Ocean thermal energy conversion (OTEC)

What is It? Ocean thermal energy conversion is a method for generating electricity which utilizes the temperature difference that exists between deep and shallow waters.

How does it work? Earth's oceans are continually heated by the sun and cover nearly 70% of the Earth's surface. This temperature difference contains a vast amount of potential solar energy which can be harnessed.

Open Cycle OTEC Open-cycle OTEC uses the tropical oceans' warm surface water to make electricity. When 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.

Closed Cycle OTEC Closed-cycle systems use fluid with a low boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Then, cold, deep seawater—pumped through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled through the system.

Closed Cycle Diagram

Related Technologies Air/Water Conditioning The cold seawater made available by an OTEC system creates an opportunity to provide large amounts of cooling to operations that are related to or close to the plant. Salmon, lobster, abalone, trout, oysters, and clams are not indigenous to tropical waters, but they can be raised in pools created by OTEC-pumped water; this will extend the variety of seafood products for nearby markets.

Chilled Soil Agriculture OTEC technology also supports chilled-soil agriculture. When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between plant roots in the cool soil and plant leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics.

Efficiency The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power, but the small size of the temperature difference makes energy extraction difficult and expensive. Existing OTEC systems have an overall efficiency of only 1 to 3%. However, the energy carrier, seawater, has no access cost and no cost for the material itself.