Innovation: Quest to harness power of the deep: Variations in ocean temperature promise to give the world a cheap and abundant source of electricity

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THE technology for generating electricity from the oceans is about to have a commercial application, more than 100 years after the idea was first proposed.

The Indian state of Tamil Nadu has reached an agreement with the US company Sea Solar Power to build a 100-megawatt floating generator. This follows more than 20 years of testing the technology, known as Ocean Thermal Energy Conversion (Otec), at a dozen or so experimental plants around the world.

Nearly three-quarters of the planet is covered in water. Yet, apart from fisheries and mineral extraction on the shallow continental shelves, the potential of this resource has barely been tapped. Some believe that Otec could be applied alongside other deep-ocean water technologies to solve many of the world's energy and food shortages.

An international association to promote commercial applications of these technologies was set up at the Oceanology International conference in Brighton last month.

Otec uses the temperature difference between surface and deep ocean water to generate power. The potential of such a system is greatest in the tropics, where there is a difference of about 20C between the warm surface layer and the dense, cold water at depths of 500m to 1,000m.

Don Lennard, managing director of Ocean Thermal Energy Conversion Systems of Orpington, Kent, says that the technology is straightforward. Warm water from the ocean surface is passed through a heat exchanger, turning liquid ammonia, which has a very low boiling-point, into a gas. This gas drives a turbine to produce electricity, and cold water pumped from the ocean depths then condenses the ammonia gas back to a liquid in a second heat exchanger. The cycle is then repeated.

Such a system is cheap to run, since it has no fuel costs and can operate 24 hours a day. Neither are there any big environmental drawbacks. The main problems, says Mr Lennard, are the high capital costs and the relative inefficiency of the energy conversion. Construction costs are between pounds 6,000 and pounds 10,000 per kilowatt, compared with about pounds 1,000/kw for fossil-fuel stations. The energy conversion efficiency of a normal oil-fired or coal-fired power station is about 35 per cent, compared with only 7.5 per cent for an Otec plant, and two-thirds of the output is needed to pump the water.

Despite these drawbacks, Mr Lennard is convinced the technology can be economic, especially when the hidden environmental costs of using fossil fuels or nuclear power are taken into account. 'We could achieve a payback in eight to ten years,' he says. Once the capital costs are recovered, the plant produces virtually free electricity.

The economics of Otec are much more inviting for government and private investors when systems are installed in parallel with other deep-ocean water applications. Research presented at Brighton showed that the production of fresh water, fish-farming and air-conditioning could help justify Otec investments.

The cold water pumped to the surface by Otec systems is rich in nutrients and could be piped into tanks next to the power station for raising fish and shellfish. Islands or arid regions could use some of the electricity generated by Otec systems to power evaporators to desalinate sea water. And experiments in Hawaii have shown that using cold sea water for air-conditioning systems in buildings can cut electricity costs by 80 per cent.

Although the concept of using thermal energy from the oceans was first proposed in France over a century ago, little progress was made until the oil price shocks in the 1970s prompted a search for cheaper, more reliable sources of energy. Since then, the situation has swung back, as fossil-fuel prices have collapsed due to overproduction. The technology has also been inhibited by the lack of a domestic market in North America and Europe, where ocean conditions are generally unsuitable. 'You could just about do it off Greece,' Mr Lennard says, 'but the water is not really deep enough.'

So far, research in Europe has mainly concentrated on design and feasibility studies. France invested considerable sums until the the mid-1980s. The UK is funding university research and hopes to build on offshore engineering skills developed in the North Sea. Support for further research has recently come from the European Union. Half a dozen pilot plants have been built in Japan and Hawaii.

The most significant of these is a 210kw experimental plant opened last year by the Natural Energy Laboratory in Hawaii. Other proposals on the drawing-board include a 5Mw plant in the Marshall Islands, and a combined electricity and desalination plant in the Virgin Islands.

Feasibility studies in Taiwan have also identified several suitable Otec sites along the country's east coast.

Jih Chang Yang of Taiwan's Industrial Technology Research Institute told the conference in Brighton that Otec could be a big energy source. in the future.

'In addition to conclusively proving Otec's commercial feasibility, we need to draw it to the attention of the world's energy policy makers as an inexhaustible, stable and non-polluting source of energy.'

(Photograph and graphic omitted)