Using the same assumptions as have been made for the SPS concerning the solar cell production and considering the required structure elements for the terrestrial system the energetic amortization time results to 42 - 86 months. This is considerably longer than the EAT for a GEO SPS but is much shorter than for a SPS operating in LEO. Figure 13 shows the sensitivity of the EAT of a terrestrial photovoltaic power plant with regard to the yearly average sun hours. Consequently, the EAT is highly dependent on the location (e.g. latitude) of the solar power plant, figure 14 summarizes the results of the comparison of space based and terrestrial solar power plants. According to [2] the terrestrial solar dynamic system reaches an EAT of 6.3 months. The reason that the dynamic option is more advantageous on Earth is its high specific mass compared to advanced solar panels. Summary and Conclusions The paper considered the net energy balance or energetic amortization time of various space-based solar power systems with the purpose to deliver electricity into a terrestrial net. Feasibility and economical aspects were not discussed. For a SPS which operates in GEO using solar cells to convert the solar radiation into electricity and which beams the energy down to a receiving station on Earth by microwaves the net energy balance becomes positive within 1-2 years after starting its operation in GEO. Assuming a slow but efficient transport from LEO to GEO by electric
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