Ocean Thermal Energy Conversion
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. This technology requires large volumes of water to convert a small portion of the available energy, yielding about 2.5%–3.0% of stored solar energy as net power after pumping and other power requirements are met (Avery 1994). OTEC can potentially provide very substantial amounts of power, making it attractive as a carbon-neutral baseload source. Under funding from the U.S. Department of Energy (DOE), open-cycle OTEC was successfully demonstrated with positive net energy production (up to 103 kW from 255 kW gross) from 1993 to 1998 at the Natural Energy Laboratory of Hawaii Authority (NELHA) facility at Keahole Point on the island of Hawaii (SERI 1989).
Technology
The deeper tropical and subtropical oceans have strong temperature stratification with warm water overlying cold deeper water. The temperature difference between surface waters and deeper water can reach over 25-deg. C in summer months, although usually this temperature difference exists only over water with depths of 1,000 m or greater. However, there are some places in the world with high thermal gradients, such as in the Florida Straits where these temperature differentials can be reached at depths less than 300 m. Ocean thermal energy conversion (OTEC) systems use the temperature difference between flows of warm surface water and cold-deep water into electricity. OTEC systems were first envisioned in the 1880s with a proof of concept demonstration project built in Cuba in 1930. 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 (Vega 2016; Meyer et al. 2011; William and Wu 1994). Each system uses one or a combination of thermodynamic processes to turn the temperature difference into mechanical energy.
Closed cycle OTEC systems use a working fluid with a low boiling point, most commonly ammonia. Although other working fluids are also used, many are toxic (e.g., hydrocarbons) or of environmental concern (e.g., chlorofluorohydrocarbons). The working fluid is heated to a gas by passing warm surface seawater through a heat exchanger, and is used to power an air turbine to produce electricity. Cold deep seawater is pumped through a second heat exchanger to condense the working fluid back to a liquid. The working fluid exists in a closed system and is reused throughout the life of the plant (references).
(Image Source: Ascari et al. 2012)
Open cycle OTEC plants use seawater directly, rather than a working fluid, to generate electricity. Warm surface water is placed under low pressure, distilling the water to a freshwater vapor which can be used to drive an electrical generator. The water vapor is condensed through heat exchangers with the cold deep ocean water, creating freshwater that is suitable for drinking and other uses. There are alternate open cycle processes that do not result in desalination as well. (references).
(Image Source: tethys-engineering.pnnl.gov)
In hybrid cycle ocean thermal energy conversion, 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. (references).
(Image Source: tethys-engineering.pnnl.gov)
One alternative use of cold deep-sea water is seawater air conditioning. This only requires a cold-water source; it does not need a thermal gradient like OTEC. In certain regions, SWAC has the potential to significantly reduce the electricity use of air conditioning systems by up to 90% when compared to conventional A/C units (Makai 2017). Naturally cold water is used for cooling in place of an electricity-driven compressor. This water is pumped up from depth through a heat exchanger to cool a freshwater loop that is distributed through buildings in a district (War 2011). The economics are largely determined by the infrastructure cost needed for implementation, which can be partially offset by including SWAC alongside an OTEC project, reusing the pumped deep-sea water.
(Image Source: Makai 2004)
Resource Potential
OTEC resource quality depends primarily on water temperature differences in the water column, but also on the availability of both warm and cold sources, often determined by the rate of replenishment at the plant location via local ocean currents. Thermal differences that are higher than 18-deg. C are considered suitable for OTEC, with optimal differences of around 25-deg. C. OTEC plants require access to both a large volume of warm surface water that is at least 22-deg. C and a cold deep water source (Ascari et al. 2012; William and Wu 1994).
Economics
Recent OTEC cost studies have been performed for floating plants and all studies have concluded that the cost of power is strongly driven by plant size (Vega 2012; Ascari et al. 2012). The studies have converged to a 100 MW plant size with an estimated LCOE of between $0.14 and 0.20/kWh. For smaller plants, approximately10 MW size, LCOE is estimated to be between $0.35 and 0.45/kWh (Vega, 2012). These studies assume the OTEC plant is relatively close to shore (0 km), and that the transmission export cable accounts for approximately 10% of the total capital costs. No cost studies have been performed for land based OTEC plants within the last 25 years. However, earlier studies indicate that for a very near shore cold water source, the LCOE would be $0.98/kWh for a 10 MW plant when adjusted for inflation (Vega 1992). A side product of some OTEC cycles is the production of fresh water which can be used to augment revenue streams. Learn more about Levelized Cost of Energy.
Environmental Effects
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. (references).