Renewable Systems Although it should be apparent that the potential demand is such that all sources of energy will be welcomed, we still need to address the concerns of proponents of various renewable energy technologies. It seems that although the world needs so much energy in so many different forms that all supplies would be welcomed, each technology must fight for its place. Power relay platforms and solar power satellites transmit or provide baseload energy. This is a global market that cannot be easily filled by intermittent "peak shaving" supplies from wind and terrestrial photovoltaics. As population continues to grow and human behavior and activities continue to increase the stress on our biosphere the question of biomass derived fuels will focus on the issues of food vs. fuel, and biodiversity of natural habitats and the threat of species extinction vs. the mono-cultures needed for efficient production of food. The first contender is solar energy and while solar cookers may seem neat to alternative energy advocates in industrialized nations with power on demand they are flawed in that they require users to drastically alter their behavior for very little return. In the long run the forcing functions described earlier may result in change but in the meantime people will, given no other alternatives, continue to denude their lands, creating deserts, in order to be able to cook in the evening. Also let us not forget that fire was the first energy mankind tamed and there is a long association with fire as a social function. In developing a level playing field we need to develop some basic guidelines. For instance energy from space or relayed energy has to cost less than about 6 times the cost of terrestrial photovoltaics (TPV). A simple estimate can be derived by assuming 8 hours of sunlight. Then a TPV system to supply baseload power needs to have 3x the area of solar cells plus 2x the capacity in battery or hydrogen energy storage and another equivalent array is thrown in to account for power conditioning and controls. Note that this simple approximation does not include any allowance for cloudy days, etc. A thousand megawatt TPV plant based on the above assumptions and with a module conversion efficiency of 12% would cover 25 million square meters. In other words it would be 5 kilometers by 5 kilometers or 3 miles by 3 miles. An environmental issue that is not generally considered by advocates of baseload TPV plants is the consequence of changing the albedo of the land over such a broad expanse. Energy density and conversion efficiencies are also important issues for comparison. Sunlight has an energy density of approximately 1 kilowatt per square meter or 100 milliwatts per square centimeter (0.1 w/cm^). With a current energy conversion efficiency of 12 % the result is 120 watts of electrical energy per sq. meter. The energy density of the microwave beam is 23 milliwatts per sq. cm ( 230 watts per sq. meter) and the rectenna conversion efficiency is approximately 80 %. Thus a square meter of rectenna yields 184 watts 24 hours per day. On a twenty four hour basis
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