Dossier Océan et énergie - Énergie Thermique des Mers

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Ocean Thermal Energy Conversion

D. E. Lennard,
Ocean Thermal Energy Conversion Systems Ltd.,
United Kingdom

The following article is reproduced, with permission, from the Survey of Energy Resources, 1998, published by the World Energy Council. This Survey was co-edited by Judy Trinnaman and Alan Clarke, partners in Energy Data Associates, a London-based information consultancy which provides energy supply and demand data on a worldwide basis.


As we move to the new millennium, the opportunity for this maritime resource to come into its own - more than 100 years after it was first conceived - is looking encouraging, and should be given a further push by 1998 being the World Year of the Oceans.

Ocean Thermal Energy Conversion (OTEC) is increasingly discussed alongside other potential products of deep water - Deep Ocean Water Applications (DOWA).

Three years after the UN Convention on the Law of the Sea (UNCLOS) came into force (November 1994), environmental impact associated with energy resource exploitation has assumed even greater importance. Whole-life costing and the drive to reduce capital expenditure are economic imperatives for a whole range of capital projects worldwide, with particular relevance to ocean resource activities. These factors continue to raise the profile, and the appeal, of OTEC/DOWA in terms not only of its electrical generating capacity (OTEC), but also its great range of associated products (DOWA), and its benign environmental impact characteristics.

OTEC is a means of converting into useful energy the temperature difference between the relatively warm surface water of the oceans in tropical and sub-tropical areas, and deep water down to approximately 1 000 m depth - which comes from the polar regions. A temperature difference of 20oC is adequate, which embraces very large ocean areas, and in fact OTEC locations favor islands and many developing countries.

Unlike many renewable energies, OTEC is base load - the thermal resource of the ocean ensures that the power source is available day or night, with only modest seasonal variations. It is environmentally benign, and some floating OTEC plants actually result in net absorption of CO2. A unique feature of OTEC is the additional products which can readily be provided - food (aquaculture/mariculture and agriculture), potable water, air conditioning, etc. In large part these utilize the pathogen-free, nutrient-rich deep cold water. OTEC is therefore the basis for a whole family of Deep Ocean Water Applications (DOWA).

The 200 nautical mile Exclusive Economic Zone (EEZ) associated with UNCLOS has given legal title to nations which previously might have contemplated OTEC/DOWA activities; prior to that no investor - private or public - would seriously contemplate funding a new form of capital plant. Since 1994 a number of nations have worked steadily to prepare overall ocean policies and 1998 will see a number of these introduced - for example that of Australia.

Technology transfer is a major factor in many maritime activities, and OTEC/DOWA is no exception, in this case borrowing from the offshore oil and gas industry - indeed the considerable advances of that industry into water depths beyond 1 000 m now place OTEC/DOWA within realistic reach of full commercialization by the beginning of the 21st century. A number of the components for an OTEC/DOWA plant are therefore either available, or nearly so, but the inherent simplicity of a number of key elements of OTEC/DOWA still require refinement into an effective system, which will need further R&D investment.

Despite the existence of EEZ's, the ongoing low costs of conventional energy resources have not encouraged venture capital investment in OTEC/DOWA, but the association of environmental costs with traditional energy sources, and the growing recognition of the environmental effects of fossil fuels, are rapidly changing the economics of these in relation to OTEC/DOWA - and other renewables. But before OTEC/DOWA can be realized, R&D must be completed to show clearly to potential investors, via a demonstration-scale plant, that the integrated system operates effectively, efficiently, economically, and safely.

Taiwan, China has been extensively evaluating its OTEC/DOWA resource, and a number of candidate sites on the east coast for land-based OTEC and aquaculture were evaluated prior to 1992. In 1995 a Master Plan was prepared for an extensive and ambitious floating OTEC program, again for the east coast, an early stage of which would be a demonstrator: an extensive international review of these concepts was obtained.

In Europe both the European Commission and the industrially based Maritime Industries Forum have examined OTEC opportunities with relevance to DOWA in general rather than just OTEC. In 1997 the UK published its Foresight document for the marine sector, looking 5-20 years ahead, and OTEC and DOWA were both included in the energy section of the paper. The emphasis in the recommendations from all three European groupings has been on the funding and construction of a demonstrator, in the 5-10 MW range.

A further indication of the interest in DOWA, rather than OTEC alone, is provided by Japan, where the industrial OTEC Association has been succeeded by the Japan Association of Deep Ocean Water Activities. The island opportunities have already been mentioned. In addition to Japan and Taiwan, the European work has stressed these as the best prospects. It is noteworthy that both Japanese and British evaluations have concentrated on prime sites in Fiji, the former on Viti Levu, the latter on Vanua Levu - the two largest islands in the group.

Further significant improvements in OTEC economics have occurred since 1995. In addition to developments in the offshore industry - which can lead to tension-leg platforms and other floaters moored in depths of water exceeding 1 000 m, OTEC/DOWA-specific developments are also contributing.

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Environmental impact relevance was given a considerable impetus by the Rio Summit of 1992, and follow-up actions have now ensured that a number of energy companies place much greater emphasis on this aspect. Calculations for generating costs now take much more account of "downstream" aspects - for example the cost of CO2 emissions. With such criteria included, OTEC/DOWA is becoming a very attractive option. Even without this aspect, the technological improvements referred to above - including the smaller heat exchangers now required - have contributed to significantly reduced capital costs. On top of these two factors the worldwide trend to whole-life costing benefits all renewables, when compared with those energy systems which require conventional fuels (and their associated costs), even when the undoubtedly higher maintenance costs of early OTEC/DOWA plants are included. The economic position of OTEC/DOWA when compared with conventional fuels is now very close indeed to equality, and work in Hawaii has contributed strongly to realistic comparisons.

Moreover, since the capital cost per kW is high, design and construction of OTEC plants could be a significant component of industrial output for those countries which manufacture them. When the DOWA derivatives of OTEC are also included, the technology is seen to provide very substantial opportunities for an industrial process which is also attractive on account of its environmental properties.

But the fact remains that until a representative-scale demonstrator plant is built and successfully operated, conventional capital funds are unlikely to be available for further plants. The establishment of renewable energy subsidiaries by some energy companies is therefore important, but there is no doubt that the principal hurdle remaining for OTEC/DOWA is not economic or technical but the convincing of funding agencies - such as the World Bank - that the techno-economic values are sufficiently soundly based for the funding of a demonstrator.

An opportunity exists for the construction of a significant amount of OTEC capacity, although OTEC will account for only a small percentage of global electricity generating capacity. Estimates have been made by French, Japanese, British and American workers in the field. A conservative assessment for the world-wide installed power suggests about a thousand OTEC plants by the year 2010, of which 50% would be no larger than 10 MW, and less than 10% would be of 100 MW size.

It is all these factors which have resulted in the resurgence of interest in ocean thermal energy, not least through the International OTEC/DOWA Association (IOA). Its second conference in Singapore in May 1997 reviewed progress under the headings of Advances in OTEC Technology; Deep Ocean Water Applications; and (significantly) Commercialization of OTEC Development. The Keynote Address by the Director of Taiwan's Industrial Technology Research Institute developed a powerful case for sustainable development through the interaction of economy, energy and environment.

To summarize: the key breakthrough now required for OTEC/DOWA is no longer technological or economic, but the establishment of confidence levels in funding agencies. There is an absolute requirement to build a representative-scale demonstration plant. Given that demonstrator, the early production plants will be installed predominantly in island locations where conventional fuel is expensive, or not available in sufficient quantity, and where environmental impact is a high priority. Both simple OTEC and OTEC/DOWA combined plants will be featured, depending on each country's particular requirements.

It can now realistically be claimed that the commercialization of OTEC/DOWA is close - a demonstrator plant is likely to be built around the turn of this century. The higher profile of the IOA since 1995 is an indication of the "coming of age" of OTEC/DOWA resource recovery and exploitation.


The following Country Notes on OTEC are based on those presented in the 1995 Survey of Energy Resources, as revised and updated by the editors. Use was made of information published by the National Renewable Energy Laboratory, Golden, Colorado and other sources, together with contributions from D.E. Lennard, Ocean Thermal Energy Conversion Systems Ltd.


This was the site of the first recorded installation of an OTEC plant. Georges Claude, a French engineer, built an experimental open-cycle OTEC system (22 kW gross) at Matanzas Bay in 1930. The plant did not survive very long before being demolished by a storm, having never managed to produce net electrical power (i.e., output minus own use).


This group of islands has been the subject of OTEC studies in the UK and in Japan. In 1982 the UK Department of Industry and relevant companies began work on the development of a floating 10 MW closed-cycle demonstration plant to be installed in the Caribbean or Pacific. The preferred site was Vanua Levu in Fiji.

In Japan an OTEC industrial grouping carried out conceptual design work on an integrated (OTEC/DOWA) land-based plant on the Fijian island of Viti Levu.

Neither study gave rise to a firm construction project.


Feasibility studies in France concluded that a 5 MW land-based pilot plant should be built with Tahiti as the test site. An industrial grouping (Ergocean and IFREMER) undertook extensive further evaluation (of both closed and open cycles) and operation of the prototype plant was initially expected at the end of the 1980s, but the falling price of oil caused development to be halted. IFREMER continues to keep the situation under review and has been active in the European Union.

Specifically, IFREMER with various partners has examined DOWA desalination, since a much smaller (1 m diameter) cold water pipe would be needed. Techno-economic studies have been completed but further development is on hold.


The program has focused on the development of a 1 MW closed-cycle pilot plant to be land-based in the Laccadive archipelago. No decision to build this demonstration unit has yet been made, but desk studies continue.

In 1996 it was reported that the National Institute of Ocean Technology (NIOT), located at the Indian Institute of Technology in Madras, was providing assistance to the Government of India on projects related to OTEC, including an evaluation of the whole Indian Exclusive Economic Zone (EEZ) area, and had particularly analyzed an east coast site 12oN and 81oE where environmental data were available. At the IOA conference in Singapore in 1997 it was suggested that some US$ 11 million could be made available in this connection.


A study was carried out in the Netherlands for a 100 kW (net power) land-based OTEC plant for the island of Bali, but no firm project has resulted.


In 1981 it was reported that the Swedish and Norwegian governments, along with a Scandinavian consortium of companies, had agreed to provide the finances required for feasibility studies toward an OTEC pilot plant to be located in Jamaica.

In a reference to OTEC, the National Energy Plan (around 1981) stated that "a 10 MW plant was envisioned in the late 1980s." However, the project does not appear to have survived.


The Japanese are working toward the establishment of a viable land-based plant, and are also designing a 10 MW floating plant to be operated in the Sea of Japan. The candidate site for the OTEC pilot plant is the Aguni Basin, located to the west of Okinawa Island.

An R&D project to clarify the features of an ocean-based combined OTEC-mariculture system included an at-sea test carried out off Himi in Toyama Bay, Sea of Japan in 1989-1990.

The re-establishment of the OTEC industry organization in Japan - now called the Japan Association of Deep Ocean Water Applications, JA/DOWA - illustrates the greater emphasis in that country on the complete system rather than OTEC alone. The establishment of JA/DOWA also indicates growing cooperation between industry, government and academic institutes.


An OTEC industrial grouping in Japan completed a conceptual design for an integrated (OTEC/DOWA) land-based plant on Kiribati, but no developments apparently ensued.


An evaluation of a 5-10 MW plant was carried out with US government funds, but no project resulted.


In 1981, the Tokyo Electric Power Company built a 100 kW shore-based, closed-cycle pilot plant on the island of Nauru. The plant achieved a net output of 31.5 kWe during continuous operating tests.


A feasibility study carried out by consultants from the Netherlands examined the competitiveness of a 10 MW floating OTEC plant. The research concluded that OTEC plants of 10 MW could be competitive on small islands, such as those in the Caribbean, where the cost of electricity is high.


IFREMER (the national organization also active in French Polynesia) is re-examining a previous proposal to establish a test site for OTEC/DOWA in New Caledonia.


In 1997 a new evaluation of OTEC/DOWA opportunities in Puerto Rico indicated that Punta Tuna on the south-east coast satisfied the criteria for an OTEC/DOWA plant with up to 10 MW capacity.


Interest in OTEC and DOWA has been revived by the National Aquatic Resources Agency in Colombo, in the context of making use of Sri Lanka's Exclusive Economic Zone (EEZ), being some 27 times its land area. The presence of submarine canyons off the east coast, in particular Trincomalee, and at the southern extremity at Dondra, where a second less attractive site has been identified, provide interesting opportunities for both electricity output and enhanced fish production from the cold, deep nutrient-rich water of an OTEC/DOWA facility.


Taiwan has developed a very positive interest in DOWA, with electrical power, aquaculture and desalination products. It has also been the initiator for the International OTEC/DOWA Association (IOA) as a means of ensuring a higher profile internationally for the development of DOWA.

This interest emphasized a particular concern to develop land-based OTEC plants, although consideration of the limited land resources in Taiwan has more recently reawakened interest in floating OTEC plants.

For the land-based variants, a study was completed on a Multiple Product OTEC Project, with the intention of constructing a 5 MW pilot plant. The preferred site changed because of designation by the government of the original site for other purposes. Oceanographic surveys of the new site (Chang-Yuan) were then completed.

The plans for floating plants are part of a major push by the Taiwan government, announced in 1994, to move OTEC power to the practical stage.

Phase 1 of the development of the Master OTEC Plan for Taiwan has been completed. International experts were invited to comment on it and a number of the reviews have been published in the IOA Newsletter. This Plan has four phases, spread over a total of 20 years, intending to work toward 400 MW-size units by the end of that period.


Hawaii remains the focus of US activity in OTEC/DOWA, primarily through the work of the Natural Energy Laboratory of Hawaii (NELHA) at Keahole Point.

In 1979 "Mini-OTEC," a 50 kWe closed-cycle demonstration plant, was set up at NELHA. The facility was installed on a converted US Navy barge moored 2 km offshore and produced 10 kW of net electric power - the first time this had been achieved from a floating OTEC plant.

In 1980 the Department of Energy constructed a test facility (OTEC-1) for closed-cycle OTEC heat exchangers on a converted US Navy tanker.

An experimental open-cycle OTEC plant at NELHA's Keahole Point site produced a record level of 50 kW of net power in May 1993, thus exceeding the 40 kW net produced by a Japanese OTEC plant in 1982.

The latest NELHA experiment employs a closed-cycle plant to test specially developed aluminium heat exchangers. During initial operation in May 1996, corrosion leaks developed in the heat exchanger modules; the plant had to be shut down but will be re-assembled on receipt of replacement modules (expected in March 1998). It uses the (refurbished) turbine from "Mini-OTEC¡¨ to produce 50 kW gross power.