Table 4.38 Summary of Energy Balance Data for each system is not sensitive to the gross, operating, or lifetime calculations. The conversion efficiency of the system dominates the result. The operating ratio calculation shows that the solar energy systems require substantially less operating energy per unit of electrical output. However, the lifetime ratio calculation shows that the intensive capital investment reduces their energy ratios significantly. The payback period for all systems except SPS/Si and terrestrial photovoltaic is less than 1.5 years. In both of these cases, sensitivity analysis shows 'that long payback is dominated by the energy intensity of silicon production. A number of possibilities, including reduction in the overall energy requirements of cell production, the use of solar-generated electricity in place of conventional electrical power for cell manufacture, increased cell lifetime, and decreased silicon requirements could reduce the SPS/Si payback period to a level comparable to the other systems (i.e., one to two years); the best combination of conditions for the terrestrial photovoltaic system would still result in a payback of about six years.1$$ Comparison to Other Studies. The comparison of these SPS results with those of previous studies is of particular interest. Direct comparison is difficult because each study is based on a different reference design and methodology. Some approximate comparisons can be made however, noting the major sources of difference.
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