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AbstractA new era in the technology of Ocean ThermalEnergy Conversion (OTEC), which is a system ofconverting thermal energy in ocean into electricity,has come now. The concept of OTEC was firstproposed by French physicist in 1881. Since then,although many scientists have made earnest effort tomaterialization, OTEC has never commercialized yet.However the National Institute of Ocean Technology(NIOT) in India succeeded to construct 1MW pilotOTEC plant, so called 'Indian plant', in 2001. As tothe matter of thermal engineering, the Indian plantwas fully supported by Saga University in Japan. Infact the Indian plant was pushed technical breakthrough of innovative thermal cycle, which is called'UEHARA CYCLE' invented in 1994, and itsassociated heat exchanger technology. The Indianplant has created a great sensation in the world, and itwill be demonstrated in due course of time in 2002.Now many people are concerning the consequence ofthe Indian plant's experiments, and at the same timemany countries have started to investigate thefeasibility of OTEC.OTEC provides not only power generation but alsosome solutions to the three greatest global issues weare facing in 21st century, i.e. ‘Energy’, ‘Water’ and‘Food’ associated with the fundamental problems ofenvironment destruction and population explosion.In collaboration among Saga University, Xenesys Inc.,the title holder of execution right of patents registeredby Saga University, and Hitachi Zosen Corporation,the partner in marine field, they are promotingseveral projects concerning the above technologies.As the ocean thermal energy is perfectly clean andrenewable, and the potential is very huge, a brightfuture for the new OTEC technology and the wideapplication is prospected.This paper describes an outlook and a trend of thenew OTEC technologies spreading rapidly now in theworld.

IntroductionEnergy, water and food are some of the essentialrequirements for the continuation and evolution ofmankind. Presently almost of all the energy dependson fossil fuels and nuclear power.The world population is 6.1 billion in 2000, and it isstill growing explosively. At the same time, energyconsumed by human is also increasing explosively, asshown in Fig.1. By considering future economicgrowth and environmental problems it is obvious thatin the 21st century we cannot rely on the currentmainstream resources, i.e. oil, coal, and uranium forthe world energy supply. Thus, we must face theurgent and important problem of developing analternative energy source to fossil and nuclear fuel.For the alternative energy sources we can easilyconsider, for example, such as wind, solar andgeothermal power. However, ocean energy shouldbecome also an important potential energy sourcewhich must be obtained.Among the various forms of ocean energy, the oceanthermal energy is plentiful and very stable.During the last decade, the technology of OTEC hasbeen made great strides. It is worthy of specialmention that OTEC technology is easily applicable inmany industrial fields for recovery and saving ofenergy in lower temperature range and small thermalhead.

The Present Status and Features of OTECand

Recent Aspects of Thermal Energy Conversion Technologies

Hiroki KOBAYASHIHitachi Zosen Corporation

Sadayuki JITSUHARA, Dr.Xenesys Inc.

Haruo UEHARA, Dr.Saga University

Fig.1. Populationand Energy Consumption/Person-Day.

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HistoryThe world’s first conceptual invention to utilizethermal energy stored between warm surfaceseawater and cold depth seawater was formed by aFrench physicist Mr. J. D’Arsonval. It was in 1881.In that same year 1881, the world’s first thermalpower generator was made. In 1883, Mr. De Laval, aSwede, has manufactured the world’s first thermalsteam turbine. 1880s remained however an infant eraof power generation technology in general.Another French scientist Mr. G. Claude had inheritedthe idea of D’Arsonval and had worked on it as hislife work but could not make much progress. One ofthe reasons of his repeated failures can be laid on thefact that he had attempted with an open thermal cyclesystem.In 1964, J. H. Anderson and his junior conceived of anew closed cycle OTEC plant, which overcome theweak point of Claude's system. This invention ofAnderson's attracted considerable attention andcreated the opportunity for rekindling researchconcerning OTEC. Later, the first energy crisis in1973 provided the motivation for Japan and U.S. toperform fundamental research. Since that time, actualOTEC plants had been constructed in rapidsuccession.In the meantime, a new movement had come around1980s and 90s. Before then, however, the Rankinecycle providing with single composition medium asworking fluid was the common practice being appliedfor closed cycle OTEC, an American physicist Dr.Alex. Kalina invented a new heat cycle, so calledKalina cycle, for thermal energy conversion usingammonia/water mixture as working fluid, in 1985.Subsequently, a more advanced cycle was inventedby Japanese physicist Dr. H. Uehara, who is one ofthe authors, in 1994. For coming such advanced cycletechnology, possibility of commercial OTEC hasbecome greater.

Looking at the current from another view, thenecessity due to developing practical solutions to CO2

reduction against global warming has been bringingearnest expectation on OTEC. One of typical aspectsin this trend is the world first 1MW class OTECexperiment project being proceeded in India.According to the program, the Indian Plant will bedemonstrated in 2002. Fig.2 shows snaps of theIndian plant.As to history of OTEC development, see Table 1.

Table.1. History of OTEC.1881 J.D’Alsonval (France) presented the Idea of OTEC 1981 Tokyo Electric Power Co. (Japan) succeeded to1926 G.Claude (France) started experiments of OTEC generate 120kW on Nauru1933 G.Claude (France) constructed on board type OTEC 1982 Kyushu Electric Power Co. (Japan) succeeded to

plant generate 50kW at Tokunoshima Is.1964 Anderson (USA) presented a proposal of Off-shore 1985 Saga Univ. completed 75kW experimental plant

type OTEC 1985 Kalina (USA) invented a new cycle with1970 New Energy Research Committee researched OTEC ammonia/water mixture

technology (Japan) 1988 OTEC Association was established (Japan)1974 OTEC was researched as a part of Sunshine Project 1989 Industrial and Technological Board succeeded to

(Japan) generate 3kW at Toyama Bay1974 ERDA project started to research OTEC (USA) 1990 International OTEC Association (IOA) was established1974 First OTEC conference was held (USA) (Taiwan)1977 Saga University (Japan) succeeded to generate 1kW 1993 210kW open cycle OTEC was completed in Hawaii

of power 1994 Uehara (Japan) invented an advanced thermal cycle1979 Mini-OTEC generated 50kW (USA) 2001 1MW closed cycle OTEC was constructed by NIOT1980 Saga Univ. performed off-shore experiments at sea (India)

of Japan 2002 NIOT (India) will perform 1MW experiment

Fig.2. Photo of Indian Plant“SAGAR-SHAKTHI”.

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OTEC PrinciplesOTEC is one of natural and renewable energy ofcourse. Comparing to the other natural energyresources, the ocean thermal energy has the followingcharacteristics.

1) Clean and renewable (solar source energy)2) Stable throughout a moment, a day and a year.3) Huge amount but low-density energy

Some of temperature distribution of ocean seawatermeasured in vertical direction in tropical and sub-tropical zones are shown in Fig.3. Sea watertemperature in the surface layer is a warm 20~30℃,while that is a cold 2~7℃ in the deep layer about700m in depth.

OTEC obtains the thermal energy associated with thetemperature difference between the warm seawater insurface layer and the cold seawater at greater depth.When there are any thermal head, the heat shouldtransfer from higher temperature side to lowertemperature side naturally. It is the thermal cycletechnology to convert thermal energy into power, i.e.kinetic and/or electric energy.Open cycle, closed Rankine cycle, Kalina cycle andUehara cycle are considerable for thermal cycle ofOTEC at present.Fig.4 shows a OTEC principle of closed Rankinecycle. A single pure thermal medium is used for thecycle as working fluid. The working fluid iscirculated in a closed circuit system. The systemconsists of a feed pump, an evaporator, a turbine anda condenser. The fluid is heated in the evaporator bywarm seawater, and then the fluid evaporates intovapor. The vapor is led to the turbine and works forpower generation. The fluid exhausted from theturbine is cooled down and reliquefied in thecondenser by cold depth seawater. By repeating thiscycle, power can be generated without any assists offuel.

Recent research and development to OTEC havebeen pursued using closed cycle application withammonia as working fluid.The closed Rankine cycle OTEC is essentially thesame cycle as that employed for ordinary thermalpower plants as well as nuclear power generation.There are some decisive differences, however,between ordinary power generation systems andOTEC. One is the difference of thermal head, in otherwords, temperature difference between evaporationand condensation. Another is the working thermalmedium for which water (H2O) is used in ordinarypower generation, while low temperature boilingfluid e.g. ammonia (NH3) is used in OTEC. Andabove all, no any fuel like fossil or nuclear fuel butseawater is needed for OTEC.

Looking back the history of thermal cycle technology,every approach was tried with the Rankine cycle, ofwhich theory was established by Rankine in 1851,but invented by Watt in 1769. The Rankine cycle is touse single thermal medium as said before.In 1985, Dr. Kalina put forward a new cycleemploying quite a different concept providing with

Fig.4. Principle of OTEC (Closed Rankine cycle).

Fig.3. Distribution of Seawater Temperature.

Fig.5. Block Diagram of Uehara Cycle.

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ammonia/water mixture as heat medium. The newcycle, so called ‘Kalina cycle’, attained epoch-making improvement of cycle efficiency, which wasa large jump up on previous known cycles. Forinstance, however the efficiency is varied dependingupon condition, under the condition of 28℃ warmwater and 4℃ cold water, expected thermal cycleefficiency of conventional Rankine cycle to beapprox. 3%, while approx. 5% efficiency is to beexpected by applying Kalina cycle.However, there is a demerit caused by mixturemedium employed in thermal cycle. That is theincrease of load on heat exchangers particularly oncondenser.For the purpose of relieving condenser load, a newthermal cycle using ammonia/water mixture isinvented, in 1994. That is so called ‘Uehara cycle’.Uehara cycle assures theoretically higher efficiencythan Kalina cycle by lessening the load for condenserby means of extraction of vapor from the turbine. Itshould be possible now to construct an OTEC plantof more compact in size but higher in efficiency thanbefore.A schematic block diagram of Uehara cycle is shownin Fig.5, and Fig.6 shows the experimental plant ofUehara cycle in Saga University.

Evolution of Heat ExchangerEvaporator and condenser are two vital componentsfor success of the OTEC. It is thus needed to develophighly effective evaporators and condensers in orderto improve the total efficiency and hence to improvethe economics.In OTEC a power generating system uses huge

quantity of seawater, and so pumping theseawater requires considerable amount ofelectrical power. The OTEC laboratory ofSaga University has investigated theseaspects experimentally for manyevaporators and condensers. The conclusionbased on a combined economic assessmentof heat transfer capacity, pressure loss andseawater flow rates, is that plate typeevaporators and condensers are the stylebest suited to OTEC.Ｉｎ former technology, it is said that there isa difficulty to apply conventional plate typeheat exchanger for that operated with phasealteration between vapor and liquid.However the new plate type heat exchangertechnology has overcome the difficulty andprovided much higher heat transfer

efficiency than ordinary tubular type. Consequently,required flow quantity of both warm and cold wateris reduced drastically, and thus required power forboth surface and deep seawater pumps is decreasedand total system efficiency goes up. Comparison ofrequired seawater quantity between conventionalclosed Rankine cycle with ordinary tubular type heatexchangers and latest Uehara cycle with advancedplate type ones is shown in Table 2.The final designs and other related criteria workedout by Saga University have been transferred toXenesys Inc. for their commercial undertaking.Xenesys is expected to announce shortly the deliveryavailability of evolutionary style heat exchangers fordual phases application of vapor and liquid.

Fig.6. Uehara Cycle Plant of Saga University.

Table 2. Comparison of Required Seawater.Condition : Gross output 100MW

Surface water temp. 28℃

Depth water temp. 4℃

ConventionalRankine

AdvancedUehara

Warm surfaceseawater 200 m3/s 110 m3/s

Cold Depthseawater 200 m3/s 120 m3/s

Calculated by Xenesys Inc.

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Multiple Industrial Complex with OTECIn Japan, the research and development for ‘MultipleOTEC System’, which utilizes the potential of DeepOcean Water (DOW), have been performed inprogress. The image of ‘Multi-OTEC’ is shown inFig.7 and considerable systems associated withOTEC are described as follows.

1) DesalinationThe Saga University has developed a spray-flush typedesalination system. By this system around 1% ofraw seawater quantity is to be distilled to pure freshwater. To combine the OTEC and this desalinationsystem, the capability of desalination is quite huge;i.e. the obtainable distilled water capacity is approx.10,000 m3/day with 1MW OTEC, approx. 1,000,000m3/day with 100MW OTEC.It is remarkable that the fresh water obtained can beutilized as resource of hydrogen as described later.Fig.8 shows the demonstration plant of spray-flushdesalination made by Saga University.

2) Mineral Water ProductionDOW is rich in mineral. It is possible to produce‘Mineral water’ as by-product of the OTEC.Providing with ion-exchanger and mineralizer, a partof desalinated water comes more valuable industrialproduct. Mineral water is one of most anticipated by-product of OTEC in order to promote a sustainablelocal industry, particularly for island countries.

3) Lithium ExtractionLithium is the one of important industrial metal as thematerial of battery. Extracting chloride-lithium

dissolved in seawater is one of considerable methodof industrial lithium production. For such lithiumextraction from seawater, purity of DOW provideslonger cleaning interval of seawater contact surfacematerial polluted with impurity of seawater, becauseDOW is much purer than seawater in surface layer.This is a big advantage of feasibility from theeconomical point of view.

4) Air ConditioningTropical condition suits for OTEC as a matter ofcourse. In the tropical area there is a need for coolingair conditioning for office, hotel, etc. For that purposeOTEC is useful. The temperature of depth cold waterafter utilization for OTEC is still low; e.g. thetemperature is to be around 10℃. It is cold enough touse as chilling source of air cool conditioning. Suchair conditioning system requires much less energythan that is needed by ordinary electrical refrigerationmethod. It means that OTEC makes demand ofelectricity decrease ecologically.

5) AquacultureThe characteristics of DOW are cold, pure andnutrient. These characteristics can be used effectivelyto aquaculture by getting rapid growth and lessdisease.In these years, another approach has made for fisheryfertilization by using nutrient DOW in Japan. Dr.Ouchi and his colleagues are developing anarchitectural sea area fertilizer unit incorporated withOTEC as an energy source of density currentgenerator system.

Fig.7. Image of Multiple-OTEC Station.

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6) Food, Cosmetics, Medical Science, etc.Due to surpassing characteristics DOW drawsattention in the various fields of industry and science.In Japan DOW is utilized for food processing of'Sake', 'Tofu', etc. and some cosmetics made of DOWhave got public favor.Effects of DOW attract medical scientist's attention.It has been said that DOW is efficacious againstatopic allergy dermatitis. Thalassotharapie, which is akind of medical treatment for relaxation, is also oneof DOW utilization.OTEC can provide such broad utilization.

7) Hydrogen ProductionTo look a little further future, it should be considered"Offshore Hydrogen Production Platform" by OTEC.This concept provides completely clean hydrogenproduction by using only natural renewable energy.Considering the tendencies of fuel-cell and micro-gas-turbine, the tide of shifting the basic industrialfuel from fossil to hydrogen would possibly comesooner than expected in common. As a consequenceof such thinking, it may be not so far dream thathundreds MW class OTEC for hydrogen generatingstation to be built as a kind of fundamental energysupplier.

Economical Feasibility of OTECIt goes without saying that economy is one the keyelements for verification of OTEC power plant.Advanced studies made thus far on thermal cycle andheat exchanger have brought promising results of farimproved efficiency of OTEC system as a whole.Although the efficiency of system itself variesdepending upon temperature conditions, Uehara cyclecan attain a 30~50% higher efficiency as compared toRankine cycle. Thanks to the highly effective plateheat exchanges newly developed by Saga University,the power consumption of pumps for cold and warmseawater can be lowered to 30~40% of theconventional case. Considering all new achievements,we can easily predict the latest OTEC technologyproduce twice as much net power from the same heatsource as the conventional OTEC.In addition to such great improvement of thecapability, the reduction of cold depth water quantitywith advanced condenser provides smaller sizedconfiguration of piping for DOW riser piping, andthus the economical performance is much improved.

Various accounting models have been applied todetermine the cost for the OTEC system. As exampleof trial calculation, the cost of electricity generatedby the OTEC is estimated by NIOT (India), who isnow proceeding experiment with ‘Indian plant’, as

Fig.8. Desalination Plant.

Fig.11. Hydrogen Generation.

Fig.10. Lithium Extraction.

Fig.9. Mineral Water Production.

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shown in Table 3.According to several accounting models, it has beendetermined that for a large plant of 50~100MW, theunit cost would be competitive with a coal-firedpower station, while for a small plant of 1~5MW theunit cost would be about the same or less than that ofa diesel power station.

However OTEC is valuable not only in powergenerating, but in additional activities. The expectedquantities of main by-products of OTEC are shown inTable 4. In case of applying 20m3/day mineral waterproduction facility with OTEC, for example, totalamount of mineral water output is estimated approx.600 million JP¥ per year based on unit price of 100JP¥ per litter and 85% operation rate.

New Aspects of OTEC DevelopmentOne of the major causes for global warming is said tobe attributable to heavy dependence upon fossil fuelfor electricity. Nevertheless, a number of countrieshave been relying on diesel generators for theirelectricity, since they have had no alternative to takeup. But now, the story is getting different turn. Someof island countries in the south Pacific region, whoselife is being threatened by rising sea level, havestarted to look into the OTEC more seriously thanever. For instance, a technical collaboration

agreement was signed in April 2001 by and amongthe Republic of Palau, Saga University and XenesysInc. The President of the Republic of Palau expressedat a news conference his determination to replacetheir diesel power stations with OTEC power plantsat an earliest possible date.Positive movement is recognized in Japan, too. Japanis well known as world-leading country inshipbuilding industry. In order to realize the OTEC,technologies developed and accumulated inshipbuilding industry are very helpful. Hitachi ZosenCorporation, one of the world-most advancedshipbuilding companies in Japan, has started todevelop conceptual design for several OTECapplications with the latest technologies in alliancewith Saga University and Xenesys Inc.

For the sake of commercialization of OTEC, researchand development program should be followed up bydemonstration and dissemination activities. For theprogress of commercialization, practical strategies areconsidered as follows.

Step 1: Construction of a 1MW class OTEC fordemonstration. The project should be conductedgovernmentally sponsored body to providepractical usefulness. Image illustration of thisclass OTEC is shown in Fig.12.

Step 2: Construction of a prototype OTEC plantwith 5~10MW for dissemination. The plantshould be built somewhere in island countries inthe Pacific Ocean probably sponsored bygovernment to government official developmentassistance. The OTEC is very beneficial for theisland nations, since it is to provide not onlyclean energy but also local industries by multipleusage of DOW resource. Image illustration of theOTEC in this step is shown in Fig.13.

Step 3: Promotion of commercial scale OTEC with50~100MW. The plant should be built as basicenergy source. At this step, the designing ofstandardized offshore type OTEC power stationwill be worked out for verifying the economiccompetitiveness in this range. Image illustrationof the OTEC in this step is shown in Fig.14.

Needless to say, assessment of environmentalconditions by introducing an OTEC power plant isalso a part of the majors concerns in executing eachof the above steps.According to preliminary prediction, in regard toenterprise, it would be expected that total 1,000MWof Multi-OTEC station would be built in Japan

Table 3. Estimation of Unit Cost of Electricityfrom OTEC Power in India (1999)

Power Output Gross (MW) 1 25 50 100

Power Output Net (MW) 0.617 15.39 30.88 64.23

Heat exchanger cost (Mill.US$) 1.70 44.40 88.22 152.58

Cost of cold w. pipe (Mill.US$) 0.69 1.74 2.67 4.65

Cost of barge (Mill.US$) 0.69 2.33 4.65 9.30

Mooring cost (Mill.US$) 2.09 3.49 4.65 5.81

Turbine + Inst. Cost (Mill.US$) 1.16 17.44 34.48 69.76

Total cost (Mill.US$) 6.42 69.42 134.67 242.10

Cost of electricity (US$/kWh) 0.189 0.082 0.079 0.068

Table 4. Expected Output of By-products.Gross Power Output (MW) 1 10

Net Power Output (MW) 0.7 7.5

Net Electricity (MWh/year) 4,900 52,500

Up-welled DOW (t/h) 4,700 43,300

Fresh Water (t/h) (*1) 1,100 10,000

Hydrogen (Nm3/h) (*1) 2,000 22,000

Chloride Lithium (kg/day) 30 260

Mineral Water (bottle/day)(*2) 16,000 150,000

Calculated by Xenesys Inc.Remarks : *1) All generated electricity to be used for

hydrogen production as basis.*2) 500ml PET bottle equivalent

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annually in near future. At that time, it means that anew industry having 1.5 trillion JP¥ annualproduction amount and 10 thousand employment willarrive in Japan.

Forecasting FutureAlthough the density of energy is comparatively poor,the ocean provides us a huge amount of thermalenergy. Today, the new OTEC technology madepossible to extract the energy practically from theocean realistically. A suitable area for OTEC spreadsover very wide range of the world from the tropics tosemi-tropics. An advantage of the OTEC technologyshould be emphasized on not only its tremendouspotential for power generation but also theconvenient feature that can disperse the power plantwith proper cost of the electricity generation. Thecost will become competitive with that of theconventional fossil fuel burning power plant as wellas nuclear in the near future.The desalination technology as by-product of theOTEC can produce a large amount of fresh waterfrom seawater for which the island nations and theother desert countries longing eagerly.Besides, the DOW exhausted from the OTEC plantwould contribute to world food security with fisheryfertilization.

It is the OTEC, which has a potential to becomepowerful solution to three greatest global issues ofEnergy, Water and Food without vital harmfulinfluence on irreplaceable earth environment.In conclusion, the new OTEC and the derivativetechnologies would offer the best promise to bringhumankind prosperity beyond measure in the 21stcentury.

References1) H. Uehara, Y. Ikegami, “Optimization of a closed

cycle OTEC system”, ASME Journal of SolarEnergy Engineering, Vol.112, No.4 (1990)

2) H. Uehara, Y. Ikegami, et al, “PerformanceAnalysis of OTEC System Using Kalina Cycle”,Journal of The JSME, No.93-1693 (1994)

3) H. Uehara, Y. Ikegami, T. Nishida, “PerformanceAnalysis of OTEC System Using a Cycle withAbsorption and Extraction Processes”, Journal ofThe JSME, No.96-1696 (1998)

4) Joseph R.Vadus & Patrick K.Takahashi, “Strategiesfor OTEC/DOWA Commercialization”, TheInternational OTEC/DOWA Conference’99 (1999)

5) Y. Ikegami, H. Uehara, “New Aspect and FutureView of OTEC Development”, Journal of theM.E.S.J., Vol.34, No.11 (1999)

6) M. Ravindran, “The Indian 1 MW Floating OTECPlant – An Overview”, IOA Newsletter Vol.11NO.2/Summer 2000, International OTEC/DOWAAssociation (2000)

7) M. Takahashi, “Umi-ni nemuru shigen Kaiyo-shinso-sui”, Asunaro-shobo (Japanese) (2000)

8) Ouchi, et al.,”A Feasibility Study on the EnergySource for Ocean Nutrient Enhancer”, The Societyof Naval Architects of Japan, 16th OceanEngineering Symposium July 18-19 (2001)

Fig.12. Image of 1MW Class OTEC.

Fig.13. Image of Barge Mounted OTEC.

Fig.14. Image of Large Offshore OTEC.