Ocean Thermal Technology
Ocean thermal energy conversion (OTEC) technology produces electricity by taking advantage of the temperature difference between the ocean's warm surface layer and cold deeper layer. All systems require a large-diameter intake pipe to pump cold water to the surface. The deep water must be at least 20°C cooler than the surface water for power to be generated.
In closed-cycle OTEC systems, warmer seawater is pumped through a heat exchanger to vaporize a low-boiling-point fluid such as ammonia. The expanding gas is run through a turbine and then condensed back into a liquid by cooler seawater pumped through a second heat exchanger.
In open-cycle OTEC systems, warmer seawater is pumped into a low-pressure container, where it boils. Expanding steam drives a turbine and is then condensed back into a liquid through exposure to deep-ocean water. If the steam is isolated from seawater during condensation, freshwater can be a by-product of the energy production process.
In hybrid OTEC systems, warm seawater is pumped into a vacuum chamber and flash-evaporated into steam. The steam is then run through a heat exchanger to vaporize a low-boiling-point fluid within a closed-cycle loop.
Small-scale OTEC systems and individual system components have been tested successfully off the coast of Hawaii. No OTEC facilities are generating electricity, but at the National Energy Laboratory of Hawaii Authority, a tube conveys cold seawater to the surface from a depth of 3000 feet to meet a portion of the air conditioning load and to reduce peak electricity demand. The limited applicability of OTEC technology has constrained public R&D investments and commercial interest.