See Photovoltaic cells Solar chimneys Category: Energy resources Solar chimneys have long been used both to aid in nat-ural cooling of homes and for passive solar heating.. In recent tim
Trang 1Physical Testing and Analysis
Physical testing investigates the physical
properties and behavior of soils The most
fundamental level of physical testing
in-cludes estimation of specific gravity,
deter-mination of moisture content, sieve
analy-sis, and hydrometer analysis
Specific gravity is defined as the ratio of
“unit weight of soil solids only” to “unit
weight of water.” For sandy soils the
spe-cific gravity ranges from 2.63 to 2.67, while
for clays it is between 2.67 and 2.90 The
specific gravity of organic soils is less than
2.0 Moisture content is defined as the
ra-tio of “the weight of water present in a soil
sample” to “the weight of dry soil.”
Mois-ture content is about 15 to 20 percent for
coarse soils and 80 to 100 percent for clays
Organic soils may have moisture content
in excess of 500 percent
In order to determine the grain-size
dis-tribution of coarse soils, the soil sample is
subjected to sieve analysis This analysis is
conducted by using a stack of sieves that
have a decreasing sieve diameter from the
top to the bottom The sieves are usually
made of woven wires After the sieves are
shaken for a specified period of time, the
amount of sediment retained at each sieve
is weighted Based on these data, each
par-ticle diameter is plotted in terms of the
percentage of material that is finer than this
particu-lar diameter For the plot, a semilogarithmic paper is
used For the determination of particle size
distribu-tion of fine soils, the hydrometer analysis is applied
The lower limit of the particle diameter that can be
detected by this analysis is 0.001 millimeter The
hy-drometer measures the particle diameter indirectly
This analysis is based on the principle that the
hy-drometer will be subject to higher buoyancy forces in
a well-mixed water-sediment system However, as the
suspended solid particles settle, the density of the
water-sediment mixture decreases, and the
hydrome-ter tends to sink The grain-size distribution is integral
to studies involving sediment transport, protection
against erosion, and soil contamination
Engineering Testing and Analysis
Tests that are most useful in geotechnical and
founda-tion engineering are those used to define the
so-called Atterberg limits for clay soils: liquid limit, plas-tic limit, and shrinkage limit The liquid limit is the moisture content that causes clay soils to behave as vis-cous liquids Liquid limit is estimated by counting the number of “blows” required (using a Casagrande de-vice) to close a groove made in the soil sample The plastic limit of moisture content is the point at which the soil sample will turn from a plastic state to a semisolid state The plastic limit is determined by the ability of the sample to roll and form threads 3.18 mil-limeters in diameter without crumbling When clay soils are losing moisture, their volume generally de-creases The moisture content, expressed as a per-centage, at which the soil volume ceases to decrease is called the shrinkage limit
To determine the performance of soils under vari-ous loading conditions a number of soil tests are nec-essary These tests include the Proctor compaction test, the unconfined compaction test, the shear test
A soil scientist inspects a field of canola plants, which will be used for cleansing soils rich in selenium (United States Department of Agriculture)
Trang 2on sands, the consolidation test, and the triaxial tests
in clay The Proctor compaction test identifies the
op-timum moisture content that would result in a
maxi-mum dry unit weight of soil This information is
applied to the design of airports, highways, and
struc-tural foundations The unconfined compaction test
determines the stress-strain relation of a soil
speci-men and is useful in studies regarding retaining walls
and landslides
The shear test in sands is used to estimate the
abil-ity of sands to sustain shear loading It is directly
re-lated to its angle of internal friction Quantification of
the time-dependent settling of saturated clays
sub-ject to increased loading is achieved by the
consolida-tion test Finally, the triaxial tests in clays define the
general stress relationships for unconsolidated
drained, consolidated drained, or consolidated
un-drained specimens
Permeability tests are integral to hydrogeological
studies They define the flow of water through a soil
sample under either a constant hydraulic head or a
falling hydraulic head Permeability tests are
indica-tive not of soil porosity but of the connectivity among
the pores and their ability to form conduits that allow
the water to flow freely through the soil
Environmental Testing and Analysis
Soils and sediment can be subjected to contamination
by a variety of pollutants Fine sediments and organic
soils, because of their large specific surface, show a
particular affinity to adsorb or absorb chemicals in
dissolved or particulate form The most common
chemical testing of soils involves estimation of the pH
value; carbonate, chloride, sulfate, and organic
con-tent; and total dissolved solids The organic content
can be defined easily through a loss-on-ignition test
The total dissolved solids can be estimated by
evapora-tion
Panagiotis D Scarlatos
Further Reading
Budhu, Muni Soil Mechanics and Foundations 2d ed.
Hoboken, N.J.: Wiley, 2007
Das, Braja M Soil Mechanics Laboratory Manual 5th ed.
Austin, Tex.: Engineering Press, 1997
Day, Robert W Soil Testing Manual: Procedures,
Classifi-cation Data, and Sampling Practices New York:
McGraw-Hill, 2001
Head, K H Soil Technicians’ Handbook New York:
Halsted Press, 1989
Kézdi, Árpád Soil Testing Vol 2 in Handbook of Soil
Me-chanics New York: Elsevier Scientific, 1974-1990.
Liu, Cheng, and Jack B Evett Soil Properties: Testing,
Measurement, and Evaluation 6th ed Upper Saddle
River, N.J.: Prentice Hall, 2008
Mudroch, Alena, and José M Azcue Manual of Aquatic
Sediment Sampling Boca Raton, Fla.: Lewis, 1995.
Web Site University of Massachusetts, Amherst, Department of Plant and Soil Sciences Soil and Plant Tissue Testing Laboratory: Results and Interpretation of Soil Tests
http://www.umass.edu/plsoils/soiltest/interp1.htm See also: Aggregates; Bureau of Land Management, U.S.; Bureau of Reclamation, U.S.; Clays; Erosion and erosion control; Peat; Sand and gravel; Sedimentary processes, rocks, and mineral deposits; Soil; Soil man-agement
Solar cells See Photovoltaic cells
Solar chimneys
Category: Energy resources
Solar chimneys have long been used both to aid in nat-ural cooling of homes and for passive solar heating In recent times, a device similar to the traditional solar chimney has been used together with a solar collector and wind turbine to create a means of generating elec-tricity from solar energy.
Background There are three different applications for the device called a solar chimney In all three cases the applica-tion is called a solar chimney The use that has been around the longest is for enhanced ventilation and cooling of living space A second use is a particular type of passive solar-heating system, also called a thermosyphon The third application is for genera-tion of electricity from solar energy
The operation of a chimney in general is based on the fact that heated air will rise because of its decreased
Trang 3density A chimney is used to exhaust flue gases from
combustion in a stove, furnace, or fireplace; the air in
the chimney is heated by the combustion at the base
of the chimney and thus rises, creating an updraft
through the chimney With a solar chimney, however,
nothing burns at the base of the chimney; rather, the
air in the chimney is heated by the Sun, causing it to
rise up the chimney and create an updraft
For enhanced ventilation and natural cooling, the
updraft caused by solar-heated air rising up the
chim-ney is used to draw air through the house, increasing
the ventilation rate and ideally drawing cool air into
the house The hot air is exhausted out the top of the
chimney Solar chimneys for natural cooling were in
use hundreds of years ago in Rome and the Middle
East
The device that is sometimes called a solar chimney
for passive solar heating is simply a rather standard
air-heating solar collector mounted vertically on a
house wall, facing the equator (facing south if in the
Northern Hemisphere, and facing north if in the
Southern Hemisphere) Just as in any solar chimney,
there will be an upflow of solar-heated air For this
ap-plication, the heated air is sent into the living space to provide heating, rather than being sent out the top of the “chimney.” Cool air is drawn into the bottom of the vertical collector from the living space
In the late 1970’s, use of a large solar chimney, to-gether with an extensive solar collector at the base of the chimney and a wind turbine or turbines at the top
to drive a generator, was proposed as another way to generate electricity from solar energy (in addition to photovoltaic cells and heating a fluid with solar en-ergy for use in generating electricity) The term “solar chimney” has come to be used for this latter system, as well as for the traditional type used for enhanced cool-ing or for passive heatcool-ing This concept has been dem-onstrated, and large facilities of this type are in the planning stages
Enhanced Ventilation The solar chimney for this application could be as simple as a traditional chimney painted black on the outside to increase the heating of the air in the chim-ney by the Sun shining on it A more effective design would be to use glass for the side of the chimney
fac-Solar chimneys provide the potential for creating clean energy at minimal cost (AP/Wide World Photos)
Trang 4ing the Sun, with a black absorbing surface on the
back wall of the chimney, so the air between the glass
and the black absorber would be heated There must
be a vent into the living space at the bottom of the
chimney, so that the air flow up the chimney will draw
air from the living space into the bottom of the
chim-ney to be heated This will cause increased air flow or
ventilation through the living space, making it more
comfortable Increased natural cooling can be
ac-complished if the air drawn into the living space is
cool In some cases, the air into the house is drawn
through an underground tunnel to cool it before it
enters the house
Passive Solar Heating
For a solar-chimney-type passive solar heater, the
col-lector consists of insulation against the house and
then a rectangular enclosure with a black absorber at
the back, glazing in the front facing out, and an
air-space between The air between the glazing and the
black absorber is heated by the solar radiation and
rises up the airspace Instead of venting the heated air
out the top, as is done for enhanced ventilation, the
heated air is directed into the living space through a
vent that passes through the building wall at the top of
the solar chimney collector Another vent through the
building wall draws cool room air into the bottom of
the collector This type of device could also be used to
increase summertime ventilation through the living
space, by closing the vent into the living space at the
top of the collector and opening the top of the
collec-tor so that the hot air is exhausted out the top For this
use, the top of the collector should be near the top of
the house, or else the heated air going out of the
lector will heat the portion of the house above the
col-lector This type of passive solar heating system is also
called a thermosiphon
Electricity Generation
The type of solar chimney used to generate electricity
is a combination of three established technologies:
the greenhouse, the chimney, and the wind turbine
The chimney, which is a long tubular structure, is
placed in the center of the circular greenhouse, and
the wind turbine is mounted inside the chimney The
solar-to-electric energy conversion involves two
inter-mediate stages In the first stage, conversion of solar
energy into thermal energy is accomplished in the
greenhouse (also known as the collector) In the
sec-ond stage, the chimney converts the generated
ther-mal energy into kinetic energy and ultimately into electric energy by using a combination of a wind tur-bine and a generator
In its simplest form, the collector is a glass or plastic film cover stretched horizontally and raised above the ground This covering serves as a trap for reradiation from the ground The ground under the cover is heated and, in turn, heats the air flowing radially above it The height of the collector cover gradually increases toward the center of the collector, providing
a smooth transition for the hot air flowing from the collector into the chimney A flat collector of this kind can convert a significant amount of irradiated solar energy into heat The soil surface under the collector cover is a convenient energy storage medium During the day, a part of the incoming solar radiation is ab-sorbed by the ground; it is later released during the night The mechanism is capable of providing a con-tinuous supply of power all year round
The chimney itself is the actual thermal engine The upthrust of the air heated in the collector is pro-portional to the rise in air temperature in the collec-tor and the volume of the air flowing The latter de-pends on the height of the chimney Mechanical output in the form of rotational energy can be ex-tracted from the vertical air current flowing in the chimney by using a suitable turbine (or turbines) This mechanical energy can be converted into elec-tric energy by coupling the turbines to the generators Solar chimneys do not necessarily need direct sun-light They can exploit a component of the diffused radiation when the sky is clouded The system also has
an advantage over traditional systems that use wind to provide power in that it does not depend on the natu-ral occurrence of wind, which always fluctuates More-over, because the direction of air movement is fixed, the complicated tracking mechanism necessary for
a horizontal-axis wind turbine is not needed Solar chimneys are relatively reliable and simple to build, and they do not have adverse effects on the environ-ment The necessary materials are readily available, and maintenance costs are minimal
Less than four years after the solar chimney was first proposed in the late 1970’s, construction of a pi-lot plant began in Manzanares, Spain A 36-kilowatt pilot plant was built; it produced electricity for seven years, thus proving the efficiency and reliability of the technology The chimney tower was 194.6 meters high, and the collector had a radius of 122 meters
S A Sherif, updated by Harlan H Bengtson
Trang 5Further Reading
Afonso, Clito, and Oliveira, Armando “Solar
Chim-neys: Simulation and Experiment.” Energy and
Buildings 32, no 1 (2000): 71-79.
Dai, Y J., H B Huang, and R Z Wang “Case Study of
Solar Chimney Power Plants in Northwestern
Re-gions of China.” Renewable Energy 28, no 8 (2003):
1295-1304
Dai, Y J., K Sumathy, R Z Wang, and Y G Li
“En-hancement of Natural Ventilation in a Solar House
with a Solar Chimney and a Solid Adsorption
Cooling Cavity.” Solar Energy 74, no 1 (2003): 65-75.
El-Haroun, A A “The Effect of Wind Speed at the
Top of the Tower on the Performance and Energy
Generated from Thermosyphon Solar Turbine.”
International Journal of Solar Energy 22, no 1 (2002):
9-18
Pretorius, J P., and D G Kröger “Critical Evaluation
of Solar Chimney Power Plant Performance.” Solar
Energy 80, no 5 (2006): 535-544.
Schlaich, Jörg, Rudolf Bergermann, Wolfgang Schiel,
and Gerhard Weinrebe “Design of Commercial
Solar Updraft Tower Systems—Utilization of Solar
Induced Convective Flows for Power Generation.”
Journal of Solar Energy Engineering 127, no 1 (2005):
117-124
See also: Electrical power; Photovoltaic cells; Solar
energy; Wind energy
Solar energy
Category: Energy resources
Present patterns of nonrenewable energy usage cannot
be sustained; diminishing supplies, increased prices,
and concerns over global warming have made the
move to sustainable sources of energy inevitable Solar
is the only renewable energy resource plentiful enough
to serve as the foundation of a new global energy
econ-omy Not only is it abundant and clean, but also it has
become both economically competitive and politically
viable.
Background
Solar energy has provided continuously almost all of
Earth’s energy, which humans have exploited,
di-rectly or indidi-rectly, since prehistoric times The steady
evolution of solar technology, however, has been in-terrupted sporadically by the discovery of plentiful and inexpensive nonrenewable fuels, such as coal, oil, and uranium Successive civilizations have shortsight-edly overexploited each new resource by treating it as
an inexhaustible supply governed only by price and availability
Designing buildings to optimize the use of the Sun during various seasons began in ancient Greece Dur-ing the fourth century b.c.e., a severe shortage of wood for fuel necessitated that homes be designed to take advantage of the abundant sunlight during the moderately cool winters while taking advantage of shade during the summer Because glass or other transparent materials for doors and windows did not yet exist, houses had to be designed to collect as much sunlight as possible during the short winter days This was achieved by designing houses with (in the North-ern Hemisphere) south-facing covered porches that were closed to the north During the winter the low-angled sunlight streamed through the porch and warmed the interior rooms During summer, rooms were shaded from the high Sun by the porch roof The ancient Romans used wood at such a prodi-gious rate that by the first century b.c.e., timber had
to be imported from more than 1,000 kilometers away The high cost of imported wood led the Romans
to copy Grecian solar architecture, but they added clear glass window coverings to keep solar energy trapped within a house They also expanded the uses
of solar heating to include greenhouses and public baths Solar design became so entrenched in the Roman state that guarantees of “sun rights” were in-corporated into Roman law After the fall of Rome, however, solar architecture was forgotten until the Re-naissance During the sixteenth century there was a revival of horticulture and greenhouses were used to grow exotic fruits and vegetables in northern Europe
By the eighteenth century, new glass-manufacturing techniques allowed the production of large windows for greenhouses Experimenters also developed “hot boxes,” glass-enclosed devices for achieving tempera-tures up to 88° Celsius
During the nineteenth century, the greenhouse evolved, at least for wealthy people, into a place for the ostentatious display of exotic plants While the Industrial Revolution was fueled by coal, some far-sighted individuals saw the vast energy of the Sun as an untapped source of power Focusing collectors, con-sisting of concentrating mirrors, developed during
Trang 6the seventeenth century, were being used by the latter
half of the nineteenth century to focus sunlight to
produce steam for operating small steam engines
In the early twentieth century, the perfection of
large solar steam engines for pumping water and for
irrigation occurred Concurrently, active systems were
invented and marketed to heat water for domestic
use, and passive systems were rediscovered as an
en-ergy-efficient way to help heat homes in a variety of
cli-mates The discovery of convenient and inexpensive
natural gas, however, virtually eliminated the solar
in-dustry By the end of World War II, not one of the
doz-ens of active solar energy manufacturers remained
During the 1970’s and continuing through 1985,
the interest in solar energy for residential and
com-mercial purposes grew This growth was caused by
lim-ited supplies and steeply rising costs of oil, coal, and
electricity Additional incentives for installing active
solar-heating systems were provided for homeowners
by federal tax credits for alternate energy devices
However, by the mid-1980’s, the price of gasoline was
dropping, electricity rates had stabilized, and the tax
credit program expired without renewal The effects
on the solar-energy market were immediate and
dev-astating On the positive side, the decade of high
growth had created an enhanced public awareness of
solar energy
Because fossil fuels remained relatively
inexpen-sive during the 1990’s, and because of the high initial
cost of solar-energy systems, solar did not significantly
impact world energy consumption However, during
the first decade of the twenty-first century, the price of
fossil fuels began to escalate again because of the
de-pletion of oil reserves The U.S Office of Technology
Assessment projected that at the 2009 rate of use all
known oil reserves would be depleted by 2037 As
fossil-fuel prices continue to rise, solar heating has
in-creased in economic viability In addition, the cost of
electricity from photovoltaic (PV) cells decreased
dra-matically after the 1990’s Future technological
break-throughs and economies of scale will undoubtedly
provide additional economic advantages to solar PV
energy
Nature of Solar Energy
Every day, the Earth receives ten thousand times more
energy from the Sun than humans derive from all
other alternative energies and nonrenewable fuels
combined Above Earth’s atmosphere, 170 billion
megawatts of power are available, but the energy’s
intensity is diluted to 430 British thermal units per hour per square foot (Btu/hr/ft2); this is attenuated
to between 100 and 200 Btu/hr/ft2at Earth’s surface, thus requiring large collector areas to capture signifi-cant amounts of usable energy Nevertheless, industri-alized societies have come to realize that past patterns
of energy consumption cannot be sustained A viable energy future requires not only that solar energy be harnessed but also that technologically advanced so-cieties modify their lifestyles to live in closer harmony with nature before an energy crisis of global propor-tions decimates humanity
Utilizing solar energy as a major source of energy for the world has several advantages First, solar en-ergy is virtually inexhaustible and is constant (at least above the Earth’s atmosphere) Second, it is clean; the only direct environmental impact may be aes-thetic—some active collection systems are conspicu-ously ugly However, more attractive large-scale de-vices could be designed, and smaller units could be integrated into residential structures so as to be less obvious Finally, the collected energy is “free” after the initial cost of purchase and installation
On the other hand, there are several disadvantages associated with solar energy First, being diffuse, the Sun’s energy must be collected and concentrated Second, it is intermittent It is only available during daylight hours, and even then it may be obscured
by cloud cover Hence it must be stored, and storage
is neither convenient nor efficient Finally, active col-lection devices are constructed of relatively expen-sive nonrenewable resources such as aluminum and copper
Types of Solar Energy Systems All solar heating systems share two common elements:
a device for collecting energy from the Sun to provide electricity, heat, or air-conditioning and a facility for storing the energy when it is not needed PV systems convert the Sun’s radiant energy directly into electric-ity, while thermal solar units provide heat for interior spaces or hot water for domestic uses Concentrating collectors are used to create steam that can power air-conditioning units or generate electricity
Thermal solar energy is captured in active or in passive systems Active systems require electricity to power pumps or fans, while passive systems convert sunlight directly into interior space heating Active systems may be subdivided into those that use flat-panel stationary collectors and systems that focus
Trang 7coming solar rays in order to achieve temperatures
high enough to create steam Active solar heating
sys-tems transfer a Sun-heated medium (air or water)
from an exterior south-facing collector (north-facing
if located in the Southern Hemisphere) to the point
of use or to a storage facility For air systems the
stor-age facility is a bed of rocks, while water systems store
heat in tanks of water Active concentrating systems
focus sunlight by one of two possible means: power
towers consisting of mirror arrays, which reflect
sun-light from a large area into a small central region, or
troughs of parabolic mirrors, which focus sunlight to
a central axis
A solar furnace is a specialized type of focusing
col-lector that uses large arrays of parabolic mirrors to
concentrate sunlight to a focal point where extremely
high temperatures are achieved The world’s largest
solar furnace was constructed in 1970 in the French
Pyrenees, where sunny weather occurs three hundred
days annually An array of sixty-three flat, movable
mirrors on a hillside reflects sunlight into a huge
curved mirror with an area of 1,800 square meters,
which then focuses the light into a 0.09-square-meter
spot This delivers 1,000 kilowatts of power and
cre-ates a temperature that can exceed 2,980° Celsius
Furnaces of this type are primarily used for materials
research
Passive solar-heating systems use no external power;
they collect light from the Sun and transform it into
heat, which warms a building by natural convection
The collectors are glass windows on the south wall, and the room’s interior air mass stores the heat There are three main types of passive systems: direct gain, in-direct gain, and attached gain Direct gain systems re-quire large expanses of south-facing glass to admit sunlight into an interior space where ample mass has been incorporated to avoid overheating Indirect gain systems require a massive wall to be positioned di-rectly behind the south-facing glass The third system, attached gain, consists of a greenhouse attached to the exterior south wall, but accessible to the interior through insulated doors The doors can be opened to heat the house when the greenhouse is warm and closed when it is cold A fourth system, the thermo-siphon, uses flat-plate collectors for domestic hot water production, but because no electricity is used, it
is technically passive A water storage tank is located at
a higher elevation than the top of the collector Water
in the collector, heated by the Sun, rises by convection and enters the storage tank, creating a siphon effect that keeps the fluid circulating
PV cells use the photoelectric effect to transform solar radiation directly into electric current The cells are made of semiconducting materials that act like in-sulators until impinging sunlight puts electrons in the conducting state, effectively making each cell a small battery When large arrays of such cells are con-nected, sufficient power can be generated to power individual residences or produce electricity for a cen-tralized power plant
Comparison of Two Types of Solar Energy Collectors
Converts solar energy directly into electricity
for immediate use
Collects heat from solar energy for conversion into electricity
Electricity can be converted into heat for
thermal use
Heat is used directly
Solar radiation of only a very small range
of energy can be utilized
Radiation of a wide range of energy can be used
Requires additional storage devices that
are costly and inefficient
Some have built-in storage devices that are relatively inexpensive and efficient
Ideal for micropower and small appliances Unsuitable for micropower and small appliances
Trang 8Active Solar Heating Systems
These types of systems, first developed in the early
de-cades of the twentieth century, were largely forgotten
(after some considerable initial interest) when
inex-pensive fossil fuels became widely available The oil
embargo of 1973 renewed interest in these systems,
and government tax credits helped homeowners
de-fray the initial high cost of purchasing and installing
active systems
All active solar systems have the following
compo-nents: a collector, a pump or fan to circulate the heat
transfer fluid, and a means of storing excess energy
Active systems have two basic uses Either they are
used to heat the interior of a building (or at least to
preheat the circulating fluid), or they are used for
do-mestic hot water (DHW) The circulating fluid for
DHW systems is always water, and the storage unit is a
water tank, typically having a 50-gallon capacity Space
heating units may use either circulating air or
circulat-ing water, but air systems are more common
Solar flat-plate collectors consist of an enclosed
rectangular box containing a metal plate with a flat
black surface covered by nonreflecting tempered glass
Tubes to conduct water across the collector are
sol-dered to the plate for water systems, while in air
sys-tems channels direct the airstream The entire box
must be watertight and well insulated When sunlight
strikes the black surface, light is changed into heat,
which is transferred to the fluid moving across the heated surface Water systems typically require pro-pylene glycol in the circulating fluid to prevent freez-ing A heat exchanger transfers the heat to the storage tank; the hot water created is used for DHW or pre-heated water for hydronic heating systems
A differential thermostat, which compares the tem-perature of the collector to the storage tank tempera-ture, is employed so the collector fluid flows only when the flat-plate collector is warmer than the stor-age water Whenever the collector is at least 10° Cel-sius warmer than the storage tank, the pump is acti-vated until the temperatures equalize
When air is used as the working fluid, excess heat is stored either in smooth rocks or in a phase-change material Rock storage consists of a large bin of 1-inch-diameter rocks, filling a 280-cubic-foot volume and weighing 6.3 metric tons When the collector is warmer than the house and heat is desired, a fan pumps the air directly from the collector into the house When the residence reaches the desired temperature, the air is directed through the rock bin, transferring the heat into the rocks At night, air from the house can
be circulated through the rocks to reclaim the stored heat
Phase-change materials store heat by changing from
a solid to a liquid at a temperature somewhere be-tween 27° and 32° Celsius The most common
Sunlight
Collector
Pump
Heat exchanger
Domestic hot water out
Cold water in
Hot water storage tank
Active Solar Domestic Hot Water System
Trang 9stance used in solar applications is Glauber’s salt
(so-dium sulfate decahydrate) Heat from the Sun is
absorbed by the Glauber’s salt as it melts In the
eve-ning, when the temperature drops below the melting
point, the salt resolidifies and releases its stored heat
to the room Because there are several unsolved
prob-lems with phase-change storage systems, most
air-heating systems use rock bin storage
Air systems have several advantages over
space-heating water systems They are less expensive, the
fluid is not subject to freezing, and leaks are not
cata-strophic Air, however, is not as efficient a heat-transfer
medium as water, the storage facility is considerably
larger and heavier, fans require more electricity to
op-erate than pumps, and air ducts require much more
space than water lines Also, air blowing directly into a
room from the collectors may have a temperature of
only 27° Celsius Although this will heat the room, it
feels cold and drafty because of the higher human
body temperature
An auxiliary heater is required for most
applica-tions of solar energy in the United States Comparing
DHW and space-heating requirements, DHW has one
major advantage: It can be used all year During the
summer months when the Sun is high in the sky, DHW
systems are most efficient and can often provide most
of a family’s hot water needs Typically, for a family of
four, two or three solar collectors of 1.2 meters by 2.4
meters apiece can provide at least one-half of the
an-nual hot water needs In order to heat the interior
space of an average home in middle or northern
lati-tudes, however, twenty or thirty collectors are
re-quired Thus, using solar collectors for DHW costs
more than most homeowners can afford, while active
space-heating systems are often too expensive for the
average homeowner to consider
Solar thermal power plants concentrate sunlight in
order to produce high-temperature steam that drives
a turbine to produce electricity in the conventional
manner Two types of collector technologies are in
use: parabolic troughs and central receivers Trough
systems use parabolic reflectors to concentrate
sun-light onto an oil-filled tube positioned along the focal
line The heated oil is piped to a central location,
where it produces the steam required to drive the
tur-bine A system of this type located in Southern
Califor-nia supplies 350 megawatts of power on clear days,
with a conversion efficiency of 25 percent and at a
price competitive with electricity produced by
fossil-fuel power plants
Central receivers, or “power towers,” use a large array of movable Sun-tracking mirrors to reflect sun-light to a central location on top of a tower At this point, the concentrated sunlight produces tempera-tures ranging from 538° to 1,480° Celsius, which va-porizes a working fluid that drives the turbine A 10-megawatt pilot plant in Southern California, later modified to produce 200 megawatts of power, pro-vides electricity at a cost comparable to that of elec-tricity from conventional power plants After the suc-cessful conclusion of these demonstration projects, many commercial plants in the 30- to 50-megawatt range were designed for the southwestern United States, Spain, Italy, Egypt, and Morocco By the end of
2006, fifteen large thermal solar generating stations were operational in the United States alone, ten in California and five in Arizona
Passive Solar Space Heating This type of heating is achieved entirely by natural means—heat circulates by natural convection without pumps or fans A well-designed passive system should include these elements: south-facing insulated (dou-ble-paned) windows to collect the winter sunlight, interior thermal mass to prevent overheating, night insulation to cover the windows, overhangs above the windows to keep out the summer sunlight, and suffi-cient insulation to minimize heat loss
Passive systems may be categorized as one of three types, depending on the relative locations of the win-dows and thermal mass These are termed direct-gain, indirect-gain, and attached-gain (or greenhouse) sys-tems In direct-gain systems large south-facing win-dows admit sunlight, which falls on the thermal mass, usually brick, tile, or concrete The mass, which may
be incorporated into a floor, a wall, or even an earth-filled planter, must be sized to the total area of south-facing glass—the greater the area, the greater the mass required to prevent overheating and to store heat efficiently for evening Without sufficient ther-mal mass, indoor temperatures can exceed 32° Cel-sius during the day while plummeting to uncomfort-ably low temperatures at night
Indirect-gain systems have a massive wall, con-structed of brick or barrels of water, located close to the south-facing glass The outside-facing surface of the mass is painted black, which transforms sunlight into heat The heat is released through vents into the interior living space by natural convection and is ab-sorbed by the thermal mass During the night the
Trang 10vents are closed, preventing heat loss by convection,
while the mass radiates heat into the interior space
Attached-gain, or greenhouse, systems are added
to an exterior south-facing wall of a house The
struc-ture is usually completely glass, while the mass is
con-crete and/or soil for growing plants During a sunny
day, heat from the greenhouse is admitted into the
residence, while at night the openings are sealed
When properly designed, an attached greenhouse
can be used to provide food as well as heat during the
winter season
When carefully designed and appropriately
inte-grated into a residence during construction, passive
solar systems can save 50 percent on heating expenses
for a 5 percent increase in construction costs During
the 1990’s, approximately 7 percent of new homes
built in the United States incorporated passive solar
heating features; this percentage increased during
the first decade of the twenty-first century
Photovoltaic Systems
Although passive and active solar technologies are
well understood, the direct conversion of sunlight
into electricity by means of photocells is still being
de-veloped Once too expensive for typical homeowners,
new technologies are reducing the cost so that it is be-coming a cost-effective and viable energy alternative, particularly in remote areas where the cost of running conventional power lines would be prohibitive Solar PV cells installed on individual residences are particularly beneficial for providing intermediate load demand because they provide electricity during the sunniest and hottest part of the day, when the de-mand for air-conditioning is maximum Such systems are also cost-effective for utility companies because if less electricity needs to be provided, the cost of up-grading transmission lines and associated equipment
is reduced When individual PV systems are tied to the grid, excess energy produced during the day can be fed back into the grid, while nighttime requirements are available from the grid Home systems connected
to the grid eliminate the need for large banks of stor-age batteries, which are expensive and potentially dangerous Cost-effective grid-connected systems are becoming common in Japan and Germany
As the world market for PV systems increases from less than 1 percent of new generating systems to hun-dreds of times this level by the mid-twenty-first cen-tury, these systems will also become significantly less expensive When PV systems are installed during
As of 2009, this commercial solar tower in Sanlucar la Mayor, Spain, was the most powerful on the planet (AP/Wide World Photos)