Ocean thermal energy conversion

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Ocean thermal energy conversion (OTEC or OTE[1]) uses the difference between cooler deep and warmer shallow waters to run a heat engine. As with any heat engine, greater efficiency and power comes from larger temperature differences. 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 small temperature ratios. Modern designs allow performance approaching the theoretical maximum Carnot efficiency.

OTEC offers total available energy that is one or two orders of magnitude higher than other ocean energy options such as wave power;[citation needed] but the small temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency. Earlier OTEC systems were 1 to 3% efficiency, well below the theoretical maximum of between 6 and 7%.[2] Current designs are expected closer to the maximum. The energy carrier, seawater. Expense comes from the pumps and pump energy costs.OTEC plants can operate continuously as a base load power generation system. Accurate cost-benefit analyses include these factors to assess performance, efficiency, operational, construction costs, and returns on investment.

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. Much of the energy used by humans passes through a heat engine.[citation needed]

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.

The Earth's oceans are heated by the sun and cover over 70% of the Earth's surface.[3]

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