Renewable Energy From the Deep Ocean

Ocean thermal energy conversion (OTEC or OTE[1]) uses the temperature difference that exists between deep and shallow waters to run a heat engine. As with any heat engine, the greatest efficiency and power is produced with the largest temperature difference. This temperature difference generally increases with decreasing latitude, i.e. near the equator, in the tropics. Historically, the main technical challenge of OTEC was to generate significant amounts of power efficiently from this very small temperature ratio. Changes in efficiency of heat exchange in modern designs allow performance approaching the theoretical maximum efficiency.

The Earth's oceans are continually heated by the sun and cover over 70% of the Earth's surface[2]; this temperature difference contains a vast amount of solar energy, which can potentially be harnessed for human use. If this extraction could be made cost effective on a large scale, it could provide a source of renewable energy needed to deal with energy shortages and other energy problems. The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power; but the small magnitude of the temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency. Earlier OTEC systems had an overall efficiency of only 1 to 3% (the theoretical maximum efficiency lies between 6 and 7%[3]). Current designs under review will operate closer to the theoretical maximum efficiency. The energy carrier, seawater, is free, though it has an access cost associated with the pumping materials and pump energy costs. Although an OTEC plant operates at a low overall efficiency, it can be configured to operate continuously as a Base load power generation system. Any thorough cost-benefit analysis should include these factors to provide an accurate assessment of performance, efficiency, operational, construction costs, and returns on investment.
View of a land based OTEC facility at Keahole Point on the Kona coast of Hawaii (United States Department of Energy)

The concept of a heat engine is very common in thermodynamics engineering, and much of the energy used by humans passes through a heat engine. A heat engine is a thermodynamic device placed between a high temperature reservoir and a low temperature reservoir. As heat flows from one to the other, the engine converts some of the heat energy to work energy. This principle is used in steam turbines and internal combustion engines, while refrigerators reverse the direction of flow of both the heat and work energy. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the sun's warming of the ocean surface.

The only heat cycle suitable for OTEC is the Rankine cycle using a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that are typically thought of as refrigerants such as ammonia or R-134a. Open-cycle engines use the water heat source as the working fluid.