Wondering whether the sea’s heat can really keep the lights on? Ocean thermal energy conversion turns the temperature gap between warm surface water and cold deep water into electricity, mostly in the tropics, using a heat engine with no fuel burned.
This guide sits within the wider topic of ocean renewable energy, and it explains the basic idea without the jargon.
| Item | Value |
|---|---|
| Temperature gap needed | At least 20° (ideally 30-40°) |
| Best latitude band | About 20°N to 20°S |
| Depth of the cold water | 500-1000 m (1000-3000 ft) |
| Boiling point of ammonia, a common working fluid | -33°C (28°F) |
| Main cycle types | Three (closed, open, hybrid) |
| Year the idea was first suggested | 1881 |
These figures come from Explain That Stuff’s OTEC guide (last updated September 2023) and Global OTEC’s June 2025 explainer on the OTEC cycles. Figures can change, so check the source before you rely on them.
What is ocean thermal energy conversion?
OTEC is short for ocean thermal energy conversion. Global OTEC’s June 2025 explainer describes it as a renewable energy technology that harnesses the natural temperature difference between warm surface seawater and cold deep seawater in the tropical area of the planet. The idea is not new: it was first suggested in 1881.
To see why a temperature difference is useful, start with the heat engine. This is a machine that cycles between one hot and one cold temperature, and it usually takes its heat from a fuel of some kind. In a steam turbine, for example, coal heats water into high-pressure steam, which then expands and cools as it does its work.
OTEC works in a broadly similar way, except that no fuel is burned. The ocean already holds a natural temperature gradient, so nothing has to be burned to create one.
That gradient exists year-round near the equator. Global OTEC describes the result as clean baseload power that can run day and night.
Why the temperature gap and the tropics matter
The greater the temperature difference, the more energy a heat engine can extract, and the more efficient it is. For OTEC, that means engineers look for the biggest vertical temperature gradient they can find.
Explain That Stuff sets out what a good site looks like:
- A gradient of at least 20°, and ideally more like 30-40°.
- Surface water that is typically 20°C (68°F) or more.
- Deep water close to freezing, around 4°C (39°F), perhaps 500-1000 m (1000-3000 ft) beneath the surface.
- A location in the tropics, between the latitudes of about 20°N and 20°S.
Between the warm top layer and the cold depths sits the thermocline, the middle region where water temperature changes rapidly with depth. Explain That Stuff points to the tropics as the area with the best potential for OTEC, while polar and temperate regions are colder.
How closed-cycle OTEC works
The closed cycle is the easiest one to picture. A loop of pipe is filled with a working fluid that has a very low boiling point, such as ammonia, which boils at -33°C (28°F). The fluid never leaves the pipe: it circles the loop again and again, picking up heat and giving it up.
The steps run like this:
- Warm surface seawater flows past a heat exchanger and heats the working fluid until it vaporizes.
- The high-pressure vapor expands through a turbine, which spins a generator and produces electricity.
- Cold seawater pumped up from the ocean depths condenses the vapor back into a liquid in a second heat exchanger.
- The liquid returns to the start of the loop and the cycle repeats, while a cable carries the electricity ashore.
A helpful comparison is a refrigerator. Its coolant also picks up heat in one place and carries it somewhere else through a closed loop. In OTEC the warm sea plays the part of the chiller cabinet, and the cold deep water does the cooling afterwards.
Open-cycle and hybrid-cycle systems
Closed-cycle is one of three main cycles that OTEC systems use. According to Global OTEC, the other two are the open cycle and the hybrid cycle. Each has its own design, advantages and technical considerations.
- Closed cycle: a low-boiling working fluid such as ammonia spins the turbine, as described above.
- Open cycle: seawater itself is the working fluid, instead of a separate substance like ammonia. Warm surface seawater is placed in a low-pressure container, where it flash-evaporates into steam that drives the turbine. Cold deep seawater then condenses the steam back into liquid water.
- Hybrid cycle: it combines elements of both, aiming to maximise energy production and freshwater output. Low-pressure steam heats a working fluid in a closed loop, and the steam is then condensed with cold seawater, which produces freshwater.
The open cycle brings its own engineering demand. Global OTEC notes that running turbines on low-pressure steam calls for specialised designs.
How much energy is in the ocean’s heat?
The raw numbers are large. Explain That Stuff reports an estimate from the US Department of Energy’s National Renewable Energy Laboratory (NREL): on a typical day, the tropical oceans absorb heat energy equivalent to 250 billion barrels of oil. Converting a mere 0.005 percent of that into electricity would, by the same estimate, be enough to power the whole of the United States.
The same guide also says that, by some estimates, the upper layers of the oceans hold enough heat to meet humankind’s energy needs hundreds of times over.
Treat those headline figures with care. Explain That Stuff adds that such estimates do not take account of the tremendous practical difficulties involved in harvesting ocean energy. The size of the resource and the amount you can realistically capture are two different things.
Where OTEC fits among ocean energy options
OTEC is only one way to draw power from the sea. Other options, such as floating offshore wind turbines, are covered in their own guide. A separate guide on salinity gradient power looks at a different approach again.
When you read a claim about any OTEC project, a few questions help you judge it:
- Is the site in the tropics, with a large enough temperature gap between the surface and the deep water?
- Which cycle does it use: closed, open or hybrid?
- Is the figure a measured result, or a theoretical estimate of the ocean’s total heat?
Asking these keeps the temperature gradient at the centre of the picture, because it is what makes the whole technology possible.
Frequently asked questions
What does OTEC stand for?
Show answer
OTEC stands for ocean thermal energy conversion. It means making power from the temperature difference between warm surface seawater and cold deep seawater.
Does OTEC burn any fuel?
Show answer
No. Explain That Stuff notes that no fuel is burned, because the temperature gradient already exists naturally in the oceans.
Where can OTEC work best?
Show answer
The best place is the tropics, between the latitudes of about 20°N and 20°S. There, surface water is typically 20°C (68°F) or more and the deep water is close to freezing.
What is the difference between the three cycles?
Show answer
The closed cycle uses a low-boiling fluid such as ammonia. The open cycle uses seawater itself as the working fluid. The hybrid cycle combines both and can also produce freshwater.
