use issues in comparison number one suggest that terrestrial PV will be unable to provide for the needs of regions like large cities. As in a wide range of investments in hardware, the duty cycle of the hardware can completely drive its economic viability. SSP has everything over terrestrial PV on this count. The SSP can be selling power 24 hours per day, to someone. This gives it a 4:1 advantage immediately. The problem of storage and weather compounds the advantage of SSP to at least 12:1. This means that if we can install an SSP for the same cost as terrestrial PV + storage, we can beat most other forms of power generation. The difficulty is that terrestrial PV will still have the advantage of incremental installation, to a higher degree than SSP. This advantage is only good up to a point, which probably cuts off at the 100 MW level Conclusions • The trade-off between terrestrial solar power and space solar power is primarily a trade-off of storage of energy versus continuous power. Space solar power can supply continuous power, while terrestrial solar power plants will require elaborate storage facilities. • Storage options range from pumped/reservoirs, which are large, require large areas of land, and significant amounts of maintenance to batteries, which are very expensive and short-lived. Batteries also present other environmental problems related to release of lead. • Solar hydrogen and fuel cells seem to have the best long term promise, but will require development (and perhaps development costs) on the same order as space solar power. • Where the energy needs are greater than 100 MWh/sq. km., it will not be possible for local terrestrial PV to meet the load. This is due to the fundamental limitations in PV efficiency and the amount of sunlight per day. High-efficiency (>25%) PV which is manufacturable for a reasonable cost is still years away. • High energy needs areas such as large cities (New York City) and industrializing regions (industrial regions of India and China) will be ideal markets for space solar power, while terrestrial PV can readily serve residential and rural markets. • Both space solar power and solar hydrogen/fuel cells should be given priority for development.
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