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Ocean Thermal Energy: The Future of Renewable Power?
Haley Zaremba

Ocean thermal energy conversion (OTEC) is the oldest renewable energy technology you’ve never heard of. The idea for the technology, which exploits the differing temperatures in different layers of ocean water to create energy, is almost 150 years old, but is only now gaining traction for practical application. While OTEC will likely never take over the energy industry, it could become an essential source of energy for island nations and other coastal communities in the decarbonization era.

London company Global OTEC plans to bring the first commercial-scale ocean thermal energy generator online as soon as 2025. And just last week oil supermajor Shell Technology signed a deal with Makai Ocean Engineering to test and develop “potentially transformative proprietary technologies” including an OTEC system. 

OTEC harvests warm water close to the surface of the ocean which has been heated by the sun’s energy. This warm water is then pumped through an evaporator which contains a fluid ready to be vaporized (such as ammonia or water). The vapor then turns a turbine to create electricity. Next, cold water is pumped from lower layers of the ocean further away from the sun’s warmth to cool down the vaporized fluid so that the whole process can be repeated. For the technology to function, the difference in ocean water temperatures has to be at least 20° Celsius or 36° Fahrenheit

Source: U.S. Energy Information Administration

In theory, this process could produce a ridiculous amount of power, since there is a ridiculous amount of ocean water out there – and a ridiculous amount of solar energy. Tropical oceans absorb about 278 petawatt-hours of solar energy each and every day. Just 1/4000th of that energy could meet the entire world's daily electricity demand. Indeed, research published in the scientific journal Renewable Energy back in 2013 calculated that there is enough ocean heat worldwide to produce 7000 gigawatts each power a year though OTEC without harmfully impacting ocean water flows. That’s an astronomical amount of energy – enough to supply the entire globe’s annual energy demand and then some.

The issue is actually accessing all that energy. “While the basic heat engine technology at the heart of OTEC can be found in every thermal power plant today, practical seawater temperature differences of the order of only 20°C [the minimum temperature difference to make the technology viable] have made OTEC implementation very challenging,” the Renewable Energy article states. “With thermodynamic efficiencies of about 3%, OTEC cycles must compensate with seawater flow rates as large as several cubic meters per second per megawatt of net electricity produced. This and other difficulties typical of deepwater marine environments have so far prevented OTEC from being economically competitive. Interest in this renewable technology has been sustained, however, with a growing worldwide energy demand, the prospect of declining fossil fuel reserves and serious concerns about climate change.”

Due to these practical challenges and a lukewarm investing environment (so to speak), OTEC has seen a lot of false starts since its conception. Although the technology was first conceived in the 19th century, there have been very few practical applications of the theory to date. The idea all but languished until the 1970s oil crisis revived hopes for innovative alternative energies, and then faded away until the climate crisis re-revived such hopes. In the last few years small-scale rollouts of OTEC projects have proliferated. “It’s the most promising it’s been in many decades,” Andrea Copping at the Pacific Northwest National Laboratory in Washington, recently told NewScientist.

While there is a veritable OTEC renaissance underway right now, it’s still not the sea change that some hopefuls wanted to see. Most projects until now have been tiny. But even if there’s only a minor resurgence of the sector to fuel island nations and select coastal communities, this could still be an important sub-section of the global decarbonization effort. And it does appear that it’s finally the right time for such a resurgence. 

By Haley Zaremba for





Haley Zaremba is a writer and journalist based in Mexico City. She has extensive experience writing and editing environmental features, travel pieces, local news in the Bay Area, and music/culture reviews.

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