Ocean thermal energy conversion

In plain language

Ocean thermal energy conversion, or OTEC, makes electricity from the difference in temperature between warm water near the sea surface and cold water far below it. It is a heat engine: warm water heats a fluid until it turns to vapour, the vapour spins a turbine, and cold water cools the vapour back into liquid so the cycle can repeat. At the site of the 1979 Mini-OTEC test off Hawai'i, the average temperature difference was 21 degrees Celsius.

The trouble is that the temperature gap is small, so a great deal of seawater has to be moved to make useful power. Pumps that lift cold water from depth, and pumps that move warm water, use some of the electricity the turbine makes. In 1979 a small floating test plant produced about 50 kW before pumping and about 15 kW after it, and its longest continuous run was 10 days. A 2026 prototype has reported short sea trials, so the question of whether OTEC can run for months and deliver power to a grid is still open.

Technical explanation

Most OTEC designs use a closed Rankine cycle. A working fluid, ammonia in the 1979 Mini-OTEC plant and R134a in a 2026 containerised prototype, is evaporated by warm surface seawater, expands through a turbine and is condensed by cold deep seawater that has been pumped up through an insulated intake pipe. The fluid stays in a sealed loop, so large heat exchangers and seawater pumps are the main plant components. An open cycle instead flash-evaporates warm seawater under low pressure to drive a low-pressure turbine, and the resulting steam is condensed to give desalinated water. Hybrid designs combine the two. The Carnot limit sets the ceiling for any heat engine. For a 21 degree Celsius difference, with warm water at about 26 degrees Celsius and cold water at about 5 degrees Celsius (assumed temperatures for this calculation), the ideal limit is roughly 7 per cent. This is a calculation, not a measured efficiency, and practical plants run below it because of heat-exchanger temperature differences, friction, pumping and heat leakage.

The 1979 Mini-OTEC plant was a barge-mounted ammonia plant off Keahole Point, Hawai'i. Its first at-sea power came on 2 August 1979. It produced about 50 kW gross at an average temperature difference of 21 degrees Celsius, and drew cold water from about 670 metres depth through a 0.61 metre polyethylene pipe. Its longest continuous run was 10 days. The 2026 containerised prototype used a radial-inflow turbine, stainless-steel heat exchangers, circulation pumps and a programmable logic controller. Its land tests reached 11 kW in one run and about 5 kW for 11 minutes in another. Sea trials in the South China Sea reached a peak of 16.4 kW, with 4 hours 47 minutes of generation accumulated across separate runs.

Intake piping is a central engineering problem, not an accessory. A 2025 study of surface seawater intake piping for a hybrid OTEC pilot at UPM I-AQUAS, Port Dickson, Malaysia, covered a 336 metre pipeline and reported pump cavitation and biofouling as challenges. Deep cold-water pipes for offshore plants would be expected to be longer and to face larger mechanical loads. Fouling on heat-exchanger surfaces would be expected to reduce heat transfer over time. The sources reviewed for this profile do not include long-term measurements of that effect on a full-scale offshore plant.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• The 1979 Mini-OTEC run produced about 50 kW gross and about 15 kW net, because pumping for seawater and ammonia used much of the gross output.
• The longest continuous run reported for Mini-OTEC was 10 days. The 2026 prototype accumulated 4 hours 47 minutes of generation across separate offshore runs.
• The 2026 paper reports peak electrical outputs and run totals. Its abstract does not give a full net power balance for the plant, including deep-water pumping, and it names longer-duration testing as a priority.
• Intake pipes face pump cavitation and biofouling, as reported for the 336 metre surface intake studied in Malaysia.
• The 2010 NOAA-sponsored workshop could not reach conclusions on cumulative or secondary environmental impacts, so site-specific baseline and monitoring data are needed before deployment.
• The sources reviewed for this profile give no verified cost figures for OTEC plants.

Possible future uses

Timeline

1. 2 Aug 1979Mini-OTEC produces the first at-sea OTEC power, from a barge about 2.2 km off Keahole Point, Hawai'i. Sources: [1]
2. 18 Nov 1979The Mini-OTEC programme ends. Its longest continuous period of operation had been 10 days. Sources: [1]
3. 22 Jun 2010NOAA-sponsored workshop report on the physical, chemical and biological impacts of OTEC is published. Sources: [4]
4. 3 Oct 2025Study of a 336 metre surface seawater intake for a Malaysian hybrid OTEC pilot is published. Sources: [3]
5. 28 Sept 2026Paper reports integration and sea trials of a containerised 20 kW-class OTEC prototype in the South China Sea. Sources: [2]

Futures this could help bring about

How far away?

Power from ocean temperature differences

Pilots · 2 of 7 steps done

Glossary

Working fluid

The liquid or vapour that carries heat around a closed power cycle, such as ammonia or R134a.

Rankine cycle

A heat-engine cycle in which a liquid is boiled, the vapour drives a turbine, and the vapour is condensed back into liquid.

Carnot limit

The highest efficiency a heat engine can reach between two temperatures. Real engines always fall below it.

Biofouling

Growth of marine organisms on pipes, screens and heat-exchanger surfaces, which can reduce flow and heat transfer.

Sources

1. peer reviewed paper
   Mini-OTEC Operational Results
   W. L. Owens, L. C. Trimble. Journal of Solar Energy Engineering (ASME), Aug 1981. DOI: 10.1115/1.3266245. Accessed 21 Oct 2026.
2. peer reviewed paper
   System Integration and Sea Trials of a Containerized Ocean Thermal Energy Conversion Prototype
   Jingyi Liu, Fuzhen Xing, Yawei Wang et al.. Machines (MDPI), 28 Sept 2026. DOI: 10.3390/machines14101112. Accessed 21 Oct 2026.
3. peer reviewed paper
   Design Optimization of Surface Seawater Intake Piping for Hybrid Ocean Thermal Energy Conversion Pilot Plant
   Shamsul Sarip, Abu Bakar Jaafar, Mohd Khairi Abu Husain et al.. Energy Science & Engineering, 3 Oct 2025. DOI: 10.1002/ese3.70316. Accessed 21 Oct 2026.
4. government report
   Ocean Thermal Energy Conversion: Assessing Potential Physical, Chemical and Biological Impacts and Risks
   J. Cunningham, Z. Magdol, N. Kinner. NOAA Office of Ocean and Coastal Resource Management (report prepared by the University of New Hampshire), 22 Jun 2010. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.