trial solar energy, geothermal and tidal energy, but there are only three new concepts visible at present that could potentially produce larger quantities of energy, i.e., 20% or more of our demand. These are the fast-breeder reactor, thermonuclear fusion, and the satellite power system. The lack of technical data on these systems does not allow us to make a rational assessment of the economic, environmental, and health aspects of any of these concepts at this moment and many more technical and economical data are needed to compare the benefits and penalties of the three concepts in a European context. The potential economic benefits appear to be large enough, however, to justify a continuation of the present investigations or, in the case of the SPS, to initiate more detailed theoretical and experimental investigations in order to collect the data necessary for a comparison of the alternatives. Only independent studies and the associated research will enable Europe to assess the viability of the concept for Europe and to build a solid basis for becoming a strong partner in a multinational development programme if the SPS concept should later be generally accepted as a viable large scale energy source. Acknowledgements — The author wishes to thank D. Kassing and J. Ruth for many useful discussions. REFERENCES I. The Final Proceedings of the Solar Power Satellite Program Review, 22-25 April 1980. Lincoln, Nebraska. 2. K.K. Reinhartz, An Overview on European SPS — Activities, in Ref. 1, pp 78-87. 3. DOE, SPS Concept Development and Evaluation Program — Preliminary Assessment. September 1979, DOE/ER 0041. 4. T.J. Connolly, G. Dantzig, and S.C. Parikh, The Stanford Pilot Energy/Economic Models, Stanford University, California. 1977. 5. Commission of the European Communities, Brussels, 1978, Report COM(78)613. 6. Conservation Commission of the World Energy Conference 1978. World Energy Demand to 2020, April 1978. 7. R.O. Piland, Reference System Characterisation and Cost Review, DOE/NASA SPS Program Review, Lincoln. Nebraska, 22-25 April 1980. 8. Round Table Discussion on Space Power Generation for Europe?. Proceedings of a European Symposium on Photovoltaic Generators in Space, Noordwijk. The Netherlands, 11-13 September 1978, ESA SP-140. 9. P. Rappaport, Overview of Photovoltaics, 14th IEEE Photovoltaic Specialists Conference, 7-10 January 1980, San Diego, California. 10. F.C. Schwenk, The SPS Concept — An Overview of Status and Outlook, Proceedings of the 15th IECEC. Seattle, Washington. August 1980.
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