Green Energy Technology, Economics and Policy Part 5 pptx

11.3.2 Ocean thermal energy Ocean thermal energy conversion OTEC uses the temperature difference that existsbetween deep and shallow waters to run a heat engine.. effi-As long as the tem

Ocean currents represent a significant, currently untapped, reservoir of energy Thetotal worldwide power in ocean currents has been estimated to be about 5,000 GW,with power densities of up to 15 kW/m2.

In large areas with powerful currents, it would be possible to install water turbines

in groups or clusters to create a marine current facility Turbine spacing would bedetermined based on wake interactions and maintenance needs A 30 MW demonstra-tion array of vertical turbines in a tidal fence is being investigated in the Philippines(WEC, 2001)

However a number of potential problems need to be addressed, including avoidance

of drag from cavitations (air bubble formation that creates turbulence and decreases theefficiency of current-energy harvest), prevention of marine growth build up, corrosioncontrol, and overall system reliability Because the logistics of maintenance are likely

to be complex and the costs potentially high, system reliability is of high importance.Ocean currents flow relatively steadily throughout the year and in some cases theflow is considerable An example is the Straits of Florida where the Gulf Stream flowsout of the Caribbean Sea and into the North Atlantic on its way to northern Europe.The speed of the current is around 7.4 km/h at the surface, but it decreases with depth.There is a potential extractable power of 1 kW/m2near the surface

A 300 kW full-scale plant installed by Marine Current Turbines (MCT) has beenoperating at Lynmouth, Devon (UK) since May 2003 MCT has also been planningdeep sea marine current systems, which could be constructed in large farms and thus useeconomies of scale both in construction and maintenance and in the infrastructure forbringing the electricity to shore Another approach which has identified the potential

of the Gulf Stream is the Gorlov helical turbine, a vertical-axis turbine which is beingcurrently prototyped in South Korea

No currently operating commercial turbines are connected to an electric-powertransmission or distribution grid; however, a number of configurations are being tested

on a small scale Because no commercial turbines are currently in operation, it is cult to assess the costs of current-generated energy and its competitiveness with otherenergy sources Initial studies suggest that for economic exploitation, velocities of atleast 2 m/s would be required, although it is possible to generate energy from velocities

diffi-as low diffi-as 1 m/s

Major costs of these systems would be the cables to transport the electricity to theonshore grid There are many similarities and common problems with tidal-currentenergy extraction

Potential environmental impacts of ocean current energy extraction include:

• Impacts on marine ecology and conflicts with other potential uses of the same area

of the ocean;

• Resource requirements associated with the construction and operation; and

• Protection of species, particularly fish and marine mammals

The slow blade velocities should allow water and fish to flow freely and safely throughthe structure Protective fences and sonar-activated brakes could prevent larger marinemammals from harm In the siting of the turbines, consideration of impacts on shippingroutes, and present as well as anticipated uses such as commercial and recreationalfishing and recreational diving, would be required

The need to introduce possible mitigating factors, such as the establishment of fisheryexclusion zones has to be considered Concerns have been raised about risks fromslowing the current flow by extracting energy Local effects, such as temperature andsalinity changes in estuaries caused by changes in the mixing of salt and fresh waters,would need to be considered for their potential impact on estuary ecosystems (Charlierand Justus 1993).

Damage to seabed flora is also potentially dangerous and designs are being exploredwhich are anchored to the seabed but operate at a distance, rather than having towersbuilt on the bed Since there are at present no firm plans for deployment of thesedevices, it is difficult to evaluate whether this will be a serious problem

11.3.2 Ocean thermal energy

Ocean thermal energy conversion (OTEC) uses the temperature difference that existsbetween deep and shallow waters to run a heat engine The greatest efficiency andpower is produced with the largest temperature difference This temperature differencegenerally increases near the equator The ocean surface contains a vast amount of solarenergy, which can potentially be harnessed for human use If this extraction could bemade cost effective on a large scale, it could be a source of renewable energy (Averyand Wu, 1994)

The technical challenge of OTEC is to generate significant amounts of power ciently from this very small temperature ratio Changes in efficiency of heat exchange

effi-in modern designs allow performance approacheffi-ing the theoretical maximum efficiency.The earth’s oceans are continually heated by the sun and cover nearly 70% of thesurface This makes them the world’s largest solar energy collector and energy storagesystem On an average day, 60 million km2of tropical seas absorb an amount of solarradiation equal in heat content to about 250 billion barrels of oil

The total energy available is one or two orders of magnitude higher than otherocean energy options such as wave power But the small magnitude of the temperaturedifference makes energy extraction comparatively difficult and expensive, due to lowthermal efficiency Earlier OTEC systems had an overall efficiency of 1 to 3% Thetheoretical maximum efficiency lies between 6 and 7%

Current designs under review will operate closer to the theoretical maximum ciency The energy carrier, seawater, is free, though it has an access cost associated withthe pumping materials and pump energy costs An OTEC plant can be configured tooperate continuously to supply base load power

effi-As long as the temperature between the warm surface water and the cold deep waterdiffers by about 20◦C, an OTEC system can produce a significant amount of power.The oceans are thus a vast renewable resource, with the potential to help us producebillions of watts of electric power The cold, deep seawater used in the OTEC process

is also rich in nutrients, and it can be used to culture both marine organisms and plantlife near the shore or on land

This cold seawater is an integral part of the three types of OTEC systems: cycle, open-cycle, and hybrid To operate, the cold seawater must be brought to thesurface This can be accomplished through direct pumping A second method is todesalinate the seawater near the sea floor; this lowers its density, which will cause it

closed-to rise up through a pipe closed-to the surface

Working fluid-Ammonia Pump

Water in 15’C

Water

in 5’C

Water out 10’C

OTEC

Condenser Turbine

Vaporizer Ocean surface

Figure 11.3.1 Scheme of closed cycle OTEC plant

Closed-cycle systems use fluid with a low boiling point, such as ammonia, to

rotate a turbine to generate electricity Warm surface seawater is pumped through

a heat exchanger where the low-boiling-point fluid is vaporized The expanding vaporturns the turbo-generator Then, cold, deep seawater—pumped through a second heatexchanger—condenses the vapor back into a liquid, which is then recycled through thesystem (Fig 11.3.1)

In 1979 the Natural Energy Laboratory (NEL) and several private-sector partnersdeveloped the mini OTEC experiment, which achieved the first successful at-sea pro-duction of net electrical power (Trimble and Owens, 1980) The mini OTEC vesselwas moored 2.4 km off the Hawaiian coast and produced enough net electricity toilluminate the ship’s light bulbs, and run its computers and televisions NEL in 1999tested a 250 kW pilot closed-cycle plant

Open-cycle OTEC uses the tropical oceans’ warm surface water to make electricity.

When warm seawater is placed in a low-pressure container, it boils The expandingsteam drives a low-pressure turbine attached to an electrical generator The steam,which has left its salt and contaminants behind in the low-pressure container, is purefresh water It is condensed back into a liquid by exposure to cold temperatures fromdeep-ocean water This method has the advantage of producing desalinized fresh water,suitable for drinking water or irrigation

In 1984 National Renewable Energy Laboratory developed a vertical-spout rator to convert warm seawater into low-pressure steam for open-cycle plants Energyconversion efficiency of 97% was achieved for the seawater-to-steam conversion pro-cess The overall efficiency of an OTEC system was few per cent In 1993, an open-cycleOTEC plant at Keahole Point, Hawaii, produced 50 000 watts of electricity during anet power-producing experiment

evapo-Hybrid cycle combines the features of both the closed-cycle and open-cycle

sys-tems In a hybrid OTEC system, warm seawater enters a vacuum chamber where

it is flash-evaporated into steam, similar to the open-cycle evaporation process The

steam vaporizes the ammonia working fluid of a closed-cycle loop on the other side

of an ammonia vaporizer The vaporized fluid then drives a turbine to produce city The steam condenses within the heat exchanger and provides desalinated water.The electricity produced by the system can be delivered to a utility grid or used tomanufacture methanol, hydrogen, refined metals, ammonia, and similar products.OCEES International, Inc is working with the U.S Navy on a design for a proposed

electri-13 MW OTEC plant in Diego Garcia, which would replace the current power plantrunning diesel generators The OTEC plant would also provide 1.25 MGD of potablewater to the base Another U.S company has proposed building a 10 MW OTEC plant

in Guam

Lockheed Martin’s Alternative Energy Development team is currently in the finaldesign phase of a 10 MW closed cycle OTEC pilot system which will become oper-ational in Hawaii during 2012–2013 This system is being designed to expand to

100 MW commercial systems in the near future

OTEC has important benefits other than power production The 5◦C cold seawatermade available by an OTEC system creates an opportunity to provide large amounts ofcooling to operations that are related to or close to the plant The cold seawater from anOTEC plant can be used in chilled-water coils to provide air-conditioning for buildings.OTEC technology also supports chilled-soil agriculture When cold seawater flowsthrough underground pipes, it chills the surrounding soil The temperature differencebetween plant roots in the cool soil and plant leaves in the warm air allows manyplants that evolved in temperate climates to be grown in the subtropics

Aquaculture can be a byproduct of OTEC Deep ocean water contains highconcentrations of essential nutrients that are depleted in surface waters due to bio-logical consumption This “artificial upwelling’’ mimics the natural upwelling that isresponsible for fertilizing and supporting marine ecosystems

Desalinated water can be produced in open- or hybrid-cycle plants using surfacecondensers In a surface condenser, the spent steam is condensed by indirect contactwith the cold seawater Studies indicate that a 2 MWe net plant could produce about

4 300 m3of desalinated water each day

Hydrogen can be produced via electrolysis using electricity generated by the OTECprocess The steam generated can be used as a relatively pure medium for electrolysiswith electrolyte compounds added to improve the overall efficiency

It will be possible to extract many elements contained in salts and other forms anddissolved in sea water In the past, most economic analyses concluded that mining theocean for trace elements dissolved in solution would be unprofitable, in part becausemuch energy is required to pump the large volume of water needed The Japaneserecently began investigating the concept of combining the extraction of uraniumdissolved in seawater with wave-energy technology

The economics of energy production today have delayed the financing of a nent, continuously operating OTEC plant OTEC is very promising as an alternativeenergy resource for tropical island communities that rely heavily on imported fuel.OTEC could provide the islands with much-needed power, as well as desalinated waterand a variety of aquaculture products

perma-Because OTEC systems have not yet been widely deployed, estimates of their costsare uncertain One study estimates power generation costs as low as US $0.07 perkilowatt-hour, compared with $0.05–$0.07 for subsidized wind systems

Future research needed to accelerate the development of OTEC systems include:

• Characterization of cold-water pipe technology;

• Advanced heat exchanger systems to improve heat transfer performance anddecrease costs; and

• Innovative turbine concepts for the large machines required for open-cycle systems

11.3.3 Salinity gradient power

Salinity gradient power is the energy retrieved from the difference in the salt tration between seawater and river water Two practical methods for this are reverseelectro-dialysis (RED) and pressure retarded osmosis (PRO) Both processes rely onosmosis with ion specific membranes Osmotic pressure is the chemical potential ofconcentrated and dilute solutions of salt

concen-All energy that is proposed to use salinity gradient technology relies on the ration to separate water from salt Solutions with higher concentrations of salt havehigher osmotic pressure

evapo-The technologies have been tested in laboratory conditions evapo-They are being loped on commercial scales in the Netherlands (RED) and Norway (PRO) Though thecost of the membrane is quite high, a new cheap membrane, based on an electricallymodified polyethylene plastic, has been proposed The world’s first osmotic plant withcapacity of 4 kW was established in 2009 in Tofte, Norway

deve-Other methods have been proposed and are currently under development includethat based on electric double layer capacitor and vapor pressure difference technolo-gies (Olsson et al, 1979; Brogioli, 2009)

The osmotic pressure difference between fresh water and seawater is equivalent

to 240 m of hydraulic head Theoretically a stream flowing at 1 m3/s could produce

1 MW of electricity The worldwide fresh to seawater salinity resource is estimated at2.6 TW This is comparable to the ocean thermal gradient estimated at 2.7 TW Inlandhighly saline lakes have higher potential The Dead Sea osmotic pressure differentialcorresponds to a head of 5 000 m, which is almost twenty times greater than seawater.Salinity gradient power is a specific renewable energy alternative that creates renew-able and sustainable power by using naturally occurring processes This practice doesnot contaminate or release CO2 emissions Vapor pressure methods will release dis-solved air containing CO2 at low pressure, but these non-condensable gases can bere-dissolved

In PRO, a membrane separates two solutions, salt water and fresh water Only watermolecules can pass the semi-permeable membrane As a result of the osmotic pressuredifference between both solutions, fresh water will diffuse through the membrane inorder to dilute the solution The pressure drives the turbines and powers the generatorthat produces the electrical energy (Brauns, 2007)

RED is the salinity gradient energy retrieved from the difference in the salt centration between seawater and river water A salt solution and fresh water are letthrough a stack of alternating cathode and anode exchange membranes The chemicalpotential difference between salt and fresh water generates a voltage over each mem-brane and the total potential of the system is the sum of the potential differences overall membranes

con-RED process works through difference in ion concentration instead of an electricfield, which has implications for the type of membrane needed As in a fuel cell, thecells are stacked A module with a capacity of 250 kW has the size of a shippingcontainer.

In the Netherlands more than 3 300 m3 fresh water runs into the sea per second

on average The membrane halves the pressure differences which results in a watercolumn of approximately 135 meters The energy potential is 4.5 GW

There has generally been a lack of systematic research and development activity inthis area Early technical advances were not considered promising, mainly becausethey relied on expensive membranes Membrane technologies have advanced, but todate, they remain the technical barrier to economical energy production Efforts areunderway to address those issues and alternatively develop designs that eliminate mem-brane Additional challenges include high capital costs and low efficiency (Jones andRowley, 2003)

Principal advantages are no fuel cost, no CO2emissions or other significant effluentsthat may interfere with global climate Inefficient extraction would be acceptable aslong as there is an adequate return on investment Salts are not consumed in the process.Systems could be non-periodic, unlike wind or wave power Systems can be designedfor large or small-scale plants and could be modular in layout

11.3.4 Tidal power

Tidal power is a form of hydropower that converts the energy of tides into electricity orother useful forms of power Tidal power has potential for future electricity generation.Tides are more predictable than wind energy and solar power (Baker, 1991)

Tidal power is the only form of energy which derives directly from the relativemotions of the earth–moon system, and to a lesser extent from the earth–sun system.The tidal forces produced by the moon and sun, in combination with earth’s rotation,are responsible for the generation of the tides

For producing significant amount of energy out of tidal water turbines, range oftides should be high Substantial amount of water should be there for pushing waterthrough the turbine Approximately 4 to 5 r meters range of tides is require producingsignificant amount electricity

It is significantly important to spot the appropriate place which provides suitable andsustainable conditions to produce tidal energy There are plenty of places around theglobe which provide good conditions for installing water turbines The Bay of Fundy

in Canada and the Bristol Channel between England and Wales are two particularlynoteworthy examples

The magnitude of the tide at a location is the result of the changing positions of themoon and sun relative to the earth, the effects of earth rotation, and the local shape ofthe sea floor and coastlines The stronger the tide, either in water level height or tidalcurrent velocities, the greater the potential for tidal electricity generation (Hammons,1993)

Tidal power can be classified into three main types:

• Tidal stream systems make use of the kinetic energy of moving water to power

turbines

Table 11.3.2 Operating and proposed tidal power facilities

Capacity

Station, Nova Scotia

Demonstration Project,Vancouver Island

Barents Sea

constructionRepublic of Korea Jindo Uldolmok Tidal Tidal stream 90 2009

Power PlantRepublic of Korea Sihwa Lake Tidal Power Tidal stream 254 Under

Philippines San Bernardino Strait Tidal stream 2200 Proposed

• Barrages make use of the potential energy in the difference in height or head

between high and low tides

• Tidal lagoons can be constructed as self contained structures, not fully across an

estuary

Tidal stream generators draw energy from currents in much the same way as windturbines Tidal stream turbines may be arrayed in high-velocity areas where naturaltidal current flows are concentrated such as the west and east coasts of Canada, theStrait of Gibraltar, the Bosporus, and numerous sites in Southeast Asia and Australia.Some of the operating and proposed facilities are shown in Table 11.3.2

The higher density of water means that a single generator can provide significantpower at low tidal flow velocities Water velocities at about one-tenth of the speed ofwind provide the same power for the same size of turbine system However this limitsthe application in practice to places where the tide moves at speeds of at least 1m/seven at neap tides (Lecomber, 1979)

Tidal stream generators are an immature technology Only a few commercial scaleproduction facilities are yet routinely supplying power No standard technology hasyet emerged as the clear winner But large varieties of designs are being experimentedwith, some very close to large scale deployment

Several prototypes have shown promise, but they have not operated commerciallyfor extended periods to establish performances and rates of return on investments The

Table 11.3.3 Prototype tidal stream generators

Device Principle/Description Examples

Axial Turbines Similar to the concept of 1 Kvalsund, south of Hammerfest,

traditional windmills; Norway with 300 kW capacity

operating under the sea 2 Seaflow, off the coast of Lynmouth, Devon,

England with 300 kW capacity

3 Verdant Power, in the East Riverbetween Queens and Roosevelt Island,New York City

4 SeaGen, in Strangford Lough in NorthernIreland has connected 150 kW into the grid

5 OpenHydro, being tested at the EuropeanMarine Energy Centre (EMEC), in Orkney,Scotland

Vertical and Deployed either vertically 1 Gorlov turbine being commercially

horizontal axis or horizontally piloted on a large scale in S Korea; starting withcross-flow a 1 MW plant that started in May 2009 and

4 Trials in the Strait of Messina, Italy, started in

2001 of the Kobold concept

Oscillating No rotating component 1 Stingray, tested off the Scottish

devices Aerofoil sections which are coast with 150 kW capacity

pushed sideways by the flow 2 Pulse Tidal, in the Humber estuary

Venturi effect Uses a shroud to increase the 1 Tidal Energy, commercial trials in the

flow rate through the turbine Gold Coast, Queensland (2002)

Mounted horizontally or 2 Hydro Venturi, is to be tested in San Francisco

Waves are generated by wind passing over the surface of the sea As long as thewaves propagate slower than the wind speed just above the waves, there is an energytransfer from the wind to the waves Both air pressure differences between the upwind

30

100 70 50

20 10 20

40 50 100 70 50 40 30

30 20 15

15

15 10 20 30

50 4020

15 15 40

40 60

20

15 40

40 20

100 100

Figure 11.3.2 Approximate global distribution of wave power levels (kW/m of wave fuel)

and the lee side of a wave crest, as well as friction on the water surface by the windcauses the growth of the waves (Cruz, 2008)

Wave height is determined by wind speed, the duration of time the wind has beenblowing, fetch or the distance over which the wind blows and by the depth and topo-graphy of the seafloor The depth and topography of the sea floor can focus or dispersethe energy of the waves A given wind speed has a matching practical limit over whichtime or distance will not produce larger waves

In general, larger waves are more powerful but wave power is also determined bywave speed, wavelength, and water density When an object bobs up and down on

a ripple in a pond, it experiences an elliptical trajectory This oscillatory motion ishighest at the surface and diminishes exponentially with depth

The waves propagate on the ocean surface, and the wave energy is also transportedhorizontally with the group velocity The group velocity of a wave is the velocity withwhich the overall shape of the wave’s amplitudes propagates through space The meantransport rate of the wave energy through a vertical plane of unit width, parallel to awave crest, is called the wave energy flux or wave power (McCormick, 2007).Wave energy can be considered as a concentrated form of solar energy Winds,generated by the differential heating of the earth, pass over open bodies of water,transferring some of their energy to form waves The amount of energy transferred,and hence the size of the resulting waves, depends on the wind speed, the length

of time for which the wind blows and the distance over which it blows The useful

worldwide resource has been estimated at >2 TW (WEC, 1993) The approximate

global distribution of wave power levels is given in Fig 11.3.2

Wave power generation is not currently a widely employed commercial technologyalthough there have been attempts at using it since at least 1890 The world’s firstcommercial wave farm is based in Portugal, at the Aguçadoura Wave Park, whichconsists of three 750 kilowatt Pelamis devices

There is a large amount of ongoing work on wave energy schemes The devices could

be deployed on the shoreline, near the shore and offshore:

Shoreline Devices: These devices are fixed to or embedded in the shoreline itself It

has the advantage of easier maintenance and/or installation These would not requiredeep water moorings or long lengths of underwater electrical cable However, theywould experience a much less powerful wave regime This could be partially com-pensated by natural energy concentration The deployment of such schemes could belimited by requirements for shoreline geology, tidal range and preservation of coastalscenery

One major class of shoreline device is the oscillating water column (OWC) It sists of a partially submerged, hollow structure, which is open to the sea below thewater line This structure encloses a column of air on top of a column of water Aswaves impinge upon the device they cause the water column to rise and fall, whichalternatively compresses and depressurizes the air column If this trapped air is allowed

con-to flow con-to and from the atmosphere via a turbine, energy can be extracted from thesystem and used to generate electricity (Falnes, 2002)

Nearshore Devices: The main prototype device for moderate water depths (i.e.

<20 m) is the OSPREY developed by Wavegen This is a 2 MW OWC, with vision for inclusion of a 1.5 MW wind turbine Since there could be environmentalobjections to large farms of wind or wave energy devices close to the shore, this systemaims to maximize the amount of energy produced from a given amount of near shorearea (Thorpe, 1999)

pro-Offshore Devices: This class of device exploits the more powerful wave regimes available in deep water (>40 m depth) before energy dissipation mechanisms have

had a significant effect In order to extract the maximum amount of energy from thewaves, the devices need to be at or near the surface and so they usually require flexiblemoorings and electrical transmission cables More recent designs for offshore deviceshave also concentrated on small, modular devices The McCabe wave pump, OPTwave energy converter, Pelamis and Archimedes wave swing are some of the examples.Some examples of wave power systems are given in Table 11.3.4

The major technical challenges in deploying wave power devices are:

• The device needs to capture a reasonable fraction of the wave energy in irregularwaves, in a wide range of sea states

• There is an extremely large fluctuation of power in the waves The peak absorptioncapacity needs to be much (more than 10 times) larger than the mean power Forwave power the ratio is typically 4

• The device has to efficiently convert wave motion into electricity Wave power isavailable at low speed and high force, and the motion of forces is not in a singledirection Most readily-available electric generators operate at higher speeds, andmost readily-available turbines require a constant, steady flow

• The device has to be able to survive storm damage and saltwater corrosion

At present, the main stumbling block to deployment of wave energy devices is funding.The capital costs are the problem, as it is hard to get companies to invest in technologiesthat have not yet been completely proved The position is similar to other forms ofrenewable energy sources

Table 11.3.4 Some examples of wave power systems

Portugal Pelamis Wave Energy Aguçadoura Wave Park/Póvoa Offshore 2.25 MW

Converter de Varzim

Denmark Wave Dragon Danish Wave Energy Test Center/ Offshore 4–11 MW

Nissum Bredning fjord

Australia CETO Wave Power Biopower/Carnegie Corporation/ Offshore –

Fremantle,Western AustraliaAustralia Oceanlinx Near Port Kembla, near Sydney Offshore 2 MWe

UK Wavebob Galway Bay near Galway in Ireland Offshore –

UK Pelamis Wave Energy European Marine Energy Centre/ Offshore 3 MW

UK Anaconda Wave Engineering and Physical Sciences Offshore ∼1 MW

Energy Converter Research Council (EPSRC)/

Checkmate SeaEnergy

UK Oyster wave energy Aquamarine Power/European Marine Nearshore 100 MW

Sweden WEC (wave energy Centre for Renewable Electric Energy Offshore 10 kW

converter) with a Conversion, Uppsala University/

linear generator Lysekil Project/Lysekil

USA EPAM SRI International/Santa Cruz, Calif Offshore –USA PowerBuoy Pacific Northwest Generating Offshore 150 kW

Cooperative/Reedsport, Oregon

Until the technology matures, estimates of the cost of power from wave energydevices represent a snapshot of the status and costs of the designs at the current stages

of their development

That review found support for this proposition, with the predicted generating costs

of several devices being reduced by factors of two or more as part of the reviewactivities

The electricity costs of a number of devices have been evaluated more recently usingthe same peer-reviewed methodology developed for the last UK review of wave energy.These figures show that there have been significant improvements in the predictedgenerating costs of devices, so that there are now several with costs of about 5 p/kWh(US 8 c/kWh) or less at 8% discount rate (if the devices achieve their anticipatedperformance) (Thorpe, 1999)

Wave devices that are on-shore have social implications for the surrounding area.They can be integrated within harbour walls, which can affect shipping and cause noisepollution They can create employment in the area and attract visitors

Offshore devices have an effect on navigation and consultation with affected bodiesmust be undertaken The experiences of other offshore industries, such as oil, shouldaid this part of planning for wave devices

There can be environmental impacts resulting from wave powered devices Devicesthat are on-shore can have environmental benefits, such as helping to reduce the erosion

of the landscape Any devices off shore can have an effect on the aquatic life in thatarea but this again is very site specific and hard to predict But anchoring systems canbecome almost like artificial reefs, creating a place for new colonization

Orthogonal rotor turbines equipped with blades of a symmetric profile can beregarded as a prospective type of free-flow hydraulic machine which can be installedeither in the free flow in a river channel and ocean or in the channels of chutes,spillways, and irrigation systems.

A low-head hydro project usually is an installation with a fall of water less than 5 m.Since no dam is required, low-head hydro has the following advantages:

• No safety risks of having a dam, avoiding the risk of a flash flood caused by abreached dam;

• Environmental and ecological complications such as submergence of large tracts

of forested and inhabited areas, need for fish ladder, silt accumulation in basin;

Low-head units are necessarily much smaller in capacity that conventional large hydroturbines So many units must be built for a given annual energy production Some ofthe costs of small turbine – generator units are offset by lower civil construction cost(Curtis and Langley, 2004)

Not every site can be economically and ecologically developed Sites may be toofar from customers to be worth installation of a transmission line, or may lie in areasparticularly sensitive for wildlife

A hydrokinetic turbine is an integrated turbine generator to produce electricity in

a free flow environment In-stream Energy Generation Technology (IEGT) turbinescould be used in rivers, man made channels, tidal waters, or ocean currents Theseturbines use the flow of water to turn them, thus generating electricity for the powergrid on nearby land

A 35 kilowatt hydrokinetic turbine has been installed in the Mississippi River nearHastings, Minnesota If the viable river and estuary turbine locations of the US aremade into hydroelectric power sites it is estimated that up to 130 000 gigawatt-hoursper year – about half the yearly production of the country’s dams – could be produced.The axial flow rotor turbine consists of a concentric hub with radial blades, resem-bling a wind mill Either a built-in electrical generator or a hydraulic pump which turns

an electrical generator on land provides the electricity The open center fan turbine sists of two donut shape turbines which rotate in the opposite direction of the current.This in turn runs a hydraulic pump that in turn drives a standard electrical generator

con-A helical turbine has hydrofoil sections that keep the turbine oriented to the flow

of the water The leader edge of the blades turns in the direction of the water Thecycloidic turbine resembles a paddle wheel, where the flow of the water turns thewheel with lift and drag being optimized Hydroplane blades are made to oscillate bythe flowing water, thus generating electricity The FFP turbine generator uses a rim-mounted, permanent magnet, direct-drive generator with front and rear diffusers andone moving part (the rotor) to maximize efficiency

The turbines can be installed in a variety of ways, multiple banks set on pilingsdriven into the river beds or mounted on existing river structures such as bridge piers.The turbine generators can be attached to bridge abutments or pilings, which minimizedisruption to river beds.

Turbines are to be deployed in arrays of multiple units spaced no less than 15 mapart where the site conditions, depth, and needed infrastructure are suitable Exactdepth and spacing is determined based on site conditions, including current flows andwater depth Since the turbines do not block waterways, and the water passing throughthe device is not subject to high pressure, these systems are designed to not impede ordamage fish or other wildlife

Another approach is to suspend the turbines from a floating barge The turbinessuspended from the bottom of a floating barge can accommodate changes in flow Thebarges can be deployed and have the generators come on line more quickly with fewerdisturbances to the river bed The obvious disadvantage to the barge system would beinterference with navigation and recreational use of the waterway

Concerns have been raised about the danger to marine animals, such as seals andfish, from wave and tidal devices There is no evidence that this is a significant problem.Such devices may actually benefit the local fauna by creating non-fishing ‘havens’ andstructures such as anchoring devices may create new reefs for fish colonization

11.4 E N H A N C E D G E OT H E R MA L SY ST E M S

Enhanced Geothermal Systems (EGS) are a new type of geothermal power technologiesthat do not require natural convective hydrothermal resources Present geothermalpower systems depend on naturally occurring water and rock porosity to carry heat

to the surface Majority of geothermal energy within drilling reach is in dry and porous rock EGS technologies “enhance’’ and/or create geothermal resources in thishot dry rock (HDR) through hydraulic stimulation (Armstead, 1987)

non-EGS offer great potential for expanding the use of geothermal energy Presentgeothermal power generation comes from hydrothermal reservoirs, and is somewhatlimited in geographic application to specific ideal places

EGS utilise new techniques to exploit resources that would have been ical in the past These systems are still in the research phase, and require additionalresearch, development and deployment for new approaches and to improve con-ventional approaches, as well as to develop smaller modular units that will alloweconomies of scale on the manufacturing level

uneconom-Several technical issues need further government-funded research and close oration with industry in order to make exploitation of geothermal resources moreeconomically attractive for investors These are mainly related to exploration of reser-voirs, drilling and power generation technology, particularly for the exploitation oflow-temperature cycles

collab-When natural cracks and pores will not allow for economic flow rates, the meability can be enhanced by pumping high pressure cold water down an injectionwell into the rock The injection increases the fluid pressure in the naturally frac-tured granite which mobilizes shear events, enhancing the permeability of the fracturesystem

per-Water travels through fractures in the rock, capturing the heat of the rock until it isforced out of a second borehole as very hot water, which is converted into electricityusing either a steam turbine or a binary power plant system All of the water, nowcooled, is injected back into the ground to heat up again in a closed loop.

EGS technologies, like hydrothermal geothermal, are expected to be baseloadresources which produce power 24 hours a day like a fossil plant Distinct fromhydrothermal, EGS may be feasible anywhere in the world, depending on the economiclimits of drill depth

EGS is one of the few renewable energy resources that can provide continuousbase-load power with minimal visual and other environmental impacts Geothermalsystems have a small footprint and virtually no emissions, including carbon dioxide.Geothermal energy has significant base-load potential, requires no storage, and, thus,

it complements other renewables – solar (CSP and PV), wind, hydropower – in alower-carbon energy future

The accessible geothermal resource, based on existing extractive technology, is largeand contained in a continuum of grades ranging from today’s hydrothermal, convec-tive systems through high- and mid-grade EGS resources Improvements to drillingand power conversion technologies, as well as better understanding of fractured rockstructure and flow properties, benefit all geothermal energy development scenarios.Field studies conducted worldwide for more than 30 years have shown that EGS

is technically feasible in terms of producing net thermal energy by circulating waterthrough stimulated regions of rock at depths ranging from 3 to 5 km

EGS systems are versatile, inherently modular, and scalable from 1 to 50 MWe fordistributed applications to large “power parks,’’ which could provide thousands ofMWe of base-load capacity EGS also can be easily deployed in larger-scale districtheating and combined heat and power (cogeneration) applications to service bothelectric power and heating and cooling for buildings without a need for storage on-site.Favourable locations are over deep granite covered by a thick (3–5 km) layer ofinsulating sediments which slow heat loss HDR wells are expected to have a usefullife of 20 to 30 years before the outflow temperature drops about 10 degrees Celsiusand the well becomes uneconomic If left for 50 to 300 years the temperature willrecover

11.4.1 Technical considerations

The EGS concept is to extract heat by creating a subsurface fracture system to whichwater can be added through injection wells Creating an enhanced or engineered,geothermal system requires improving the natural permeability of rock

Geothermal energy consists of the thermal energy stored in the Earth’s crust Thermalenergy in the earth is distributed between the constituent host rock and the naturalfluid that is contained in its fractures and pores at temperatures above ambient levels.These fluids are mostly water with varying amounts of dissolved salts; typically, intheir natural in situ state, they are present as a liquid phase but sometimes may consist

of a saturated, liquid-vapor mixture or superheated steam vapor phase

The source and transport mechanisms of geothermal heat are unique to this energysource Heat flows through the crust of the Earth at an average rate of 59 mW/m2 The

intrusion of large masses of molten rock can increase this normal heat flow locally;but for most of the continental crust, the heat flow is due to two primary processes:(i) Upward convection and conduction of heat from the Earth’s mantle and core, and(ii) Heat generated by the decay of radioactive elements in the crust, particularlyisotopes of uranium, thorium, and potassium.

Local and regional geologic and tectonic phenomena play a major role in determiningthe location (depth and position) and quality (fluid chemistry and temperature) of aparticular resource For example, regions of higher than normal heat flow are asso-ciated with tectonic plate boundaries and with areas of geologically recent igneousactivity and/or volcanic events (younger than about 1 million years)

Certain conditions must be met before one has a viable geothermal resource The firstrequirement is accessibility This is usually achieved by drilling to depths of interest,frequently using conventional methods similar to those used to extract oil and gas fromunderground reservoirs

The second requirement is sufficient reservoir productivity For hydrothermal tems, one normally needs to have large amounts of hot, natural fluids contained in

sys-an aquifer with high natural rock permeability sys-and porosity to ensure long-term duction at economically acceptable levels When sufficient natural recharge to thehydrothermal system does not occur, which is often the case, a reinjection scheme isnecessary to ensure production rates will be maintained

pro-Thermal energy is extracted from the reservoir by coupled transport processes vective heat transfer in porous and/or fractured regions of rock and conduction throughthe rock itself) The heat extraction process must be designed with the constraintsimposed by prevailing in situ hydrologic, lithologic, and geologic conditions Typically,hot water or steam is produced and its energy is converted into electricity, process heat,

(con-or space heat

Rocks are permeable due to minute fractures and pore spaces between mineral grains.Injected water is heated by contact with the rock and returns to the surface throughproduction wells, as in naturally occurring hydrothermal systems (Fig 4.3.4.1) Themain technological details are:

• Injection Well: A well drilled into hot basement rock that has limited permeabilityand fluid content

• Injecting Water: Water is injected at sufficient pressure to ensure fracturing, oropen existing fractures within the developing reservoir and hot basement rock

• Hydro-fracture: Pumping of water is continued to extend fractures some tance from the injection wellbore and throughout the developing reservoir andhot basement rock This is a crucial step in the EGS process

dis-• Doublet: A second production well is drilled with the intent to intersect the ulated fracture system created in the previous step, and circulate water to extractthe heat from the previously “dry’’ rock mass

stim-• Multiple Wells: Additional production-injection wells are drilled to extract heatfrom large volumes of rock mass to meet power generation requirements.EGS technologies are being developed and tested in France, Australia, Japan, Germany,the U.S and Switzerland (Table 11.4.1) The largest EGS project in the world is a

potential to increase this to over 2 000 ZJ with technology improvements — sufficient

to provide all the world’s current energy needs for several millennia

With a modest R&D investment of $1 billion over 15 years, 100 GWe (gigawatts ofelectricity) or more could be installed by 2050 in the United States The “recoverable’’resource (that accessible with today’s technology) is between 1.2–12.2 TW for theconservative and moderate recovery scenarios respectively (MIT, 2006)

11.4.2 Economic considerations

EGS could be capable of producing electricity at 3.9 cents/kWh EGS costs were found

to be sensitive to four main factors:

(i) Temperature of the resource;

(ii) Fluid flow through the system measured in liters/second;

(iii) Drilling costs; and

(iv) Power conversion efficiency

EGS energy which is transformed into delivered energy (electricity or direct heat) – is anextremely capital-intensive and technology-dependent industry The capital investmentmay be characterized in three distinct phases:

• Exploration and drilling of test and production wells

• Construction of power conversion facilities

• Discounted future redrilling and well stimulation

Estimates of capital cost by the California Energy Commission (CEC, 2006), showedthat capital reimbursement and interest charges accounted for 65% of the total cost ofgeothermal power The remainder covers fuel (water), parasitic pumping loads, laborand access charges, and variable costs

By way of contrast, the capital costs of combined-cycle natural gas plants are mated to represent only about 22% of the levelized cost of energy produced, with fuelaccounting for up to 75% of the delivered cost of energy

esti-Given the high initial capital cost, most EGS facilities will deliver base-load power

to grid operations under a long-term power purchase agreement (typically greater than

10 years) in order to acquire funding for the capital investment

There is a positive correlation between the development of new EGS fields andcontinued declines in delivered costs of energy This reflects not only the economiesfrom new techniques and access to higher value resources, but also the inevitable cost

of competitive power sources

For the US it is suggested that with significant initial investment, installed capacity

of EGS could reach 100 000 MWe within 50 years, with levelized energy costs at paritywith market prices after 11 years It is projected that the total cost, including costs forresearch, development, demonstration, and deployment, required to reach this level

of EGS generation capacity ranges from approximately $600–$900 million with anabsorbed cost of $200–$350 million

Center for Geothermal Energy Excellence at the University of Queensland, has beenawarded $18.3 million (AUS) for EGS research, a large portion of which will be used todevelop CO2EGS technologies Research conducted at Los Alamos National Labora-tories and Lawrence Berkeley National Laboratories examined the use of supercritical

CO2, instead of water, as the geothermal working fluid with favorable results CO2has numerous advantages for EGS:

• Greater power output

• Minimized parasitic losses from pumping and cooling

• Carbon sequestration

• Minimized water use

11.4.3 Further studies required

Further research is required in three areas:

• Drilling technology – both evolutionary improvements building on conventional

approaches to drilling such as more robust drill bits, innovative casing ods, better cementing techniques for high temperatures, improved sensors, andelectronics capable of operating at higher temperature in down-hole tools; andrevolutionary improvements utilizing new methods of rock penetration willlower production costs These improvements will enable access to deeper, hot-ter regions in high-grade formations or to economically acceptable temperatures

meth-in lower-grade formations

• Power conversion technology – improving heat-transfer performance for

lower-temperature fluids, and developing plant designs for higher resource lower-temperatures

to the supercritical water region would lead to an order of magnitude (or more)gain in both reservoir performance and heat-to power conversion efficiency

• Reservoir technology – increasing production flow rates by targeting specific zones

for stimulation and improving downhole lift systems for higher temperatures, andincreasing swept areas and volumes to improve heat-removal efficiencies in frac-tured rock systems, will lead to immediate cost reductions by increasing output perwell and extending reservoir lifetimes For the longer term, using CO2as a reser-voir heat-transfer fluid for EGS could lead to improved reservoir performance as aresult of its low viscosity and high density at supercritical conditions In addition,using CO2 in EGS may provide an alternative means to sequester large amounts

of carbon in stable formations

11.4.4 Induced seismicity

Some seismicity is expected in EGS, which involves pumping fluids at pressure toenhance or create permeability through the use of hydraulic fracturing techniques.Depending on the rock properties, and on injection pressures and fluid volume, thereservoir rock may respond with tensile failure, as is common in the oil and gas industry,

or with shear failure of the rock’s existing joint set, as is thought to be the mainmechanism of reservoir growth in EGS efforts

Seismicity associated with hydraulic stimulation can be mitigated and controlledthrough predictive siting and other techniques Based on substantial evidence collected

so far, the probability of a damaging seismic event is low