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A 140-year-old idea to exploit the temperature differences between layers of the ocean is gaining new interest as a way to provide clean power for people living on islands.

Called ocean thermal energy conversion (OTEC), the idea is to use surface water warmed by the sun to heat a fluid such as ammonia or water, which then spins a turbine as it evaporates, just like a power plant driven by steam. Cold water from deeper layers of the ocean would then be piped up to cool the fluid and repeat the cycle.

Theoretically, oceans could provide vast amounts of energy in this way. Researchers estimate that there is enough ocean heat to supply of power a year without affecting the ocean鈥檚 circulation, enough to easily supply the entire world鈥檚 annual electricity demand 鈥� if we can access it.

The idea was first proposed in 1881, but went largely unexplored until the 1970s oil crisis drove a search for new sources of energy. A few were built at the time, then the 2000s again brought some interest 鈥� including an effort to design bigger pipes for large-scale plants. But progress has largely been 鈥渄isappointing鈥�, says at SIDS DOCK, an organisation that promotes clean energy development in small island countries.

Now, the fresh urgency to transition away from fossil fuels, plus some technological advances in offshore energy, have brought hints of an OTEC resurgence. 鈥淚t鈥檚 the most promising it鈥檚 been in many decades,鈥� says at the Pacific Northwest National Laboratory in Washington.

A small OTEC plant has been supplying energy to the grid in Hawaii since 2015, and another has been running in Okinawa, Japan, since 2016. In 2019, a Korean company tested an OTEC system to be installed on the Pacific island of Kiribati. And Chinese researchers recently 聽a small, ship-based system in the South China Sea.

These are all tiny projects, but at the University of Hawai鈥檌 at M膩noa says any demonstration that the technology works could help get past a 鈥減sychological barrier鈥� to investing in OTEC after past flops. 鈥淲hat鈥檚 lacking is operational record and visibility.鈥�

If UK-based company Global OTEC has its way, a demonstration of commercial viability could come soon. The firm has designed a 1.5-megawatt floating system, which is planned to supply power to the islands of S茫o Tom茅 and Pr铆ncipe off the coast of West Africa by 2026. The company has been in talks with other interested island governments, such as Fiji. 鈥淭his isn鈥檛 just a bunch of guys tinkering around in a garage,鈥� says , the company鈥檚 CEO.

Tropical islands in particular are strong candidates for OTEC because of current high energy costs 鈥� diesel generators are common there 鈥� and the presence of deep cold water not far from shore, says at Florida Atlantic University. Islands also have less space for other types of renewables and fewer options for sources of continuous power, such as a natural gas power plant.

鈥淸With OTEC] we keep our ecosystems, we keep our land and we keep our water resources,鈥� says Binger, who is working with Global OTEC to promote this form of energy. In addition to generating electricity, he says the systems could be used to desalinate seawater and produce hydrogen fuel.

A major barrier for past projects was the cost of installing kilometres of large-diameter pipes to reach deep water. Global OTEC鈥檚 design aims to avoid that problem by extending a shorter pipe from a platform floating in deep water 10 kilometres offshore, then transmitting electricity back via a cable. Grech says advances in offshore wind and in oil and gas technology have already done some of the 鈥渉eavy lifting鈥� to improve designs.

鈥淥TEC will be part of providing an equitable energy transition so these places aren鈥檛 depending on diesel imports from the richest petrol states in the world,鈥� he says.