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Ocean thermal energy conversion (OTEC) can generate electricity from the temperature difference between warm surface water and cold deep seawater. But that difference is modest, so a plant must move enormous volumes of water—and the pipes, pumps, heat exchangers, offshore structures and discharge system needed to do it are central engineering challenges, not minor supporting equipment.
How OTEC turns seawater temperature into electricity
OTEC uses solar-warmed surface water as the hot side of a heat engine and colder deep water as the cold side. The U.S. Energy Information Administration (EIA) says the temperature difference must be at least 20°C (36°F) for an OTEC system to power a turbine. That is a relatively small temperature span for a heat engine, which limits how much energy the plant can extract from each unit of seawater.
Closed-cycle systems
In a closed cycle, warm seawater passes through a heat exchanger and vaporizes a separate working fluid, often ammonia in standard Rankine-cycle descriptions. The vapor drives a turbine, then cold deep water cools another heat exchanger and condenses the working fluid so the cycle can repeat. Seawater supplies and removes heat; it is not the fluid driving the turbine.
Open-cycle systems
In an open cycle, warm seawater itself is flashed into low-pressure steam, which drives a turbine before cold seawater condenses it. Because the steam comes from seawater, condensation can also produce desalinated water. The two cycles use different working fluids and equipment, but both depend on moving warm and cold seawater through the system.
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Why does OTEC need so much seawater?
With only a modest temperature difference between its hot and cold sides, OTEC needs a large heat-transfer surface and high water flow to move enough heat through the plant. Ocean Energy Systems (OES) gives an approximate cold-seawater flow of 2.5–3 cubic metres per second per net megawatt; warm-water flow is usually greater. This is an approximate technical figure, not a specification for every plant design.
The word net matters: pumps consume electricity to move the water, so a plant’s usable output depends on what remains after those loads. Higher flows can help transfer more heat, but they also require larger intakes, conduits and heat exchangers and can increase pumping demand. OTEC’s engineering problem is therefore to make the complete flow-and-heat-transfer system deliver more useful power than it consumes.
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The cold-water pipe is an offshore system, not just a large tube
The cold-water pipe (CWP) must reach deep enough to access suitably cold water and deliver it to the plant. OES identifies pipe deployment and survival as central challenges; for designs above 10 MW, it calls the CWP a technological frontier. The scale of a historical deployment illustrates the task: OES reports a pipe 1.4 metres in diameter and 2.8 kilometres long, deployed off Hawaiʻi to a depth of 900 metres in 2001. That example demonstrates a significant engineering effort, not current commercial-scale operation.
Pipe design cannot be separated from the rest of the installation. The route from deep water to the plant, the platform or shore connection, pumping equipment, heat exchangers and return-water outlets must work together. Layout differs among land-based, seabed-moored and floating configurations, and each faces its own demands in reaching cold water and routing very large flows. IRENA’s 2014 technology brief identifies large water ducting as a major technical challenge at scale.
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What a design must account for
- Intake: access to water cold enough for the intended cycle and plant design.
- Conduit and platform: a pipe arrangement compatible with the plant’s location and architecture.
- Pumping and heat transfer: the electricity used to move seawater must be weighed against the heat transferred in the exchangers.
- Return flow: used seawater needs a discharge location and depth suited to the installation and local conditions.
Plant configurations change the engineering trade-offs
There is no single pipe-and-platform arrangement that suits every site. The distance to deep cold water, the plant’s location and the route seawater must travel all affect the system that has to be built.
| Configuration | What it means for the flow system | Evidence and qualification |
|---|---|---|
| Land-based | Water is routed between the ocean and an onshore plant; the intake and discharge layout must fit the coast and access to deep water. | IRENA describes land-based systems as one OTEC platform option; the best layout depends on the site. |
| Seabed-moored | The plant is moored to the seabed, so the pipe and plant arrangement must be designed together. | IRENA describes seabed-moored platforms as an option; the cited brief does not establish a universal design or performance figure. |
| Floating | The plant operates on a floating platform, making the connection to the deep-water intake part of the offshore system design. | IRENA describes floating platforms as an option; the cited material does not establish one standard pipe arrangement for all floating plants. |
OTEC demonstrations show feasibility, not broad commercial maturity
The EIA reports U.S. research involvement beginning in 1974, a 250 kW demonstration that operated for six years in the 1990s, and a Navy-supported 105 kW facility that became operational in 2015. These projects show that OTEC has been built and operated, but they do not establish widespread utility-scale operation.
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IRENA’s 2014 brief said plants of up to 1 MW had been built at that time and identified a lack of operating experience at larger scale. That is a dated account of technology status, not a statement of what has or has not happened since 2014. The cited material does not establish current commercial-scale operating performance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What historical cost estimates can—and cannot—tell you
IRENA’s 2014 technology brief reported the following estimates. They are expressed in 2010 U.S. dollars and are not current prices, bids or verified costs for a present-day project.
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| Estimate in IRENA’s 2014 brief | Reported amount | How to interpret it |
|---|---|---|
| Small-scale OTEC installation costs | USD 16,400–35,400 per kW | Historical installation-cost range, stated in 2010 dollars. |
| Feasibility-study estimates for larger plants | USD 5,000–15,000 per kW | Historical estimates, stated in 2010 dollars; not evidence of a current project price. |
The brief says the estimates depend on financing, deployment scale and learning assumptions. It also notes that small systems may need valuable co-products, such as freshwater or cooling, to make their economics work. These qualifications matter because a cost per kilowatt by itself cannot show whether a particular site, plant layout or combination of services is viable.
Discharge is part of the environmental and engineering design
After transferring heat, the plant must return seawater to the ocean. The depth and location of that discharge are consequential design choices: the water’s temperature and, for deep water, nutrient content differ from those of surrounding surface waters. It is not justified to label deep-water discharge universally harmless or universally damaging.
A 2026 assessment by Pacific Northwest National Laboratory (PNNL) focused on Kona, Hawaiʻi, and identified discharge depth as a major environmental concern. It developed an open-source model to examine discharge depth and location. IRENA’s 2014 brief also noted uncertainty about marine-life effects from moving large volumes of cold, nutrient-rich water upward. Together, these sources support site-specific assessment; they do not establish a single impact outcome for all locations.
Can co-products improve OTEC’s value?
Using the same site or infrastructure for more than electricity may strengthen the value proposition, but the fit depends on local conditions and does not remove the need to solve the intake, pumping, pipe and discharge challenges.
- Seawater air conditioning (SWAC) and industrial cooling: PNNL’s Kona assessment, published September 22, 2026, identifies these as promising onshore combinations with OTEC power.
- Aquaculture cooling: PNNL identifies cooling for aquaculture in warm climates as a potential use.
- Freshwater: Desalination shows promise alongside open-cycle OTEC, in which condensation can produce desalinated water.
- Other proposed uses: PNNL describes offshore critical-mineral extraction and e-fuels as future uses with promise, not as established market outcomes.
PNNL’s findings concern Kona and should not be treated as proof that these combinations are practical at every OTEC site. Their potential value has to be considered alongside the local demand, required infrastructure and operating conditions.
Quick Recap
What to examine when judging an OTEC proposal
- Temperature resource: Is the available surface-to-deep-water temperature difference suitable for the intended plant?
- Flow and net output: What cold- and warm-water flows does the design require, and what is left after pumping loads?
- Intake route and plant architecture: How far must water travel, and how does the land-based, moored or floating layout affect the conduits?
- Discharge plan: Where and at what depth will return water be released, and how will local effects be assessed?
- Value beyond electricity: Are cooling, aquaculture or freshwater functions a genuine fit for the site, or merely an assumed benefit?
- Evidence at the proposed scale: Do the cited operating examples match the size and configuration being proposed?
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