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Marshall DP (1997) Subduction of water masses in an eddying ocean. Journal of Marine Research 55: 201}222. Marshall JC, Nurser AJG and Williams RG (1993). Inferring the subduction rate and period over the North Atlantic. Journal of Physical Oceanography 23: 1315}1329. McDowell S, Rhines PB and Keffer T (1982) North Atlan- tic potential vorticity and its relation to the general circulation. Journal of Physical Oceanography 12: 1417}1436. Pedlosky J (1996) Ocean Circulation Theory. New York: Springer. Pollard RT and Regier LA (1992) Vorticity and vertical circulation at an ocean front. Journal of Physical Oceanography 22: 609}625. Price JF (2001) Subduction. In: Ocean Circulation and Climate: Observing and Modelling the Global Ocean, G. Siedler, J. Church and J. Gould (eds), Academic Press, pp. 357d371. Rhines PB and Schopp R (1991) The wind-driven circula- tion: quasi-geostrophic simulations and theory for non- symmetric winds. Journal of Physical Oceanography 21: 1438}1469. Samelson RM and Vallis GK (1997) Large-scale circula- tion with small diapycnal diffusion: the two-thermo- cline limit. Journal of Marine Research 55: 223}275. Stommel H (1979) Determination of watermass properties of water pumped down from the Ekman layer to the geostrophic Sow below. Proceedings of the National Academy of Sciences of the USA 76: 3051}3055. Williams RG (1991) The role of the mixed layer in setting the potential vorticity of the main thermocline. Journal of Physical Oceanography 21: 1803}1814. Williams RG, Spall MA and Marshall JC (1995) Does Stommel’s mixed-layer ‘Demon’ work? Journal of Physical Oceanography 25: 3089}3102. Woods JD and Barkmann W (1986) A Lagrangian mixed layer model of Atlantic 183C water formation. Nature 319: 574}576. OCEAN THERMAL ENERGY CONVERSION (OTEC) S. M. Masutani and P. K. Takahashi, University of Hawaii at Manoa, Honolulu, HI, USA Copyright ^ 2001 Academic Press doi:10.1006/rwos.2001.0031 Ocean thermal energy conversion (OTEC) generates electricity indirectly from solar energy by harnessing the temperature difference between the sun-warmed surface of tropical oceans and the colder deep waters. A signiRcant fraction of solar radiation inci- dent on the ocean is retained by seawater in tropical regions, resulting in average year-round surface tem- peratures of about 283C. Deep, cold water, mean- while, forms at higher latitudes and descends to Sow along the seaSoor toward the equator. The warm surface layer, which extends to depths of about 100}200 m, is separated from the deep cold water by a thermocline. The temperature difference, T, between the surface and thousand-meter depth ranges from 10 to 253C, with larger differences occurring in equatorial and tropical waters, as de- picted in Figure 1. T establishes the limits of the performance of OTEC power cycles; the rule-of- thumb is that a differential of about 203C is neces- sary to sustain viable operation of an OTEC facility. Since OTEC exploits renewable solar energy, recurring costs to generate electrical power are minimal. However, the Rxed or capital costs of OTEC systems per kilowatt of generating capacity are very high because large pipelines and heat ex- changers are needed to produce relatively modest amounts of electricity. These high Rxed costs dom- inate the economics of OTEC to the extent that it currently cannot compete with conventional power systems, except in limited niche markets. Consider- able effort has been expended over the past two decades to develop OTEC by-products, such as fresh water, air conditioning, and mariculture, that could offset the cost penalty of electricity generation. State of the Technology OTEC power systems operate as cyclic heat engines. They receive thermal energy through heat transfer from surface sea water warmed by the sun, and transform a portion of this energy to electrical power. The Second Law of Thermodynamics pre- cludes the complete conversion of thermal energy in to electricity. A portion of the heat extracted from the warm sea water must be rejected to a colder thermal sink. The thermal sink employed by OTEC systems is sea water drawn from the ocean depths by means of a submerged pipeline. A steady-state control volume energy analysis yields the result that net electrical power produced by the engine must equal the difference between the rates of heat trans- fer from the warm surface water and to the cold deep water. The limiting (i.e., maximum) theoretical Carnot energy conversion efRciency of a cyclic heat OCEAN THERMAL ENERGY CONVERSION (OTEC) 1993
7

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Page 1: OCEAN THERMAL ENERGY CONVERSION (OTEC) › ... › Ency_Oceans › OTEC.pdf · Ocean thermal energy conversion (OTEC) generates electricity indirectly from solar energy by harnessing

Marshall DP (1997) Subduction of water masses in aneddying ocean. Journal of Marine Research 55:201}222.

Marshall JC, Nurser AJG and Williams RG (1993).Inferring the subduction rate and period over theNorth Atlantic. Journal of Physical Oceanography23: 1315}1329.

McDowell S, Rhines PB and Keffer T (1982) North Atlan-tic potential vorticity and its relation to the generalcirculation. Journal of Physical Oceanography 12:1417}1436.

Pedlosky J (1996) Ocean Circulation Theory. New York:Springer.

Pollard RT and Regier LA (1992) Vorticity and verticalcirculation at an ocean front. Journal of PhysicalOceanography 22: 609}625.

Price JF (2001) Subduction. In: Ocean Circulation andClimate: Observing and Modelling the Global Ocean,G. Siedler, J. Church and J. Gould (eds), AcademicPress, pp. 357d371.

Rhines PB and Schopp R (1991) The wind-driven circula-tion: quasi-geostrophic simulations and theory for non-symmetric winds. Journal of Physical Oceanography21: 1438}1469.

Samelson RM and Vallis GK (1997) Large-scale circula-tion with small diapycnal diffusion: the two-thermo-cline limit. Journal of Marine Research 55: 223}275.

Stommel H (1979) Determination of watermass propertiesof water pumped down from the Ekman layer to thegeostrophic Sow below. Proceedings of the NationalAcademy of Sciences of the USA 76: 3051}3055.

Williams RG (1991) The role of the mixed layer in settingthe potential vorticity of the main thermocline. Journalof Physical Oceanography 21: 1803}1814.

Williams RG, Spall MA and Marshall JC (1995) DoesStommel’s mixed-layer ‘Demon’ work? Journal ofPhysical Oceanography 25: 3089}3102.

Woods JD and Barkmann W (1986) A Lagrangian mixedlayer model of Atlantic 183C water formation. Nature319: 574}576.

OCEAN THERMAL ENERGY CONVERSION(OTEC)

S. M. Masutani and P. K. Takahashi,University of Hawaii at Manoa, Honolulu, HI, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0031

Ocean thermal energy conversion (OTEC) generateselectricity indirectly from solar energy by harnessingthe temperature difference between the sun-warmedsurface of tropical oceans and the colder deepwaters. A signiRcant fraction of solar radiation inci-dent on the ocean is retained by seawater in tropicalregions, resulting in average year-round surface tem-peratures of about 283C. Deep, cold water, mean-while, forms at higher latitudes and descends toSow along the seaSoor toward the equator. Thewarm surface layer, which extends to depths ofabout 100}200m, is separated from the deep coldwater by a thermocline. The temperature difference,T, between the surface and thousand-meter depthranges from 10 to 253C, with larger differencesoccurring in equatorial and tropical waters, as de-picted in Figure 1. T establishes the limits of theperformance of OTEC power cycles; the rule-of-thumb is that a differential of about 203C is neces-sary to sustain viable operation of an OTEC facility.

Since OTEC exploits renewable solar energy,recurring costs to generate electrical power areminimal. However, the Rxed or capital costs ofOTEC systems per kilowatt of generating capacity

are very high because large pipelines and heat ex-changers are needed to produce relatively modestamounts of electricity. These high Rxed costs dom-inate the economics of OTEC to the extent that itcurrently cannot compete with conventional powersystems, except in limited niche markets. Consider-able effort has been expended over the past twodecades to develop OTEC by-products, such as freshwater, air conditioning, and mariculture, that couldoffset the cost penalty of electricity generation.

State of the TechnologyOTEC power systems operate as cyclic heat engines.They receive thermal energy through heat transferfrom surface sea water warmed by the sun, andtransform a portion of this energy to electricalpower. The Second Law of Thermodynamics pre-cludes the complete conversion of thermal energy into electricity. A portion of the heat extracted fromthe warm sea water must be rejected to a colderthermal sink. The thermal sink employed by OTECsystems is sea water drawn from the ocean depthsby means of a submerged pipeline. A steady-statecontrol volume energy analysis yields the result thatnet electrical power produced by the engine mustequal the difference between the rates of heat trans-fer from the warm surface water and to the colddeep water. The limiting (i.e., maximum) theoreticalCarnot energy conversion efRciency of a cyclic heat

OCEAN THERMAL ENERGY CONVERSION (OTEC) 1993

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Less than 18°C

Depth less than 1000 mMore than 24°C

40°S

30°S

20°S

10°S

Equator10°N

20°N

40°N

30°N

40°E 80°E 120°E 160°E 160°W 120°W 80°W 40°W 0°W

Latit

ude

18°_20°C20°_22°C

22°_24°C

Longitude

Figure 1 Temperature difference between surface and deep sea water in regions of the world. The darkest areas have thegreatest temperature difference and are the best locations for OTEC systems.

engine scales with the difference between the tem-peratures at which these heat transfers occur. ForOTEC, this difference is determined by T and isvery small; hence, OTEC efRciency is low. Althoughviable OTEC systems are characterized by CarnotefRciencies in the range of 6}8%, state-of-the-artcombustion steam power cycles, which tap muchhigher temperature energy sources, are theoreticallycapable of converting more than 60% of theextracted thermal energy into electricity.

The low energy conversion efRciency of OTECmeans that more than 90% of the thermal energyextracted from the ocean’s surface is ‘wasted’ andmust be rejected to the cold, deep sea water. Thisnecessitates large heat exchangers and seawaterSow rates to produce relatively small amounts ofelectricity.

In spite of its inherent inefRciency, OTEC, unlikeconventional fossil energy systems, utilizes a renew-able resource and poses minimal threat to theenvironment. In fact, it has been suggested thatwidespread adoption of OTEC could yield tangibleenvironmental beneRts through avenues such as re-duction of greenhouse gas CO2 emissions; enhanceduptake of atmospheric CO2 by marine organismpopulations sustained by the nutrient-rich, deepOTEC sea water; and preservation of corals andhurricane amelioration by limiting temperature risein the surface ocean through energy extraction andartiRcial upwelling of deep water.

Carnot efRciency applies only to an ideal heatengine. In real power generation systems, irrevers-ibilities will further degrade performance. Given itslow theoretical efRciency, successful implementationof OTEC power generation demands careful engin-eering to minimize irreversibilities. Although OTECconsumes what is essentially a free resource, poor

thermodynamic performance will reduce thequantity of electricity available for sale and, hence,negatively affect the economic feasibility of anOTEC facility.

An OTEC heat engine may be conRgured follow-ing designs by J.A. D’Arsonval, the French engineerwho Rrst proposed the OTEC concept in 1881, orG. Claude, D’Arsonval’s former student. Their de-signs are known, respectively, as closed cycle andopen cycle OTEC.

Closed Cycle OTEC

D’Arsonval’s original concept employed a pureworking Suid that would evaporate at the temper-ature of warm sea water. The vapor would sub-sequently expand and do work before beingcondensed by the cold sea water. This series of stepswould be repeated continuously with the sameworking Suid, whose Sow path and thermodynamicprocess representation constituted closed loops} hence, the name ‘closed cycle.’ The speciRc pro-cess adopted for closed cycle OTEC is the Rankine,or vapor power, cycle. Figure 2 is a simpliRed sche-matic diagram of a closed cycle OTEC system. Theprincipal components are the heat exchangers,turbogenerator, and seawater supply system, which,although not shown, accounts for most of the para-sitic power consumption and a signiRcant fractionof the capital expense. Also not included are ancil-lary devices such as separators to remove residualliquid downstream of the evaporator and subsys-tems to hold and supply working Suid lost throughleaks or contamination.

In this system, heat transfer from warm surfacesea water occurs in the evaporator, producinga saturated vapor from the working Suid. Electricity

1994 OCEAN THERMAL ENERGY CONVERSION (OTEC)

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Working fluidcondensate

Warmseawaterdischarge

Evaporator

Workingfluid

vapor

Warm seawater in

Turbogenerator

Working fluidpressurizer

(boiler feed pump)

Condenser

Cold seawater in

Coldseawaterdischarge

Figure 2 Schematic diagram of a closed-cycle OTEC system. The working fluid is vaporized by heat transfer from the warm seawater in the evaporator. The vapor expands through the turbogenerator and is condensed by heat transfer to cold sea water in thecondenser. Closed-cycle OTEC power systems, which operate at elevated pressures, require smaller turbines than open-cyclesystems.

is generated when this gas expands to lower pres-sure through the turbine. Latent heat is transferredfrom the vapor to the cold sea water in the conden-ser and the resulting liquid is pressurized witha pump to repeat the cycle.

The success of the Rankine cycle is a consequenceof more energy being recovered when the vaporexpands through the turbine than is consumed inre-pressurizing the liquid. In conventional (e.g.,combustion) Rankine systems, this yields net electri-cal power. For OTEC, however, the remaining bal-ance may be reduced substantially by an amountneeded to pump large volumes of sea water throughthe heat exchangers. (One misconception aboutOTEC is that tremendous energy must be expendedto bring cold sea water up from depths approaching1000 meters. In reality, the natural hydrostatic pres-sure gradient provides for most of the increase inthe gravitational potential energy of a Suid particlemoving with the gradient from the ocean depths tothe surface.)

Irreversibilities in the turbomachinery and heatexchangers reduce cycle efRciency below the Carnotvalue. Irreversibilities in the heat exchangers occurwhen energy is transferred over a large temperaturedifference. It is important, therefore, to selecta working Suid that will undergo the desired phasechanges at temperatures established by the surfaceand deep sea water. Insofar as a large number ofsubstances can meet this requirement (because pres-sures and the pressure ratio across the turbine andpump are design parameters), other factors must beconsidered in the selection of a working Suid includ-

ing: cost and availability, compatibility with sys-tem materials, toxicity, and environmental hazard.Leading candidate working Suids for closed cycleOTEC applications are ammonia and variousSuorocarbon refrigerants. Their primary disadvan-tage is the environmental hazard posed by leakage;ammonia is toxic in moderate concentrationsand certain Suorocarbons have been banned bythe Montreal Protocol because they depletestratospheric ozone.

The Kalina, or adjustable proportion Suid mix-ture (APFM), cycle is a variant of the OTEC closedcycle. Whereas simple closed cycle OTEC systemsuse a pure working Suid, the Kalina cycle proposesto employ a mixture of ammonia and water withvarying proportions at different points in the sys-tem. The advantage of a binary mixture is that, ata given pressure, evaporation or condensation oc-curs over a range of temperatures; a pure Suid, onthe other hand, changes phase at constant temper-ature. This additional degree of freedom allows heattransfer-related irreversibilities in the evaporatorand condenser to be reduced.

Although it improves efRciency, the Kalina cycleneeds additional capital equipment and may imposesevere demands on the evaporator and condenser.The efRciency improvement will require some com-bination of higher heat transfer coefRcients, moreheat transfer surface area, and increased seawaterSow rates. Each has an associated cost or powerpenalty. Additional analysis and testing are requiredto conRrm whether the Kalina cycle and assortedvariations are viable alternatives.

OCEAN THERMAL ENERGY CONVERSION (OTEC) 1995

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Warm seawater in

De-aeration(Optional)

Vacuumchamber flash

evaporatorTurbogenerator

Desalinatedwater vapor

Cold seawaterdischarge

Condenser

Noncondensablegases

Vent compressor

Desalinatedwater

(Optional)

Coldsea water in

Noncondensablegases

Warm seawaterdischarge

Figure 3 Schematic diagram of an open-cycle OTEC system. In open-cycle OTEC, warm sea water is used directly as theworking fluid. Warm sea water is flash evaporated in a partial vacuum in the evaporator. The vapor expands through the turbine andis condensed with cold sea water. The principal disadvantage of open-cycle OTEC is the low system operating pressures, whichnecessitate large components to accommodate the high volumetric flow rates of steam.

Open Cycle OTEC

Claude’s concern about the cost and potential bio-fouling of closed cycle heat exchangers led him topropose using steam generated directly from thewarm sea water as the OTEC working Suid. Thesteps of the Claude, or open, cycle are: (1) Sashevaporation of warm sea water in a partial vacuum;(2) expansion of the steam through a turbine togenerate power; (3) condensation of the vapor bydirect contact heat transfer to cold sea water; and(4) compression and discharge of the condensateand any residual noncondensable gases. Unless freshwater is a desired by-product, open cycle OTECeliminates the need for surface heat exchangers. Thename ‘open cycle’ comes from the fact that theworking Suid (steam) is discharged after a singlepass and has different initial and Rnal thermo-dynamic states; hence, the Sow path and processare ‘open.’

The essential features of an open cycle OTECsystem are presented in Figure 3. The entire system,from evaporator to condenser, operates at partialvacuum, typically at pressures of 1}3% of atmo-spheric. Initial evacuation of the system and re-moval of noncondensable gases during operation areperformed by the vacuum compressor, which, alongwith the sea water and discharge pumps, accountsfor the bulk of the open cycle OTEC parasiticpower consumption.

The low system pressures of open cycle OTEC arenecessary to induce boiling of the warm sea water.Flash evaporation is accomplished by exposing thesea water to pressures below the saturation pressurecorresponding to its temperature. This is usuallyaccomplished by pumping it into an evacuatedchamber through spouts designed to maximize heatand mass transfer surface area. Removal of gasesdissolved in the sea water, which will come out of

solution in the low-pressure evaporator and com-promise operation, may be performed at an inter-mediate pressure prior to evaporation.

Vapor produced in the Sash evaporator is rela-tively pure steam. The heat of vaporization isextracted from the liquid phase, lowering itstemperature and preventing any further boiling.Flash evaporation may be perceived, then, asa transfer of thermal energy from the bulk of thewarm sea water of the small fraction of mass that isvaporized. Less than 0.5% of the mass of warm seawater entering the evaporator is converted intosteam.

The pressure drop across the turbine is establishedby the cold seawater temperature. At 43C, steamcondenses at 813 Pa. The turbine (or turbine dif-fuser) exit pressure cannot fall below this value.Hence, the maximum turbine pressure drop is onlyabout 3000Pa, corresponding to about a 3:1 pres-sure ratio. This will be further reduced to accountfor other pressure drops along the steam path anddifferences in the temperatures of the steam andseawater streams needed to facilitate heat transfer inthe evaporator and condenser.

Condensation of the low-pressure steam leavingthe turbine may employ a direct contact condenser(DCC), in which cold sea water is sprayed over thevapor, or a conventional surface condenser thatphysically separates the coolant and the condensate.DCCs are inexpensive and have good heat transfercharacteristics because they lack a solid thermalboundary between the warm and cool Suids. Surfacecondensers are expensive and more difRcult to main-tain than DCCs; however, they produce a market-able freshwater by-product.

EfSuent from the condenser must be discharged tothe environment. Liquids are pressurized to ambientlevels at the point of release by means of a pump,or, if the elevation of the condenser is suitably high,

1996 OCEAN THERMAL ENERGY CONVERSION (OTEC)

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can be compressed hydrostatically. As noted pre-viously, noncondensable gases, which include anyresidual water vapor, dissolved gases that havecome out of solution, and air that may have leakedinto the system, are removed by the vacuum com-pressor.

Open cycle OTEC eliminates expensive heat ex-changers at the cost of low system pressures. Partialvacuum operation has the disadvantage of makingthe system vulnerable to air in-leakage and pro-motes the evolution of noncondensable gases dis-solved in sea water. Power must ultimately beexpended to pressurize and remove these gases. Fur-thermore, as a consequence of the low steam den-sity, volumetric Sow rates are very high per unit ofelectricity generated. Large components are neededto accommodate these Sow rates. In particular, onlythe largest conventional steam turbine stages havethe potential for integration into open cycle OTECsystems of a few megawatts gross generating capa-city. It is generally acknowledged that higher capa-city plants will require a major turbine developmenteffort.

The mist lift and foam lift OTEC systems arevariants of the OTEC open cycle. Both employ thesea water directly to produce power. UnlikeClaude’s open cycle, lift cycles generate electricitywith a hydraulic turbine. The energy expended bythe liquid to drive the turbine is recovered from thewarm sea water. In the lift process, warm seawateris Sash evaporated to produce a two-phase,liquid}vapor mixture } either a mist consisting ofliquid droplets suspended in a vapor, or a foam,where vapor bubbles are contained in a continuousliquid phase. The mixture rises, doing work againstgravity. Here, the thermal energy of the vapor isexpended to increase the potential energy of theSuid. The vapor is then condensed with cold seawater and discharged back into the ocean. Flow ofthe liquid through the hydraulic turbine may occurbefore or after the lift process. Advocates of the mistand foam lift cycles contend that they are cheaper toimplement than closed cycle OTEC because theyrequire no expensive heat exchangers, and aresuperior to the Claude cycle because they utilize ahydraulic turbine rather than a low pressure steamturbine. These claims await veriRcation.

Hybrid Cycle OTEC

Some marketing studies have suggested that OTECsystems that can provide both electricity and watermay be able to penetrate the marketplace morereadily than plants dedicated solely to power gen-eration. Hybrid cycle OTEC was conceived as a

response to these studies. Hybrid cycles combine thepotable water production capabilities of open cycleOTEC with the potential for large electricity genera-tion capacities offered by the closed cycle.

Several hybrid cycle variants have been proposed.Typically, as in the Claude cycle, warm surfaceseawater is Sash evaporated in a partial vacuum.This low pressure steam Sows into a heat exchangerwhere it is employed to vaporize a pressurized,low-boiling-point Suid such as ammonia. Duringthis process, most of the steam condenses, yieldingdesalinated potable water. The ammonia vaporSows through a simple closed-cycle power loop andis condensed using cold sea water. The uncondensedsteam and other gases exiting the ammonia evapor-ator may be further cooled by heat transfer to eitherthe liquid ammonia leaving the ammonia condenseror cold sea water. The noncondensables are thencompressed and discharged to the atmosphere.

Steam is used as an intermediary heat transfermedium between the warm sea water and the am-monia; consequently, the potential for biofouling inthe ammonia evaporator is reduced signiRcantly.Another advantage of the hybrid cycle related tofreshwater production is that condensation occurs atsigniRcantly higher pressures than in an open cycleOTEC condenser, due to the elimination of theturbine from the steam Sow path. This may, in turn,yield some savings in the amount of power con-sumed to compress and discharge the noncondens-able gases from the system. These savings (relativeto a simple Claude cycle producing electricity andwater), however, are offset by the additional back-work of the closed-cycle ammonia pump.

One drawback of the hybrid cycle is that waterproduction and power generation are closelycoupled. Changes or problems in either the water orpower subsystem will compromise performance ofthe other. Furthermore, there is a risk that the pot-able water may be contaminated by an ammonialeak. In response to these concerns, an alternativehybrid cycle has been proposed, comprising de-coupled power and water production components.The basis for this concept lies in the fact that warmsea water leaving a closed cycle evaporator is stillsufRciently warm, and cold seawater exiting thecondenser is sufRciently cold, to sustain an indepen-dent freshwater production process.

The alternative hybrid cycle consists of a conven-tional closed-cycle OTEC system that produces elec-tricity and a downstream Sash-evaporation-baseddesalination system. Water production and electric-ity generation can be adjusted independently, andeither can operate should a subsystem fail or requireservicing. The primary drawbacks are that the

OCEAN THERMAL ENERGY CONVERSION (OTEC) 1997

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ammonia evaporator uses warm seawater directlyand is subject to biofouling; and additional equip-ment, such as the potable water surface condenser,is required, thus increasing capital expenses.

Environmental Considerations

OTEC systems are, for the most part, environ-mentally benign. Although accidental leakage ofclosed cycle working Suids can pose a hazard, undernormal conditions, the only efSuents are the mixedseawater discharges and dissolved gases that comeout of solution when sea water is depressurized.Although the quantities of outgassed species may besigniRcant for large OTEC systems, with the excep-tion of carbon dioxide, these species are benign.Carbon dioxide is a greenhouse gas and can impactglobal climate; however, OTEC systems release oneor two orders of magnitude less carbon dioxide thancomparable fossil fuel power plants and those emis-sions may be sequestered easily in the ocean or usedto stimulate marine biomass production.

OTEC mixed seawater discharges will be at lowertemperatures than sea water at the ocean surface.The discharges will also contain high concentrationsof nutrients brought up with the deep sea water andmay have a different salinity. It is important, there-fore, that release back into the ocean is conducted ina manner that minimizes unintended changes to theocean mixed layer biota and avoids inducing long-term surface temperature anomalies. Analyses ofOTEC efSuent plumes suggest that discharge atdepths of 50}100m should be sufRcient to ensureminimal impact on the ocean environment. Con-versely, the nutrient-rich OTEC discharges could beexploited to sustain open-ocean mariculture.

Economics of OTEC

Studies conducted to date on the economic feasibil-ity of OTEC systems suffer from the lack of reliablecost data. Commercialization of the technology isunlikely until a full-scale plant is constructed andoperated continuously over an extended period toprovide these data on capital and personnel andmaintenance expenses.

Uncertainties in Rnancial analyses notwithstand-ing, projections suggest very high Rrst costs forOTEC power system components. Small land-basedor near-shore Soating plants in the 1}10 MW range,which would probably be constructed in ruralisland communities, may require expendituresof $10 000}$20000 (in 1995 US dollars) per kWof installed generating capacity. Although thereappears to be favorable economies of scale, larger

Soating (closed cycle) plants in the 50}100 MWrange are still anticipated to cost about$5000kW�1. This is well in excess of the$1000}$2000kW�1 of fossil fuel power stations.

To enhance the economics of OTEC power sta-tions, various initiatives have been proposed basedon marketable OTEC by- or co-products. OTECproponents believe that the Rrst commercial OTECplants will be shore-based systems designed for usein developing PaciRc island nations, where potablewater is in short supply. Many of these sites wouldbe receptive to opportunities for economic growthprovided by OTEC-related industries.

Fresh Water

The condensate of the open and hybrid cycle OTECsystems is desalinated water, suitable for humanconsumption and agricultural uses. Analyses havesuggested that Rrst-generation OTEC plants, in the1}10MW range, would serve the utility powerneeds of rural PaciRc island communities, with thedesalinated water by-product helping to offset thehigh cost of electricity produced by the system.

Refrigeration and Air Conditioning

The cold, deep sea water can be used to maintaincold storage spaces, and to provide air conditioning.The Natural Energy Laboratory of Hawaii Author-ity (NELHA), which manages the site of Hawaii’sOTEC experiments, has air-conditioned its buildingsby passing the cold sea water through heat ex-changers. A new deep seawater utilization test facil-ity in Okinawa also employs cold seawater airconditioning. Similar small-scale operations wouldbe viable in other locales. Economic studies havebeen performed for larger metropolitan and resortapplications. These studies indicate that air condi-tioning new developments, such as resort com-plexes, with cold seawater may be economicallyattractive even if utility-grid electricity is available.

Mariculture

The cold deep ocean waters are rich in nutrients andlow in pathogens, and therefore provide an excellentmedium for the cultivation of marine organisms.The 322-acre NELHA facility has been the base forsuccessful mariculture research and developmententerprises. The site has an array of cold waterpipes, originally installed for the early OTEC re-search, but since used for mariculture. The coldwater is applied to cultivate Sounder, opihi (limpet;a shellRsh delicacy), oysters, lobsters, sea urchins,abalone, kelp, nori (a popular edible seaweed usedin sushi), and macro- and microalgae. Although

1998 OCEAN THERMAL ENERGY CONVERSION (OTEC)

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many of these ongoing endeavors are proRtable,high-value products such as biopharmaceuticals, bi-opigments, and pearls will need to be advanced torealize the full potential of the deep water.

The cold sea water may have applications foropen-ocean mariculture. ArtiRcial upwelling of deepwater has been suggested as a method of creatingnew Rsheries and marine biomass plantations.Should development proceed, open-ocean cages canbe eliminated and natural feeding would replaceexpensive feed, with temperature and nutrientdifferentials being used to keep the Rsh stock inthe kept environment.

Agriculture

An idea initially proposed by University of Hawaiiresearchers involves the use of cold sea water foragriculture. This involves burying an array of coldwater pipes in the ground near to the surface tocreate cool weather growing conditions not found intropical environments. In addition to cooling thesoil, the system also drip irrigates the crop via con-densation of moisture in the air on the cold waterpipes. Demonstrations have determined that straw-berries and other spring crops and Sowers can begrown throughout the year in the tropics using thismethod.

Energy Carriers

Although the most common scenario is for OTECenergy to be converted into electricity and delivereddirectly to consumers, energy storage has been con-sidered as an alternative, particularly in applicationsinvolving Soating plants moored far offshore. Stor-age would also allow the export of OTEC energy toindustrialized regions outside of the tropics. Long-term proposals have included the production ofhydrogen gas via electrolysis, ammonia synthesis,and the development of shore-based mariculture

systems or Soating OTEC plant-ships as ocean-going farms. Such farms would cultivate marinebiomass, for example, in the form of fast-growingkelp which could be converted thermochemicallyinto fuel and chemical co-products.

See also

Carbon Dioxide (CO2) Cycle. Geophysical HeatFlow. Heat and Momentum Fluxes at the SeaSurface. Heat Transport and Climate.

Further ReadingAvery WH and Wu C (1994) Renewable Energy from the

Ocean: A Guide to OTEC. New York: Oxford Univer-sity Press.

Nihous GC, Syed MA and Vega LA (1989) Conceptualdesign of an open-cycle OTEC plant for the productionof electricity and fresh water in a PaciRc island. Pro-ceedings International Conference on Ocean EnergyRecovery.

Penney TR and Bharathan D (1987) Power from the sea.ScientiTc American 256(1): 86}92.

Sverdrup HV, Johnson MW and Fleming PH (1942)The Oceans: Their Physics, Chemistry, and GeneralBiology. New York: Prentice-Hall.

Takahashi PK and Trenka A (1996) Ocean ThermalEnergy Conversion; UNESCO Energy EngineeringSeries. Chichester: John Wiley.

Takahashi PK, McKinley K, Phillips VD, Magaard L andKoske P (1993) Marine macrobiotechnology systems.Journal of Marine Biotechnology 1(1): 9}15.

Takahashi PK (1996) Project blue revolution. Journal ofEnergy Engineering 122(3): 114}124.

Vega LA and Nihous GC (1994) Design of a 5 MWOTEC pre-commercial plant. Proceedings Oceanology94: 5.

Vega LA (1992) Economics of ocean thermal energy con-version. In: Seymour RJ (ed.) Ocean Energy Recovery:The State of the Art. New York: American Society ofCivil Engineers.

OIL POLLUTION

J. M. Baker, Clock Cottage, Shrewsbury, UK

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0055

Introduction

This article describes the sources of oil pollution,composition of oil, fate when spilt, and environ-mental effects. The initial impact of a spill can vary

from minimal to the death of nearly everything ina particular biological community, and recoverytimes can vary from less than one year to more than30 years. Information is provided on the range ofeffects together with the factors which help to deter-mine the course of events. These include oil typeand volume, local geography, climate and season,species and biological communities, local economicand amenity considerations, and clean-up methods.With respect to clean-up, decisions sometimes have

OIL POLLUTION 1999