OTEC Works. The Seawater Flow Is The Problem.

OTEC can turn tropical ocean heat into electricity, but the small temperature difference requires enormous seawater flows and deep-water infrastructure.

October 2, 20261 minute

			Michael Barnard			

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Ocean thermal energy conversion has an appealing proposition. Tropical oceans can have surface water around 25°C while water roughly a kilometre down is near 4°C to 5°C. Put a heat engine between the two reservoirs, use the warm water to vaporize a working fluid, run it through a turbine, condense it with cold deep water and repeat. The fuel is free, the temperature difference is available around the clock, and none of this requires speculative physics. OTEC has produced net electricity, including in Hawaii, and engineers have been refining the concept for decades.

The water volume involved is enormous because the temperature difference is small. A heat engine operating between 25°C and 5°C seawater has a theoretical efficiency ceiling of only about 6.7%, and practical OTEC cycles land in the low single digits once real heat exchangers, turbines, pumps and internal loads are included. Low efficiency is not automatically fatal when the heat itself is free, but it means every useful MWh requires processing very large quantities of thermal energy. The physical consequence is not an abstract efficiency penalty. It is a civil-engineering problem.

The full TFIE Strategy Briefing analysis looks at the denominator OTEC advocates usually leave out: how much seawater, pipe, heat-exchanger area and offshore infrastructure are required for each useful megawatt.

A detailed 100 MW net reference design makes the scale tangible. It requires about 235 cubic metres of deep cold water every second and another 470 cubic metres of warm surface water, for a combined flow of 705 m³/s. That is 705,000 litres every second, equivalent to an Olympic-size pool roughly every 3.5 seconds. Over a day the plant moves the equivalent of more than 24,000 such pools. The water is returned to the ocean, but every liter still has to enter through an intake, pass through pipes and heat exchangers without creating excessive pressure losses, and then be discharged again.

The cold-water pipe turns that flow rate into something easier to visualize. Engineering reviews put a 100 MW-class intake at roughly 10 metres in diameter and around a kilometre long. The depth does not mean pumping water vertically a kilometre against its full hydrostatic head, because pressure inside and outside the pipe largely balances. The problem is friction, screens, bends, heat exchangers and every other pressure loss along a flow path carrying hundreds of tonnes of seawater each second. Even modest pressure losses become major parasitic loads at that scale, which is why the pipes need to be so large in the first place.

Moving the plant offshore does not remove the engineering burden; it changes its form. A floating OTEC system can shorten the horizontal run to deep water, but then a kilometer-scale intake hangs beneath a platform exposed to currents, storms, fatigue, corrosion and stationkeeping loads. The machinery itself is substantial because several gigawatts of thermal energy have to cross the heat exchangers to produce around 100 MW of useful electricity. Offshore engineering can build structures of this scale. The question is whether it can do so cheaply and repeatedly enough to compete with energy systems that are manufactured in large numbers and installed incrementally.

The deployment record is the other useful denominator. OTEC produced net electricity in Hawaii in 1979, and later systems in Hawaii, Japan and elsewhere demonstrated more components and operating configurations. Makai’s Hawaiian plant even sent electricity to a US utility grid. Yet the expected commercial progression never followed: larger plants leading to repeat orders, standardized equipment, multiple suppliers and a growing body of operating data. Solar, wind and batteries have moved into annual deployments measured in hundreds of gigawatts. Utility-scale commercial OTEC remains absent.

That does not mean OTEC has no plausible application. Its strongest case is likely to be a steep tropical island where very deep water lies close to shore, electricity is expensive, land is constrained and cold seawater can provide additional value for cooling or water systems. But those locations also force OTEC to compete with whatever else is available locally: solar, batteries, flexible demand, wind, direct seawater cooling, geothermal where geology allows it, and conventional desalination. A large theoretical ocean resource is not the same thing as a large economically accessible resource. As an example, in my roadmap to a decarbonized domestic energy system for Hawai’i, OTEC isn’t part of the solution stack, but ocean urban center cooling certainly is.

OTEC does not need another demonstration that a warm ocean and a cold ocean can run a heat engine. It needs a multi-megawatt plant in one of its best possible locations that publishes net output after pumping loads, complete installed cost, maintenance and availability, then leads to another commercial order without heroic bespoke engineering. Until that happens, the most useful number is not how much heat tropical oceans contain. It is how much marine infrastructure must be built for every megawatt delivered.

For the deeper infrastructure, niche-market and competing-technology analysis, read the full TFIE Strategy Briefing assessment.

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Source: https://cleantechnica.com/2026/10/02/otec-ocean-thermal-energy-seawater-flow/