OTEC systems use the ocean's natural thermal gradient—the fact that the ocean's layers of water have different temperatures—to drive a power-producing cycle. In deep tropical waters ocean surface temperature can be as high as 27 ℃; and at depths as close as 1000 ft below the surface, temperature can be as low as 4 ℃.

OTEC system

• There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. 

• Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia.