From: Mike Lorrey (mlorrey@datamann.com)
Date: Mon Oct 22 2001 - 10:42:20 MDT
Adrian Tymes wrote:
>
> jeff davis wrote:
>
> > I envision the dish as spin stabilized to keep it
> > pointed at the sun, with steam generator, and dual,
> > counter-rotating turbine/generator units axially
> > mounted, and the waste heat radiator located in the
> > shadow behind the dish.
>
> Wouldn't direct photovoltaic be more efficient than a steam
> generator?
No. PV, even in multi layer/ multi spectrum cells, peaks out at about
35% efficiency. Thermal systems can be 80-90% efficient. This is
efficiency in terms of energy conversion.
After that, you need to consider energy density efficiency: If your PV
system weighs 100kg and your thermal system weighs 300 kg, where both
receive the same amount of flux but have different conversion
efficiencies, the PV system is more efficient to launch with, since you
can produce 15-25% more power for the same amount of weight. If,
instead, you have a 200 kg thermal system to receive the same power, it
is the more efficient choice.
So, there, you are dealing with launch cost efficiency. Then you also
have to consider maintenance and reliability. Generally, the rule is
that the fewer moving parts, the more reliable and therefore the least
cost in terms of maintenance. Since PV has no moving parts, it would
seem to be the most reliable, while thermal systems generally require at
least a turbine generator and a pump or two to function. This is fine if
the actual MTBF is longer than the amount of time you expect between
opportunities for maintenance inspections. This is not so great for a
device which is not expected to return to human hands any time in
decades.
As you can see, there are a lot of factors to consider.
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