What Is Ocean Thermal Energy Conversion?

Ocean thermal energy conversion, generally abbreviated OTEC, is an energy conversion technology that generates electricity from the temperature difference between warm surface seawater and cold water drawn from the deep ocean. It is a heat engine in the strict thermodynamic sense, with the sun-warmed mixed layer serving as the hot reservoir and water pumped from 800 to 1,000 meters serving as the cold sink. Practical operation requires a temperature difference of about 20 degrees Celsius, a condition met year-round in tropical waters roughly between 20 degrees north and 20 degrees south, which makes OTEC one of the few marine renewable resources available as continuous baseload rather than an intermittent supply.

Georges Claude demonstrated the principle off Cuba in 1930, and interest revived after the oil shocks of the 1970s. The PNNL Tethys knowledge base entry on OTEC traces the modern development record, including the open-cycle plant operated at the Natural Energy Laboratory of Hawaii Authority at Keahole Point from 1993 to 1998, which produced up to 103 kilowatts net from 255 kilowatts gross.

Thermodynamic Cycles

Three cycle configurations dominate the engineering literature. Closed-cycle systems circulate a working fluid with a low boiling point, typically ammonia, through an evaporator heated by warm seawater, an expander driving a generator, and a condenser cooled by deep water. Open-cycle systems flash the warm seawater itself into steam in a partial vacuum, expand that low-pressure steam through a turbine, and condense it, which yields desalinated fresh water as a byproduct. Hybrid designs combine the two to produce power and water together. Because the available temperature difference is small, the ideal Carnot efficiency is only about 6 to 7 percent and net efficiency after parasitic loads falls to roughly 3 percent, so plants must move enormous volumes of water. A 100-megawatt facility circulates on the order of several hundred cubic meters of seawater per second.

Engineering Components

The low efficiency shifts the design problem toward heat exchangers and pumping. Evaporators and condensers must transfer very large thermal loads across small temperature differences, which favors plate or shell-and-tube exchangers in titanium or aluminum alloys and makes biofouling control a continuous operating concern. The cold water pipe is the signature component and the hardest to build: a pipe several meters in diameter and around a kilometer long, suspended from a floating platform or laid along a steep seabed slope, subject to current-induced vibration, wave loading, and thermal contraction. Turbine design also departs from conventional practice, since ammonia turbines run at modest pressure ratios and open-cycle steam turbines must handle extremely low-density vapor with rotor diameters measured in meters. The PRIMRE technology summary maintained through Open Energy Information documents these subsystems and the platform configurations under study.

Resource Potential and Constraints

Estimates of the extractable resource depend on how much deep cold water can be withdrawn before the thermal structure of the ocean itself is altered, since discharged water returns heat and nutrients to intermediate depths. A published assessment of OTEC resources and climate change mitigation potential evaluates that ceiling and the associated mitigation contribution. Capital cost is the main barrier to deployment: the plant is offshore, the cold water pipe is expensive, and no commercial-scale unit has yet been built, so cost estimates carry wide uncertainty. Environmental review focuses on entrainment of marine organisms, the plume of cold nutrient-rich discharge, and any working fluid release.

Applications

Ocean thermal energy conversion has applications in a range of settings, including:

• Baseload electricity for tropical islands and coastal communities
• Seawater desalination through open-cycle and hybrid plants
• Seawater air conditioning using the cold water supply
• Aquaculture and mariculture fed by nutrient-rich deep water
• Hydrogen and ammonia production for export from remote sites
• Military and civil installations seeking local energy security