California such a system might be very profitable since the by-product, water, would be essentially free. We can calculate what the water flow would be for a one megawatt electric fuel cell substation. Operating data on the General Motors Electrovan taken from Appleby and Foulkes 1989, showed a hydrogen flow of 45 grams per kW-hr. This is 44.6 grammoles of hydrogen. Two gram-moles of hydrogen reacts with one gram-mole of oxygen to produce one gram-mole of water. Therefore a fuel cell operating at 100 % efficiency will produce 22.6 gram-moles of water per kW-hr or 0.406 liters per kW-hr. Consequently a 1 MW(e) fuel cell substation would produce 406 liters or 107 gallons of fresh water per hour per 1,000 kW-hr's. The sale of this by-product might be enough to compensate for the re-conversion losses in the fuel cell thus making a energy from space/hydrogen/fuel cell system more cost effective and environmentally compatible with dense urban population centers located in arid areas such as San Diego. Table 1, taken from "A Hydrogen Economy" lists the cost of electrolytic hydrogen as a function of the cost of electricity. The costs are in 1973 dollars. The coefficients are a measure of plant efficiency. Table I Cost of electrolytic hydrogen in 1973 $ from page 9 of Gregory, 1973. Cost estimates indicate that the cost of electricity accounts for over 50% of the cost of electrolytic hydrogen delivered to a consumer located some 1,600 km away from the production site. Table 2 in 1990 dollars tabulates the cost of electrolytic hydrogen using terrestrial photovoltaics, i.e. 8 hours of operation. Ogden and Williams, 1990, reported on electrolytic hydrogen from thin-film solar cells. Their cost data indicates electrolytic hydrogen produced from DC electricity costing $0,035 kW-hr and which was obtained from 12% efficient cells would cost $14.00/GJ. This yields a product cost to power cost coefficient of 400 which can be considered a measure of cost for the balance of plant. A low coefficient indicates a low plant cost.
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