PETER BOND'S 'Quest to harness power of the deep' (Business, 17 April) quotes a 7.5 per cent efficiency for the Ocean Thermal Energy Conversion (Otec) plant. This 7.5 per cent is the maximum theoretical or 'Carnot' efficiency. If a comparable Carnot efficiency calculation is performed for a coal-fired power station, the figure is approximately 55 per cent. As coal-fired power station have efficiencies in practice of 35 per cent (due to unavoidable losses in the real world of engineering) the Otec plant could be expected, on a pro rata basis, to have an actual efficiency of only 4.7 per cent.

As two-thirds of the energy output is needed to pump the sea water around the plant, the net efficiency could be as low as 1.5 per cent. This does not leave much room for error or for recovering the substantial capital costs of such a project.

Professor Andrew Porteous

The Open University

Milton Keynes