http://apps1.eere.energy.gov/solar/cfm/faqs/third_level.cfm/name=Photovoltaics/cat=Applications#Q50…
Q: How much space would be needed for photovoltaic systems to meet the entire electrical needs of the United States?
A: If PV were a primary energy source, what would the world look like? Would PV collectors cover every square inch of available land? Contrary to some popular notions, the landscape of a world relying on PV would be almost indistinguishable from the landscape we know today. The impact of PV on the landscape would be low, for three reasons. First, PV systems have siting advantages over other technologies; for example, PV can be put on roofs and can even be an integral part of a building, such as a skylight. Second, even ground-mounted PV collectors are efficient from the perspective of land use. Third, adequate sunlight is ubiquitous and often abundant, and present in predictable amounts almost everywhere. As we move away from fossil-fuel energy, PV will become important because of its land-use advantages:
PV's low-impact siting for flat-plate systems. In the United States, cities and residences cover about 140 million acres of land. We could supply every kilowatt-hour of our nation's current energy requirements simply by applying PV to 7% of this area—on roofs, on parking lots, along highway walls, on the sides of buildings, and in other dual-use scenarios. We wouldn't have to appropriate a single acre of new land to make PV our primary energy source!
PV's efficient ratio of produced energy to land use. Even if it isn't installed on rooftops, flat-plate PV technology is the most land-efficient means to produce renewable energy.
PV has a competitive conversion efficiency, a high capacity factor, and can be "packed" densely in a given area. We still wouldn't have a land use issue, even if we didn't use roofs for PV. We would need only 10 million acres of land — only four-tenths of one percent of the area of the United States — to supply all of our nation's energy using PV. Is that a lot of land? Not for something as important as producing electricity, and not in comparison to some of the other ways we use land.
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http://apps1.eere.energy.gov/solar/cfm/faqs/third_level.cfm/name=Concentrating%20Solar%20Power/cat=Applications#Q84…
Q: Do concentrating solar power (CSP) plants require a lot of land? How much, exactly?
A: Relatively speaking, no. Consider Hoover Dam, for example. Nevada's Lake Mead, which is home to the dam, covers nearly 250 square miles. In contrast, a CSP system occupying only 10 to 20 square miles could generate as much power annually as Hoover Dam did in one recent year. And if we take into consideration the amount of land required for mining, CSP plants also require less land than coal-fired power plants do.
It's hard to say exactly how much land is required for a CSP plant, however, because this depends on its generating capacity and the particular technology used. For example, a 250-kilowatt plant composed of ten 25-kilowatt dish/engine systems requires less than an acre of land. And a parabolic trough system uses about 5 acres for each megawatt of installed capacity. But in any case, the solar resource needed to generate power using CSP systems is quite plentiful. Imagine being able to generate enough electric power for the entire country by covering about 9 percent of Nevada — a plot of land 100 miles on a side — with parabolic trough systems!
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