OTEC (Ocean Thermal Energy Conversion)
Power generation from the temperature difference between warm tropical surface seawater and cold deep water, typically about 20 to 25 °C, using a heat engine cycle.
OTEC exploits the ocean as a vast solar collector. In the tropics surface water is 25–29°C while water at 800–1,000 m depth is 4–6°C. A temperature difference of at least about 20°C is needed for useful operation.
The available efficiency is fundamentally low. The Carnot limit for a 24 K difference is:
Real cycles achieve about 3–4% gross, and pumping huge volumes of cold water consumes 20–30% of gross output. A 100 MW net plant needs cold water flows on the order of a few hundred m³/s through a pipe several metres wide reaching to about 1,000 m depth.
Cycle types:
- Closed cycle: warm water evaporates a working fluid such as ammonia, which drives a turbine, as in an Organic Rankine / binary cycle
- Open cycle: warm seawater flash-evaporates under vacuum; the condensed steam doubles as desalinated freshwater
- Hybrid combinations
Demonstrations remain small: Makai Ocean Engineering's 100 kW plant in Hawaii (2015) and the Kumejima plant in Japan (about 100 kW), alongside earlier tests at Keahole Point. Co-products such as freshwater, seawater air conditioning and aquaculture improve economics. OTEC's attraction is a baseload-like output for tropical islands, with capacity factors potentially above 90%, but costs of large cold-water pipes and floating platforms remain unproven at scale.
Related terms
All terms →- Wave energy
Energy captured from the motion of ocean surface waves using devices such as point absorbers, oscillating water columns and attenuators; still at pre-commercial stage.
- Binary cycle power plant
A geothermal power plant in which hot brine heats a separate low-boiling-point working fluid that drives the turbine, enabling generation from resources of roughly 100 to 180 °C.
- Baseload
The minimum level of electricity demand on a grid over a day, season or year, historically served by plants that run continuously at high output.
- Tidal stream power
Electricity generated by underwater turbines placed in fast tidal currents, analogous to wind turbines but driven by water about 800 times denser than air.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.