First commercial-scale ocean thermal energy generator slated for 2025

In an average day, tropical oceans absorb about 278 petawatt-hours of solar energy. Harvesting just 1/4000th of that energy would supply the entire world's daily electricity – and ocean thermal energy conversion provides a possible method.

Harvesting energy from the temperature differential between the warm surface and the cold deep ocean is certainly not a new idea – indeed, it was first trialed 142 years ago in 1881, and a 22-kilowatt OTEC plant was built in Cuba in 1930.

The basic idea is this: you go to an area with a consistently large temperature differential – somewhere in the tropics, reasonably close to land, where you can tap a cool 4 °C (39 °F) at a depth around 800 m (2,625 ft) and grab warm water at over 25 °C (77 °F) from the surface, all through the year.

You then set up a floating barge, moored to the sea floor and equipped with a closed loop power system, using a refrigerant-style liquid like ammonia with a boiling point in between those two temperatures. The warm surface water boils this liquid, which expands as a gas and drives a turbine to generate electricity, then you cool and condense the liquid using cold water brought up from the deep ocean through an insulated pipe, and cycle it through again.

Such a system generates electricity reliably, 24/7, at a constant rate suitable for baseload usage. It can be ramped up and down within seconds to account for demand peaks and troughs. And if deployed en masse via thousands and thousands of floating OTEC barges, it could potentially do something to limit the rise of sea surface temperatures, which have been spiking wildly in 2023.

Historically though, OTEC has also used the vast majority of the power it generates to run the pumps bringing the cold water to the surface; a plant in Nauru was switched on in 1981 by the Tokyo Electric Power Company. It generated about 120 kW of energy, for example, but it used about 90 kW of that to run the plant. So while the energy resource in question is virtually unlimited, the efficiency of extraction is low.

There are other challenges: storms, which can be brutal in these tropical areas, can damage the floating barge, or toss it around enough to break the long, insulated pipe leading all the way to the sea floor. And as with any ocean-based project, there's biofouling and the corrosive properties of seawater to contend with.

And the economics are tough; this remains an early-stage technology that's yet to benefit from economies of scale. It's long been assumed that in order to be cost-competitive with more established power options, you'd need to build an OTEC system at around the 100-megawatt scale, and nobody's built anything near that size.

Still, London company Global OTEC says that "recent changes in the global economics of energy and advancements in the efficiency of components" make now the time for another push at the idea. At the recent International Vienna Energy and Climate Forum, the company presented a new concept for an OTEC barge called Dominique, which it expects to begin commissioning in 2025 off the coast of São Tomé and Príncipe – a tiny island nation off the West coast of Africa.

The Dominique barge is designed to provide a net output of 1.5 megawatts year-round, enough to supply nearly 17% of the nation's entire 78 million-odd kilowatt hour energy consumption, by my math, and apparently enough for Global OTEC to pronounce it the "first commercial-scale OTEC platform."

“We know Dominique is a life-changer for small islands and coastal nations, and that’s why we see the pace of the project on track for success," said Global OTEC founder and CRO Dan Grech in a press release. "This is an important lesson we want to share with investors as the public-private partnership allowed the smooth undertaking of critical techno-economic, environmental and social studies to progress to this point."

It's unclear at this stage whether the project is fully funded or still seeking finance, but the company says in terms of Levelized Cost of Energy (LCoE), it expects early generation OTEC barges to deliver power at between US$150-300 per megawatt-hour. It'll struggle to compete with other power sources at that price, although its reliable 24/7 generating capacity could make it a good fit for certain renewable-based grids where geography and other factors are favorable.

Over time, the company says larger-scale plants could produce power for as little as US$50/MWh, which could be closer to the ballpark of wind and solar, depending on what's happening in those markets.

But getting there won't be easy, as evidenced by an even more ambitious project announced in 2014, which would've put a 16 MW gross, 10 MW net electricity OTEC facility to work off the Caribbean island of Martinique. Akuo Energy and DCNS secured some €72.1 million in funding for the so-called NEMO project in July 2014, but Akuo announced four years later that DCNS, which had changed its name to Naval Energies, was shelving the project due to "technical difficulties relating to the main cold-water intake pipe." The Naval Energies website explains little, as it is now a highly dubious-looking Indonesian lottery betting site featuring NSFW photos of scantily-clad women.

We'd also have to point out that another proponent, Ocean Thermal Energy Corporation, made a press release in May pointing out that "progress on OTEC is presently constrained by what is known as the Innovation Valley of Death," and that somebody would need to spend US$200-300 million building a 5-10 MW OTEC plant and run it at a "total loss" for several years just in order to prove the concept was worth taking to the hundred-plus megawatt scale. OTE Corporation's proposed solution: to combine the idea somehow with Bitcoin mining on the island of Oahu, which in this context starts sounding to us a bit like the Indonesian lottery thing, just with fewer scantily-clad women.

A 2021 review published in the Journal of Marine Science and Engineering praised the "enormous potential" of OTEC technology, but pointed out that "high infrastructure costs" and "the hostile and corrosive environment of marine waters" were preventing it from making the leap to a commercial implementation. "Many companies have been developing new and efficient solutions," it reads, and the next few decades are likely to see an increase in R&D in the area, but the kinds of remote island locations where it might work in conjunction with desalination tech typically just don't have the money to pay for it.

So while we're as excited as anyone to learn about clean energy alternatives, and we wish Global OTEC every success in delivering ocean thermal energy to São Tomé and Príncipe by 2025, we'd have to rate this one as a long shot.

Source: Global OTEC