Source: U.S. Department of Energy (public domain)
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 77o F) to power 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. The Natural Energy Laboratory and the U.S. Navy's Office of Naval Research supported the development of a 105 kilowatt demonstration OTEC plant in Hawaii.
No large-scale operation of OTEC exists today because of the 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 2016, which has projections out to 2040.