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Tropical idea: Ocean thermal energy conversion

Technology that taps into the solar energy stored in seawater may prove to be a valuable source of power

Hawaii: likely to be one of the first markets

Thanks to the development of systems that can harvest energy from its waves, tides and currents, the ocean is viewed as in increasingly valuable source of power. But according to the proponents of technology that exploits the sea’s ability to trap solar energy, we haven’t seen anything yet.

The technology, Ocean Thermal Energy Conversion (OTEC), could, they say, transform the way we use energy, allowing us to supply huge quantities of non-polluting, baseload power. It could also help desalinate water and provide refrigeration to local communities.

According to some estimates, more than 300 times the energy we currently consume is available from the solar energy that is constantly stored in the upper layers of the tropical ocean. On an average day, 60 million square kilometres of tropical seas absorb an amount of solar radiation equivalent in heat content to around 250 billion barrels of oil. Tapping into that potential has been the goal of OTEC scientists for more than a century.

Invented in 1881 by a visionary French scientist, Jacques-Arsène d’Arsonval, OTEC uses the temperature difference between the warmer water of oceans and the colder water that can be found up to 1km below the sea’s surface. In some tropical regions, this difference is enough to operate vapour turbines that drive generators and produce electricity and fresh water as a byproduct.

In the 19th century, d’Arsonval couldn’t prove the viability of OTEC. But its potential captured the imagination of a small group of researchers. The oil crisis of the 1970s brought the radical technology into the mainstream, and work in Europe, the US and Japan began to gain momentum. No sooner had it started, however, that petroleum prices dropped and investors lost interest.

But now, major players are planning to revive the technology. Two years ago, the US Navy awarded defence group Lockheed Martin an $8m (£5m) contract to develop critical OTEC components and to mature its design for a pilot plant. Since then, the navy has awarded a further $4m to build on its research. The hope is that OTEC will provide a consistent energy source for naval bases.

More than 300 times the energy we consume is available from the solar energy in the tropical ocean’s upper layers

’Some of the large bases, such as Pearl Harbour, are ideal locations,’ said Robert Varley, OTEC project manager at Lockheed Martin. ’Our fiscal-year defence programme has a separate area specifically to develop OTEC. That will be a signal to industry that the navy is serious about going forward with the technology. Of course, congress has to approve it, but I think the chances are good.’

As the technology requires a difference of 20°C in water temperature, OTEC can only operate between latitudes of 20°N and 20°S. This area includes 29 territories and 66 developing nations, as well as OTEC’s main investor: the US. The first markets are likely to be the small island nations of the South Pacific and Molokai in Hawaii. By 2015, Lockheed Martin hopes to have developed a 5MW pilot plant in this area in collaboration with Hawaii’s Makai Ocean Engineering.

’The challenge is designing the plant for minimal capital cost,’ said Joe Van Ryzin, vice-president of Makai. ’We’ve spent some time developing large low-cost and corrosion-resistant heat exchangers. Makai has a heat exchanger test facility in Hawaii where we are testing various heat exchangers in both surface and deep seawater.’

Revival: Lockheed Martin builds pilot plant

Revival: Lockheed Martin builds pilot plant

In Lockheed Martin’s planned closed-cycle system, warm water from the surface of the sea is pumped into a heat exchanger that acts as a boiler for the system’s working fluid, ammonia. As the ammonia vapour expands, it spins a turbine coupled to a generator to produce electricity. The vapour is condensed in another heat exchanger using cold seawater from below the surface to remove the heat. Then, the working fluid is pumped back to the evaporator to repeat the cycle.

Varley believes that, while the process is non-polluting, the movement of the seawater could cause concern among environmental groups. ’We’re pumping about 1,000 gallons per second per megawatt. That’s a lot of water, and the impact needs to be assessed. In the next five to 10 years, I would like to see a full-scale OTEC system in the water that can make these environmental measurements.’

In some cases, the change in seawater could benefit the environment. The cold water is rich in nutrients and could help to culture marine and plant life near the shore. An open-cycle system that uses warm seawater as the working fluid instead of ammonia could also produce desalinated water using surface condensers. The water could be used by local communities where freshwater supplies are limited. In addition, the cold water (around 4ºC) could provide cooling near the plant and supply air conditioning in buildings.

“We’re pumping a lot of water, and the environmental impact needs to be assessed”


’Despite its benefits, OTEC is a hard sell in the US,’ said Jan War, operations manager of the Natural Energy Laboratory of Hawaii Authority. ’For the 5-10MW demonstration plant, we are talking at least $200m of investment, and the return of investment is going to be quite bad because it is not on an economical scale…It’s a hard pill to swallow by just one entity, so we’re working with Japan and Taiwan to see if they want to share the IP.’

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The future for OTEC, however, could be in open grazing plants. If the technology can be proven on a full-scale floating plant, engineers will develop a system that is free to drift across the ocean. The plant would use the power generated from thermal differences to split seawater into liquid hydrogen and liquid oxygen. The hydrogen could be offloaded into tankers and taken to countries that need liquid hydrogen as part of their infrastructure.

’This could happen within the next 10 years,’ said Varley. ’While the UK’s waters aren’t warm enough for direct use, a grazing plant could help to import much-needed alternative fuels for transport.’ If this happens, it would require huge investment at the cost of other renewable technologies. But Varley believes the return will be worth it. ’The real advantage is in autonomy,’ he said. ’Do you want to import oil from our friends - or not-so friends - in other countries, or from developments that Great Britain controls? That’s the question that needs to be asked.’

Good for the solar

From a radical concept to a serious idea awaiting congress approval

  • 1881 French scientist Jacques-Arsène d’Arsonval reveals his idea to tap into the thermal energy of the ocean.
  • 1926 Georges Claude, one of d’Arsonval’s students, begins research into OTEC for commercial use.
  • 1935 Claude attempts to build an open-cycle plant aboard a 10,000-tonne cargo vessel, but fails.
  • 1956 French researchers design a 3MW open-cycle plant for Abidjan on Africa’s west coast. The plant was never completed owing to high costs.
  • 1974 The Natural Energy Laboratory of Hawaii is established as a test facility for OTEC technologies.
  • 1981 Japan demonstrates a shorebased, closed-cycle plant in the Republic of Nauru in the Pacific Ocean.
  • 2009 The US Navy awards Lockheed Martin $8m to develop a 5MW OTEC pilot plant.

Readers' comments (8)

  • Great idea and possible energy solution for small users but, again, it is a matter of scale and control. Grazing plants sound good but how do you control accurate positioning, location and visibility. Security is another problem. Maintenance and damage control requirements also need to be considered. These problems will be sorted but the initial cost for such a small return may stop the project again.

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  • I think the greatest problem with OTEC is the numbers. The theoretical efficiency is only 7%. At that narrow margin a relatively complex plant like OTEC loses its ability to create an appreciable net power output with our current technology. There needs to be some breakthrough technology that will allow for the harvesting of this potential other than current vapor turbines.

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  • Back in the 1980's, I was involved in an extensive OTEC study sponsored by the US government. Two issues were raised, that pretty much killed the project back then:

    1. Excessive cost. Imagine a pipe large enough, and pumps big enough, to raise sea water from 1 km depths- all needing to be fabricated from corrosion resistant materials.

    2. Viable sites, where sufficient temperature differences occur, are, for the most part, quite a distance from population centers- i.e., the market for the energy. Hawaii was one exception to this.

    It seems nothing has much changed, except the price of oil has gone up again. I can understand the US Navy's interest in this technology- think in terms of rapid deployment to areas lacking sufficient infrastructure to support operations (not only war- disaster recovery also comes to mind). But this is a far cry from a panacea for solutions to the energy situation. I do not see this having any real commercial potential until after tidal and wave energy have been exploited to the maximum practical potential. Better put your money to good use, rather than chasing "pipe" dreams...

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  • The removal of vast amounts of heat energy 'stored' in sea-water would surely lead to some cooling of the tropical ocean(s). Should we consider this a blessing as a sure-fire method to end 'global warming' or should we begin to worry about the effects that cooling of tropical seas would have on the populations of food fish depending on the warmer waters. IMHO I would guess that the vast Pandora's box of "unintended cpnsequences" should make us think long and hard before going large scale on this idea.

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  • I recall experiments for tapping energy from ocean temperature differentials using Hot/Cold thermocouples converting the temperature difference directly into electrical current. This removes the potential hazard, and cost, of shifting vast amounts of water into a different temperature region. Why is this not being considered?

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  • Generating Energy Storage & A Combination Renewable Energy System

    “It is cheaper to save energy than make energy”

    Any renewable energy system that is installed should have extra capacity and be able to convert water into hydrogen which will be used to power a hydrogen generator as a back-up power source.

    We should install a renewable energy system that utilizes solar & wind, when possible add geothermal to the mix.

    A design is needed for a renewable energy system that can generate electricity and heat water with a step down mixer allowing the system to provide water hot enough for radiant heating and at the same time utilize a step down mixing valve to reduce the water temperature to be able its use for hot water in normal consumption.

    A thermal renewable energy system may be able to provide both.

    Prior to sizing up a renewable energy system, an energy audit should be conducted and energy efficiency recommendations should be implemented, that includes changing habits in utilizing energy and utilities in general.

    Habitual changes can save between 20 to 50% of energy & utility consumption.
    When people are considerate not to waste, they save resources and money.

    YJ Draiman, Energy/Utility Analyst

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  • Thanks for an inspiring and well-written feature article. As someone mentioned in the user comments, it's impossible to know just how profitable electrical power from OTEC platforms will be, but that fresh water for emergency areas will be of interest nonetheless.

    This is the approach we have been taking for the OTEC Africa project ( where we suggest that OTEC pilot plants are to be built outside the coasts of Kenya and Tanzania. These days many people are dying from starvation and lack of fresh water in Kenya, Tanzania, and Somalia. Using pipelines with fresh water from OTEC pilot plants, lives can be saved in the future.

    If the pilot plants turn out to generate a good income, then the involved parties have a proven technology to sell world-wide. If not, then at least they have sponsored a fantastic project that might save the lives of thousands of people.

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  • Tinian Island sits on the edge of the Marianas Trench. Cold Seawater is less than 1KM away. Population is about 4,000 people, so electricity, and fresh water needs are minimal. small scale say 5MW would be practical size. Land based, cost of construction could be kept down.

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