one of my facebook friends, send me this (winning!) essay about harnessing a very powerful energy source from the sea. I had never heard about this before.

This is exactly how I would like my website to work, sharing ideas and getting smarter together.

Following is the essay:

“Energy from the Oceans 

“I get everything from the ocean. It produces electricity, and electricity supplies the Nautilus with light – in a word, with life.” Jules Verne, Twenty Thousand Leagues under the Sea, 1870. 

The oceans are the world’s largest solar energy collector. Every day, 60 million square kilometres of tropical seas absorb one quadrillion megajoules of solar energy, which is equivalent to 170 billion barrels of oil. This is more than 1000 times greater than the current total global energy consumption. The oceans are a vast renewable resource and if this energy could be tapped, not only would it provide huge amounts of electrical power, but it would also reduce the amount of fossil fuel which is burnt, and so help mitigate the disastrous effects of climate change. 

Ocean thermal energy conversion (OTEC) provides a way of harnessing this power. In tropical and sub-tropical regions, the upper surface water is warmed by the sun and mixed to a depth of about 50 or 100 metres by wave motion. Deeper in the ocean, beyond about 500 to 1000 metres, the water has flowed from the Polar Regions and is very cold. This temperature difference can be used to generate electricity in the following way. In one arrangement warm water from the ocean surface is introduced into a low-pressure container where it boils and the steam is used to drive a turbine and generate electricity. In another system warm water from the surface is passed through a heat exchanger which vaporises, for example, ammonia, and the expansion is used to drive a turbine. Cold water is pumped through a second heat exchanger to condense the ammonia. 

OTEC has many advantages over other sources of energy. OTEC does not produce carbon dioxide or radioactive waste. The amount of energy available is vast; none of the other alternatives to fossil fuel can even come close to the magnitude of the OTEC resource. And, unlike wind or wave power, OTEC offers energy in constant supply, available day and night regardless of the weather and with only a small seasonal variation. 

Unlike other clean energy technologies, OTEC has some useful by-products. OTEC plants can be used to produce fresh water from seawater. When the warm surface water is vaporised under low pressure the salt is left behind and the condensed steam is almost pure water. The desalination produced by OTEC in this way is effectively free. 

To run an OTEC generating plant, cool deep water has to be pumped to the surface where it can be put to other uses, such as air conditioning. Or the cold water can be used in so-called chilled-soil agriculture. Food crops which can only grow in temperate regions can be successfully grown in tropical regions if the soil is cooled by passing cold seawater through underground pipes. The United States Natural Energy Laboratory has a test facility in Hawaii, where 100 different fruits and vegetables, mostly from temperate climates, are growing in chilled soil. 

For an OTEC plant to work efficiently there must be a temperature difference of more than 20°C between the surface water and the cool deep water and this is only found in tropical and sub-tropical oceans. However, in this zone are the southern states of the USA and parts of Australia, as well as many developing nations including India which is increasingly using more fossil fuel to generate electricity. Tropical islands would particularly benefit from OTEC, because oil-fired power is expensive and there is usually a need for desalinated water. 

OTEC could be used by the highly developed countries which lie in the world’s temperate zones by means of grazing generating plants. These would slowly drift through the warm tropical and sub-tropical oceans, where they generate electricity which is used to split sea water into hydrogen and oxygen. The hydrogen, and possibly the oxygen too, would be picked up by transport vessels and shipped back to the developing countries for use as fuel. 

The idea of Ocean Thermal Energy Conversion is not new. In 1881, the French physicist, Jacques-Arsene d’Arsonval, suggested generating electricity using the temperature gradients in the tropical oceans. In 1930 one of d’Arsonval’s students constructed the first ever OTEC plant in Cuba, which produced 22 kW of electricity. In 1979 the United States Natural Energy Laboratory constructed an OTEC system on a vessel moored off the coast of Hawaii, which produced enough electricity to power the ship’s lights, computers and television. Then in 1999 they tested a 250 kW closed-cycle OTEC plant. Since then the USA has ceased work on OTEC and interest has waned and the technology has apparently been overlooked or forgotten. 

There is no denying that the construction of an OTEC plant faces some major obstacles. The actual amount of energy which can be produced per cubic metre of seawater is fairly small. Therefore enormous quantities of water have to be processed, and construction of an OTEC plant will be a huge engineering project. It has been estimated that a 100 megawatt unit would require an intake tube 11 metres in diameter and pumps of a similar scale. No doubt this has put off potential investors. 

In the past the development of OTEC has been further deterred by the ready availability of cheap fossil fuel. However, we now live in different times. The economics of energy production are changing rapidly. The cost of fossil fuels continues to increase and today we have to take account of the associated environmental costs as we face the catastrophic changes in the climate of the Earth caused by burning fossil fuels. 

There are questions about OTEC that cannot be answered without further development and testing. What is required now is a mid-size demonstration plant to give some confidence to the funding agencies in this neglected energy technology. Some experts believe that OTEC could be competitive with other clean energy technologies such as wind power, but without the test facility it is impossible to be certain. A number of clean energy sources are currently under development around the world, yet little attention is being paid to one completely untapped source with the potential to become a giant energy supply. 

As Rick Dworsky wrote in the Energy Bulletin “OTEC is a technology of oceanic magnitude. To ameliorate the enormous problems of global warming, peak oil, fresh water and food supplies, we are going to need proportionally large solutions. OTEC may be one of our best hopes…. In combination with other renewable sources, efficiency gains, conservation and adequate voluntary population management, we may be able to maintain a semblance of world civilisation.” 

Chris Holt 

chrisholt@fastmail.fm

This essay won first prize in a competition organised by the Institute of Physics and was published in Physics Education”