supplier by not having to add expensive capacity. This is a most promising development. However, at a political level, there is frequently a disconnect between rational energy policy and other policies. For example, veiy large users of electricity are frequently given preferential rates which do not induce efficiency. There are encouraging signs at the level of national governments that external effects of energy acquisition and use are being taken seriously. At the United Nations Conference on Economic Development and the Environment (Rio ’92) a number of nations agreed to limit their CO2 emissions. Other nations have undertaken such a commitment since Rio. Just how these commitments will be fulfilled is not clear. The typical goal, which is to limit carbon emissions in the year 2000 to the levels of 1990, is very optimistic. Various countries are experimenting with policy and tax measures to reflect external and environmental energy costs [7,8], In Denmark, for example, subsidies are provided for certain renewables. In over half of the states in the U.S.A., electric utilities either include environmental costs [24] in their planning for new generating capacity or plan to incorporate such costs. Some countries have instituted a carbon tax and others plan to do so. Such taxes have been seriously proposed for all the member nations of the Organization for Economic Cooperation and Development (OECD) and discussed at a global level in the World Energy Congress. There are concerns over the effectiveness of such taxes and the extent to which countries which impose them will be at an economic disadvantage to those which do not. It will be some time before agreement and general action results. However, the steps taken to date are promising, if only as indications of serious concerns. Implications for Power from Space The data presented in the previous sections indicates that renewable resources as a class are not at a disadvantage in comparison to traditional sources if all costs of acquisition and utilization of energy are accounted for. The next step in the analysis should be to explore whether solar power from space can also compete along with the other renewables, particularly terrestrial solar. Understandably, figures for the probable cost of solar energy imported from space are not as well based as for current sources. However, several analyses have been done for the cost of imported energy, starting with the U.S. Department of Energy/NASA study carried out in the late 1970s and early 1980s [15], This study made low, nominal, and high estimates of capital costs for the so-called Solar Power Satellite (SPS) "Reference System". Nominal estimates indicated costs some three times higher than the corresponding costs of fossil and nuclear plants. The cost of delivered power was asserted to be competitive with conventional plants of the time, approximately 5 cents/kWh for base load power. Work by these agencies was discontinued following an evaluation by the National Academy of Sciences which stated that launch costs were likely to be higher than estimated by NASA and that space solar power systems should be studied rather than implemented at that time.
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