Using the Moon The Moon has been proposed quite often as a source for the materials needed for a Earth-orbiting SPS. Due to the high percentage of silicon (20 % in weight) and aluminum (around 10 % in weight in the lunar crust) the on Moon construction of solar panels with lunar materials is technically feasible. The lower gravity on the Moon's surface results in a much lower delta V to reach a Earth orbit and hence significantly lower transportation effort compared to Earth-to- orbit transportation. When considering the energetic amortization time the effect is not of great importance, because the energy needed for transportation is only in the order of 20 %, whereas the big portion required for the solar cell production (ca. 65 %) is not affected when using lunar materials. But there is a big difference concerning the Earth's energy balance. When producing solar cells on Earth we consume energy there which we get back some months after the SPS is operational. In the case of lunar cell production, the energy is invested on the Moon, provided by lunar based power systems, and consequently the net energy flow to the Earth is increased. All this assumes an existing infrastructure on the Moon. Although a lunar based solar cell production seems even further in the future than a SPS, from a energetics point of view this concept is an interesting option. Another possibility is to install a lunar based power system, mainly made out of lunar materials, and transfer the energy to Earth by electromagnetic means. This concept decreases the required transportation energy to a minimum but suffers from the more than 10 times longer distance (compared to GEO) to the Earth's surface which would result in extremely large microwave antennas and/or rectennas or would require the less efficient laser transmission. Comparison with Terrestrial Solar Power Plants For a reference terrestrial solar power plant with mono-crystalline Silicon cells the following assumptions have been made: • solar cell efficiency: 15 % • average solar flux: 1 kW/m2 • average sun hours: 1000 h per year
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