The ocean thermal energy conversion technology utilizes the temperature difference between the warm surface of the ocean heated by solar energy and the colder deep waters. This technology can provide a power source for many regions. Although it has existed for over 100 years, it has not received enough attention. One of the reasons is the lack of interest in funding.
Energy Source from the Sea
The French engineer Georges Claude (1870-1960) proposed the ocean thermal energy conversion system at the Institute of France in 1926. This system was designed to use a compound like liquid ammonia (which has a very low boiling point) as the working fluid. Warm water from the ocean surface flows to a heat exchanger, where the heat causes the ammonia to evaporate. As the vapor expands, it spins the blades of a turbine connected to a generator to produce electricity. The vapor then enters another heat exchanger, where the cold water pumped from the deep ocean condenses it back. The pressure difference between the two chambers pulls the vapor around the system, spinning the turbine and generating electricity. Some of this electricity is used to power the pumps that bring water up, while the excess electricity is sent to the grid for public use.
Slow Start
In 1979, former US President Jimmy Carter signed the Ocean Thermal Energy Conversion Act, aimed at promoting the development of a range of energy sources. Researchers developed two ocean thermal energy conversion plants off the coast of Hawaii and successfully generated net energy. Japanese companies tested their own ocean thermal energy conversion systems in 1981 off the coast of Nauru in the Pacific Ocean. They generated 35 kilowatts of power before the plant was destroyed in a storm. In the 1990s, the Natural Energy Laboratory of Hawaii operated a 250-kilowatt ocean thermal energy conversion pilot plant for six years. Recently, the US Navy supported Makai Ocean Engineering in constructing a 105-kilowatt ocean thermal energy conversion pilot plant in Hawaii. This facility was connected to the grid in 2015 and provided enough energy to power approximately 120 homes.
Benefits for Tropical Islands
The best use of ocean thermal energy conversion may be providing energy to tropical islands. Despite ample sunlight, many of these islands, especially small and low-lying ones like Kiribati, suffer from a lack of land that cannot accommodate even small solar farms. Among these small island developing states, there is a significant interest in ocean thermal energy conversion. This is because the surrounding ocean water and tropical biomass make small island countries likely candidates for net carbon absorption, but they are more affected by changes in ocean temperatures and levels than others. For these countries, achieving net-zero is a matter of self-preservation—if climate change is not slowed, rising sea levels will inundate the entirety of these nations.
Challenges and Future Opportunities
It is clear that the technology works, so why aren’t coastlines filled with ocean thermal energy conversion plants generating cheap energy? One reason is that the plants require at least a temperature difference between the surface water and deep water of about 20 degrees Celsius (68 degrees Fahrenheit). These conditions occur only near the equator, far from the regions that need energy the most. Another issue is that ocean thermal energy conversion plants come with relatively high capital costs, making the technology less attractive to investors, especially with the decreasing costs of renewable energy like solar and wind. Ocean thermal energy conversion plants have been planned in Bora Bora, Martinique, and Puerto Rico, but overall, ocean thermal energy conversion technology has been outpaced by solar and wind energy.
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Also, there is relatively low efficiency for the process. Only about 2 to 3 percent of the energy in seawater is converted into electricity using ocean thermal energy conversion. This seems low, but if the pumps are powered by solar energy, the economics actually work – as long as you can pump a large volume of water. One white paper indicates that it would take the equivalent of four Niagara Falls worth of water flowing continuously through heat exchangers at the plant to make the economics work. Additionally, the cost of the equipment and piping to do this is prohibitive – hence the high startup costs.
This is doubly true for land-based facilities. Running a massive pipe long enough to find cold water costs a lot of money. One alternative is to build the plant on a platform at sea and run the pipe directly. Perhaps these plants could be parked near offshore wind energy generation and share the cable to bring power ashore. However, building at sea also increases costs.
In an article published in Eos, George Hagerman, a senior project scientist at the Coastal Ocean Sciences Center at Old Dominion University, proposed using an underwater tunnel to bring cold water to a land-based plant.
Others are working on making the technology more cost-efficient by making it lighter. In September 2023, China conducted offshore trials of a floating ocean thermal energy conversion plant developed in a project led by the Marine Geological Survey in Guangzhou. The experiment, aboard the research vessel Haiyang Dizhi-2 (“Ocean Geology No. 2”), demonstrated the feasibility of low-cost ocean thermal energy conversion technology. China is also planning to build an ocean thermal energy conversion power plant in the South China Sea in a partnership between Lockheed Martin and the Reignwood Group based in Hong Kong. The power plant will supply electricity to a resort to be developed by Reignwood.
A Blessing for Tropical Island Nations
The best use for ocean thermal energy conversion may be providing power to tropical island nations. Despite abundant sunlight, many of these islands, especially small and low-lying ones like Kiribati, suffer from a lack of land that cannot be spared even for small solar farms. Among these small island developing states, there is significant interest in ocean thermal energy conversion. This is because the surrounding ocean water and tropical biomass make these island nations potential net carbon absorbers, but they are more vulnerable than others to changing ocean temperatures and sea levels. For these nations, reaching net zero is a matter of self-preservation – if climate change is not slowed, rising ocean waters will completely cover these countries.
In 2021, SIDS DOCK, the sustainable energy and climate resilience organization for small island developing states, signed an agreement with UK-based Global OTEC Resources Limited to collaborate on the development and deployment of ocean thermal energy conversion technology. The project will start with a 1.5 megawatt (MW) plant located on a barge off São Tomé and Príncipe. This small plant will serve as a model and practical guide and can be expanded to become a larger plant or additional smaller barges. An ocean thermal energy conversion project is planned off the shores of São Tomé and Príncipe.
Other islands are considering establishing plants onshore. This is attractive because although they are associated with high startup costs, they can also support secondary industries reliant on cold water, such as seafood farming.
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Designing other types of ocean thermal energy conversion systems, called open ocean thermal energy conversion (which uses seawater as the working fluid, exposing it first to a vacuum to lower its boiling point), to desalinate seawater while it evaporates, providing freshwater – a rare and expensive commodity on many islands.
So far, so good, but there are also potential risks if this technology really proliferates. In a 2012 research paper, researchers at the University of Hawaii at Manoa found that one ocean thermal energy conversion plant would have minimal impact on the local ocean environment. However, if the seas became crowded with plants, they would convert enough seawater to eliminate the thermal gradient that makes the process work in the first place. In a more recent research paper by researchers from the University of Hawaii, published in the journal Ocean Science and Engineering, they used modeling to conclude that negative impacts could be reduced by siting plants at least 100 kilometers (km) offshore and that “the total electricity production from ocean thermal energy conversion of about 2 terawatts would have no widespread environmental effects, and 6 to 7 terawatts could be produced as long as associated impacts remain acceptable.” To put that in perspective, global electricity consumption in 2022 was about 24,400 terawatt-hours. Additionally, it was found that the number of plants needed to achieve peak production would be sufficient to affect some ocean currents, which could impact the climate.
However, researchers caution that more work is needed on these issues. For example, it is not clear how smaller disturbances will affect global warming.
Other constraints are, as expected, funding. SIDS DOCK and Global OTEC are currently seeking funding to kickstart projects. However, investors are hesitant to contribute to relatively untried projects when solar and wind energy seem like better alternatives.
At COP28 in December, a side event organized by the Alliance of Small Island States highlighted the necessity of ocean thermal energy conversion technology and the funding shortfall.
Adelino Rosa Cardoso, the Minister of Infrastructure, Natural Resources, and Environment in São Tomé and Príncipe, confirmed that their project still needs additional financial support to complement what the private sector contributes.
Other tropical island nations hope for future developments in ocean thermal energy conversion. Dr. Al Pinheiro, the Secretary-General of SIDS DOCK, drew attention in Renewable Energy Magazine to the fact that small island developing states “have no other options regarding energy; we are running out of land, and we do not have much space for solar energy. But we have one space that we can never run out of, which is our oceans.”
Indeed, the oceans themselves may also point to a solution for the funding shortfall. If ocean thermal energy conversion can be integrated with offshore solar and wind projects to provide base power and share infrastructure like cables, investors may be more interested.
Electricity storage technologies being developed to support intermittent power sources such as wind and solar can also benefit ocean thermal energy conversion. In the future, floating ocean thermal energy conversion platforms could be used to generate electricity or hydrogen fuel, which could then be shipped around the world.
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Electric ships passing by can stop at floating ocean thermal energy conversion stations to swap or charge their batteries before moving on. This can also help small island nations by providing a new source of income – controlling a major energy source.
Currently, the technology is ready for implementation; we are just waiting for the right conditions.
Source: https://interestingengineering.com/innovation/100-year-renewable-energy-source
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