OTEC on Kume Island: Deep Seawater and Renewable Energy

What is ocean thermal energy conversion (OTEC)?

Turning "warm on top, cold below" into electricity

Ocean thermal energy conversion (OTEC) generates power from the temperature difference between surface seawater warmed by the sun and cold seawater in the deep ocean where light does not reach. A thermal power plant burns fuel to boil water and spins a turbine with steam; OTEC instead evaporates a low-boiling liquid with warm seawater and condenses it again with cold seawater to spin the turbine. No fuel is needed, and it can run day and night in any weather as long as there is sea.

Solar and wind depend on the weather, but ocean temperatures do not change suddenly. OTEC has therefore long been studied as a renewable source that could serve as stable baseload power. The ideal condition is a gap between surface and deep water of about 20°C or more throughout the year. This is met mainly in tropical and subtropical waters around the equator; in Japan, the Nansei Islands including Okinawa are candidates.

How it works: the Rankine cycle

OTEC is based on a heat engine called the Rankine cycle, and comes in three main types.

The Kume Island demonstration plant uses a closed cycle with the hydrofluorocarbon R134a (boiling point −26°C) as the working fluid. Surface seawater heats and evaporates the fluid, the vapor spins a turbine generator, and deep seawater cools it back to a liquid, repeating the loop.

Why deep seawater is essential

The cold source for OTEC is deep seawater pumped up from several hundred meters. Because sunlight does not reach it, deep water stays cold all year, is rich in nutrients, and is clean, with few bacteria and little organic matter. These properties lead not only to power generation but to wide industrial uses such as fisheries, agriculture, food and cosmetics. Combining OTEC with deep seawater use is the core of the "Kumejima Model" described below.

Why Kume Island: location and the deep sea water research institute

Island power supply and a suitable site for OTEC

Okinawa has one of the lowest energy self-sufficiency rates in Japan. According to the sources, it is 4.8% nationwide (2010) versus 0.5% in Okinawa (2012). Few large rivers limit hydropower, and each remote island needs its own power grid. For islands that depend on imported fuel, OTEC, which draws electricity from the sea at their doorstep, is an attractive option.

Deep seawater drawn from 612 m

Kume Island hosts the Okinawa Prefecture Deep Sea Water Research Institute (opened in 2000). It takes deep seawater from a depth of 612 m through two intake pipes and surface seawater from 15 m through one pipe. The maximum intake is about 13,000 tons per day each for deep and surface water, said to be the largest in Japan. Surface water is about 25–30°C and deep water about 5–10°C (some sources say 10–12°C), giving a large gap all year.

Why was the demonstration placed on Kume Island?

The main reason is that existing intake infrastructure could be shared. Most of the construction cost of OTEC is the long intake pipe that draws cold water from the deep. Because the research institute already had pipes, the project could keep initial costs down and focus on long-term continuous operation and technical reliability.

Kume Island's OTEC demonstration since 2013

Overview of the demonstration plant

In April 2013, demonstration operation of OTEC in the real ocean began on the grounds of the Okinawa Prefecture Deep Sea Water Research Institute. It consists of one generating unit with a maximum output of 50 kW and one test unit equivalent to 50 kW. (Reports say "50 kW class" while the operating company sometimes describes it as "100 kW class". This may reflect differences in configuration or notation; the exact rating should be checked in official prefecture or institute materials.)

JICA's explanation introduces it as a world first of generating power with real seawater, and notes that the deep seawater after generation can be reused by other industries. In 2012, the JST Science Portal also reported the start of the OTEC trial on Kume Island.

Saga University and Japan's research legacy

OTEC research in Japan surged after the oil shocks of the 1970s but later shrank. Saga University is said to be the only one in Japan to have continued it. The Kume Island demonstration was realized through this long accumulation of research together with Okinawa Prefecture and local companies.

Worldwide: Hawaii's 105 kW OTEC

Demonstrations are advancing abroad too. At NELHA (Natural Energy Laboratory of Hawaii Authority) on Hawaii Island, Makai Ocean Engineering's 105 kW-class OTEC plant was connected to the grid in August 2015. It uses ammonia as the working fluid and was reported as the largest operating OTEC plant in the world at the time. The cost was about 5 million US dollars, funded by the Office of Naval Research and others. Some sources say it is not a commercial plant and does not run continuously, so its status should be treated with care.

The Kumejima Model: cascading use of deep seawater

How deep seawater became an island industry

On Kume Island, industries built around deep seawater have grown for a quarter century since the institute opened in 2000. Contributions by the town's vice mayor and others describe how this came to be called the "Kumejima Model". The institute supplies deep and surface water to companies, spreading aquaculture technology and attracting businesses. Kuruma prawns and sea grapes (Caulerpa lentillifera) are known aquaculture products.

The idea of cascade use

Deep seawater used in OTEC is still cold and rich in nutrients. Using it in sequence instead of discarding it is cascade (multi-stage) use, the heart of the Kumejima Model.

1. First use it for OTEC to obtain electricity from the temperature gap
2. Then use the remaining cold water for aquaculture temperature control and air conditioning
3. Finally use it for agriculture (cultivation on cooled soil) and as raw water for food and cosmetics

Even if power alone is hard to make profitable, revenue from downstream industries can improve the economics of the whole project. In fact, a 2011 survey by Kumejima Town showed the possibility of combining a 1 MW-class OTEC with deep seawater use. The idea is to lower power costs by using the water after generation for fisheries, agriculture and air conditioning.

Jobs and the local economy

Okinawa Prefecture's materials say that using deep seawater supports agriculture, forestry and fisheries on small remote islands and attracts businesses. Some sources say deep-seawater industries have become the main industry of Kume Island in little more than a decade. However, sales and employment figures of individual operators could not be confirmed from primary sources in this research. Please check the latest publications of Kumejima Town and the institute before citing numbers.

Challenges: why OTEC has not spread yet

Economies of scale are hard to reach

Because the temperature gap is small (in the 20s of °C), thermal efficiency is low, only a few percent even in theory. Pumping large volumes of seawater also takes power, so smaller plants lose more of their net output. Professor Yasuyuki Ikegami of Saga University has said that 100 kW scale is not economically viable but that a 1 MW-class plant could solve OTEC's problems. Okinawa Prefecture materials likewise assume that profitability is almost impossible with small plants under 100 kW.

The intake pipe is the biggest cost

Laying a long pipe to draw deep water accounts for most of the construction cost. The pipe must withstand typhoons, ocean currents and biofouling. Kume Island could run its demonstration by sharing existing pipes, but building pipes from scratch at a new site would greatly raise the investment.

Winter cooling and operational stability

In winter, when surface water cools, the temperature gap narrows and output may fall. Even in subtropical Okinawa, water temperatures vary by season, so whether the expected output can be achieved must be confirmed with long-term operating data.

How to think about environmental impact

OTEC pumps up large volumes of deep water and returns it to the sea after use. Because deep water is rich in nutrients and differs in temperature, the place and depth of discharge could affect surrounding ecosystems. Before scaling up, the spread of the discharge and effects on phytoplankton need to be assessed in advance. This is an ongoing research topic in Japan and abroad, and continuous environmental monitoring is needed from the demonstration stage.

Toward 1 MW: Mitsui O.S.K. Lines and Kumejima Town

What private-sector participation means

The next stage drawing attention is a 1 MW class. In October 2025, Mitsui O.S.K. Lines signed a comprehensive partnership agreement with Kumejima Town and announced it would develop and design an intake system toward a 1 MW-class OTEC in the field of "OTEC and the use of deep seawater" (as reported by LNEWS). The company already takes part in operating the 100 kW-class OTEC demonstration plant on Kume Island.

A shipping company's involvement may reflect its marine expertise and its search for new energy businesses for decarbonization. A notable feature is the move from the research stage toward bringing in private funding and operating know-how.

What would change with 1 MW?

Converted to household use, 1 MW corresponds to a few hundred households (a rough guide, as it varies greatly with usage). What matters more than the output itself is whether the smallest economically viable scale can be reached. If profitability becomes clear here, expansion to other islands and tropical island nations becomes realistic.

OTEC in the world and Japan: can it change island energy?

Tropical islands are the best sites

The best sites for OTEC are tropical and subtropical seas with a large temperature gap all year. Many island nations in the Pacific, Caribbean and Indian Ocean pay high prices for power from imported diesel, so the benefit would be large if OTEC becomes profitable. Experience from Hawaii and Kume Island could also lead to technical cooperation with such countries.

Its place among renewable energies

Japan is also developing marine renewables such as offshore wind, wave and tidal power. OTEC can complement them because its output tends to be stable. For example, offshore wind varies with the wind, whereas OTEC can run steadily as long as the temperature gap exists. Combining them could make island grids more stable.

Related efforts are also explained in these articles on this site.

• Generating power from tidal flow: Tidal current power generation and its potential in Japan
• Sailing ships with wind: Wind-assisted sails for decarbonizing ships
• Carbon absorption by seaweed: Seaweed beds and blue carbon

OTEC and diversifying marine industries

OTEC is more than a power technology. Linked with aquaculture, food, cosmetics and health industries using deep seawater, it can become a foundation for an island's whole economy. The Kumejima Model is watched in Japan and abroad as an example of growing renewable energy together with local industry.

What we can do: bringing ocean energy closer

First learn, then see it in person

OTEC is still little known to the public. The deep sea water research institute on Kume Island may offer tours or introductions to related facilities (please check availability on the official website beforehand). If you visit Kume Island, enjoying products such as sea grapes made with deep seawater is also a way to support local industry.

Connecting energy and the ocean

Being aware of where electricity comes from is the first step in climate action. Renewables include solar, wind, hydro, geothermal and ocean energy, each with strengths and weaknesses. Whether technologies like OTEC grow depends not only on researchers and companies but also on how much interest society takes.