Dossier Océan et énergie - Énergie Thermique des Mers

Sommaire IOA News Letters

Comments to "Questions & Answers of OTEC in Taiwan"

William H. AVERY
Member, IOA,
The Johns Hopkins University, Applied Physics Laboratory (ret.)
U.S.A

Question 1. Why, after the technology was demonstrated, has no OTEC commercial plant been built?

There are several reasons but the most important are that government interest in funding OTEC was withdrawn in the United States due to the change of the U.S. Administration in 1983, and that world oil prices dropped in 1984, dimming the economic prospects for OTEC and other renewable energy options. An extensive discussion of the background is given in Avery and Wu.

Question 2. Predict the OTEC potential for the world.

An assessment of the potential of OTEC can be regarded from two viewpoints: Its potential to become a profitable investment at selected sites for entrepreneurs, and its potential to become a major contributor to world energy needs through large scale commercialization. It is now evident that neither of these opportunities will be implemented without sizable support from government sources, unless oil prices double (roughly) or pollution from fossil fuels becomes unacceptable. With reasonable government support in the form of loan guarantees, investment tax credits and taxes on the disbenefits of dependence on fossil fuels, OTEC commercialization would yield enormous long-term benefits to national and world economics. Shore-based OTEC in tropical island sites could provide profitable markets, with sales, of over one billion dollars per year for electric power, mariculture, air conditioning and fuel. OTEC plantships sited on the tropical oceans could supply over 200 quads per year of inexhaustible, non-polluting ammonia motor vehicle fuel or 800 quads of methanol fuel. This would be competitive with gasoline, with appropriate economic evaluation of the environmental impacts of CO2 emissions, and the economic and national security disbenefits of dependence on middle-east oil.

A political resolve to orient national economics toward long-term goals is necessary. This will not happen until those concerned with the wellbeing of future generations gain a major role in political decisions involving energy.

3. What are the implications of CO2 emissions from OTEC power plants?

Studies of CO2 emission in OTEC closed-cycle experiments in Hawaii showed that a small fraction of the CO2 predicted from equilibrium calculations was actually evolved. There is further discussion of the subject in Avery and Wu (1994). As Lennard points out, in the worst scenario, the CO2 evolved by OTEC would be a small fraction of that generated by burning fossil fuel in power plants producing an equal amount of energy.

4. What are the considerations of the environmental impacts with OTEC power plant?

Dr. Lennard's statement is appropriate. A quantitative estimate by Martin and Roberts (1977) of the impact of siting 100 200-MWe OTEC power plants in the Gulf of Mexico predicted a drop in sea surface temperature of 0.05 deg C after 30 years, and a warming of the water column above the cold water intake of 0.8 deg C. (See the discussion in Chapter 9.2 of Avery and Wu. loc. cit.)

5. Is it possible at the present time...to state what will be the best platform and cold water pipe design?

Several designs of platform-cold water pipe combinations that could be suitable have been investigated in some detail. The "best" selection involves evaluation of trade-offs among costs, estimated risks, contractor capabilities, political factors and other items. If top priority is given to minimizing risk in operation of a moored plant in coastal waters subject to typhoons a spar design appears preferable. Construction know-how, applicable test experience, and building and deployment convenience will make the development and production costs of ship-barge configurations significantly lower.

It should be noted that a preliminary engineering design of a moored 40-50 MWe baseline OTEC plant was completed in 1981.3 The development program was ready for the next phase: construction of the demonstration plant, when funding was terminated. The 40-MWe baseline design was backed up by at-sea tests of 1/4 to 1/6 scale CWPs, and extensive water tunnel tests were completed of a moored scale configuration with a gimbled CWP under conditions representing the maximum sea states expected in a 100-year-storm off Punta Tuna, Puerto Rico. In a parallel program under NOAA support and supervision4, an at-sea test of an 8 ft. diameter CWP was successfully completed. Related tests of a power cable design were encouraging but were not completed.5 The power system of the baseline plant, including the CWP, could be incorporated as a module in larger plants of 200-400 MWe to meet Taiwan requirements.6 This configuration would be detached from the mooring when a severe storm threatened and moved out of the path of the hurricane, ensuring low risk continuing operation. It would then be reattached to the mooring when the storm abated. The same baseline plant would also provide the basic engineering and cost information for construction of commercial OTEC plantships that would provide renewable fuel to replace fossil fuel imports.

I believe that this path of OTEC development should be preferred. It would avoid a long, costly, period of engineering design and at-sea testing of the spar central body, development of new construction techniques, and extensive testing of the high-risk deployment and underwater attachment of the large power modules and large cold water pipe.

Questions 6. and 7.

Dr. Lennard's answers are well stated and seem to be appropriate. I can not comment on the background studies that he discusses.

REFERENCE

  1. Avery, W.H. and C.Wu (1994) "Renewable Energy from the Ocean: A Guide to OTEC." Oxford University Press New York
  2. Martin, P.J. and G.O. Roberts, 1977. "An estimate of the impact of OTEC operations on the vertical distribution of heat in the Gulf of Mexico." Proc. 4th Annual Conf. on Ocean Thermal Energy Conversion, New Orleans, LA. March 4, 26.
  3. George, J.F. and D. Richards, 1980. Baseline designs of moored and grazing 40-MWe OTEC pilot plants. Johns Hopkins Univ. Applied Physics Lab. SR-80-A & B.
  4. Kalvaitis, A.N., F.A. McHale, and L.Vega, 1983. "At-sea test of a large-scale OTEC pipe." Proc. Oceans '83, Aug 2,          734. (see also Avery and Wu. loc.cit. p163)
  5. Soden, J.E., R.Eaton, and J.P. Walsh 1982. "Progress in the development and testing of OTEC riser cables." Oceans '82 Conf. Records (Marine Technology Society) Washington, D.C. Sept. E, 587 Morello, A., 1981. "Constructability of submarine bottom cables for OTEC power transmission." Proc. 8th Ocean Energy Conf., Washington, D.C. June, 2,747
  6. Richards, D., E.J. Francis, and G.L. Dugger, 1980. "Conceptual designs for commercial OTEC plantships." 3rd Miami International Conf. on Ocean Thermal Energy Systems. Miami, FL