Direct Use of Solar Energy

All direct uses of solar energy for electricity generation suffer from the "dilute" nature of the solar source. The average flux of solar energy at the surface of the Earth is about 200 W/m2. Thus, it requires about 5 km2 to collect 1 GW of incident solar energy. The area required for electricity generation depends on the efficiency of conversion from solar energy to electricity.

One potential source of electricity is biomass, used as a fuel in a steam turbine plant. The main source of biomass now used in electricity generation is wastes, including wastes from the forest product industry. However, the amounts of such wastes are limited. A major increase in biomass use for electricity generation would require dedicated biomass plantations and adequate supplies of water and fertilizer. As estimated by David Hall and colleagues, the "practical maximum yields" of biomass in temperate climates corresponds to an annual average efficiency of about 1% for conversion from solar energy to chemical energy in the plants [9, p. 600]. The thermal efficiency for biomass combustion is unlikely to reach the 33% efficiency achieved for coal, and some of the plantation area must be used for nonproductive purposes such as roads. Thus, an optimistic estimate—probably unrealistically optimistic—of the area required for biomass production of electricity is 2000 km2/GWe. More typical estimates are roughly a factor of 2 higher, and some are still higher.4 In any estimate, however, the demands on land and water are high.

Solar photovoltaic power is suitable for use in remote locations, but, at present, it is too expensive to be a candidate for supplying large amounts of power to the electric grid. If the cost is eventually reduced to an acceptable level, the land requirement would be substantial, but much less than for

4 For example, an estimate made by Eric Larson for a large plantation in Brazil assumes a yield of 1000 dry tonnes of biomass/km2, 20 GJ/tonne, and a 60% use of land. This corresponds to 1.2 x 1013 J/km2 of thermal energy per year or an average electric output of about 0.12 MWe/km2, which translates to 8000 km2/GWe [10, p. 579]. In Global Energy Perspectives, projections of future biomass energy yields of 4-10 toe (tonnes oil equivalent) per hectare are indicated, corresponding to roughly 2000-6000 km2/GWe at a 33% thermal conversion efficiency [11, p. 82].

biomass. It would depend on the solar flux at the site chosen, the efficiency of the photovoltaic cells and associated electronics, the orientation of the solar panels with respect to the sun (including possible tracking of the sun), and the fraction of the land occupied by the panels. If one assumes an overall module efficiency (i.e., for conversion of sunlight to usable electricity) of 10%, an average solar flux of 250 W/m2, and a 50% coverage of the ground by the solar modules, then the area required would be 80 km2/GWe. This is only a crude, order-of-magnitude estimate and future systems may be more efficient. However, this may not be an appropriate way of looking at the matter. For the near future, solar photovoltaics are most likely to be used in niche markets, extending down to arrays as small as rooftop modules for individual homes, rather than in very large arrays. Thus, the area occupied may not be an immediate concern.

Neither of these direct sources, or other less conspicuous candidates, should be excluded from consideration, but among renewable sources of electrical power, they at present appear less promising than wind (see later subsection). This is reflected in the differences in the rates at which new facilities are being installed.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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