probably fail to meet global requirements by 2050). Although controversial, a potentially major replacement energy source is nuclear power. If nuclear fusion becomes practical, it will present a comparatively clean alternative to the fission reactors now used. As the situation stands now, fission reactors are no longer regarded as attractive power sources. Excepting nuclear fusion, none of these alternative energy sources, as they now are conceived, offers a large enough contribution to offset the enormous need for energy now existing and projected for the future. Contemporary developments in nuclear fusion have been unable to produce energy at rates greater than the energy required to sustain a reaction. While it would seem certain that fusion research must eventually produce successful results, it is not clear how long it will take to be able to meet needs on the scale confronting us now. Even combinations of the various alternative energy systems are not promising for the truly large scale replacement required. Energy use by North Americans alone is now over 300 gigajoules (GJ) per capita per year. 1985 estimates placed total world energy use at approximately 319 exajoules (EJ). A gigajoule is 109 joules; an exajoule is 1018 joules. In somewhat more familiar terms, one EJ is equivalent to 200,800,000 MWhr (megawatt-hours) of electricity or the energy produced from 22,340,000 metric tons of oil. That amount of oil would require 49 of the world's largest oil tankers, the ultra large crude carriers that cannot enter conventional ports and take almost five kilometers to stop. Estimates of future energy demands place annual world needs at 800 to 1000 EJ per year by 2030. Excepting energy created from nuclear fission or fusion, the Earth receives its energy from the Sun. Whether locked up in fossil fuels or temporarily transformed in wind, wave, heat or other energy form, it originally arrived as radiant energy from the Sun. Considering the losses in energy that always take place as one form is changed to another, it is most efficient to extract it for human use as it arrives - in sunlight. A prodigious amount of energy falls on the Earth as sunlight - about 5,600,000 EJ per year. At the distance of the Earth's orbit, a single square meter of surface facing the Sun receives 1,372 watts of energy. Over the years, attempts to use the Sun's energy to power man's works have been limited to small scale projects because of low energy conversion efficiencies. Recently, however, research in photovoltaic energy conversion, the direct conversion of light to electricity by solar cells, has yielded efficiencies of as much as 30 per cent. As production costs are driven down, photovoltaic power should now become practical for many applications. In spite of this success, two considerable difficulties face any plans for large scale development of solar energy generation systems. The first is the large commitment of Earth surface required for solar collectors. The second is the intermittent availability of sunlight, diurnally masked by the Earth itself, and unpredictably interrupted by weather.
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