location [1]. This hydropower can generate electricity at far lower unit cost than can fossil fuel or nuclear powerplants. This is offset by the necessity of exploiting these resources where they lie and of transporting the energy to its point of use. Various studies have shown that it would take decades to overcome these difficulties, so for now much of the world's available hydropower is not exploited. Using conventional technology, only a few of the largest and most economical of these resources can be tapped in the foreseeable future. The total energy available in exploitable hydropower is about 5.76 E 18 joules per year, equivalent to the average output of 2,500 gigawatt sized nuclear powerplants. The gaseous effluent of fossil-fuel-powered generating stations with this capacity would be stupendous: □ About 10 E 12 cubic metres of carbon dioxide per year; □ 5 to 10 E 9 cubic metres of sulphur oxides and nitrogen oxides per year. This quantity of combustion gases would have enormous impact on the earth through global greenhouse warming, regional rain acidification, and air pollution in relatively large areas near the powerplants. Hydroelectric and nuclear power generation do not pollute the atmosphere with these gases. Since this idea was first broached in 1974, very little progress on it has been made. We must determine how to reduce progressively the time required to exploit hydroelectric resources since implementation times traditionally increase as the powerplants become farther removed from the centres of consumption. Fig. 1 summarizes the global hydroelectric potential broken down by continent. The total hydroelectric potential of each continent is further broken down by whether it is currently being exploited, will be exploited by projects currently planned or under construction, or which exists only as a potential resource.
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