In the previous sections, it was pointed out that in order to avoid greenhouse warming, a one percent per year reduction of CO2 emissions will have to be put in place almost immediately. This reduction in CO2 emissions translates into a cap on the amount of fossil fuel combustion permissible, and this is shown in Figure 8 (the curve is labelled allowable fossil fuel use). Also shown in Figure 8 is the total fossil fuel demand that would be necessary to fuel the economy of the world. When these two curves are compared, a shortfall in energy becomes apparent and this shortfall will have to be made up for with the use of non-fossil fuel energy sources. Non-Fossil Energy Alternatives The alternative energy sources to fossil fuel combustion considered in this study are terrestrial solar photovoltaics (PV), nuclear fission, nuclear fusion, and solar power satellites (SPS). Solar energy available at the Earth's surface, when averaged over the day/night cycle, the seasons, the different latitudes, the 50% average cloud cover of the Earth, and even some attenuation in clear air, is perhaps one-tenth of that available in space. Great progress in conversion efficiency and costs of solar cell modules has been made in recent years. The highest efficiency achieved by a non-concentrating experimental cell was 30%,14 while 10 to 12% efficiency for large operational modules seems to be typical. The cost of electricity generated by photovoltaics stands at 30 cents per kWh, about a factor of five more than the cost of power generated by conventional utilities. The major problem of terrestrial PV is not the installed cost of the solar cell modules but storage cost and the intermittent nature of solar insolation. The storage battery that is most preferable for terrestrial photovoltaics is a lead-acid battery designed for deep discharge,15 and at this point is the best technology available. Costs of the lead-acid battery are almost 400 times the average price of the solar cells per kWh. Furthermore, the best of these are limited to about 1500 charge-discharge cycles. Consequently, battery lifetime is less than the lifetime of the PV modules themselves requiring that they (the batteries) be replaced approximately every four years. Since these batteries are expensive to begin with, it is likely that energy storage is the economic bottleneck for terrestrial solar energy. Hubbard16 points out that a major problem with PV is the fluctuations in the power output due to sudden changes in the cloud cover. Furthermore, during overcast sky conditions and the night, there would be no power produced, and these problems, Hubbard feels, might lead to problems in the interfacing of PV with the electrical grids. Nuclear fission power has the advantage of being a well-developed technology that is already being used. However, a great deal of controversy remains as to the safety of nuclear power plants. Waste disposal as well as security concerns about the production and shipping of ever-increasing amounts of fissionable material are also issues that remain to be solved. Most of the fissile material that is used in conventional nuclear reactors is an oxide of 235U. The abundance of 235U in natural uranium is about 0.7 % and the rest is mainly 238U. Thus it is apparent that with a limited supply, conventional reactors (henceforth referred to as burners) cannot be utilized to make up the shortfall of energy and hence, breeder reactors will have to
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