global atmospheric CO2 levels is important. A single HLLV flight will result in the emission of about 3.67 x 10^ t of C02*®^ It is estimated that two 5-GWe satellites can be constructed per year, which would entail about 500 HLLV launches.73 This means that SPS implementation and utilization will result in the emission of 1.835 x 10^ t of CO2 per MWe of system capacity. The conventional combustion of coal for electrical generation will result in an average annual emission of 2.2 x 10^ metric tons of CO2 per MWe of system capacity. Thus, the use of coal will result in CO2 emissions that are two orders of magnitude larger per unit capacity than those associated with the SPS. It appears that the CO2 emitted from HLLV launches will not be significant in comparison to other sources of CO2. 3.4 PHOTOVOLTAIC SYSTEMS 3.4.1 Waste Heat Impacts The waste heat released by any energy facility is defined as the heat released in excess of the amounts that would be released if the facility were not there. For coal and nuclear technologies, this can be easily defined as all of the heat rejected at the plant and its cooling towers. However, an array of photovoltaic cells is utilizing solar energy that would normally strike the ground, and thus it is important to assess the magnitude of the solar energy absorbed at the collector surface and rejected to the atmosphere above and beyond the amount that would occur at a natural surface. Some incoming solar radiation is absorbed by the ground, heating it and the surrounding air, and some is reflected back to the atmosphere. The important parameter that determines how much is absorbed is the albedo (or reflectivity) of the surface. Photovoltaic collector cells will have a much lower albedo than the terrestrial surface and hence absorb more solar radiation. The exact amount of waste heat released by photovoltaic collector cells depends on the difference in albedo between the cells and soil surfaces and on the thermal efficiency of the photovoltaic module. For a typical southwestern United States soil cover and a unit thermal efficiency of 13%, the waste heat rejected per unit energy produced has been estimated as 1.5 MWt-yr/MWe-yr.82 This results in an equivalent waste heat density of about 40 W/m^, which is several times larger than is expected from a SPS rectenna but of a comparable magnitude to releases from major urban areas.
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