded that a realistic
assessment of installed capacity in 1990 would be between
3786 MVIi and 5645 MW, less than half the figure suggested by
the United Nations Conference Their projected figures of
" UN Watt Committee, derived installed figure."
li UN projected figures (These UN projections were qualified by the
comment that they were 'minimum' figures.)
Dickson and Fanelli installed figure.IM'
3786 MW and 5645 MW for 1990 would represent average annual growth rates over the decade of 6.1% and 10.5%,
respectively McVeigh's comment' in 1984 that the Shaw and Robinson projection seemed reasonable compared with the nearly 20% per annum growth rate to achieve the figures suggested by the UN Conference was fully justified with the publication of the 1989 data
The risks and problems associated with geothermal projects are not unlike those in searching for oil The success rate of geothermal drilling, when measured by the proportion of wells which strike exploitable hot water or steam, is probably greater than that of oil drilling However, the rewards are much smaller and there are risks, as the first UK experience outlined above indicates
12.9.5 Hot dry rocks
Geophysicists have suggested that rock at temperatures of 200°C can be found at drillable depths, less than 10 km, over large regions of the Earth's surface.'5" This has resulted in a number of major research projects in which deep holes are drilled into these hot rocks and a system of cracks is propa- gated between them 150.154,157,159 M ost of the research is aimed
at establishing an optimum method for generating these fracture patterns The basic technique uses hydraulic fractur- ing, and the first successful tests were carried out at Los Alamos in the United States during the early 1 9 7 0 ~ ' ~ ~ A fracture system some 600 m in diameter was created between wells 3 km deep and up to 4.5 MW was removed as heat during the initial test period of 2000 hours This showed that the concept was valid In the UK the Camborne School of Mines have extended the work in the United States by a more sophisticated approach to fracturing They initiate the fracture system by explosives and then follow up with hydraulic fracturing
Formal reviews of progress were undertaken in 1984; 1987 and 1990.16' By 1984, two 2-km deep wells had been drilled, but the reservoir, or heat exchanger, between the two wells
had relatively poor hydraulic properties A third well was
drilled and the second review concluded that while consider- able progress had been made, there were several specific problems The main one was still with the reservoir This was a hundred times smaller than the size calculated to be necessary
for a commercial reservoir and a reliable reservoir design process had not been validated Three more years of experi- ments and studies still revealed that a satisfactory procedure for creating a commercial-scale reservoir had not been demonstrated There was no reliable information about the properties of the rock likely to be encountered at the 6 7 km depths necessary for commercial exploitation In a technical analysis of the work up to 1990: Parker162 stated that until holes have been drilled to these depths the uncertainty will never be removed Hot dry rock projects were unlikely to attract any private sector income in the short term.16' Never- theless, the potentially exploitable granites in south-west England alone contain the equivalent of 8000 million tonnes of coal
12.9.6 Some factors influencing developments
The economics of the applications of geothermal energy depend on the costs of competitive fuels Where there are active geothermal fields and scarce indigenous resources, such
as in Iceland or Hungary, geothermal power is already the economic choice Financial constraints and the lack of a suitable technical infrastructure can inhibit development in some of the poorer developing countries who would appear to have considerable hyperthermal field potential
Trang 1512/30 Alternative energy sources
Several other possibilities for using geothermal energy have
been discussed, includin the direct exploitation of the heat
from active volcanoes 14'.154,157 This would have very consi-
derable practical difficulties as it would involve tapping the
magma at a depth of several thousand metres below the
volcano, a technology which has not yet been developed
Among the ideas put forward for exploiting this source,
Armstead'j4 has suggested injecting water into the hot basaltic
magma to produce hydrogen by dissociation
The possible environmental problems which can arise from
geothermal exploitation have been identified'54.'5g and can
include:
1 The use of land for initial drilling operations and possible
noise and damage;
2 The long-term visual impact and use of land for the power
5 The physical effects on the geological structure of the area
The earliest geothermal operations were carried out at a
time when environmental issues were not taken into consid-
eration These early steam plants were reported to have
unsightly tangles of steam-transmission pipes, clouds of waste
steam accompanied by a strong smell of hydrogen sulphide
and, eventually, significant surface subsidence However, in
recent years these adverse effects have been minimized For
example, air pollution standards at the world's largest field,
the Geysers in the United States, have resulted in 'cleaner' air
than before the field was exploited For the low-temperature
aquifer systems the environmental impact should be negli-
gible.'59 The main problem is the safe disposal of the warm
chemically laden water after it has passed through the heat
exchangers It has become normal practice to reinject the
brine back into the other end of the aquifer as shown in Figure
12.14
12.9.7 Summary
Unlike the other alternative energy sources, geothermal ener-
gy is capable of providing continuous heat and power With
electricity generation, the plant is particularly suitable for
base-load operation The use of low-grade aquifers can pro-
vide space heating at costs comparable with or below conven-
tional systems However, the long term future is still quite
unpredictable Several authorities believe that heat mining,
the exploitation of hot dry rocks, could become a commercial
reality within the next two decades 153.154 Should this happen,
a major new energy resource, comparable in size to the
ultimate oil and gas resources, would be available, but the UK
experience at the Camborne School of Mines serves as a
warning against over-optimism
12.10 Tidal power
Earth during its path round the sun.164 The difference in length between the 24-hour solar day and the 24.813-hour tidal day causes the spring and neap tides When the sun and moon are almost in line with the Earth the tides have their maximum amplitude and are known as the spring tides When the moon-Earth-sun angle is a right angle the tides have their minimum amplitude and are known as the neap tides The ratio between the greatest spring tide and the smallest neap tide can be up to 3:1 '** The overall effect of tidal forces is surprisingly small In the open ocean the tidal range, defined
as the difference in amplitude between low and high tides, is typically about 1 metre.I2' Over the continental shelves the tidal range increases to about 2 metres and in some estuaries
or deep narrow bays it can be up to 16 metres
These increased tidal ranges in estuaries or bays are caused
by the interaction of two types of wave The first is the tidal wave advancing from the open sea and the second is the reflected waves from the sides of the estuary.I6j These two waves can reinforce each other at certain times, depending on the shape of the estuary and the period of tide, causing an amplification Peak amplification occurs at resonance Theo- retically, a channel of uniform cross section would be resonant
if its length were equal to one-quarter of the wavelength of the tidal movement In practice, this length is modified by actual variations in depth and width The rise and fall of the tide is also limited by the frictional losses caused by the action of the water over the sea bed
Scientific publications on tidal schemes date from the early eighteenth century and various designs for dams and asso- ciated turbines appeared from the end of nineteenth cen- tury Modern proposals for exploiting tidal power are based
on the use of the stored potential energy in a dam 164 The use
of the kinetic energy of the tidal current has been limited to a few very small-scale developments
12.10.2 Tidal power principles
Tidal power can be obtained from the flow of water caused by the rise and fall of the tides in partially enclosed coastal basins This energy can be converted into potential energy by enclos- ing the basins with dams This creates a difference in water level between the ocean and the basin The resulting flow of water as the basin is filling or emptying can be used to drive turbo-generators Electricity conversion removes the geogra- phical restrictions placed on the earlier uses of tidal energy.'65 The potential energy of a body of mass, m, at a height, z ,
above the datum line is mgz If the surface area of a tidal basin
is A m2 and the mean tidal range is Y m, then the maximum potential energy available during the emptying or filling of the basin is given by
The tide rises and falls twice during the tidal day of 24.814 hours, so the theoretical average power is four times the maximum potential energy divided by the total time in the tidal day or
4 X :pgAu2
12.10.1 Introduction
Tides are caused by the interaction of the gravitational and
kinematic forces of the Earth, the moon and the sun The
gravitational force at any point on the surface of the ocean
depends on the position of the moon and the sun and on their
distance from the point The period of the tides depends upon
the 29.53-day period of rotation of the moon about the Earth,
the Earth's daily rotation and upon the orientation of the
24.813 X 3600 Taking p as 1000 kg I I - ~ and g = 9.81 m s - ~ the theoretical average power becomes
0.220 Ar2
If generation is only on the ebb tide the figure is halved The actual power output is up to 25% of the theoretical average Some locations are particularly favourable for large tidal schemes because of the focusing and concentrating effect
Trang 16Tidal power 12/31
at about half the tidal range Initially, the flow is restricted to maintain a high head and to operate the turbines at maximum efficiency Later in the cycle the turbines are usually operated
at maximum power
Single-basin flood generation is the reverse of ebb genera-
tion It has a number of potential disadvantages, the main one being the prolonged periods of low tide experienced above the dam A second disadvantage is that the amount of energy would be less than with an ebb generation scheme as the surface area of the estuary decreases with depth
Two-way generation with a single-basin system generates
electricity from both the flood and ebb tides This does not result in a greatly increased power output Neither phase of the cycle can be taken to completion because of the need to reduce or increase levels in the basin for the next phase There are also economic disadvantages The turbines are more complex and less efficient if they are required to operate in both directions and the turbine water passages must be longer
An advantage is that power is available four times in the tidal day, rather than for two longer periods
Double-basin schemes often include provision for pumped
storage but in their simplest form they could operate as two independent two-way generation schemes In another form water would always flow from the higher-level basin to the second lower-level basin The second basin could only be emptied at low tide
A detailed discussion of the relative merits of the different schemes has been given by Taylor,”2 who points out that it is difficult to generalize, as a large number of variables, which vary from one site to another, need to be considered
Table 12.8 Mean tidal range in selected locations (rn)I6’
Bay of Fundy, Canada
Severn Estuary UK
Rance Estuary, France
Passamaquoddy Bay, USA
Solway Firth UK
10.8
8.8
8.45 5.46 5.1
which can be obtained from the shape of their bays or
estuaries Typical ranges are shown in Table 12.8 which
includes the world’s largest tidal range in the Bay of Fundy
and Europe‘s largest, the Severn Estuary
In an ebb generation system the use of pumps to increase
the level of water contained in the basin at high tide appears to
be attractive The principle is illustrated by a simple example
The additional energy required to raise the water level z m at
high tide is
The maximum potential energy now available during the
emptying of the basin becomes
;pgA(r -t 2)’
giving a net gain of
;pgA(r2 + 2rz + z 2 - 2’ - r 2 )
= pgArz
In practice there are two problems Pumping involves some
loss of overall efficiency and turbines capable of pumping as
well as generating are more expensive The power needed for
pumping may be required when demand on the whole electric-
ity system is high and could involve the use of an expensive
form of generation in another section of the network The
net gains in revenue from flood pumping in the proposed
Mersey barrage (UK) have been examined’68 for various
ratios off imported energy cost against exported energy value
12.10.3 Tidal power schemes
There are a number of different schemes which can be
grouped into two main combinations, depending on whether
one or two basins are used:
Single basin, generation only on the ebb tide;
Single basin, generation only 011 the flood tide;
Single basin generation with both the ebb and the flood
tides,
Single basin, generation with both tides and pumped
storage;
Double basin;
Double basin with a pumped storage system
Any particular scheme could be optimized against any one of a
nunber of different and distinct parameters These include
maximum net energy output; constant power output;
constan1 -head operation; maximum pumped storage capacity
or lowest initial capital requirements
Single-basin ebb generation allows the incoming tide to flow
through sluice gates and the turbine passageways These are
closed at high tide and the water is retained until the sea has
ebbed sufficiently for the turbines to operate This is normally
12.10.4 Tidal power sites
Many of the various design studies carried out until the early 1970s suggested that while tidal power systems were tech- nically possible they would be unable to generate electricity at
a competitive price A notable exception was the first major
report on the Severn Barrage, published in 1933,Ih9 but the
recommendations were ignored and over half a century later further feasibility studies were still being carried out Only three modern tidal power schemes were operating in
the 1980s The largest and oldest is the Rance Barrage near St Malo on the Brittany coast of France Two much smaller schemes are in the former USSR and China All three schemes have been built primarily to gain operating experience for the possible development of much larger systems ’’’
Work on the Rance site commenced in June 1960 the final closure of the estuary against the sea took place in July 1963 and the last of the 24 10 MW turbo-generators was commis- sioned in November 1967.’64 The overall width of the barrage
is 750 m The tides follow a fairly constant two-week cycle throughout the year During the first week of the cycle the tidal range is between 9 m and 12 m and in the following week between 5 m and 9 m.164 The mean tidal range is 8.45 m For the lower tidal ranges the barrage operates only on the ebb tide with the basin level increased by pumping For mean and spring tides two way generation is used, sometimes augmented
by pumping Electricit6 de France have shown’64 that the outputiinput ratio for pumping can be as high as 2.8:l The operation of La Rance is computer controlled and optimized
to match the period when it would be most expensive to generate electricity for the French national grid from conven- tional power stations The nominal average output of between
50 and 65 MW is therefore not the maximum which could be obtained
Nevertheless, it has been pointed out168 that while La Rance tidal power is the cheapest electricity on the French
Trang 1712/32 Alternative energy sources
system, Electricite de France comment that it would be too
expensive to build any further tidal power systems
In the former USSR a small 400 kW pilot scheme was
completed in 1968 at Kislogubsk on the Barents Sea The main
objective was to try out a new construction method, the use of
floated-in prefabricated caissons to form both the main power-
house and spillway structures The overall dimensions of
this structure were 36 m X 18.3 m by 15.35 m high and the
single reversible turbine was purchased from the French
company that supplied turbines for the Rance Barrage The
People’s Republic of China have a broadly similar pilot
scheme rated at 500 kW at Jangxia Creek in the East China
Sea ”’
In the UK one site has been considered to be outstanding
for nearly 70 years, the River Severn Government interest in
the Severn commenced in 1925 when the House of Commons
established a Sub-committee, which later became the Severn
Barrage Committee of the Economic Advisory Council The
main conclusion of its report in 1933 was that the cost of power
generated by a Severn Barrage with secondary storage some
20 km away would be only two thirds of the cost of that
generated at equivalent coal-fired stations 169 The scheme
included road and rail crossings and the rated output was 804
MW A further report in 1944 suggested doubling the output
of the turbines to 25 MW while maintaining the rated output at
some 800 MW A later report in the early 1950s drew attention
to the potentially high capital cost of any scheme
Several further reports which appeared up to 1975 have
been summarized by McVeigh,’ who noted that in a comment
on the various proposals that had been made over the past 50
years, the authors of an Institution of Chemical Engineers
report17’ stated in 1976: ‘A curious feature has been the
regular conclusion that the scheme would have been economic
if embarked upon on earlier occasions, but never on the
current one .’
In 1977 the Department of Energy summarized the various
proposals,16’ includin the results of their own specially
commissioned s t ~ d i e s ’ ~ ’ , ~ ~ ~ This was followed in 1978 by the
establishment of a further pre-feasibility Severn Barrage Com-
mittee which reported in 1981 17‘ that it is ‘technically feasible
to enclose the estuary by a barrage located in any position east
of a line drawn from Porlock due north to the Welsh coast’
The most cost-effective of three schemes considered in
detail was a single-basin, ebb-generation scheme with a 13 km
barrage from Brean Down, a few kilometres south of Weston-
super-Mare, to Lavernock Point, with an estimated annual
output of 13 TWh at a cost of some 2.4 pence per kWh, at that
time close to the official cost of nuclear electricity This
particular scheme was the subject of a two-year study jointly
funded by government and industry which commenced in June
1983
By 1990 all the recent studies concluded that the Severn
Barrage was technically feasible and the main design para-
meters had been agreed for the Brean Down-Lavernock Point
line outlined above These parameters included a nominal
design life of 120 years, which was selected on the basis of it
being a multiple of 30 and 60 years, time spans regarded as
periods for major refurbishments In practice, it was felt that
the barrage could have an indefinite life Other main para-
meters included an estimated total installed capacity of 8640
MW, obtained from 216 turbine generators, each 9 m in
diameter and rated at 40 MW, and an annual output now
estimated to be 17.0 TWh This represented some 7% of the
electricity consumption of England and Wales in 1989 But the
possibilities for promoting and financing the scheme had to be
delayed until after the UK electricity supply industry had been
privatized
The second lar est potential tidal project in the UK is the Mersey Barrage i% Although the potential installed capacity
is only in the order of 600 MW, it could have a major impact
on the local economy Overall in the UK, the theoretical tidal barrage capacity is approximately 25 GW Other possible smaller sites in the UK include Morecambe Bay, the Humber, the Wash and the Solway Firth
In Canada tidal ranges of up to 16 m have been recorded in the upper regions of the Bay of Fundy in north-east Canada This has been the subject of several investigations, including the working of the Atlantic Tidal Power Board’75 in 1969 (development not economically justified) and the Bay of Fundy Tidal Power Review Board’76 in 1977, who assessed the potential of 30 possible sites The three with the best pros- pects, Cobequid Bay, Shepody Bay and Cumberland Basin, had an estimated potential output of 6.4 GWe
Estimates of some 500 possible sites in the People’s Republic of China have suggested a potential of over 110 GW.”’ Those for the former Soviet Union have concentrated
on the White Sea between Murmansk and Archangel, where the potential could range from 16 to over 50 GW
Other parts of the world with potential large-scale sites include the Kimberley Region of Western Australia, and South America, India and South Korea
A series of estimates reviewed by H ~ b b e r t ” ~ suggested that the total tidal ener y dissipated in the world’s shallow seas was any estimates from the People’s Republic of China The average maximum potential power which could be recoverable from these sites was estimated to be 64 GWe in 1969 When it
is appreciated that some of the sites are very long distances away from any potential main user, this figure does not seem
to be unduly pessimistic
no more than 10’ 4 W although these data appeared to omit
12.10.5 Possible impacts of tidal schemes
It is difficult to quantify the social, industrial and environ- mental impacts which any proposed scheme in the UK or elsewhere could have These have been widely reviewed in the
and some of the main points are discussed briefly below:
Water levels both in the basin upstream of the barrage and
to seaward could be changed
Tidal flows reduce the strength of the currents upstream
of the barrage Downstream and to sea the effects could extend over 50 km
Sedimentation may occur in the basin and could lead to a
slow and possibly small reduction in basin volume To seaward the sediments previously swept out by tidal flows may stay deposited
Mixing will occur less in the water above the basin because
of reduced currents and tidal excursions
Navigation Ships could be slowed by passing through
locks; on the other hand, predictable periods of deeper water could be an advantage
Industry could benefit during construction but may have
to adopt higher standards in dealing with possible pollut- ing liquid effluents
Land drainage could be affected inside the barrage
because of higher low-water levels
Sea defences will be less liable to storm damage after the
construction of a barrage
Ecosystem The aquatic ecosystem will always be affected
by any changes in turbidity and salinity
Migratovyfish will face the obstacle of the barrage, but the
Trang 18Wave ~ o w e r 12/33
inwarsd journey will probably be straightforward through
the sluices in ebb-generation schemes
11 Recreational opportunities could be enhanced in suitable
locations, with less turbid water above the barrage
There was unanimous agreement among members of the
discussion panel at the third UK Tidal Power Conference'@
that several more years of environmental assessment would be
needed before specific plans could be brought forward for the
Severn Barrage There were still many poorly understood or
completely unknown oceanographic sedimentary, engineer-
ing or environmental issues which had not yet been pursued,
mainly because they were not 'make-or-break' issues in res-
pect of the viability of the Severn project The effect of global
warming on mean sea levels, however was not thought to
impose any problems over the next few decades
Some ideas of the scope and range of the environmental
work on the Severn Barrage Project can be found by examin-
ing the full list of the 59 Environmental Impact Studies (and
their asso8:iated contractors) during the two-yea: period from
1987 to 1089 It is difficult to see how all this work could be
replicated for the many smaller UK sites
12.10.6 The economics of UM tidal power (and of many
of the renewables)
The discussion panel at the third UK Tidal Power Conf-
erenceI6' were asked if they could assure the assembled
delegates that there was common ground between, on the one
hand, the opportunity to generate power without damaging
emissions or radiation risks and, on the other, the need to
provide power to the consumer on a commercially attractive
basis Dr John Chesshire"' responded that he had stressed in
his own publications and research work that there were
inconsistencies in approach to capital expenditure and invest-
ment appraisal in the UK energy sector A fourfold disaggre-
gation was the best way to make this point briefly, i.e
between public and private sectors, and between energy
efficiency and new energy supply projects
Thus thle financial return and depreciation period required
from tidal barrage schemes would be similar to those expected
by the UK financial sector for nuclear power and would lead
to barrage generating costs in excess of 10 p kWh-' (in 1989),
as compared with combined cycle gas turbine generating costs
of 2.2-2.5 p kWh-' It was therefore unlikely that, without
regulatory or fiscal adjustment, there can be any common
ground between the opportunity to generate power from
nuciear and n o s t renewable sources of energy and the need to
provide electricity to consumers on a commercially attractive
basis
12.1 1 \Nave power
12.11.1 Introduction
The history of wave energy conversion probably dates from
the last few months of the eighteenth century, when the first
patent for a wave energy device was filed in Paris.'8o Since
then there have been hundreds of patents filed throughout the
world with well over 300 in the UK alone between 1856 and
1973 ''I
Only a few proposals appear to have had model tests and
these showed poor efficiency Modern developments started
in 1945 when Yoshio Masuda commenced privately funded
research in Japan on a wide range of devices.'" In 1960 his
work received government support and he concentrated on
wave-activated air turbines, one of which was installed in a
lighthouse in Tokyo Bay This had a maximum output of 130
W By the early 1970s over 200 small units were operating in Japan, mostly on buoys
The UK Department of Energy wave energy programme commenced in 1974 This early work showed that the waves arriving at Britain's Atlantic coastline delivered a surprisingly large amount of e r ~ e r g y ' ~ ~ ' ~ ~ and the national programme quickly established that it was physically possible to generate useful power from ocean waves with reasonable efficiency Is5
However, after a decade of development, always exciting and sometimes frustrating, none of the devices investigated seemed to be capable of generating power at a cost which would be comparable to more conventional sources 'E Some
of the more promising concepts are discussed after the intro- duction to simple wave theory and characteristics, and the prospects for wave power in the 1990s are assessed
12.11.2 Wave theory and characteristics
The basic characteristics of wave power can be studied from standard linear wave theory This considers a simple pro- gressive sinusoidal wave of amplitude, a , wave length, A, and period T , progressing in deep water Sinusoidal waves of a single wavelength are known as monochromatic waves and 'deep water' is defined as having a depth greater than half a wavelength The velocity with which the wave propagates, the phase velocity, can be written as
Woz2 a2T
87T
Real ocean waves are quite different from the theoretical ideal wave described above Ocean waves are generated by the wind, so that wave energy is an indirect form of solar energy The ocean acts effectively as an extremely large integrator for wind energy'2'~'88 and, in addition, the inertia of the water can provide a limited amount of short-term energy storage which can compensate for variations in wind velocity with time and place.183 The waves arriving at a point can have originated from storms hundreds of kilometres away, the 'swell' sea, or from local winds, the 'wind' sea.188
The distance from the origin of swell waves is known as the fetch Swell waves can appear to be substantially plane and monochromatic so that the longer the period between waves, the faster they travel However, the local wind sea which is superimposed on it can be more complicated and random in wavelength, phase and direction Any record of sea waves is therefore very complex and is best described as 'the linear sum
of many monochromatic waves of random relative phase
distributed both in direction and across the frequency spec- trum' '" A detailed discussion of the spectral density function
has been given by Pierson and Moskowitz However, for most practical purposes a very simplified relationship has been derived which is probably accurate to within i 30% This gives the power of a wind-generated wave system for any location in terms of the significant wave height, N,, defined as the average height of the highest third of the waves, and the zero crossing period, T, defined as the time interval between successive upward movements of the water level past the mean position For the Ocean Weather Ship India (50"N, 19'W) the
Trang 1912/34 Alternative energy sources
power per metre of wave front has been shown'90 to be
approximately
0.55 Hs2T, kW
The estimates of power availability for wave energy systems
in the mid-1970s were based on India data which suggested
that an average power of 91 kW per metre of wave front was
available 184 This ranged from periods with very little power to
severe storm conditions when the power level could exceed
several megawatts '" More recent measurements near South
Uist reported by the CEGB in 1983 showed that at inshore
sites more suitable for the deployment of wave energy
systems, power levels between 40 and 50 kW m-' could be
expected in water about 50 m deep Levels of 25 kW m-'
could be expected off the north-east coasts of England or
south-west Wales
By the early 1980s a number of overall estimates had been
made for the UK, giving a total resource capacity estimated to
be 120 GW, based on a mean potential of 80 kW m-' along a
1500-km coastline When simple geographical limits were
imposed, the potential dropped to 48 GW: and with further
limitations such as device configuration, station design, cap-
ture and power train limitations, the achievable resource
would reduce to about 6 GW, according to an ETSU assess-
ment.'92 The 1988 UK review of renewable energy in the UK
gave a technical potential of 30 GW, capable of providing
some 50 TWh yr-', mainly off the Western Isles of Scotland
and the coast of C ~ r n w a l l , ~ ' but in 1991, a later report to the
CEC'93 showed only 0.23 GW theoretically available by the
year 2000
12.11.3 Types of wave energy convertor
There have been a number of different classifications in the
past decade, starting with Salter's flaps, floats, ramps, con-
verging channels and liquid pistons or air bells.187 The conce t
of 'active' and 'passive' systems has also been ~uggested,'~
active systems having parts which respond to the waves with
power generated from the relative motion of these compo-
nents while passive systems absorb wave energy by virtue of a
fixed structure One of the simplest methods for using wave
energy would be to construct an immovable structure to
capture large volumes of water which can subsequently be
used to drive a water turbine This system could also be
classified as a ramp or passive system The best-known
proposal was initiated in Mauritius in the 1950s and has been
the subject of a series of official reports, summarized by
B ~ t t ' ~ ~ in 1979 All the reports agreed that the project would
be technically feasible, but the economic viability has proved
to be the stumbling block By the early 1980s the more
generally accepted scientific classification was that of ter-
minators, attenuators and point absorbers 191
A terminator is defined as a wide structure which is aligned
perpendicular to the incident wave direction Much of the
experimental work on this type has been carried out in narrow
wave tanks and thus has resulted in good agreement with
extensive theoretical studies, so that the performance of
terminators is generally more fully understood than other
types of device
The best known is probably the 'Salter Duck', a system
originally proposed by Salter in 1974.lX7 The floating 'duck'
section is an asymmetric cam-shaped device designed to
extract energy through semi-rotary motion induced by the
incident waves 191 The system would consist of a long central
core, or spine, upon which the duck sections are mounted
Large gyroscopes would be placed inside the nose of each
duck, which rotates along its principal axis The precessing
motion of the gyroscopes could be used to drive hydraulic
pumps The concept is revolutionary, as it would be designed
to be maintenance-free over a 25-year lifetime
The Cockerell raft was based on a simpler concept, that of a series of pontoons or rafts connected by hinges, with power generated from the relative movement of the rafts Both the Duck and the Raft were taken to 1/10 scale model tests.196,197 Several systems known as oscillating water column devices and which could also be considered as variations of Masuda's original air bell concept have been studied One which was considered very promising and capable of proceeding to full-scale testing was the National Engineering Laboratory's breakwater system, which consists of a concrete structure mounted on the seabed The motion of the waves causes a column of water inside the structure to rise and fall, inducing air flow through turbines Proposals were suggested in 1982 by
a private consortium and the National Engineering Labora- tory for a 4 MW prototype to be built off the island of Lewis in the Outer Hebrides,' but this was not followed up Almost a decade later a 30 MW array of oscillating water columns was being considered in Plymouth Sound The estimated capital cost in 1991 was E35 million and the cost of the electricity generated would be 6p kWh-' 193 Other terminators have been studied in the UK? including the Russel Rectifier and the Sea Clam, and detailed descriptions are widely available 12*
An attenuator is a long, thin structure which is aligned parallel to the incident wave direction It was originally thought that energy could be progressively extracted along its entire length, but this has proved impractical because the rear element would need to extract as much energy as the front one for optimal operation 19' Two devices considered in the UK
were the Vickers Attenuator and the Lancaster flexible bag Neither device was able to show a satisfactory performance in model testing
The point absorber is an axially symmetric device, con- strained to move vertically, which can absorb wave energy from any direction Theoretically, it can absorb wave energy from an effective wave frontage of X/27r 191 This means that a number of interconnected but widely distributed point ab- sorbers could produce as much power as a continuous line absorber having the same total length
Research into the development of composite materials at Queen's University, Belfast, resulted in the development of a glass-fibre reinforced polyester resin and its utilization in the prototype of a new type of wave energy convertor, the Belfast Buoy This can be considered as an oscillating water column device with an important difference; the vertical axis air turbine rotates in the same direction, irrespective of the direction of air flow The turbine system has been named after its inventor, Professor Alan Wells lg8 Several different con- figurations have been suggested, but a good hydrodynamic performance can only be obtained from a very limited band- width.'"
In 1982 a major review of renewable energy in the UK
resulted in a recommendation that no new development work should be supported on large-scale offshore wave energy devices'92 mainly on economic grounds, as other renewables (e.g wind and tidal power) were considered to be more economically attractive Since then, wave energy work has concentrated on three systems: 199
1 Point absorber devices, mainly at Lancaster University (the
2 The Circular SEA Clam;
3 The shoreline wave energy resource, through the genera- tion of electricity from oscillating columns located in shore- line rock gullies This has included studies of the potential shoreline resource and the building by Queen's University, Belfast, of a device on Islay, which is described below Flounder, Frog and PS Frog);
Trang 20Wave power 12/35
illustrated in Figure 12.15 The device spans a natura! rock gully in relatively shallow water and is being used as a test bed for a new two-stage Wells turbine.20' The wave entering the gully oscillates a column of water inside the box, causing air to pass through the turbine in either direction The official inauguration ceremony was held in 199i
12.11.4 Shore-based systems
Small shore-based wave power systems have been used by the
Japanese to power lighthouses for nearly two decades2''
During 1984-1985 the Wells turbine was used by the Kraener
BTug company in a Norwegian 500 kW wave power station
built on a cliff edge at Tostestallen, north of Bergen This
system was partially destroyed later in a severe storm
Another Norwegian device is the Norwave Tapchan, or tap-
ering channei, invented by a mathematician, Even Meh-
lum.20" This uses a funnel-shaped channel blasted out of rock
to a predetermined profile which causes resonance with the
loca1 wave spectra At the inner end of the channel is a wall,
about 3 m above sea levei Waves enter the channel and
propagate As the walls narrow, the wave height increases
until the water reaches the top of the wall and flows over it
into a storage lagoon with a surface area of 8000 m2 Water can
then flow out of the la oon and back to the sea through a 350
kW Kaplan turbine.20'
In the mid-l980s, the Queen's University, Belfast, team
followed their earlier work with a proposal to harness wave
energy in relatively shallow natural rock gullies The island of
Islay in the Inner Hebrides was selected for the first prototype
shore-based wave power system in the UK The principle is
12.11.5 Summary
The future of wave power is particularly uncertain None of the systems tested in the past decade have been able to demonstrate that they could generate electricity at a cost which would be comparable with other, more conventional, sources, except for a few specific locations in remote islands The economics would alter in favour of wave power if conventional methods become more expensive, which many authorities believe is inevitable
However, a major reassessment of wave power was being carried out in the UK in the early 1990s, and the publication of the interim report in October 1991199 showed a new approach
with all the active members of what is described as 'the wave
energy community' participating, to produce a 'forward- looking review based on best current knowledge'
Figure 12.15 Schematic diagram of an oscillating water column device (afler Review 7"')
Trang 21Alternative energy sources
12.12.1 Introduction
The development of human life can be directly traced through
biological conversion systems, initially through the provision
of food, then food for animals, the materials for housing and
energy for cooking and heating The commencement of indus-
trial activities was followed by the development of agriculture
and forestry to their present levels The renewed emphasis on
biological conversion systems arises from the fact that solar
energy can be converted directly into a storable fuel and other
methods of utilizing solar energy require a separate energy-
storage system The carbohydrates can be reduced to very
desirable fuels such as alcohol hydrogen or methane, a
process which can also be applied directly to organic waste
materials which result from food or wood production
Biomass can be defined as all types of animal and plant
material which can be converted into energy It includes trees
and shrubs, grasses, algae, aquatic plants, agricultural and
forest residues, energy crops and all forms of wastes
Estimates of how much of the world’s energy demand is met
by biomass range from 6% to 13% .202,203 Among developing
countries, biomass is the single most important source of
energy, especially within the domestic sector, although some
local industries such as bakeries, brick firing or steam produc-
tion are also dependent on fuelwood Nine tenths of the
population in the poverty belts rely on wood as their chief
source of fuel, and although in cooler regions some is used for
heating, by far the most important energy need is for cook-
ing.204 Cooking fuel represents approximately 50% of fuel use
in many rural areas and up to 90% of energy needs in warmer
regions ’05
The quantities of biomass produced throughout the world
are very large The annual net production of organic matter
has an energy content of about 3 X lo2’ J , some eight times
the world’s annual energy use in the early 1990s In forests
alone the biomass productivity was estimated to be about
three times the world’s annual energy use at the end of the
1970s *03 At that time the potential biomass resource already
standing in the world’s forests was 1.8 X lo2’ J, a figure
comparable with the proven world oil and gas reserves Even
with the depletion of the world’s forests and the increase in
both oil and gas reserves in the past decade, biomass remains
the largest and most familiar of all the renewable energy
resources
12.12.2 Photosynthesis
Solar energy can be used by all types of plants to synthesize
organic compounds from inorganic raw materials This is the
process of photosynthesis In the process, carbon dioxide from
the air combines with water in the presence of a chloroplast to
form carbohydrates and oxygen This can be expressed by the
following equation:
Sunlight
Chloroplast
A chloroplast contains chlorophyll, the green colouring
matter of plants The carbohydrates may be sugars such as
cane or beet, C12H22011, or the more complex starches or
cellulose, represented by ( C ~ H ~ J O ~ ) ~ All plants, animals and
bacteria produce usable energy from stored carbon com-
pounds by reversing this reaction Compared with other
methods, biological or photosynthetic conversion efficiencies
are much lower, but are potentially far less expensive Photo-
synthetic efficiency is based on the amount of fixed carbon
energy produced by the plant compared with the total incident solar radiation Plants can only use radiation in the visible part
of the solar spectrum between wavelengths of 400-700 nm, known as the photosynthetically active radiation (PAR) re- gion This represents about 43% of the potentially available total radiation At the plant some of the PAR is reflected and with other losses due to internal chemical processes the maximum attainable efficiency lies between 5% and 6% Under very favourable conditions, conversion efficiencies of between 2% and 5% have been recorded in the field for growth periods of a few weeks, but considerably lower effi- ciencies are achieved over longer periods of growth Irish grasslands or forests with Sitka spruce are capable of dry matter yields greater than 16 tonnes ha-’ which represents an efficiency of about 0.7% The main reasons for these relatively low efficiencies are environmental constraints nutritional limitations and the incidence of pests and diseases 2a6 Typical environmental constraints would include a drought or daily variations in ambient temperature Nutritional limitations depend on the soil quality which, in turn, relies on the output
of fertilizers
12.12.3 Energy resources
There are five routes which can be followed to obtain the organic material or biomass which is the starting point for the energy conversion process The first and by far the simplest,
is to harvest the natural vegetation There are fertile regions in many parts of the world where the topography or some other reason makes the land unsuitable for agriculture or other valuable activities With the harvesting of natural vegetation,
no costs are involved in planting or clearing and the land would be given a new use A major disadvantage of this method is that the yields are, at best about half those which could be obtained from an energy plantation The second is through the cultivation of a specific energy crop, grown only for its energy content, or the use of agricultural surpluses, so that the stored chemical energy can be converted into useful energy by combustion or converted into a storable fuel
A land crop should have as high a conversion efficiency as possible, but it does not have to be digestible by animals or edible by humans The entire material or biomass of the crop can be used, including the leaves, stalks and roots By careful genetic selection and intensive cultivation the conversion efficiency should reach 3% under normal conditions In the third route, trees and other types of lignocellulose material are grown specifically as fuel in energy plantations Short-rotation forestry (described later) is a good example The fourth uses the wastes from agro-industrial processes or residues from agriculture, animal wastes, straw, and all forms of urban wastes The fifth route is through algae in the sea or grown in inland ponds
12.12.4 Conversion of biomass to fuels and other products
A selection from some of the main conversion processes is illustrated in Figure 12.16, which shows that there are often several different routes to the same end product Combustion
is by far the simplest and best-known technique, particularly with forestry residues and industrial and urban wastes A number of the processes are well known and are ideally suitable for producing fuels With aerobic fermentation, ma- terials containing starches and simple sugars can be used to produce ethyl alcohol or ethanol Anaerobic fermentation has the added advantage of producing a valuable by-product, the nutrient-rich fertilizer from the digested slurry, when used to treat domestic sewage or animal wastes and produce biogas In
Trang 22Biomass and energy from wastes 12/37 world is on an open fire The most basic stove is simply three stones arranged on the ground in a triangle The pan rests on the stones between which three or more pieces of wood are placed Efficiencies are low, between 2% and 10% although much depends on the rate of burning, the air convection and other factors such as the height of the pan above the fire Compared with the three-stones method, traditional woodfuel stoves are more efficient and the technology for improving their use of fuel exists With a potential 30% fuel saving through the adoption of improved designs, fuel demand would
be considerably reduced Another possibility is the adoption
of solar cookers, but these are too expensive and often local cooking habits or other sociological factors inhibit their accep- tance, and they are unlikely to provide an adequate substitute for fuelwood, although they could provide a complementary energy source
Adoption of alternative cooking methods also depends on
such factors as aesthetic appeal and even attributes of social status Each community is unique with regard to cooking modes and each will pertain to different values with con- sequent perceptions of suitability It is therefore difficult to generalize from one community to another as to the most appropriate and acceptable fuelwood stove or solar cooker '
Biogas (two thirds CH4,
one third CO2)
19 MJ 1-'
38 MJ m-' 16.9 MJ 1-' 30-40 MJ kg-' 23-30 MJ kg_' 8-15 MJ m-' 19-34.5 MJ kg-'
the pyrolysis process the organic material is heated to temp-
eratures between 500" and 900°C at ordinary pressures in the
absence of oxygen, producing methanol, which was a by-
product of charcoal in the last century Methanol was first
used as a fuel for high-performance racing cars and was
subsequently stadied as an additive in many laboratories It is
now considered to be an essential part of the future auto-
mobile fuel The typical energy content of some of
the products in Figure 12.16 is shown in Table 12.9
12.12.5 Cooking - the major application of biomass
The United Nations have warned for many ears that over
90% of wood cut in Africa is burnt as fuel.*' Deforestation
and desertification are widespread and increasing with the
southern edge of the Sahara extending by over 5 km yr-'
The scarcity of wood in some areas has meant that local
inhabitants have had to move on or turn to substitutes The
World Bank203 reported that between a half to one billion
people use agricultural or animal wastes to fuel their fires In
India, (cattle dung represents three quarters of the Indian
domestic fuel consumption, robbing the land of valuable
nutrients.*09 In parts of Africa crop residues and stubble are
uprooted and used for fuel
Cooking is a very cultural-specific acrivity However, the
most common means of cooking throughout the developing
x
12.12.6 Energy from waste materials
In the UK over 70 Mt of waste materials are generated annually2" in homes, agriculture, commerce and industry Because of changes in living patterns, especially in central heating and in the consumption of packaged goods, up to two thirds of collected domestic waste can be combustible A breakdown of waste production and the distribution of com- bustible content are given in Table 12.10 (after Jackson and TronZ1')
The calorific value of combustible waste can vary from 5.0
to 40 MJ kg-' so that even at the lowest level (which predominates) a potential primary fuel content of 364 x io6
GJ per year or an installed generating capacity of 12 GW is
available, of which some 60% is in domestic waste A recent (1991) assessment of the overall gross calorific value of UK municipal waste was about 11 MJ kg-', a figure which was said to underline the advantage for energy-recovery purposes
of burning the total combustible waste, rather than transport- ing the bulky waste and landfilling "'
The recent history of the use of waste as a fuel is not particularly encouraging The variability of content of waste materials, the cost of acquisition and clean air legislation generally precludes all but the largest schemes, which inevi- tably mean district heating schemes run by iocal authorities There are such installations in the UK at Edmonton and Nottingham, both of which are combined heat and power schemes 23
The late 1970s and early 1980s saw the development of pelleted Waste Derived Fuel (or Refuse Derived Fuel -
RDF) Some types of refuse were dried, shredded and refined
to concentrate the combustible and compressible fraction and produce a hard pellet-type fuel which could be used as a direct replacement for small coal Only a few plants were built and operated in the UK The completion of the plants coincided with the fall of conventional energy prices in the mid-1980s and new electricity tariffs made direct combustion of wastes a more economic proposition."'
Estimates for the annual average production of straw as a by-product of cereal crops in the UK range between 12 and 13.7 Mt.23,64 Probably over half (5O-60%) has been burnt in the fields to recycle minerals, but this was being phased out in the early 1990s A total of some 166 000 t is used directly for farmhouse and animal house heating annually in over 7000
Trang 2312/38 Alternative energy sources
Table 12.10 Estimated UK waste and its energy content
Gross weight Weight of combustible content Energy content
“Excludes mining, quarrying construction site wastes and power-station ash
boilers.64 Proposals have been made for a number of applica-
tions from heat production to producer-gas fuelled vehicles,
particularly tractors ’” The maximum UK potential for on-
farm straw combustion is 1.9 Mt yr-’, but 0.9 Mt yr-’ by the
end of the century is thought to be more realistic.213 Industrial
heating in small industries such as food and drink, cement and
brickmaking, and in light engineering could use a further 5 Mt,
but this was not considered to be likely in the short term.64
Domestic and commercial refuse contains large quantities of
organic matter In the UK, over half this refuse is carbohy-
drate in origin, and each of the estimated 3300 active landfill
sites can be considered as large ‘bio-reactors’ for the decompo-
sition of this organic matter In the first stage, microbial
activity is high and the rapid depletion of the available oxygen
results in anaerobic conditions Anaerobic digestion is a
complex process, involving the degradation of large organic
compounds, such as vegetable matter or paper, to simpler
substances such as sugars.214 This is followed by the produc-
tion of hydrogen, carbon dioxide and fatty acids prior to the
generation of biogas in the final stage This is, by definition,
‘landfill gas’, with a chemical composition and calorific value
indicated for biogas in Table 12.9
It has a fairly recent history, being first noticed in the
United States and Germany in the 1 9 6 0 ~ , ” ~ and its first use as
a gaseous fuel in the UK followed in the 1970s with the firing
of brick kilns.214 By the end of 1987, some 14 sites in the UK
were using landfill gas directly214 and the first five small
electricity-generation stations had commenced operation The
total capacity of those sites fitted with generating equipment
had reached 30 MW towards the end of 1991.215
In a survey of world trends at the end of the 1980s, the UK
came second only to the United States in the commercial
exploitation of landfill gas with the main UK use being in gas
kilns, furnaces and boilers World use was projected to reach
some 3.5 Mtce by 1992, with electricity generation at 440 MW
The UK potential for landfill gas is considered to be about 3
Mtce
Anaerobic digestion occurs naturally in organic swamps,
producing marsh gas Man-made digesters can provide the
best conditions for the controlled continuous production of
12.0 1.6 4.1 2.3
A basic Chinese family biogas unit is shown in Figure 12.17 This would use the wastes from the smallholding activities of a number of families Most units of this type could produce about 6-7 m3 gas daily during the summer months
It is formed from a horizontal concrete cylinder, buried about 1 m underground Square-sectioned vertical entrance and exit chambers have tight-fitting concrete lids The gas is generated in the upper section of the cylinder and the delivery pipe to the family kitchen branches to a large vertical water manometer, mounted on the kitchen wall so that a careful check can be kept on the gas pressure, normally about 250 mm
of water above atmospheric pressure Both human and animal wastes are used as raw material for the units, as well as various types of vegetable waste matter Basic loading and clearing the processed waste for use as fertilizer takes between one and two hours per week in the summer months and slightly longer
in the spring and autumn, as more care has to be taken with the quality of the wastes in colder conditions.216
India has also had considerable experience with the development of biogas systems and some countries are now basing their designs on the established Chinese and Indian systems For example, over 1000 gobar (cow-dung) plants have been built in Nepal, biogas has been used to replace diesel fuel in Botswana and plans to develop some 300 000 biogas systems in rural Thailand have been studied 217 Among the industrial countries Romania, with extensive pig farms, was using biogas in an experimental bus during the 1980s ’
Trang 24Energy
gas storage chamber
square- seethed inlet chamber
Figure 12.117 A Chinese family biogas unit (after McVeigh’)
In the UK the temperate climate is less encouraging for
biogas but careful overall system design can overcome this
problem A prototype unit for a dairy herd of 320 cows was
completeid in Kent in 1979 Electrical power was generated
from a Ford diesel generator modified for gas combustion with
spark ignition, with a continuous maximum power calculated
to be about 25 kW.*I8 A number of smaller units were also
operating in other parts of the
A leading commercial organization in the UK, Farm Gas,
developed a range of small digesters mainly for farm use, from
the mid-1970s In 1991 they were examining the use of small
digesters for municipal solid waste for the UK Department of
Energy *”
12.13.1 Short-rotation forestry
The use of trees as energy crops has been proposed in several
countries since the early 1970s Detailed feasibility studies in
the USA have shown that biofuels can be produced at
competitive costs, by choosing the appropriate plant species,
planting density and harvest schedule for each plantation site,
thus minimizing the overall cost of the plant material.221 In
Ireland about 6% of the land area consists of bogland and less
than a fifth of this area is being harvested for peat, which is
either wed directly as fuel in the home or for generating
electricity
Until recently it had been thought that bogland was unpro-
ductive, hut grass, shrv.bs and trees have all been successfully
grown Even with a conversion efficiency of 0.5% for Sitka
spruce, the same bogland area at present used for turf could
produce exactly half the quantity of electricity through the
combustion of the trees The Irish government has demon-
strated that woodchips obtained from short-rotation forestry
can provide an economic alternative to
Briefly, the short-rotation forestry concept follows the
sequence of selecting, planting, harvesting and utilizing as fuel
the woodchips obtained from coppicing hardwood trees The
chips will be left to dry naturally in the fields, then collected,
transported and burnt directly in specially modified power
stations Alternatively, they can be bagged and sold directly
An interesting statistic from the UK in 1991 pointed out that there were well over 200 woodfuelled combined heat and power plants.223 While most of these were using by-products from sawmills or joinery works, the possibility of an alterna- tive use of farmland for fuelwood production was being seriously considered, with support from the European Social Fund
12.13.2 A fuel-alcohol plant
An important factor in considering energy crop conversion is the energy needed for harvesting and for fertilizers to increase the crop yields Net energy analysis is used to assess the energy costibenefit ratio of any proposed fuel conversion process The energy inputs and outputs of the system can be measured and the net energy ratio (NER) can be defined as the ratio of the energy outputs to the energy inputs Any application of this concept requires a careful definition of the system boun- daries The NER concept has been used in the world’s first cassava (mandioca) fuel-alcohol commercial plant in Brazil *’‘
The system boundaries and energy flows are shown in Figure 12.18 The system consists of the cassava plantation, the fuel-alcohol distillery and the forest from which the fuelwood is obtained to provide process steam for the distill- ery Energy optimization of cassava distilleries could lead to the development of varieties of cassava with larger stalk-to- root ratios, so that the cassava stalks could replace the fuelwood requirement
Another self-sufficient process is the sugarcane fuel-alcohol system The bagasse or by-product can generate all the necessary process steam The NER of both systems is shown in
Table 12.11 and based on 1 m3 of anhydrous ethanol and total
on-site generation of electric power.224
12.13.3 Marine and aqueous applications
In oceans the production of organic matter by photosynthesis
is generally limited by the availability of nutrients and they have been compared to deserts because of their low productiv-
Trang 251 1 1 roots
t
ity However, there are a few areas where natural flows bring
the nutrients from the bottom of the ocean to the surface so
that photosynthesis can take place Particular interest has
been shown in the cultivation of giant kelp (Macrocystis
pyriferu), a large brown seaweed found off the west coast of
the USA
An early estimate examined the yield from an area of
600 000 km2 and concluded that the equivalent of some 2% of
the US energy supply could be p r o ~ i d e d ” ~ One of the
disadvantages of harvesting natural kelp beds would be the
relatively low output caused by the lack of nutrients Artificial
kelp ‘farms’ have been suggested226 and a 1000 m2 system,
with nutrient-rich deep water being pumped from deep water
to the surface kelp, has been developed, as the first stage of a
project which could lead to a 40 000 ha system
Aquatic weeds can easily be converted to biogas The water
hyacinth (Eichhornia crassipes) has been extensively studied
as a biogas source, particularly at the US National Aeronau-
tics and Space Administration (NASA).227 On a dry weight
basis, one kilogram of water hyacinth can produce 0.4 m3 of
biogas with a calorific value of 22 MJ m-‘ Aquatic weeds
grow prolifically in many tropical regions and are costly to
harvest However, as weed clearance is essential to keep
waterways clear, biogas production could be regarded as a
valuable by-product in these applications
Distillery
12.13.4 Choice of system
The factors affecting the choice of a particular biological
conversion system identified by Hall and CoombsZz8 include
-
considerations of agricultural capacity, environmental factors, population density labour intensity in the agricultural sector and the energy demand per capita Four regimes were dis- tinguished in their simple classification:
1 Temperate industrial areas such as North America, West- ern Europe and Japan where biomass will only produce a small fraction of the energy demand Emphasis will be placed on the production of scarce chemicals from biomass The by-products from certain industries may be used to provide heat and power, while the use of wood as a direct fuel source is also possible
2 Tropical and sub-tropical regions with good soil and high rainfall such as parts of India and Africa, Brazil, Indo- China and north Australia Energy from biomass has the greatest potential in these regions, with many examples already competing economically, e.g biogas and the more efficient use of fast-growing wood species
3 Northern polar and arid regions where biological systems are only possible in an artificial environment, e.g use of the nutrient film technique
4 Marine and aqueous regions through the use of fast- growing water weeds, seaweeds or micro-algae
The development of photobiological energy-conversion systems can take place more readily in the temperate Western countries with their high technological background However, these systems can function more effectively in the developing tropical and sub-tropical countries and could make a very significant contribution towards reducing their dependence on
increasingly scarce and expensive oil
References
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2 Hubbert, M King, Energy Resources: A Report to the
Oxford (1984)
Committee on Natural Resources, National Academy of Sciences
- National Research Council Publ 1000-D, Washington, DC (1962)
Table 12.11 Net energy analysiszz4
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Trang 3013 Nuclear engineering
Contents
13.1 Introduction
e coolant challenge 1315 st-reactor fuel processes 1318 uality assurance and control 1319 13.4 Other applications of nuclear radiation 1319 13.4 1 Gauging and detection 13/9 13.4.2 Compact power sources 1319 13.4.3 Materials and sterilization effects 1319
Trang 3213.6 initroductiorr
Advances in physics that occurred towards the end of the
nineteenth century in understanding the make-up of atoms
and their nuclei led in the twentieth century to engineering
applications of nuclear energy The two major applications
have been to nuclear weapons and to the production of
electricity There are a number of peripheral uses of the
energy released in nuclear reactions
It is the task of the nuclear engineer to exploit the energy of
the nuclens to the benefit of humankind Nuclear engineering
shares with other branches of engineering, metallurgy, chem-
istry etc the history of having been bred in war and only
subsequently applied in peace It is perhaps the most stark in
its contrasts of the benefit it might bring to humankind and the
dangers of misusing the power our command over the nucleus
of matte: can give
The en,ergy coming from nuclear reactions may be specific
in terms of the major radiations (alpha-particles, beta-particles
and gamma-rays and neutrons in particular) or may be ex-
ploited f ’ x the general energy associated with them as an
intense source of heat Table 13.1 lists some of the major facts
about the energy of nuclear radiations It is the intensity of the
nuclear energy source that is the underlying phenomenon for
both nuclear weapons and power stations
Nuclear engineering calls for many skills and arguably: is
not a basic engineering discipline It certainly has a major
element o f mechanical engineering involved in the economic
and safe development of nuclear power But it is a discipline
that must synthesize abilities in physics and chemistry, mecha-
nical and electrical engineering, metallurgy process control,
economics, biology and health physics and a broad view of the
social responsibilities of engineers The present survey seeks
to cover two points only: an illustration of the role of the
mechanical engineer in major aspects of the use of nuclear
power for the production of electricity in land-based plants;
and to summarize some of the restrictions and regulations
involved in using ionizing radiations in the workplace
It is necessary first to survey briefly the nature of the energy
arising from nuclear reactions and the radiations produced
We also llist some of the other applications involving mechan-
ical engineers in nuclear engineering
Table 13.2 Nuclear radiation examples
uclear radiations and energy
A number of naturally occurring elements are radio-active,
particularly the higher mass elements immediately starting
with uranium and thorium The transformation of the nucleus
is accompanied by the emission of energy (Table 13.2)
Table 13.6
Natural chain alpha-emission 238U + ’34Th + He (a) + y Beta decay - cobalt source
60Co + 60Ni + p + y + neutrino Fusion
D + T = 2H + ‘H 4 ‘He + ‘n + Q: 17.6 MeV Fission
2 3 5 ~ + 1, 2 3 6 ~ 1 : (typically) +I4’La + 87Br + 2 ‘n + Q: approx 200 MeV Fission fragments
1 q P 135Xe P mcs
9,2 long-lived: half-lives (h) 6.7
Common forms for this energy to take are:
0
0
Alpha-particles (the nucleus of helium) - recoil energy; Beta-particles (an electron; more rarely its complement, the positron) - recoil energy;
Gamma-radiation; in nature identical to X-rays and char- acterized by a wave-frequency value proportional to the energy of the gamma photon
Three chains exist in nature for the decay of the heavy elements to lead: the fourth has already decayed One or two other naturally occurring radioactive elements include the potassium-40 isotope (present in fertilizers) and carbon-14 produced in the atmosphere by interaction with cosmic or
solar radiation Each radioactive species is characterized by its half-life (i.e the mean time for half the nuclei to decay) For these chains, the shorter half-lives go with increasing kinciic energy of the radiation
Neutrons, which with protons (the nucleus of hydrogen) form the building blocks or nucleons of the nuclei, can be obtained by stripping the proton out of deuterium, the naturally occurring heavy isotope of hydrogen, in a charged particle accelerator However neutrons are formed in more copious quantities in fusion and fission reactions
I n fusion, light elements have their nuclei brought together
to fuse into a heavier nucleus that is more stable, thus releasing energy In particular, the deuterium-deuterium fusion can proceed either to a nucleus of helium-3 and a neutron or to a nucleus of hydrogen-3 (tritium) and a proton
u : one unified atomic mass unit 1.66057 X IO-” kg a: linear energy transfer
e: one atomic charge 1.60219 x lo-’’ C
Trang 3313/4 Nuclear engineering
Such a process occurs in the stars and is being investigated
(so-called cold fusion) in the laboratory The competing fusion
reaction being studied in magnetic and inertial confinement
systems is the more favourable deuterium-tritium (d-t) re-
action Such a reaction yields 17 MeV (megaelectron-volts) of
energy The energy taken off by the neutron is 14 MeV,
which therefore forms the principal source of energy to be
exploited in making electricity, a formidable engineering task
that has not yet been demonstrated The favoured d-t route to
fusion requires tritium This can be bred locally from lithium
and the excess neutrons leaving the fusion plasma Tritium, a
further isotope of hydrogen, is notoriously difficult to contain
and is radioactive with a half-life of some 12 years It therefore
forms a hazard in such fusion reactors
Fission, discovered in late 1938 and exploited in reactors
and weapons in World War 11, is the process of disrupting a
heavy element, typically uranium-235 or plutonium-239, with
a neutron to give neutron-rich fission products In the process
two or three neutrons are released making a chain reaction
feasible The energy release in fission is about 200 MeV of
which 160 MeV are found in the kinetic energy of the fission
products These can be expected to be slowed down in the
solid fuel which thus becomes hot Coolants are essential and
the hot coolant in turn can be made to raise steam to produce
electricity The fragments are unstable and decay largely by
intense beta emission, thus heating the fuel even after the
neutron reaction is shut down This decay or ‘after-heat’ is a
major safety implication of fission reactors calling for highly
reliable design to ensure safety during an accident
The excess neutrons that are not absorbed in a further
fission will be captured by other elements in the fuel or
structure or leak into the surrounding shield This material, in
turn, has radioactivity induced in it which may last, depending
on the half-lives of the isotopes produced, for hundreds of
years, a major factor in decommissioning fusion or fission
reactors
Capture in uranium-238 (the common isotope) leads to the
production of plutonium-239 The former is called a ‘fertile’
material and is converted to the latter, a ‘fissile’ material, Le
one that is readily fissioned by a slow or thermal neutron
(Essentially, any heavy element would fission if struck by a
sufficiently energetic or fast neutron but there might then be
no energy gain.) Breeder reactors (FBR) exploit this conver-
sion of fertile to fissile material particularly using neutrons
around 1 MeV to cause fission at an energy that produces
about three neutrons per fission
13.3 Mechanical engineering aspects of
nuclear power stations and associated plant
In this section the broad outline of the nuclear electricity
industry is described with reference to mechanical engineering
aspects that embrace: pressure vessel design, heat transfer and
fluid flow, remote manipulators and robotics, automatic con-
trol, etc
13.3.1 Fuel provision
The only significant fuel currently used is natural uranium
Uranium is widely distributed throughout the world (and in
sea water) and economic supplies are found particularly in
Canada, the USA, Southern Africa, the former USSR and
Australia That these areas are in part distinct from oil-
producing regions is a strategic attraction Commercial ores
are exploited to around 1% uranium so that mining, often
open-cast, is accompanied by an on-site concentration pro- cess
Thorium, more widely distributed than uranium, is a poten- tial fertile fuel since it can, in a reactor breed the fissile uranium-233 isotope It is not yet exploited commercially Natural uranium undergoes considerable further processes from the imported yellow cake (oxide) before final fabrication into fuel elements Natural uranium has little risk of leading to
a spontaneous critical reaction (except in some liquid pro- cesses) but has a natural radioactivity that may be an industrial hazard, particularly in the mining stage, where it is associated with radon gas as well as other decay daughters In general, however the metal needs more protection from the sweat of its handlers than vice versa Impurities in the fuel and in structural or moderating materials used in reactors must be kept to exceptionally low levels to avoid competition for neutrons in the chain reactions
Most commercial reactors now use slightly enriched ura- nium where the natural concentration of uranium-235 (around
0.7%) is raised to between 2.5% and 3% Such enrichment is needed to offset the neutron capture in fuel clad with stainless steel or zirconium and to provide for long fuel life before the fissile material is burnt up
The technology of uranium enrichment is well advanced The major route in World War I1 and immediately after was through gaseous diffusion (more properly gaseous fusion) with preferential separation through membranes of the low- pressure gas uranium hexafluoride or ‘hex’ Major mechanical problems with these plants are the large runs of pipes at low pressures, the corrosive nature of fluorine compounds and the design of pumps The high pumping costs of this process fostered the development of the rival ultra-centrifuge process
for enrichment in which hex is separated by a thermally enhanced centrifuge Major mechanical problems here are the reliability of the centrifuges, operating at a rim speed of some
300 mis in vast cascade halls where the mechanical failure of one unit risks knock-on damage to the remainder Gas and magnetic bearings have been developed to meet this challenge (Figures 13.1 and 13.2)
The UK’s enrichment takes place at Capenhurst, where the centrifuge process has superceded the diffusion process since
1982 These plants are operated in conjunction with plants in Germany and Holland by URENCO in which British Nuclear Fuels plc have a third interest
The enrichment plants, of necessity, produce depleted uranium There is some prospect of this being used in fast breeder reactors and thus, like thorium, offering a greatly increased availability of fission fuel Large stocks of depleted uranium are available from store in the UK Its exploitation rests on rising fuel prices until the extra capital expense of the fast breeder reactors is offset by their cheap fuel At least the availability of depleted uranium and the demonstrated ability
of fast breeders to produce electricity puts a ceiling on competing fuels
Enrichment also plays a part in preparing supplies of other items for the nuclear industry, particularly heavy water, and the isotopes of boron and lithium
Plutonium bred in a nuclear reactor by the capture of neutrons in uranium-238 is an artificial source of nuclear fuel available from thermal and fast reactors The immediate product of such breeding is the fissile isotope plutonium-239
It should be known that, like highly enriched uranium-235, this isotope of plutonium is a potential weapons material Fuel removed from a reactor after a short exposure contains principally this weapons-grade plutonium However, fuel left longer - with economic advantage in peaceful use - allows further neutron capture to the successive 240,241 and perhaps
242 isotopes of plutonium Plutonium-241 is fissile but the
Trang 34Mechanical engineering - nuclear power stations and plants 1315
intermediate plutonium-240 is not Thus such fuel is far less suitable for weapons and is known as commercial fuel Plutonium is separated in the reprocessing stages If it is to
be incorporated into fuel elements, either for the core of fast reactors or as a substitute for uranium-235 in thermal reactors, account must be taken of the greater radio-toxicity (some ten times more than uranium) and the risks of criticality accidents
in the concentrated fissile plutonium It is essential therefore for production to be undertaken in a controlled atmosphere, usually at lower than ambient pressures inside glove boxes to direct the leakage of air safely The major risk is of inhalation with deposit in the lungs or of entry to the bloodstream via cuts Ingestion is appreciably less risky as plutonium is not easily absorbed by the gut
The fissile material is used in the dioxide form in most reactors since such ceramics can operate at considerably higher temperatures than the metal Final preparation of fuel, usually after enrichment in uranium-235 or as a mixed oxide with plutonium (MOX), involves fabrication and cladding Cladding is generally in a zirconium alloy (or in stainless steel for higher operating temperatures) with the major purpose of
retaining fission products whose radioactivity would otherwise contaminate the plant Extremely high standards of manufac- turing integrity are required of fuel elements so that quality control and assurance, including non-destructive testing and the documentation of individual fuel elements, is essential Major mechanical implications of the special nature of producing nuclear fuel include therefore: the radiation and criticality hazard, the reliability of high-speed centrifuges, the leaktightness of many miles of vacuum pressure piping in enrichment plant, corrosion of hexafluoride material, the use
of glove boxes and the control of quality in the final fuel element assembly
Figure 13.2 Centrifuge hall (courtesy BNFL and URENCO)
13.3.2 Reactor types
Reactors may be classified in several ways, first by the predominant energy of the neutrons causing fission Fast reactors produce neutrons that directly induce fission They generally require highly enriched uranium or plutonium cores Alternatively, in thermal reactors, the fast neutrons produced
in fission are slowed down or moderated to thermal energies The better yield of neutrons in fission allows natural or only slight enrichment of fuel
Hydrogen is an effective moderating material and finds a particular role in light (ordinary) water-cooled reactors (LWR) which may be pressurized (PWR) or boiling (BWR) Alternatively, heavy water (deuterium oxide) is used The other major category of thermal reactor has gas cooling and graphite moderation, as in the UK Magnox and Advanced
Gas-Cooled Reactors (AGR)
Thus a second categorization of reactors is in their coolant and/or moderator: Light Water Reactors (LWR), Heavy Water Reactors, Gas-Cooled Reactors and Metal-Cooled Reactors where sodium or sodium potassium alloy (NaK) are used for good heat transfer in fast reactors in which modera- tion is not wanted
13.3.3 The coolant challenge
Reactors are operated at high energy density to make them economic despite high capital costs The energy density in a PWR is comparable to the boiler of an oil-fired station; in a FBR it is comparable to a chemical rocket It is essential that coolant capacity is maintained throughout operation In this respect, there is competition between a gaseous coolant with poorer heat transfer capability and a liquid coolant with
greater heat transfer but the risk of two-phase loss of heat
Trang 35Figure 13.3 Cross-section of PWR building
pressure vessel
transfer on depressurization Nuclear power has made consid-
erable demands on the prediction of heat transfer in one- and
two-phase flow
Coolant capacity must be provided not only during normal
operation but also on shutdown, particularly in accident
conditions Whereas the neutron chain reaction can be termi-
nated rapidly with neutron-absorbing control rods, etc., the
after-heat from the decay of previously formed fission pro-
ducts cannot be turned off Roughly, this after-heat is at 10%
of previous steady operating power for a few minutes from
shutdown, decaying slowly until, say, 1% after a week, 0.1%
after a month Thus a reactor that had been designed for
operation at 3000 MW (thermal) must be provided with a
Personnel /access
- T-+21.028 m 0 D operating floor
-Reactor coo la n t
A major failure of the vessel of a PWR would be catastro- phic It would imply no availability of coolant water and the overheating and probably meltdown of the fuel with the release of its residual radioactivity Suffice it to say that 29 of the initial 31 casualties at Chernobyl (1986) died from radia-