The Division of Solar Technology of the Department of Energy is investigating the feasibility of harvesting the solar energy absorbed by the ocean and converting this energy into electricity. Four power cycle concepts have been identified and researched. They are: closed, binary; open; hybrid, and lift cycles.
The paper gives the status of each cycle and the plans for its further development.
The Department of Energy (DOE) and earlier the National Science Foundation (NSF) and the Energy Research and Development Administration (ERDA) have embarked on a program to extract power from the thermal gradient found in tropical waters. The program is known as Ocean Thermal Energy Conversion, OTEC, and is currently one of a number of solar options being managed by the Division of Solar Technology within DOE.
The surface water in the tropics can o be as high as 40° F above the coldwater found at 3,000 - 4,000 ft-depth. This temperature difference (?T) can be utilized to drive a heat engine. If a lossless, ideal cycle could harness the available (?T), a theoritical efficiency nmax 40/(460+80)=074 or 7.4% could be realized. In a real system, not all the available (?T) can be harnessed. Part of the (?T), roughly 50%, is required to force the heat flux through the walls of the heat exchangers; evaporators and condensers. The realizable efficiency is further reduced because of the pumping power required to move the huge volumes of warm and cold water through the heat exchangers and to raise the cold water from the depth where its density is high to the condenser (approximately 200 ft below the surface) where the density is low. These losses account for another 30% of the gross power generated. As a consequence, the final system efficiency of a well designed and economically optimized system can be no higher than 2.5%.
While efficiency by itself has little significance in a fuel-free power system, the importance of efficiency manifests itself in the capital cost of the power plant. Huge heat exchangers are required and enormous volumes of warm and cold water must be processed by the plant in order to produce the power desired.
The OTEC development program focuses on the reduction of the capital cost of the power plant. The trade offs among heat exchanger and pump cost with performance are the main concerns of the power system designer. Other trade offs between the power system and the platform are the domain of the system integrator. It is recognized at the outset that the OTEC power system cannot be developed independently of site characteristics and thermal resources, platform/hull shape, materials for components exposed to the ocean environment, biofouling of heat exchangers, and finally, the mission as a whole. These interrelationships are highlighted in the paper.
