OTEC Can Make Power From the Ocean — But the Water It Must Move Is the Real Obstacle

Ocean thermal energy conversion has long held a seductive pitch: tropical surface water sits near 25°C while water a kilometre below hovers around 4–5°C, and a heat engine placed between those two reservoirs can spin a turbine. The temperature gap never disappears, so the fuel is effectively endless and available day and night. Net electricity has already been produced this way, including in Hawaii, and the underlying physics is settled rather than speculative.

The catch lies in the arithmetic of low efficiency. A heat engine running between 25°C and 5°C tops out at roughly 6.7% in theory, and real-world OTEC cycles fall into the low single digits once pumps, turbines, heat exchangers and internal loads are counted. Free heat makes that tolerable in principle, but it means each useful megawatt demands an extraordinary volume of water. A 100 MW net design, for instance, would need about 235 cubic metres per second of deep cold water plus roughly 470 m³/s of warm surface water — a combined 705,000 litres every second, or one Olympic swimming pool about every three and a half seconds.

Turning that flow into hardware is where OTEC stops being an energy story and becomes a civil-engineering one. A 100 MW-class cold-water intake is measured in the order of 10 metres in diameter and about a kilometre long. The depth itself is not the killer, since pressure inside and outside the pipe mostly balances, but friction, screens, bends and heat exchangers all bleed pressure, and those losses turn into heavy parasitic loads at such volumes. Going offshore swaps one burden for another: a floating platform shortens the horizontal reach to deep water, yet leaves a kilometre-scale intake suspended under a structure facing currents, storms, corrosion and stationkeeping forces.

The deployment record is equally sobering. OTEC delivered net power in Hawaii in 1979, and later projects in Hawaii, Japan and elsewhere exercised more components and configurations — one Makai plant even fed a US utility grid. Yet the expected commercial chain never materialised, and utility-scale OTEC remains absent while solar, wind and batteries now ship in the hundreds of gigawatts annually. The technology’s best remaining case is likely a steep tropical island where deep water sits close to shore, power is costly and land is scarce, and cold seawater can double as a cooling resource. Even there it must outcompete solar, batteries, wind, flexible demand and conventional desalination. What OTEC needs is not another proof of concept but a multi-megawatt plant at a favourable site that publishes net output, full installed cost and availability — and then wins a follow-up order without bespoke engineering.