The expanding vapor turns the turbo-generator. Then, cold, deep seawater—pumped through a second heat exchanger—condenses the vapour back into a liquid, which is then recycled through the system.
Open-cycle OTEC uses the tropical oceans’ warm surface water to make electricity. When warm sea water 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 and contaminants behind in the low-pressure container, is pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water. This method has the advantage of producing desalinization fresh water, suitable for drinking water or irrigation.
A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In a hybrid OTEC system, warm sea water enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process.
The steam vapourizes the ammonia working fluid of a closed-cycle loop on the other side of an ammonia vapourizer. The vapourized fluid then drives a turbine to produce electricity. The steam condenses within the heat exchanger and provides desalinization fresh water.