20,000 megawatts under the sea: Oceanic steam engines

Jules Verne mused about getting energy from heat in the ocean (Image: Marc Pagani/Getty)

Jules Verne imagined this limitless power source in Victorian times – now 21st-century engineers say heat trapped in the oceans could provide electricity for the world

IF ANY energy source is worthy of the name "steampunk", it is surely ocean thermal energy conversion. Victorian-era science fiction? Check: Jules Verne mused about its potential in Twenty Thousand Leagues Under the Sea in 1870. Mechanical, vaguely 19th-century technology? Check. Compelling candidate for renewable energy in a post-apocalyptic future? Tick that box as well.

Claims for it have certainly been grandiose. In theory, ocean thermal energy conversion (OTEC) could provide 4000 times the world's energy needs in any given year, with neither pollution nor greenhouse gases to show for it. In the real world, however, it has long been written off as impractical.

This year, a surprising number of projects are getting under way around the world, helmed not by quixotic visionaries but by hard-nosed pragmatists such as those at aerospace giant Lockheed Martin. So what's changed?

It's possible that Verne dreamed up the idea for OTEC to help out Captain Nemo, the protagonist of Verne's deep-sea yarn who needed electricity to power his submarine, the Nautilus – it is the first written mention of the idea. "By establishing a circuit between two wires plunged to different depths, [it should be possible] to obtain electricity by the difference of temperature to which they would have been exposed," Nemo told his shipmate. Eleven years after the book was published, French physicist Jacques-Arsène d'Arsonval proposed the first practical design for a power plant that does exactly that. Instead of using wires, he used pipes to exploit the temperature difference between the cold deep ocean and the warm surface waters to generate steam energy.

The idea is brilliant. The ocean is a massive and constantly replenished storage medium for solar energy. Most of that heat is stored in the top 100 metres of the ocean, while the water 1000 metres below – fed by the polar regions – remains at a fairly constant 4 to 5 °C.

To make energy from that heat difference, modern-day systems pump warm surface water past pipes containing a liquid with a low boiling point, such as ammonia. The ammonia boils and the steam is used to power a turbine, generating electricity. Cold deep-ocean water is then piped through the steam, causing the ammonia to condense back into a liquid, ready to begin the cycle again (see diagram). Steam-powered turbines drive nearly every coal and nuclear power plant in the world, but their steam is produced by burning polluting coal or generating long-lived nuclear waste. OTEC, by contrast, provides steam in a clean and theoretically limitless way.

Electric ocean

That's in an ideal world. In reality, what the ocean's thermal gradient gives, the equipment takes away. The main problem is accessing the cold deep water: pumping the vast amounts of water needed requires 1000-metre-long pipes that are wide enough and strong enough to handle several cubic metres of seawater per second for every megawatt of electricity produced. Tally all the inefficiencies in the process and the theoretical efficiency of an OTEC plant drops to a dismal 4 to 6 per cent.

Thanks to this and other factors, the process needs a temperature difference of at least 20 °C between the surface and deep water to work. Such conditions exist in a relatively narrow band around Earth's equator that includes the tropics and subtropics (see map).

Despite these constraints, the 20th century was filled with fitful efforts to make OTEC work. The most ambitious of these, in the 1970s, was sparked by an oil crisis, after which the US president Jimmy Carter signed into law the production of 10,000 megawatts of electricity using the technology by 1999. However, the price of oil then fell again, and alternatives to petroleum sank once more to the bottom of the to-do list.

So when Lockheed Martin last year announced that it would begin construction on a 10-megawatt plant off the coast of southern China, the news was met with a marked lack of interest. We had been here before.

A closer look, however, reveals that the project may signal a sea change for OTEC. The time may finally have come for this 19th-century technology to become part of the 21st century's renewable energy mix, thanks to a strange partnership of other renewables, the oil industry – and perhaps even climate change.

Many calculations are changing. OTEC's efficiency may be low, but since it uses seawater, which is abundant and free, it still makes economic sense if done on a large-enough scale. Oil prices are unstable and climate change is becoming an increasingly urgent driver of alternative energy sources. The shortcomings of intermittent renewables such as wind and solar energy, which only produce electricity when the sun is shining or the wind is blowing, are still keeping these on the margins. By contrast, OTEC plants can operate 24 hours a day, says Ted Johnson of Ocean Thermal Energy Corporation, which plans to commercialise the technology. Round-the-clock power means an OTEC plant could simply be plugged directly into a municipal grid to replace fossil fuel power plants, without the adjustments and balances necessary to integrate unpredictable solar and wind power.

But what use is that power if the equipment needed to harness it costs more than the electricity it provides? Here, too, advances have been made. Lockheed Martin borrowed techniques from bridge and wind-turbine manufacturing – both of which use advanced fibreglass and resin composites to make their ultra-light, ultra-strong materials – to design a cheap pipe that is strong and flexible enough to withstand the stresses and strains of ocean currents. Even better, it can be assembled on the ocean-surface platform of the OTEC plant itself and gradually lowered in as it is made, eliminating the risk of transporting the huge structure into position – and dropping it. A promising OTEC project in the Bay of Bengal had to be scrapped in 2003, after engineers building a 1-megawatt plant lost not only their first pipe but also its replacement.

Then there are myriad lessons from the offshore oil and gas industry, where it has become commonplace to operate in ocean depths greater than 1000 metres. These have made equipment available for commercial purchase that just 20 years ago would have needed to be designed from scratch.

Thanks to such developments, a 100-megwatt plant would cost about $790 million to build, says Luis Vega, who researches OTEC at the Hawaii Natural Energy Institute at the University of Hawaii at Manoa. Taking the costs of building and running an OTEC plant into account, Vega reckons the price of the electricity produced would come in at around 18 US cents per kilowatt hour, not far from US Department of Energy estimates of 14 cents for coal with carbon capture and storage, and 14 to 26 cents for solar energy.

In this changed landscape, OTEC projects have begun to pop up all over the world. Last year, a 50-kilowatt pilot OTEC plant began operating on Kume Island in Okinawa, Japan. Meanwhile in Hawaii, Makai Ocean Engineering is building a 100- kilowatt plant at its Ocean Energy Research Center in Kailua-Kona on the Big Island. This year, Bluerise, a spin-out from Delft University of Technology in the Netherlands, is planning to start building a 500-kilowatt OTEC plant close to Curaçao International Airport in the Carribbean. "These smaller islands are likely to be the first market, as they are all suffering from a dependency on expensive imported fuels," says Remi Blokker, CEO of Bluerise.

But they won't be the last. Recent advances promise to bring OTEC into the mainstream.

Various research groups have investigated the possibility of combining OTEC with solar power. Paola Bombarda at the Polytechnic University of Milan in Italy has modelled the output of an OTEC plant that uses solar power to increase the temperature of the warm ocean water before it is used to boil the ammonia. She found that even a low-cost solar collector – a simple device that traps heat in lenses or tubes – could triple a plant's daytime electricity output (Journal of Engineering for Gas Turbines and Power, vol 135, p 42302).

Similar techniques could help plants in countries that lie a bit too far north to rely on OTEC all year round, such as South Korea. In the summer months, the temperature difference between the surface and deep water around South Korea exceeds the all-important 20 °C minimum, but that isn't the case in winter. So to make it work year-round, engineers at the Korea Ocean Research & Development Institute (KORDI) in Goseong-gun are beginning to modify a 20-kilowatt demonstration plant so that heat from solar power, wind farms and waste incineration plants can pre-heat the incoming surface water before it meets the ammonia.

An even better idea would be to combine OTEC with another 24-hour power source. Hyeon-Ju Kim and his colleagues at KORDI are looking to geothermal energy, which taps heat deep underground, to boost the temperature of the seawater that boils the ammonia in a combined "GeOTEC" plant. Such tweaks could expand the "equatorial waistband" for productive OTEC plants by a factor of two.

In light of these rapid developments, OTEC has become promising enough that the prospect of its expansion has begun to ring alarm bells among environmentalists. Concerns have been raised by the US National Oceanic and Atmospheric Administration, among others, about the risk of algal bloomsforming as nutrient-rich, bacteria-free water from the sunless depths is introduced to the hungry algae in warmer, sunlit waters. But computer modelling suggests that as long as the cold water is returned to the ocean at depths lower than 60 metres, the risk of algal blooms should be minimal, says Vega.

To eliminate even this modest risk, London-based Energy Island has patented a design for an OTEC plant in which the ammonia vapour is no longer condensed into liquid at the surface but at depth. This means nutrient-rich water would never need to be pumped up to the surface, says founder Dominic Michaelis.

Another question being posed echoes previous concerns about the large-scale take up of other renewables: does OTEC have local and global effects on the environment, such as changing global temperatures?

Happily, research suggests we can ramp up OTEC production without affecting the ocean. Researchers at the University of Hawaii's Ocean and Resources Engineering department in Honolulu modelled the effect of widespread, commercial-scale OTEC production on the seas, including the global thermohaline circulation – the network of slow currents that transport deep water throughout the oceans. They found that OTEC plants could safely extract the equivalent of 7 terawatts of electricity, or nearly 50 per cent of global energy consumption, before they would have any noticeable effect on ocean temperatures (Journal of Energy Resources Technology, vol 135, p 41202). However, the authors acknowledge the difficulties of drawing strong conclusions about the environmental effects of OTEC.

It is certainly a good time to add a new form of renewable-energy generation to the mix, since climate change may have unforeseen circumstances for some existing clean technologies. In July, the US Department of Energy released a report on the energy sector's vulnerability to climate change, which found that higher temperatures could reduce the amount of fresh water available for both hydropower generation and concentrated solar power plants, whose superheated equipment requires water cooling.

By comparison, OTEC sweet spots don't appear to be vulnerable to climate change, says Robert Thresher, a research fellow at the National Renewable Energy Laboratory in Golden, Colorado. "Most of the OTEC resources are along the equator, and you wouldn't expect the sea surface temperature to dramatically change there," he says.

Out of the blue

Indeed, climate change might even increase the global output for OTEC by expanding the OTEC-friendly zone: "As the oceans warm with climate change, you might find warmer [surface] water further north and south from the equator," he says. Though the idea has also been proposed elsewhere, he hastens to add that this is "an intuitive notion" that would need to be confirmed by rigorous modelling.

More problematic is the suggestion that the deep oceans may have absorbed a great deal of the heat of climate change, which could reduce the all-important temperature difference of surface and deep water (New Scientist, 7 December 2013, p 34). However, according to research published last year by Magdalena Balmaseda and colleagues at the European Centre for Medium Range Weather Forecasts in Reading, UK, it is far from clear where exactly that heat is going. "The heat absorption is not uniform in space, depth and time," says Balmaseda (Geophysical Research Letters, vol 40, p 1754).

Whether or not the warm equatorial waistband OTEC relies on expands, the technology might not be limited to countries in the tropics for much longer. At the Offshore Symposium in Houston, Texas, in February 2013, SBM Offshore, which develops technology for oil exploration and drilling, revealed that it has been investigating designs for a 10-megawatt OTEC ship as a means of providing power to remote oil wells. OTEC plants become more expensive the further they are built from shore, but ships, which are cheaper to build, have no such constraints. OTEC ships could roam the seas in search of spots with the best temperature ratios, tethering to submarine cables to return power to shore.

Indeed, proponents of the technology believe the future lies in OTEC ships that "graze" the oceans for electricity. To get around the problem of delivering it to shore by submarine cables, the electricity generated could be used in situ to split seawater into hydrogen and oxygen, with the hydrogen stored in fuel cells before being transported for use around the world. A 100-megawatt OTEC ship could produce 1.3 tonnes of liquid hydrogen per hour, says Vega, albeit at a present cost of about three times what a barrel of oil costs today. The hydrogen economy, after all, is still finding its feet.

Nonetheless, it appears, after all this time, that Jules Verne may have been onto something. If anything, he was thinking too small. Instead of a ship powered by the ocean, a fleet of ships may bring the ocean's energy to the world. Steampunk indeed.

This article appeared in print under the headline "Sea change"

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Helen Knight is a writer based in London