Overview of the thermodynamics
OTEC operates as cyclic heat engines, receiving thermal energy through heat transfer from surface of the sea water warmed by the sun and converting that into usable electric energy.
OTEC works best when the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water is about 36°F (20°C). These conditions exist in tropical coastal areas, roughly between the Tropic of Capricorn and the Tropic of Cancer. To bring the cold water to the surface, ocean thermal energy conversion plants require an expensive, large-diameter intake pipe, which is submerged a mile or more into the ocean's depths.
The Second Law of Thermodynamics prevents 100% conversion of energy; a portion of the heat is rejected into the cold sink at the bottom of the ocean due to entropy increase. The net electrical power produced by the engine must equal the difference between the rates of heat transfer from the warm surface water to the cold deep water.
The Carnot efficiency of the cycle changes with the temperature difference of the surface of the ocean versus the bottom of the ocean. Average viable efficiency ranges from 6-8% for OTEC, very low percentage compared to other power plants. Also, Carnot efficiency only apply to ideal heat engines; real life heat engines have further heat losses that degrade the performance through irreversibility. Even though OTEC consumes, from what it looks, an unlimited free resource energy, poor thermodynamic performance reduces the economic feasibility of an OTEC facility.
In a closed-cycle system, a fluid with low boiling temperature, such as ammonia, is used inside the engine. The warm surface seawater is pumped into the heat exchanger which vaporizes the low boiling point fluid. As the vaporized fluid expands, it rotates the turbo-generator. Cold water, which is pumped from the bottom of the ocean then cools the fluid in a second heat exchanger, condensing it back into liquid form. The process is recycled and operated continuously.
Open-cycle OTEC is different from Closed-cycle in that it directly uses seawater as the working fluid instead of a low-boiling point substitute. The warm seawater is first pumped into a low-pressure container, which causes it to boil. The steam operates a low-pressure turbine attached to an electrical generator. The steam which is pure fresh water after vaporizing and leaving its salt content in the low pressure container is condense back to liquid from after exposure to cold temperatures from deep-ocean water. The apparent byproduct of this process is fresh water, which can even be suitable for drinking purposes.
A hybrid cycle combines the features of the closed- and open-cycle systems. In a hybrid system, warm surface seawater enters vacuum chamber, where the water is flash-evaporated to produce steam. The steam then continues onto vaporize a low boiling point fluid that is used to drive a turbine to generate electricity. The steam is condensed in the heat exchanger and produces fresh water as a byproduct.
OTEC is an environmentally friendly technique. Its benefits include chilled agriculture, aquaculture, and fresh water production. The byproducts of the process are usually seawater discharges and gases from depressurized seawater. Its carbon dioxide uses are also one to two orders of magnitude less than comparable fossil fuel power plants.
The effluent of OTEC heat engine is discharged at lower temperatures than the earth's surface. The pipes that these waters flow into can be placed underground which can chill the soil to a point where many plants that evolved in temperate climates to be grown in the subtropics. The Natural Energy Laboratory maintains a demonstration garden near its OTEC plant with more than 100 fruits and vegetables, many of which would not normally survive in Hawaii.
The deep seawater is oxygen deficient and generally 20-40 times more nutrient rich (in nitrate and nitrite) than shallow seawater. When these plumes are mixed, they are slightly denser than the ambient seawater. Mixing of deep ocean water with shallower water brings up nutrients and makes them available to shallow water life. Cold-water delicacies, such as salmon and lobster, thrive in the nutrient-rich, deep seawater culled from the OTEC process.