Ocean thermal energy conversion (OTEC) is a type of renewable energy that uses the temperature difference between warm ocean surface water and cold deep water to operate a machine that produces electricity. It is a special way of creating clean energy that can provide a reliable and ongoing power source. Even though there are challenges to solve, OTEC can still offer a steady and long-lasting form of clean energy, especially in tropical areas where deep ocean water is available.

Description

OTEC uses the temperature difference between cooler deep ocean water and warmer surface water to operate a heat engine and create electricity. OTEC systems can run continuously and provide steady power supply.

Cold water masses form in specific areas of the North Atlantic and Southern Ocean when surface water interacts with cold air. These dense waters sink to the deep ocean and move across the ocean floor through thermohaline circulation. Cold water rising from the deep ocean is replaced by cold surface water sinking from above.

OTEC is a renewable energy source that can supply power continuously. It has a large potential to generate electricity, with estimates suggesting up to 10,000 TWh per year could be produced without harming ocean temperatures.

OTEC systems can be closed-cycle or open-cycle. Closed-cycle systems use refrigerants like ammonia or R-134a, which have low boiling points and help generate electricity. The Rankine cycle, using a low-pressure turbine, is the most common method for OTEC. Open-cycle systems use seawater vapor as the working fluid.

OTEC also produces cold water as a by-product, which can be used for cooling and refrigeration. Nutrient-rich deep ocean water can support biological technologies. Fresh water can also be produced by distilling seawater.

The idea of OTEC was first proposed in the 1880s. The first small-scale model was built in 1926. Today, pilot OTEC plants operate in Japan, managed by Saga University, and in Hawaii, managed by Makai Ocean Engineering.

History

Attempts to develop and improve OTEC technology began in the 1880s. In 1881, Jacques Arsene d'Arsonval, a French scientist, suggested using the ocean's heat energy. His student, Georges Claude, built the first OTEC plant in Matanzas, Cuba, in 1930. The system produced 22 kW of electricity using a low-pressure turbine. The plant was later destroyed by a storm.

In 1935, Claude built another OTEC plant on a 10,000-ton cargo ship near Brazil. Strong weather and waves damaged the plant before it could create enough electricity to be useful. Net power is the electricity produced after subtracting the energy needed to run the system.

In 1956, French scientists planned a 3 MW OTEC plant in Abidjan, Ivory Coast. The project was not completed because large supplies of cheap oil were found, making the plant too expensive to build.

In 1962, J. Hilbert Anderson and James H. Anderson, Jr. worked to improve the efficiency of OTEC parts. They patented a new "closed cycle" design in 1967. This design improved upon an earlier system and was used in a plan to produce electricity at a lower cost than oil or coal. At the time, their research received little attention because coal and nuclear energy were seen as the future of power.

Japan has played an important role in OTEC development. Starting in 1970, the Tokyo Electric Power Company built a 100 kW closed-cycle OTEC plant on Nauru. The plant began operating on October 14, 1981, producing about 120 kW of electricity. Ninety kW was used to power the plant, and the remaining electricity was used to power a school and other buildings. This was the first time an OTEC system produced electricity for a real power grid.

In 1981, Russian engineer Dr. Alexander Kalina used a mixture of ammonia and water to generate electricity. This improved the efficiency of the power cycle. In 1994, the Institute of Ocean Energy at Saga University built a 4.5 kW plant to test a new system called the Uehara cycle. This system performed slightly better than Kalina's design.

The 1970s saw more OTEC research after the 1973 Arab-Israeli War caused oil prices to rise. The U.S. government invested $260 million in OTEC research after President Carter set a goal to produce 10,000 MW of electricity from OTEC by 1999.

In 1974, the U.S. created the Natural Energy Laboratory of Hawaii Authority (NELHA) on the island of Hawaii. Hawaii is a good place for OTEC because it has warm surface water, access to deep cold water, and high electricity costs. NELHA became a major testing site for OTEC technology. That same year, Lockheed received funding to study OTEC. This led to a project by Lockheed, the U.S. Navy, and other companies to build the first OTEC plant that produced more electricity than it used, called "Mini-OTEC." A small amount of electricity was generated for three months in 1979. NELHA operated a 250 kW demonstration plant from the 1990s to the early 2000s. In 2015, a 105 kW plant at NELHA began supplying energy to the local power grid with help from the U.S. Navy.

A European group called EUROCEAN, made up of nine companies, promoted OTEC from 1979 to 1983. They studied large offshore OTEC facilities and later explored a 100 kW land-based system that combined OTEC with desalination and aquaculture. This idea was based on a small aquaculture project on St. Croix, which used deepwater pipes. They also studied a shore-based open-cycle OTEC system on Curaçao.

Research to develop open-cycle OTEC began in 1979 at the Solar Energy Research Institute (SERI) with funding from the U.S. Department of Energy. SERI developed and patented evaporators and condensers for the system. An early experiment called the 165-kW project was described in a lecture. Later, a team at the National Renewable Energy Laboratory (NREL) designed a 210 kW open-cycle OTEC experiment. This design combined all parts of the system into one vacuum vessel made of concrete. Attempts to use low-cost plastic materials for some parts were not fully successful. Engineers later worked to improve the design, which was renamed the Net Power Producing Experiment (NPPE) and built at NELHA.

In 2002, India tested a 1 MW floating OTEC plant near Tamil Nadu. The project failed because the deep-sea cold water pipe broke. India continues to support OTEC research.

In 2006, Makai Ocean Engineering was asked by the U.S. Navy to study how OTEC could produce hydrogen in floating plants in tropical waters. Makai partnered with Lockheed Martin to explore OTEC's potential. Lockheed resumed OTEC work in 2007 and helped Makai with their projects.

In March 2011, Ocean Thermal Energy Corporation signed an agreement with the Baha Mar resort in the Bahamas to build the world's largest seawater air conditioning system. The project was paused in 2015 due to financial issues but is expected to restart in 2017.

In July 2011, Makai Ocean Engineering completed a facility at NEL

Currently operating OTEC plants

In March 2013, Saga University and several Japanese companies finished building a new OTEC plant. On April 15, 2013, Okinawa Prefecture began testing the OTEC system on Kume Island. The goal was to check if computer models work correctly and to show the public how OTEC operates. Testing and research will continue with Saga University’s help until the end of fiscal year 2016. IHI Plant Construction Co. Ltd, Yokogawa Electric Corporation, and Xenesys Inc built the 100 kilowatt plant on the grounds of the Okinawa Prefecture Deep Sea Water Research Center. The location was chosen because it already has deep and surface seawater intake pipes built in 2000 for research, fishing, and farming. These pipes are used to bring deep seawater into the research center. The plant has two 50 kW units that work together in a double Rankine setup. The OTEC facility and research center allow free public tours by appointment in English and Japanese. This is one of only two fully working OTEC plants in the world. It runs continuously unless specific tests are being conducted.

In 2011, Makai Ocean Engineering built a heat exchanger test facility at NELHA. This facility tests different heat exchange technologies for OTEC. Makai received money to install a 105 kW turbine, making it the largest operational OTEC facility. However, the Open Cycle plant in Hawaii still holds the record for the most power produced.

In July 2014, DCNS group and Akuo Energy announced funding for their NEMO project through NER 300. If successful, the 16 MW gross (10 MW net) offshore plant would have been the largest OTEC facility at the time. DCNS planned to operate NEMO by 2020. However, in April 2018, Naval Energies stopped the project indefinitely because of technical problems with the main cold-water intake pipe.

In August 2015, an OTEC power plant built by Makai Ocean Engineering started operating in Hawaii. Hawaii’s governor, David Ige, activated the plant. This is the first closed-cycle OTEC plant connected to the U.S. electrical grid. It is a demonstration plant that can produce 105 kilowatts of power, enough to supply about 120 homes.

Thermodynamic efficiency

A heat engine works better when there is a large temperature difference. In the oceans, the temperature difference between surface water and deep water is largest in the tropics, though this difference is still small, about 20 to 25 °C. Because of this, OTEC (ocean thermal energy conversion) has the greatest potential in the tropics. OTEC could provide 10 to 100 times more energy globally compared to other ocean energy methods, such as wave power.

OTEC plants can operate nonstop, offering a steady power supply for electricity generation.

The main technical challenge of OTEC is to produce large amounts of power efficiently using small temperature differences. OTEC is still considered a developing technology. Early OTEC systems had thermal efficiency of 1 to 3 percent, which is much lower than the theoretical maximum of 6 to 7 percent for this temperature range. Modern OTEC designs now achieve performance close to this theoretical maximum Carnot efficiency.

Power cycle types

Cold seawater is used in all three types of OTEC systems: closed-cycle, open-cycle, and hybrid. To work, cold seawater must be brought to the surface. The main methods are using pumps or desalination. Desalinating seawater near the ocean floor reduces its density, causing it to rise naturally.

An alternative to using expensive pipes to bring cold water to the surface is to pump vaporized fluid with a low boiling point into the deep ocean. This fluid condenses underwater, reducing the amount of water that needs to be pumped and lowering costs and environmental issues.

Closed-cycle systems use a fluid with a low boiling point, such as ammonia (which boils at about -33°C at normal pressure), to power a turbine that generates electricity. Warm surface seawater is pumped through a heat exchanger to turn the fluid into vapor. The expanding vapor spins a turbine. Cold water, pumped through a second heat exchanger, cools the vapor into liquid, which is then reused in the system.

In 1979, the Natural Energy Laboratory and private partners tested the "mini OTEC" system. This experiment was the first to successfully produce net electricity from closed-cycle OTEC at sea. The mini OTEC vessel was anchored 1.5 miles (2.4 km) off Hawaii’s coast and generated enough electricity to power the ship’s lights, computers, and television.

Open-cycle OTEC uses warm surface water directly to create electricity. Warm seawater is pumped into a low-pressure container, causing it to boil. In some setups, the steam from this process turns a turbine connected to a generator. The steam, now free of salt and other impurities, is cooled by deep ocean water into liquid fresh water, which can be used for drinking, farming, or fish farming.

In other setups, the rising steam is used to lift water high using a gas lift technique. Depending on the design, this steam can power a hydroelectric turbine before or after lifting water.

In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory) created a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle OTEC. This process had an efficiency of up to 97% for converting seawater to steam (though only a small percentage of the incoming water became steam). In 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced nearly 80 kW of electricity, breaking the previous record of 40 kW set by a Japanese system in 1982.

Hybrid systems combine features of closed- and open-cycle systems. Warm seawater enters a vacuum chamber and quickly turns into steam, like in open-cycle systems. This steam then vaporizes ammonia in a closed-cycle loop on the other side of a heat exchanger. The vaporized ammonia drives a turbine to generate electricity. The steam cools in the heat exchanger, producing fresh water.

Ammonia is often used as a working fluid because it is easy to obtain, inexpensive, and moves well through systems. However, it is toxic and flammable. Fluorinated chemicals like CFCs and HCFCs are not toxic or flammable but harm the ozone layer. Hydrocarbons are also possible but are highly flammable, which could compete with their use as direct fuels. The size of an OTEC power plant depends on the vapor pressure of the working fluid. Higher vapor pressure reduces the size of turbines and heat exchangers but increases the thickness of pipes and heat exchangers to handle high pressure, especially on the evaporator side.

Land, shelf and floating sites

OTEC can generate gigawatts of electricity, and when used with electrolysis, it could create enough hydrogen to replace all expected global fossil fuel use. However, lowering costs is still a challenge that hasn't been solved. OTEC plants need a long, large pipe that is submerged more than a kilometer into the ocean to bring cold water to the surface.

Land-based and near-shore facilities have three main benefits compared to those in deep water. Plants built on or near land do not need complex mooring systems, long power cables, or the extra maintenance required in open-ocean areas. They can be placed in protected areas, making them safer from storms and rough seas. Electricity, desalinated water, and cold, nutrient-rich seawater could be sent from near-shore facilities to land via trestle bridges or causeways. Additionally, land-based or near-shore sites allow plants to work with other industries, such as mariculture or those that need desalinated water.

Preferred locations include areas with narrow shelves (like volcanic islands), steep offshore slopes (15–20 degrees), and smooth sea floors. These sites reduce the length of the intake pipe. A land-based plant could be built far inland from the shore, offering more protection from storms, or on the beach, where the pipes would be shorter. In both cases, easier access for construction and operation helps lower costs.

Land-based or near-shore sites can also support mariculture or chilled water agriculture. Tanks or lagoons built on land allow workers to monitor and control small marine environments. Mariculture products can be sent to market using standard transportation.

One challenge of land-based facilities is the rough wave action in the surf zone. OTEC discharge pipes should be placed in protective trenches to avoid extreme stress during storms and long periods of rough seas. Also, the mixed discharge of cold and warm seawater may need to be carried several hundred meters offshore to reach the proper depth before release, which increases construction and maintenance costs.

One way OTEC systems can avoid some problems and costs of operating in the surf zone is by building them just offshore in waters 10 to 30 meters deep (Ocean Thermal Corporation 1984). These plants would use shorter (and less expensive) intake and discharge pipes, avoiding the dangers of rough surf. However, the plant itself would need protection from the marine environment, such as breakwaters and erosion-resistant foundations, and the electricity produced would need to be sent to shore.

To avoid the surf zone and get closer to the cold-water source, OTEC plants can be placed on the continental shelf at depths up to 100 meters (330 ft). A shelf-mounted plant could be towed to the site and attached to the sea floor. This type of construction is already used for offshore oil rigs. However, operating OTEC plants in deeper water may be more expensive than land-based options. Challenges include the stress of open-ocean conditions and the difficulty of delivering products. Dealing with strong ocean currents and large waves increases engineering and construction costs. Platforms need strong pilings to stay stable. Power delivery may require long underwater cables to reach land. For these reasons, shelf-mounted plants are less preferred.

Floating OTEC facilities operate offshore. While they could be ideal for large systems, floating plants face several difficulties. Mooring plants in very deep water makes power delivery complicated. Cables attached to floating platforms are more likely to be damaged, especially during storms. Cables at depths greater than 1,000 meters are hard to maintain and repair. Riser cables, which connect the sea bed to the plant, must be built to resist entanglement.

Like shelf-mounted plants, floating plants need a stable base for continuous operation. Major storms and rough seas can break the cold-water pipe or interrupt the warm water intake. To help prevent these issues, pipes can be made of flexible polyethylene attached to the platform’s bottom and connected with gimballed joints or collars. Pipes may need to be disconnected from the plant to avoid storm damage. As an alternative to a warm-water pipe, surface water can be drawn directly into the platform, but the intake flow must be protected from damage or interruption caused by violent sea movements.

Connecting a floating plant to power delivery cables requires the plant to stay relatively still. Mooring is a possible method, but current mooring technology is limited to depths of about 2,000 meters (6,600 ft). Even at shallower depths, mooring costs may be too high.

Political concerns

OTEC facilities are mostly stationary platforms on the ocean's surface. Their exact location and legal standing may be influenced by the United Nations Convention on the Law of the Sea (UNCLOS). This treaty gives coastal countries zones extending 12 and 200 nautical miles (22 and 370 km) from their land, with different levels of legal control. These zones can create possible problems and rules that need to be followed. OTEC plants and similar structures are treated as artificial islands under the treaty, which means they do not have their own legal rights. These plants might be seen as either a threat or a helpful partner to fishing activities or seabed mining projects managed by the International Seabed Authority.

Cost and economics

Because OTEC systems are not used much yet, it is hard to know their costs. A study from the University of Hawaii in 2010 found that electricity from OTEC would cost 94.0 cents per kilowatt hour (kWh) for a 1.4 megawatt (MW) plant, 44.0 cents per kWh for a 10 MW plant, and 18.0 cents per kWh for a 100 MW plant. A 2015 report by Ocean Energy Systems, part of the International Energy Agency, estimated about 20.0 cents per kWh for 100 MW plants. Another study found power generation costs as low as 7.0 cents per kWh. Compared to other energy sources, a 2019 study by Lazard found that large solar power plants cost 3.2 to 4.2 cents per kWh without government help, and wind power cost 2.8 to 5.4 cents per kWh without government help.

A 2014 report by IRENA said that OTEC technology can be used in different ways to produce electricity. Small OTEC plants could provide power for small communities (5,000–50,000 people), but they would need to produce valuable by-products, like fresh water or cooling, to be cost-effective. Larger OTEC plants would cost more to build and operate.

Important factors to consider include OTEC’s use of no waste or fuel, its availability near the equator (within 20° latitude), its ability to reduce reliance on oil from other countries, its ability to work with other ocean energy types like wave and tidal power, and other uses for seawater, such as cooling or fresh water production.

Some proposed projects

Ocean Thermal Energy Conversion (OTEC) projects are being considered for various locations. One example is a small OTEC plant planned for the U.S. Navy base on Diego Garcia, an island in the Indian Ocean that is part of the British overseas territory. Ocean Thermal Energy Corporation (OTE), formerly known as OCEES International, Inc., is working with the U.S. Navy to design a 13-megawatt (MW) OTEC plant to replace current diesel generators. This plant would also provide 1.25 million gallons of drinkable water each day. The project is waiting for changes in U.S. military contract policies. OTE has also proposed building a 10-MW OTEC plant on Guam.

OTE currently plans to install two 10-MW OTEC plants in the U.S. Virgin Islands and a 5–10-MW OTEC facility in the Bahamas. OTE has also designed the world’s largest Seawater Air Conditioning (SWAC) plant for a resort in the Bahamas. This system uses cold deep seawater to cool buildings. In mid-2015, the project was temporarily paused due to financial and ownership issues at the resort. On August 22, 2016, the government of the Bahamas announced a new agreement to complete the resort. On September 27, 2016, the Bahamian Prime Minister announced that construction had resumed, with the resort expected to open in March 2017. However, the project is now on hold and may not continue.

Lockheed Martin’s Alternative Energy Development team partnered with Makai Ocean Engineering to complete the final design of a 10-MW closed-cycle OTEC pilot system. This system was planned to operate in Hawaii between 2012 and 2013 and could expand to 100-MW commercial systems later. In November 2010, the U.S. Naval Facilities Engineering Command awarded Lockheed Martin a $4.4 million contract to develop key components for the plant, adding to a $8.1 million contract from 2009 and two Department of Energy grants totaling over $1 million in 2008 and 2010. A small OTEC plant was completed in Hawaii in August 2015. This 100-kilowatt facility was the first closed-cycle OTEC plant connected to the U.S. power grid.

In April 2013, Lockheed Martin signed a contract with the Reignwood Group to build a 10-MW OTEC plant off the coast of southern China to power a resort on Hainan Island. A plant of that size could supply electricity to thousands of homes. The Reignwood Group acquired Opus Offshore in 2011, forming its Reignwood Ocean Engineering division, which also works on deepwater drilling projects.

The only continuously operating OTEC system is in Okinawa Prefecture, Japan. Government support, community backing, and research by Saga University helped contractors, including IHI Plant Construction Co. Ltd., Yokogawa Electric Corporation, and Xenesys Inc., complete the project. Work is ongoing to develop a 1-MW OTEC facility on Kume Island, which requires new pipelines. In July 2014, over 50 members formed the Global Ocean reSource and Energy Association (GOSEA), an international group aiming to promote the Kumejima Model and advance plans for larger seawater pipelines and a 1-MW OTEC facility. Companies involved in OTEC projects, along with other interested parties, are also planning offshore OTEC systems.

On March 5, 2014, OTE and the 30th Legislature of the U.S. Virgin Islands (USVI) signed a Memorandum of Understanding to study the feasibility of installing onshore OTEC power plants and SWAC facilities in the USVI. The study will assess benefits such as 24/7 clean electricity, fresh water, energy-saving air conditioning, sustainable aquaculture, and agricultural projects on St. Thomas and St. Croix.

On July 18, 2016, OTE received approval from the Virgin Islands Public Services Commission to apply for a Qualifying Facility status. OTE also received permission to begin negotiating contracts for the project.

South Korea’s Research Institute of Ships and Ocean Engineering (KRISO) received approval in principle from Bureau Veritas for a 1-MW offshore OTEC design. No timeline was provided for the project, which will be located 6 kilometers offshore from Kiribati.

Akuo Energy and DCNS received NER300 funding on July 8, 2014, for their NEMO (New Energy for Martinique and Overseas) project. This 10.7-MW offshore facility is expected to be completed by 2020, with a total development funding of 72 million euros.

On February 16, 2018, Global OTEC Resources announced plans to build a 150-kilowatt OTEC plant in the Maldives, specifically designed for hotels and resorts. Director Dan Grech stated that many resorts currently rely on diesel generators, which consume large amounts of fuel and produce significant carbon dioxide emissions annually. The European Union provided a grant, and Global OTEC Resources launched a crowdfunding campaign to fund the remaining costs.

Related activities

OTEC has uses other than power production.

Desalinated water can be made in open- or hybrid-cycle plants. These plants use surface condensers to turn evaporated seawater into drinkable water. Studies show that a 2-megawatt OTEC plant can make about 4,300 cubic meters of fresh water every day. Another system, patented by Richard Bailey, creates condensate water by controlling the flow of deep ocean water through surface condensers. This process uses no extra energy and has no moving parts.

In 2015, Saga University opened a Flash-type desalination demonstration facility on Kumejima. This facility is part of their Institute of Ocean Energy. It uses deep seawater from the Okinawa OTEC Demonstration Facility and raw surface seawater to make desalinated water. A vacuum pump removes air from the closed system. When raw seawater is pumped into the flash chamber, it boils, allowing pure steam to rise. Salt and leftover seawater are removed. The steam is cooled back into liquid using cold post-OTEC deep seawater. This desalinated water can be used for hydrogen production or drinking water (if minerals are added).

The NELHA plant, started in 1993, produces about 7,000 gallons of freshwater daily. KOYO USA, formed in 2002, uses this water to make bottled water in Hawaii. With the ability to produce 1 million bottles daily, KOYO is now Hawaii’s largest exporter, earning $140 million in sales.

Cold seawater at 41 °F (5 °C) from an OTEC system can provide cooling for industries and homes. This water can be used in chilled-water coils to cool buildings. A 1-foot (0.30 m) diameter pipe can deliver 4,700 gallons of water per minute. Water at 43 °F (6 °C) could cool a large building. If used instead of electrical air conditioning, which costs 5–10¢ per kilowatt-hour, it could save $200,000–$400,000 yearly in energy costs.

The InterContinental Resort and Thalasso-Spa on Bora Bora uses an SWAC system to cool its buildings. This system passes seawater through a heat exchanger, where it cools freshwater in a closed loop. The cooled freshwater is then pumped to buildings to directly cool the air.

In 2010, Copenhagen Energy opened a district cooling plant in Copenhagen, Denmark. This plant sends cold seawater to commercial and industrial buildings, reducing electricity use by 80%. Ocean Thermal Energy Corporation (OTE) designed a 9800-ton SDC system for a vacation resort in The Bahamas.

OTEC technology supports chilled-soil agriculture. When cold seawater flows through underground pipes, it cools the surrounding soil. The temperature difference between cool soil and warm air allows plants that grow in temperate climates to be grown in subtropical areas. Scientists patented this process and tested it at NELHA. Research showed over 100 different crops can be grown using this system. Many of these crops would not survive in Hawaii or at Keahole Point.

Japan has studied agricultural uses of deep sea water since 2000 at the Okinawa Deep Sea Water Research Institute on Kume Island. Their facilities use regular water cooled by deep sea water in a heat exchanger. This cooled water is run through pipes in the ground to cool soil. These techniques now allow the island to grow spinach commercially all year. In 2014, Kumejima Town expanded its deep seawater agriculture facility near the OTEC Demonstration Facility to study the economic benefits of chilled-soil farming on a larger scale.

Aquaculture is a well-known benefit of OTEC. It reduces the cost and energy needed to pump large amounts of deep ocean water. Deep ocean water has high levels of nutrients that are missing in surface waters. This artificial upwelling mimics natural processes that support large marine ecosystems and high life density.

Cold-water animals like salmon and lobster thrive in this nutrient-rich water. Microalgae such as Spirulina, a health supplement, can also be grown. Deep ocean water can be mixed with surface water to create water at an ideal temperature.

Non-native species like salmon, lobster, abalone, trout, oysters, and clams can be raised in pools using OTEC-pumped water. This increases the variety of fresh seafood available for nearby markets. Low-cost refrigeration from OTEC helps keep fish fresh in warm tropical regions. In Kona, Hawaii, aquaculture companies working with NELHA generate about $40 million annually, a major part of Hawaii’s GDP.

Hydrogen can be made using OTEC electricity through electrolysis. Adding electrolyte compounds to steam improves efficiency. OTEC can be scaled to produce large amounts of hydrogen. The main challenge is the cost compared to other energy sources.

The ocean contains 57 trace elements in salts and dissolved forms. In the past, mining these elements from the ocean was seen as unprofitable because of the energy needed to pump water. Mining usually targets minerals that are easy to extract, like magnesium. With OTEC plants providing water, the only cost is for extraction. Japanese researchers studied extracting uranium and found that advances in other technologies are improving the possibility.

Ocean thermal gradients can help increase rainfall and lower summer temperatures in tropical areas. When sea surface temperatures are high, lower pressure forms over the ocean compared to nearby land. This creates winds from land to sea, which are dry and warm. These winds do not help rainfall as much as moist winds from the ocean. To improve rainfall and keep summer temperatures comfortable (below 35 °C), moist winds from the ocean are needed. Creating high-pressure zones through artificial upwelling can guide monsoon winds toward land. Artificial upwelling also boosts fish growth in tropical and temperate regions. It can increase carbon absorption by oceans through more algae growth and help glaciers gain snow, reducing sea level rise or global warming. Tropical cyclones avoid high-pressure zones because they rely on warm surface water to grow stronger.

Cold deep seawater (below 10 °C) is pumped to the ocean surface to lower surface temperatures (above 26 °C) using electricity from large floating wind turbines. Lower surface temperatures can improve local weather conditions.

Thermodynamics

A detailed study of OTEC shows that a 20 °C temperature difference between warm and cold ocean water can produce as much energy as a hydroelectric plant with 34 meters of water height for the same amount of water flow. However, because the temperature difference is small, extremely large volumes of water must be moved to collect enough heat. A 100 MW OTEC power plant would need to pump about 12 million gallons (44,400 tonnes) of water every minute. For comparison, this is more than the weight of the battleship Bismarck (41,700 tonnes) moved each minute. This large water movement uses a significant amount of energy, reducing the overall efficiency of OTEC systems. For example, one Lockheed design used 19.55 MW of energy just for pumping water to generate 49.8 MW of electricity. In OTEC systems that use heat exchangers, these devices must be extremely large compared to those in traditional power plants, making them a key and expensive part of the system. A 100 MW OTEC plant would need 200 heat exchangers, each larger than a 20-foot shipping container.

The total solar energy absorbed by the oceans (which cover 70% of Earth’s surface) can be calculated using the formula: 5.45×10 MJ/yr × 0.7 × 0.5 × 0.15 = 2.87×10 MJ/yr. This calculation considers factors like the ocean’s clearness and how much energy it retains.

Solar energy absorption by water can be measured using Beer–Lambert–Bouguer’s law, which relates the depth of water (y), the intensity of light (I), and the absorption coefficient (μ). The absorption coefficient varies depending on water’s salt content, ranging from 0.05 m for very clear fresh water to 0.5 m for very salty water. Because light intensity decreases exponentially with depth, most heat absorption happens in the top layers of the ocean. In tropical regions, surface water temperatures often exceed 25 °C (77 °F), while at 1 kilometer deep, temperatures drop to about 5–10 °C (41–50 °F). The warm surface water is less dense, so it does not mix with colder water through convection. Because temperature differences are small, heat transfer by conduction is too slow to balance temperatures. This makes the ocean both an almost endless source of heat and an almost endless sink for heat.

Temperature differences for OTEC are greatest in tropical, subtropical, and equatorial waters, making these regions the best locations for OTEC systems.

In an OTEC system, warm surface water (about 27 °C or 81 °F) enters an evaporator at a pressure slightly below its boiling point, causing it to vaporize. The enthalpy of liquid water at the inlet temperature (T₁) is used to calculate energy changes. The vaporized water partially turns into steam, creating a two-phase mixture (liquid and gas) in the evaporator. The pressure inside the evaporator is kept at the saturation pressure (T₂), and the fraction of water that vaporizes is represented by x₂. The ratio of warm water flow to turbine flow is 1/x₂.

A vacuum pump maintains low pressure in the evaporator and removes non-condensable gases. The steam is separated from the water and sent to a turbine, while the remaining water is returned to the ocean. The steam, which has low pressure and high volume, expands in a turbine to produce electricity. The enthalpy of steam at T₂ (Hg) is used to calculate turbine work. For an ideal turbine, the exhaust temperature (T₅) and vapor fraction (x₅,s) are determined using thermodynamic equations. The actual turbine work depends on the turbine’s efficiency.

After the turbine, the exhaust steam is mixed with cold ocean water in a condenser, creating near-saturated water that is returned to the ocean. The enthalpy at the exhaust temperature (T₅) is lower than at the turbine inlet. The turbine work is calculated as the difference in enthalpy between the turbine inlet and exhaust.

The cold water flow rate and warm water flow rate are related to the turbine’s work requirements. In the Anderson closed cycle, heat (Q_H) is transferred from warm seawater to the working fluid in the evaporator. The working fluid exits as a gas near its dew point and expands in the turbine to produce work (W_T). The turbine typically has an efficiency of 90%. After the turbine, the working fluid releases heat (-Q_C) to cold seawater in the condenser. The condensate is then pumped back to the evaporator, completing the cycle.

The working fluid’s thermodynamic cycle can be analyzed using the first law of thermodynamics. The net work (W_N) for the cycle is the sum of turbine work (W_T) and condensate pump work (W_C). In an ideal system with no pressure drops in heat exchangers, the net work is calculated based on energy transfers. Subcooled liquid enters the evaporator, where it evaporates into superheated vapor. This vapor drives the turbine, and the two-phase mixture enters the condenser. Subcooled liquid exits the condenser and is pumped back to the evaporator, completing the cycle.

Environmental impact

Carbon dioxide that is dissolved in deep, cold, and high-pressure layers of the ocean is brought to the surface and released when the water warms.

When deep ocean water mixes with shallow water, it brings nutrients to the surface, making them available for life in shallow areas. This process could help aquaculture of important fish and shellfish species, but it might also disrupt the balance of the ecosystem near power plants.

OTEC plants use large amounts of warm surface seawater and cold deep seawater to produce continuous, renewable energy. The deep seawater has very little oxygen and is usually 20–40 times richer in nutrients like nitrate and nitrite than shallow seawater. When these water flows mix, they become slightly heavier than the surrounding seawater. Although no large-scale physical testing of OTEC has been done, computer models have been created to study its effects.

In 2010, a computer model was developed to study the ocean effects of one or more 100-megawatt OTEC plants. The model shows that OTEC plants can be designed to operate continuously, with temperature and nutrient changes that match natural ocean conditions. Studies suggest that if the water flows from OTEC plants are released below 70 meters, the nutrients are diluted enough to allow 100-megawatt OTEC plants to operate sustainably over time.

If nutrients from OTEC discharges collect in the photic zone, they might increase biological activity. In 2011, a biological part was added to the computer model to study how 100-megawatt OTEC plants affect ocean life. In all modeled scenarios (discharges at 70 meters or deeper), no unnatural changes occurred in the top 40 meters of the ocean. The increase in picoplankton in the 70–110 meter layer was about 10–25%, which is within natural ocean variability. Nanoplankton changes were not significant, and the increase in diatoms (microplankton) was small. The slight increase in phytoplankton from OTEC plants suggests that larger biochemical effects would be minimal.

A Final Environmental Impact Statement (EIS) from 1981 by the U.S. NOAA is available, but it needs updates to match current ocean and engineering standards. Studies have proposed best practices for monitoring the environment around OTEC plants, focusing on ten chemical ocean parameters. NOAA held OTEC workshops in 2010 and 2012 to evaluate the physical, chemical, and biological impacts of OTEC and identify gaps in knowledge.

The Tethys database offers access to scientific research and general information about the potential environmental effects of OTEC.

Technical difficulties

The performance of direct contact heat exchangers in OTEC systems is important for the Claude cycle. Early designs often used surface condensers because their performance was better understood. However, direct contact condensers have disadvantages. When cold water rises in intake pipes, pressure drops, causing gas to form. If large amounts of gas are released, placing a gas trap before the heat exchanger may be needed. Experiments showed that about 30% of dissolved gas forms in the top 8.5 meters (28 ft) of the intake pipe. The balance between removing gas from seawater before it enters the system and managing gas removal in the condenser depends on factors like gas release speed, deaerator efficiency, pressure loss, vent compressor efficiency, and energy use. Studies found that vertical spout condensers work about 30% better than falling jet types.

Since seawater must flow through the heat exchanger, maintaining good heat transfer is important. Thin biofouling layers (25 to 50 micrometers) can reduce heat exchanger performance by up to 50%. A 1977 study showed that even small microbial growth on heat exchangers reduced thermal conductivity, even though fouling levels were low. This happens because microbial growth traps a thin layer of water, which affects heat transfer. Another study found that fouling worsens over time. Regular brushing removed most microbial layers, but tougher layers formed later that could not be removed by brushing alone. Sponge rubber balls were used to reduce fouling, but they did not stop growth completely. Brushing was still needed occasionally. Microbial growth increased more quickly after cleanings, likely due to changes in the microbial population.

Studies tested daily use of chlorine (0.1 mg per liter for 1 hour) to control microbial growth. Chlorine slowed but did not stop growth. However, it may help maintain long-term operation. The study found that while microbial fouling affected warm water heat exchangers, cold water heat exchangers had little to no biofouling and only minor inorganic fouling.

Besides water temperature, microbial fouling depends on nutrient levels. Fouling increases in nutrient-rich water. The material used for heat exchangers also affects fouling. Aluminum tubing slows microbial growth but makes cleaning harder and causes greater efficiency loss. Titanium tubing allows faster fouling but is easier to clean.

The evaporator, turbine, and condenser operate under partial vacuum, with pressure as low as 1% of atmospheric pressure. The system must be sealed tightly to prevent air from entering, which could disrupt operation. In closed-cycle OTEC, low-pressure steam takes up much more space than pressurized working fluid. Components must have large flow areas to avoid excessive steam speeds.

One method to reduce exhaust compressor energy use is to pass non-condensible gas and steam through a counter-current region after most steam is condensed. This increases gas-steam interaction by five times, reducing exhaust pumping power needs by 80%.

Cold air/warm water conversion

In winter, coastal Arctic areas can have a temperature difference of up to 40 °C (72 °F) between seawater and the air. Closed-cycle systems can use this difference between air and water temperatures. Removing pipes that extract seawater might make systems based on this idea less expensive than OTEC. This technology was developed by H. Barjot, who proposed using butane as a cryogen. Butane has a boiling point of −0.5 °C (31.1 °F) and does not mix with water. If the system operates with 4% efficiency, calculations show that one cubic meter of water at 2 °C (36 °F) in a place with air temperatures of −22 °C (−8 °F) could generate the same amount of energy as flowing through a hydroelectric plant with a height of 4000 feet (1,200 meters).

Barjot Polar Power Plants could be built on islands in polar regions or designed as floating platforms attached to ice caps. For example, the weather station Myggbuka on Greenland’s east coast, located 2,100 km from Glasgow, records average monthly temperatures below −15 °C (5 °F) during six winter months each year.

Application of the thermoelectric effect

In 1979, SERI suggested using the Seebeck effect to create power with a total conversion efficiency of 2%. In 2014, Liping Liu, an Associate Professor at Rutgers University, imagined an OTEC system that uses solid-state thermoelectric materials instead of fluid cycles that were usually used.