Could sea power solve the energy crisis?

Ocean Thermal Energy Conversion (OTEC) may be an idea whose time has come. The theory is very simple: OTEC extracts energy from the difference in temperature between the surface of the sea (up to 29C in the tropics) and the waters a kilometre down, which are typically a chilly 5C. This powers a "heat engine": think of a refrigerator in reverse, in which a temperature difference creates electricity.

The first OTEC plant was constructed by the French inventor Georges Claude in 1930 in Matanzas Bay, Cuba, where the sea plunges to great depths close to the shore. After several failures - including the loss of two of the pipes used to suck up cold water from the sea - Claude produced 22,000 watts of electricity from this early prototype. This is a very small amount, enough to heat five well insulated houses. But it confirmed that his calculations worked, and that both electricity and drinking water could be produced by the sea. When a later version was installed on a converted boat in Brazil, Claude found a use for this desalinated water: he froze it to make ice, a valuable commodity at the time. The boat, however, was damaged in a storm and the project abandoned.

There are two basic versions of the technology. The first operates in a "closed cycle", using warm surface water to heat ammonia, which boils at a low temperature. This expands into vapour, driving a turbine that produces electricity. Cold water from the depths is used to cool the ammonia, returning it to its liquid state so the process can start again. The second, "open cycle" version offers the added benefit of producing drinking water as a by-product. Warm seawater is introduced into a vacuum chamber, in which it will boil more easily, leaving behind salt and generating steam to turn a turbine. Once it has left the turbine, the steam enters a condensing chamber cooled by water from the depths, in which large quantities of desalinated water are produced - 1.2 million litres for every megawatt of energy. A modern OTEC plant could meet much of the world's energy needs, without generating polluting clouds of carbon and sulphur dioxide. A 250MW plant (a sixth of the capacity of the new coal-fired power station that has recently won planning permission in Kent) could produce 300 million litres of drinking water a day, enough to fill a supertanker. Using electrolysis, it would also be possible to produce hydrogen fuel.

Difficulties have beset many subsequent attempts to turn principle into practice, but with oil at $100 a barrel, and with pressure growing to arrest the effects of climate change, the conditions are right for OTEC to make a comeback. And - if you are going to build an offshore platform, it makes sense to harness its space to create an "energy island" - a facility that uses a variety of alternative energies, such as wind, wave and solar, to generate enough power to pump the huge quantities of water from the sea and run the vacuum pumps of the OTEC plant.

Not only would these "islands" be self-sufficient, but several could be linked to generate energy outputs of around 1,000MW, rivalling the output of a typical nuclear plant. The cost, according to our models, would be roughly double that of a nuclear power station. This might seem expensive, but an OTEC plant would not involve the waste-treatment or astronomical decommissioning costs of a nuclear facility. Also, it would offset its expense through the sale of the desalinated water. This is in addition to the other advantages: carbon-free energy production and no generation of heat. In fact, the plants would cool the seas and oceans by the same amount as the energy extracted from them.

OTEC could produce enough drinking water to ease drought-stricken tropical areas like Ethiopia

The energy islands would mainly be built from reinforced concrete, using corrosion-resistant metals, but they would come in different sizes and with different functions: some would be simple energy-generating platforms, while others would be larger installations, linked to the land by cable, along which the electricity could be distributed (this would involve superconductors, materials that lose all resistance to electricity when cooled, minimising energy loss in transmission).

And energy generation would not be their only function. Larger islands could support fish farms that used the plankton pumped up along with the cold seawater, as well as pods for heliports, greenhouses, accommodation and maintenance areas, facilities for producing sea salt and harbours and mooring for supertankers to collect the desalinated water or the hydrogen produced by electrolysis to ship to energy- and water-scarce parts of the world.

We also have a clear idea where to build these powerful islands. For the system to function, the gap between the surface and the depths must be at least 20C. Maps of the oceans show which waters contain the necessary temperature gradients -and China, India and Brazil, the three countries projected to have the greatest economic and population growth this century, and therefore the most pressing energy requirements, all have OTEC potential.

Many groups are now sponsoring serious research on renewable marine power. 50,000 energy islands could meet the all world's energy requirement while providing two tons of fresh water per person per day for its six billion inhabitants. That would be a legacy of which Georges Claude could be proud.

• Dominic Michaelis is an architect and engineer. He and his architect son Alex are developing the energy island concept with Trevor Cooper-Chadwick of Southampton University.

Ttaken from Daily Telegraph 8th Jan 2008

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