1998 Year of the Ocean Ocean Energy and Minerals 



Outlook for Development of Renewable Ocean Energy Resources 



The time scale to renew fossil fuels by geochemical processes is long compared to the 

 time in which the modern world can consume known fossil fuel reserves. Consequently, the 

 world's supply of fossil fuels is non-renewable for all practical purposes, and many nations are 

 looking seriously at other alternative energy sources to provide part of their expanding energy 

 needs. Many of the alternative energy sources are derived directly or indirectly from the radiant 

 energy of the sun. The heat content of the world's ocean and the mechanical energy manifested 

 by the various ocean water motions (e.g., surface waves, tides, and mean currents) are among the 

 renewable energy sources available to many countries. This section examines some of these 

 oceanic alternative energy resources. 



One way of using the abundant heat content of the surface ocean is to exploit the 

 temperature difference between the warm surface layer of the ocean and the colder, deeper ocean 

 lying below the penetration depth of sunlight. Engines operate between temperature differences 

 by extracting heat from a higher temperature "reservoir" and ejecting "waste" heat into a lower 

 temperature reservoir. The upper and lower layers of the ocean could function as the reservoirs 

 for an engine that could, for example, power an electric generator. In this fashion, the heat of the 

 surface ocean could be converted into a more usable form of energy (e.g., electricity). 



The mechanical motions of the ocean such as surface waves and mean currents are 

 indirect manifestations of solar energy and, like the heat content of the ocean, constitute a large 

 source of energy potentially available for human use. This likewise applies to the gravitationally 

 driven tidal motion of the sea. Engineers in many countries have developed various devices for 

 generating electricity from these forms of mechanical energy. Specially designed turbines 

 mounted in dams or on moorings can capture some of the energy manifested in elevated sea level 

 or strong currents. Other mechanical devices move under the influence of surface waves and 

 capture some of their energy. All of these modem devices are coupled with electric generators. 



Coastal areas having large amplitude tides and narrow channels or embayments that can 

 be dammed or support moorings with turbines are candidates for practical ocean energy 

 conversion. Devices moored in persistent and strong ocean currents like the Gulf Stream could, 

 in principle, also extract usefiil energy. Other coastal areas where the tidal energy may be small, 

 but where high wave action is common, are candidates for wave energy conversion. 



There are practical problems with mechanical energy converters. Not all coastal areas 

 have the high tidal amplitudes or the frequency of high waves necessary to provide the needed 

 amount of energy for conversion. Tides are periodic with quiescent periods one or two times per 

 day, and high wave activity is not always present even in frequently windy areas. Energy storage 

 thus becomes an important consideration for using some of these devices but also increases 

 operating costs. 



In addition, damming bays and straits significantly alters the circulation of water with 

 possible adverse environmental consequences, and rapidly rotating turbines can kill fish. 



D-11 



