burning about 100 tons of coal each year. The large pile of coal in Fig. 3 is scaled to 3000 tons which corresponds to a 30 year energy output delivered to Earth by an advanced section of solar panel. Alternatively, with fully developed SPS technology two small panel sections like “D” (Fig. 7) could supply approximately 10 kw of power. Space power systems can supply more than sufficient power to mine the common rocks of Earth for the non-carbon component of Demandite. Demandite would likely be redefined as carbon is used far more extensively in production of high value plastics and composites rather than burned. Power from space could build rather than deplete Earth resources. For example, iron scrap is a higher grade resource than taconite (7). A flourishing tree or even sawdust are generally higher grade resources than wood ash. Power from space can produce the scrap iron from taconite and displace the trees as power sources with no readily identifiable depletion of Earth resources. SPS ground-mass per person need consist of no more than 2000 kg of concrete and steel supports (about the mass of a car and much simpler) and 2 kg of antennas and associated electronic/electric equipment. SPS systems are presently conceived to receive approximately 2 GW of power per square kilometer of receiver area. The line under the small pile of coal in Fig. 2 depicts the length of the side of the per capita area for receiving 10 kW of useful power (5 m2). Laser energy reception equipment could be far more compact. There are other options. Creation of a new type of basic power system to supply world scale levels of power would be an extremely massive undertaking. We have seen that on a per capita basis SPS appears to be an efficient use of mass. Table 6 compares several operating and construction features of power systems proposed to supply major fractions of the world's energy needs in the 21st century. Reviews are available of major power flows and energy reserves of Earth and their relations to projected rates of power usage (20, 21, 51, 54). Table 6 concentrates on the larger scale systems denoted in column A appropriate to a world population of 6 E+9 people using 10 kW each. Column B indicates the mass transport for both fuel and tailings or Earth-area affected by collection or processing (solar energy). Column C contains estimates of the mass of the facility(ies) per unit of produced power (tons/GW). This number should be as low as possible assuming complexity of construction does not increase quickly as tons/GW decreases. Column D gives the total facilities mass estimated to input power to the appropriate distribution systems. The facilities production rate (tons/year) is simply the total in column D divided by 30 years. This corresponds to the installation and maintenance rate of the power system for a steady state world consumption of 60,000 GW. We aim at the power level that could create a materially rich world population. Why aim for less? We note the preferable power system should require: the least facility (capital) mass if it can be provided at a reasonable cost; the minimal annual mass disturbance (mining, burning, burial, recycling, etc.); the least use of Earth surface area; and should preferably contribute to control over and nurturing of the biosphere rather than intense husbandry activities. The reader is encouraged to examine Table 6 in light of these suggested power source characteristics. Several points are worth noting. Both the coal (1) and biomass (2) systems would involve carbon manipulation on the scale of the biosphere carbon cycle (55,56). Coal use would perturb the biosphere carbon cycle over a 200-year period (16). We do not currently know the implications of introducing such large quantities of fossil carbon as CO2. We are affecting a major component of the atmosphere of the Earth, not a trace component.
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