The ocean thermal energy is the heat energy generated from the differnce in temperature of the ocean water layer at the different depth.
There are three systems to harness the OTE (ocean thermal energy) to generate electricity.
1) Closed System : This system uses a fluid ammonia which has very low boiling point. Ocean's warm water is pumped through heat exchanger which vaporizes the low boiling liquid (ammonia). The expanding vapors generate electricity.
2) Open System : Warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt behind in the low-pressure container, is almost pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water.
3) Hybrid System : These systems combine the features of both the closed-cycle and open-cycle systems. In a hybrid system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle loop) that drives a turbine to produce electricity.