[extropy-chat] Nanoengineered terrestrial solar vs. nanoengineered space solar power

[Robert Bradbury]

On 4/7/07, Eugen Leitl <eugen at leitl.org> wrote:
>
>
> > Do you assume SPS based on photovoltaics? Is that really a good model?
> > What about using sunlight to heat a working fluid for a generator?
>
> Bad idea. With PV you have high efficiency and potential separation
> in a semiconductor, which is one step removed from reradiating this
> towards a particular space segment, where your rectenna is.


This may be one of the few times that I disagree with Eugen.  Even with
quadruple layer cells (and boy are those going to be pricey unless you are
producing many square km of them) you are probably only going to be able to
get  maybe 60-65% efficiencies.  The problem is that you lose the UV energy
and the far IR energy.  The only way to recover those is something like a
system of dielectric mirrors [1] and/or dichroic filters [2] which
efficiently split the light so it can be directed at systems optimized for
dealing with photons with specific energies.  You have to remember that UV
photons have enough energy to break atomic bonds and IR photons, especially
far IR, can't do much more than make the atom vibrate a bit (you have to
have a system for adding the energy of a number of IR photons to get
something that can free up an electron).  There may be a reasonable argument
that "heat engines" are the best way to harvest the IR photons.  But Eugen
is right in that using such approaches to harvest the visible light photons
is likely to be inefficient.  Something to keep in mind is that the overall
efficiency of "plants" is 2-4% (sugarcane in Brazil can approach 3-4%, corn
in the U.S. is probably closer to 2% or less).  Pricey satellite cells are
in the 34-38% range, home solar is probably in 16-22% range and cheap low
end solar (calculators?) is probably in the 6-8% range.

Now of course you could argue that if you you are making sq. km of the cells
on an industrial scale then harvesting the UV and IR photons isn't worth the
excess effort.  But if you look at the history solar energy it has been one
of continually harvesting the available energy at increasing efficiencies.
Though I haven't read it in detail, [3] would appear to be a good place for
people unfamiliar with these topics to start.

Robert

1. http://en.wikipedia.org/wiki/Dielectric_mirror
2. http://en.wikipedia.org/wiki/Dichroic_filter
3. http://en.wikipedia.org/wiki/Solar_cells
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