However, a recent reassessment by Duncan [5] indicates that some of these numbers may be too optimistic. We should not be complacent that alternate energy sources can be left for a later generation to develop. The simple conclusion to the supply side of the situation is that, over the next 50 years or so, we should accomplish a major shift to something other than a fossil-fuel-based energy system. The environmental considerations for doing so will be developed a little later. Supply Outlook - Additional Sources The sources for meeting increased energy demand can be categorized as follows: Hydro Nuclear - fission and fusion Terrestrial solar Wind "Modern[21]" biomass Geothermal Ocean energy - tidal, thermal, wave, etc. Solar imported from space The prospects for these various energy sources, looking ahead to the year 2020, have been assessed by a number of researchers [1,6]. A very brief summary of this work indicates the following: • Hydroelectric supply will grow significantly by 2020. This will be an important contribution, but not a replacement for much of the fossil supply • Nuclear fission will increase only moderately by 2020. The most optimistic projections indicate perhaps 50-70%. There is at least equally good rationale to believe that growth will stop and the nuclear contribution will actually decline. Again, this will not be a replacement for much of the fossil supply • Nuclear fusion will not be a factor in the projected time frame • The renewable sources, which contribute less than 2% of world energy now, are projected to contribute about 4% by 2020. Figures 4 and 5 show the present (1990) contributions and those expected in 2020 according to WEC projections. The share which the various renewables contribute to supply will shift markedly over the next few decades. While dominated by biomass, solar and wind are growing very rapidly. Figures 6 and 7 show the present (1990) contributions and those expected in 2020 according to WEC projections. It is important to note, however, that while some of these sources are growing vigorously, they are starting from such a small base that their penetration in the next few decades will be modest.
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