Figure 10 illustrates the energetic amortization time for the considered options, each for conservative and optimistic assumptions. All systems are very sensitive to the size of the SPS, i.e. to the design electrical power output which is fed to the electrical net on Earth. This effect is illustrated in figure 11 for the mono-crystalline Si photovoltaic option. The reason for this sensitivity is that microwave antenna and rectenna are not scaleable with good performance if the net electrical power output is less than 2 GW. All these estimations were based on an assumed transmission efficiency of 63 % which, according to several studies [2], [3], is realistic for the midterm future for microwave transmission. For laser transmission the efficiency which seems to be achievable is in the order of 20 %. Additionally, the receiving station on Earth, although being smaller in size due to the less diverging laser beam, requires more power for production and installation. For these reasons a SPS using laser beam transmission needs 4 to 10 times longer to recover the energy which was invested in it compared to the microwave transmission option. Therefore, the laser beam transmission is not considered any further in this paper.
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