Ocean thermal energy conversion (OTEC) uses the temperature differences between deep cold ocean water and warm surface water to run a heat engine and generate electricity. There are three main types of OTEC systems - closed cycle uses ammonia as a working fluid, open cycle uses seawater, and hybrid cycle combines aspects of both. OTEC could provide clean, renewable electricity for tropical coastal regions while also producing fresh water and enabling aquaculture. However, OTEC is still an experimental technology that faces challenges related to high capital costs and impacts on marine environments.