Ocean thermal energy conversion (OTEC) technology converts solar energy stored in the layers of the tropical and subtropical oceans. Thermal heat engines use the temperature difference between the sun-warmed surface water and cold water in the deep ocean. The temperature difference between surface waters and deeper water can reach over 25 oC in summer months, although usually this temperature difference exists only over water with depths of 1,000 m or greater. Many different OTEC archetypes have been investigated, some on floating offshore platforms and some built onshore with pipelines to transport the water to and from the ocean. Each system uses one or a combination of thermodynamic processes to turn the temperature difference into mechanical energy.
- Closed-cycle: A working fluid with a low-boiling point, such as ammonia or propane, is circulated. Warm, surface seawater is pumped through a heat exchanger where the working fluid is vaporized and driven through a turbine, which then generates electricity. Cold, deep seawater is then pumped through a second heat exchanger where the working fluid vapor condenses back into a liquid and is recycled through the system.
- Open-cycle: Warm, surface seawater is placed in a low-pressure container that causes the water to boil, turning a turbine as the steam expands. The steam is then exposed to cold, deep seawater and condenses into freshwater.
- Hybrid: Warm, surface seawater enters a vacuum chamber where it is flash-evaporated into steam. The steam is used to vaporize a working fluid with a low-boiling point, which then turns a turbine to generate electricity. Hybrid configurations also produce freshwater, which can have other uses such as agriculture and potable drinking water.
The primary environmental concerns associated with OTEC technologies typically encompass changes in water quality. A major environmental concern arises as cool, nutrient rich water is withdrawn from the deep ocean and discharged higher in the water column. The effects of this cooler water and transport of nutrients on ecosystem processes could vary but may affect nearby habitats through temperature changes or increased biological growth. Screens will cover the intake tubes, but there is still a chance that marine organisms could be fatally entrapped. In order to maintain the efficiency of the heat exchangers, the warm water will need to be treated with chlorine, which will cause changes to water quality.
Photo Credit: Okinawa Prefecture Industrial Policy Division