Ocean thermal energy conversion, or OTEC, is a method of generating electricity from the natural temperature difference between warm surface seawater and cold deep seawater. It works in tropical and subtropical regions where surface water sits around 25°C or warmer while water 1,000 meters below hovers near 4–5°C. That gap of at least 20°C (36°F) is enough to drive a turbine and produce continuous, around-the-clock power, something most other renewables can’t do.
How OTEC Generates Power
OTEC operates on the same basic principle as any power plant: heat a fluid, use the expanding vapor to spin a turbine, then cool it back down and repeat. The difference is that OTEC draws its heat and cold from the ocean itself rather than burning fuel.
In the most common design, called a closed-cycle system, warm surface water (typically drawn from the top few meters of the ocean) passes through a heat exchanger containing ammonia. Ammonia has a boiling point of about negative 33°C at normal pressure, so even the modest warmth of tropical seawater is enough to vaporize it. That expanding ammonia vapor spins a turbine connected to a generator. Cold water pumped up from roughly 1,000 meters deep then flows through a second heat exchanger, cooling the ammonia vapor back into liquid so the cycle can start again. No fuel is consumed and no combustion byproducts are released. The only hazardous substance involved is the ammonia itself, which stays sealed inside the loop.
An alternative approach, called an open-cycle system, skips the ammonia entirely and uses the seawater as the working fluid. Warm surface water enters a low-pressure chamber where it flash-evaporates into steam. That steam drives the turbine, then gets condensed by contact with cold deep water. Because the evaporation process leaves salt and impurities behind, the condensed output is freshwater. This makes open-cycle OTEC a combined power-and-desalination system, a particularly appealing feature for island nations with limited freshwater supplies.
Where OTEC Can Work
The 20°C temperature differential requirement confines OTEC to a band roughly between 20°N and 20°S latitude, covering large stretches of the Pacific, Atlantic, and Indian Oceans. Hawaii, the Caribbean, Southeast Asia, and the western coast of Africa are among the most promising locations. Even within these zones, local ocean currents create meaningful variation. Detailed mapping around Hawaii, for instance, found that average temperature differentials run about 1°C higher on the western (leeward) side of the islands, thanks to a narrow eastward-flowing counter current. Seasonal cycles also matter: temperature gaps shrink slightly in winter and widen in summer, though in equatorial waters the variation is small enough that year-round operation is feasible.
The Engineering Challenge
OTEC’s core concept is simple, but the infrastructure is demanding. The cold water pipe alone, which must reach down to around 1,000 meters, can be 9 meters in diameter and requires materials that withstand enormous pressure, constant seawater exposure, and the mechanical stress of ocean currents and waves. Wall thickness on these pipes ranges from 0.07 to 0.6 meters depending on the material used.
Heat exchangers present another persistent problem. Seawater promotes both corrosion and biofouling, the gradual buildup of algae, bacteria, and marine organisms on surfaces. Even a thin layer of biological growth insulates the exchanger and degrades performance. Decades of research at Hawaii’s Natural Energy Laboratory have focused on identifying corrosion-resistant alloys (particularly aluminum-based ones) and developing cleaning methods that keep exchangers efficient without shutting down the plant. These aren’t unsolvable problems, but they add cost and complexity that conventional power plants don’t face.
Thermal efficiency is inherently low because the temperature difference driving the system is so small compared to a fossil fuel plant. OTEC converts only a fraction of the thermal energy into electricity, and a significant portion of the power generated gets consumed by the pumps that move enormous volumes of water through the system. This means plants need to be large to produce meaningful net output.
Current State of OTEC
Despite decades of development, OTEC remains in the pilot stage. Only two land-based plants are currently operational worldwide. Saga University in Japan runs a 50-kilowatt demonstration system using a double-cycle design, primarily for research and model validation. In Hawaii, the Natural Energy Laboratory hosts a 105-kilowatt closed-cycle plant that powers roughly 120 homes. For context, a single modern wind turbine produces 2,000 to 5,000 kilowatts.
Cost is the primary barrier. OTEC’s levelized cost of energy (the total lifetime cost divided by total energy produced) remains significantly higher than wind, solar, or even other ocean energy technologies like tidal and wave power. Industry groups project that tidal energy could reach roughly €100–150 per megawatt-hour by 2025–2030, with wave energy following by 2030–2035, but OTEC cost projections remain less certain. The massive upfront investment in deep-water pipes, large heat exchangers, and corrosion-resistant materials makes it difficult to compete with renewables whose costs have plummeted over the past decade.
Benefits Beyond Electricity
What makes OTEC more interesting than its raw power numbers suggest is the bundle of secondary uses that come from pumping cold, nutrient-rich deep ocean water to the surface. Once that water has passed through the condenser, it’s still cold and still useful.
Seawater air conditioning uses the cold discharge water to cool buildings near the coast, replacing conventional compressor-based systems and cutting electricity use for cooling dramatically in tropical climates. Aquaculture operations can use the cold, pathogen-free deep water to raise cold-water species like salmon and lobster in tropical locations where they wouldn’t normally survive. Open-cycle systems produce desalinated water as a direct byproduct of the power generation process. And the nutrient content of deep ocean water (rich in nitrogen and phosphate) can support agriculture and algae cultivation.
For small island developing states that import diesel fuel for electricity, truck in freshwater, and rely on energy-intensive air conditioning, an OTEC plant offering power, freshwater, and cooling from a single system addresses multiple needs at once. This multi-product value proposition is often what keeps OTEC development moving forward despite the cost challenges.
Environmental Considerations
OTEC produces no greenhouse gas emissions during operation, but large-scale deployment would alter ocean conditions in ways that are still being studied. The core issue is that OTEC moves massive volumes of deep ocean water, which is cold, nutrient-rich, and more acidic, into the sunlit surface layer where photosynthesis happens.
Modeling studies looking at widespread OTEC deployment through the year 2100 found that introducing these deep-water nutrients to the surface boosts biological productivity, essentially fertilizing surface waters and increasing the growth of phytoplankton. This enhanced productivity increases the ocean’s uptake of CO₂ from the atmosphere, which is a climate benefit. It also raises oxygen levels in the upper 600 meters of the water column, potentially shrinking shallow oxygen-depleted zones that stress marine life.
The tradeoffs cut the other way at greater depths. Oxygen concentrations between 600 and 1,900 meters decrease under OTEC scenarios, potentially worsening deeper oxygen minimum zones. The discharge of cold, more acidic water also alters local temperature and chemistry at the surface, with possible effects on nearby coral reefs and ecosystems. At a global scale, models suggest OTEC-induced mixing could even cause minor localized warming near the poles. These effects scale with the size of deployment: a few small plants are negligible, but a network large enough to meaningfully contribute to global energy supply would require careful management of discharge locations and volumes.