Here's a quote about nanotech based solar energy from Unbounding the
Future by Drexler et al, at
http://www.foresight.org/UTF/Unbound_LBW/chapt_7.html:
External walls, roofs, and paving surfaces are exposed to sunlight, and
sunlight carries energy. A proven ability of molecular machinery is the
conversion of sunlight to stored energy: plants do it every day. Even
now, we can make solar cells that convert sunlight into electricity
at efficiencies of 30 percent or so. Molecular manufacturing could
not only make solar cells much cheaper, but could also make them
tiny enough to be incorporated into the mobile building blocks of a
smart paint.
To be efficient, this paint would have to be dark-that is, would have
to absorb a lot of light. Black would be best, but even light colors
could generate some power, and efficiency isn't everything. Once
the paint was applied, its building blocks would plug together to
pool their electrical power and deliver it through some standard
plug. A thicker, tougher form of this sort of material could be used
to resurface pavement, generate power, and transmit it over large
distances. Since smart solar-cell pavement could be designed for
improved traction and a similar roofing material could be designed
for amazing leak-resistance, the stuff should be popular.
On a sunny day, an area just a few paces on a side would generate
a kilowatt of electrical power. With good batteries (and enough
repaved roads and solar-cell roofing), present demands for electrical
power could be met with no coal burning, no oil imports, no nuclear
power, no hydroelectric dams, and no land taken over for solar power
generation plants.
Another page I found calculates how much area would be needed by solar
collectors using present-day efficiencies to collect enough energy to
equal all the energy usage of the United States. (Of course this is
only good for comparison purposes, since not all energy uses could be
converted easily to electrical power.)
http://www.nmsea.org/Curriculum/7_12/The_Solar_Resource.htm:
We find that we would need an area of 1.29 x 10^4 square miles, which
is an area 100 miles by 129 miles, to completely power the US.
That's a very modest size. Solar energy has many disadvantages, but
it is a mistake to suggest that a substantial part of the country would
have to be covered with solar cells in order to generate enough energy.
Hal
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