Energy from the sun heats the surface water of the ocean. In tropical regions, surface water can be much warmer than deep water. This temperature difference can be used to produce electricity. The Ocean Thermal Energy Conversion (OTEC) system uses a temperature difference (of at least 77°F) to operate a turbine to produce electricity.
The United States became involved in OTEC research in 1974 with the establishment of the Natural Energy Laboratory of Hawaii Authority. The laboratory is one of the world's leading test facilities for OTEC technology.
There is no large-scale operation of OTEC today, mainly because there are many challenges associated with the technology. The U.S. Energy Information Administration does not project any generation of electricity with OTEC systems in the Annual Energy Outlook 2015, which has projections out to 2040. The U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy have supported OTEC technology research and development.