Ocean thermal energy conversion, or OTEC, generates electricity by exploiting the temperature difference between warm surface seawater and cold water drawn from deep in the ocean. It needs a minimum gap of 20°C (36°F) between the two layers to work, which limits viable locations to tropical and subtropical waters roughly between 35°S and 40°N latitude. The concept is simple in principle: warm water provides the heat source, cold water provides the heat sink, and the difference between them drives a turbine. In practice, turning that small temperature gap into usable power requires some clever engineering.
The Temperature Gradient That Powers It All
In tropical oceans, surface water heated by the sun typically sits around 25–28°C. Meanwhile, water at depths of about 1,000 meters stays near 4–5°C year-round, largely untouched by sunlight. That 20-plus degree difference is the entire energy source for an OTEC plant. Both warm and cold water are pumped continuously through the system: warm water heats a working fluid (or becomes the working fluid itself), cold water cools it back down, and the cycle repeats.
The theoretical maximum efficiency for this kind of heat engine is low. With surface water at 28°C and deep water at 4.4°C, the ideal Carnot efficiency tops out at about 7.8%. Real-world losses from friction, heat exchange, and the energy needed to pump massive volumes of water push actual efficiency much lower, sometimes below 4%. That sounds discouraging, but the fuel is free, available 24 hours a day, and the ocean stores an enormous amount of thermal energy. Global OTEC power potential is estimated at 8 to 10 terawatts, and climate models project that figure could rise to nearly 13 terawatts by the end of this century as ocean surface temperatures climb.
Closed-Cycle Systems
The most common OTEC design is the closed cycle. Instead of using seawater directly, it circulates a separate working fluid with a very low boiling point, most often ammonia. Warm surface water flows through a heat exchanger, warming the ammonia enough to vaporize it. The pressurized ammonia vapor spins a turbine connected to a generator, producing electricity. The vapor then passes through a second heat exchanger where cold deep-ocean water absorbs its heat, condensing it back into liquid. The liquid ammonia is pumped back to the warm side, and the loop starts again.
Researchers have also tested propane and various refrigerants as working fluids, but ammonia remains the standard because of its favorable heat-transfer properties and availability. The closed cycle’s main advantage is that everything stays sealed. The turbine handles a relatively dense vapor at manageable pressures, so it can be compact compared to the alternative.
Open-Cycle Systems
Open-cycle OTEC uses the seawater itself as the working fluid. Warm surface water enters a chamber where the pressure has been dropped well below atmospheric levels. At that low pressure, a small fraction of the seawater, roughly 0.5%, flash-evaporates into steam without needing to reach 100°C. This low-pressure steam flows through a turbine, generating electricity, then enters a condenser cooled by deep-ocean water.
The open cycle has a valuable side benefit: if the condenser uses a surface type (where the steam doesn’t touch the cold seawater directly), the condensed vapor is essentially fresh water. A 10-megawatt open-cycle plant could produce around 4 million gallons of desalinated water per day. For island communities that struggle with freshwater supply, this alone could justify the investment. The tradeoff is that the steam operates at extremely low pressures, requiring very large turbines and careful vacuum management.
Hybrid Designs
Hybrid OTEC systems combine both approaches. Warm seawater flash-evaporates in a vacuum chamber (the open-cycle part), but instead of sending that steam through a turbine, it’s used to vaporize ammonia in a closed loop. The ammonia drives the turbine. This arrangement captures the freshwater production of the open cycle while keeping the turbine hardware more compact and efficient, like the closed cycle. These designs remain largely theoretical but represent a path toward maximizing multiple outputs from a single plant.
The Cold Water Pipe Challenge
Every OTEC plant, regardless of cycle type, needs to pull enormous volumes of water from roughly 1,000 meters below the surface. The cold water intake pipe is one of the biggest engineering hurdles. Current designs call for pipes up to 9 meters in diameter and 1,000 meters long, with wall thicknesses ranging from 0.07 to 0.6 meters depending on the material. Deploying and anchoring a structure like that in open ocean, where it must withstand currents, wave motion, and its own weight, is a serious construction challenge.
Pumping all that water takes a significant bite out of the electricity the plant generates. In OTEC systems, the energy consumed by pumps can eat up half or more of the gross power output. If the system isn’t carefully optimized, pump power consumption can actually exceed generation, producing a net energy loss. This parasitic load is the central engineering constraint: every component must be tuned to minimize the energy cost of moving water while maximizing heat transfer.
Beyond Electricity
The cold deep-ocean water doesn’t have to go to waste after it passes through the condenser. One of the most commercially promising spinoffs is seawater air conditioning, or SWAC, which pipes the cold discharge water through building cooling systems. SWAC is already operating in Hawaii, Bora Bora, and French Polynesia, replacing conventional air conditioning that runs on imported diesel fuel. Pairing SWAC with an OTEC plant turns the cold water pipe into a dual-revenue asset. Recent economic simulations suggest this combination can raise overall revenues by 25 to 40% compared to electricity generation alone.
The nutrient-rich deep water also opens doors for aquaculture, agriculture (irrigating crops with cold-water condensation), and even cosmetic and therapeutic uses marketed as thalassotherapy. For small island nations, an onshore OTEC plant with these co-products creates an entire cluster of economic activity from a single infrastructure investment.
Environmental Effects
OTEC’s most significant environmental impact comes from moving huge volumes of deep water to the surface. Deep ocean water is rich in nutrients like nitrate and phosphate that are scarce in sunlit surface layers. Releasing this water near the surface stimulates plankton growth, which in turn increases biological productivity. Modeling studies project that widespread OTEC deployment could boost ocean primary productivity measurably in tropical regions by 2100.
That nutrient upwelling also affects ocean chemistry in potentially beneficial ways. Higher surface alkalinity helps buffer against ocean acidification, and increased photosynthetic activity draws more carbon dioxide from the atmosphere into the water. The cold water discharge also cools surface temperatures locally, with models showing reductions of up to 3.1°C in some scenarios. About 60% of the cooling effect comes from the physical mixing of cold deep water, with the rest attributable to displacing fossil fuel emissions. Whether large-scale shifts in surface temperature and nutrient levels would disrupt existing marine ecosystems is still an open question, but the direction of the chemical changes tends toward counteracting some effects of climate change.
Where OTEC Stands Today
Despite decades of research, OTEC remains in the demonstration phase. The world’s largest operational facility is a 100-kilowatt closed-cycle plant at the Hawaii Ocean Science and Technology Park, built by Makai Ocean Engineering. Connected to the U.S. electrical grid in 2015, it generates enough power for about 120 Hawaiian homes and runs continuously, day and night. It’s a functioning prototype, not a commercial power station.
The core barrier is cost. OTEC’s levelized cost of energy ranges from $0.15 to $0.63 per kilowatt-hour, compared to about $0.12 for solar and $0.07 for onshore wind. The massive underwater infrastructure, specialized heat exchangers, and low thermodynamic efficiency all contribute to that price gap. For large mainland grids with access to cheap solar and wind, OTEC is hard to justify economically.
The calculus changes for tropical island nations that import expensive diesel fuel, face limited land for solar farms, and need freshwater and cooling alongside electricity. In those settings, an OTEC plant that simultaneously generates power, produces drinking water, and provides air conditioning could compete on total value even if the electricity alone costs more. Whether that potential translates into commercial-scale plants will depend on whether the engineering costs of building and maintaining kilometer-long deep-water pipes can be brought down through larger-scale manufacturing and deployment experience.