From: Adrian Tymes (wingcat@pacbell.net)
Date: Mon Oct 22 2001 - 20:15:39 MDT
Mike Lorrey wrote:
> 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.
Not discounting thermal in general, but I thought steam engines in
practice tend to be about 30-35% efficient too. But that just makes
them equal (barring your data about power per mass), I guess...
> 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.
One could imagine RC maintenance bots, drifting from solar sat to solar
sat, skimming reaction mass from the atmosphere for ion engines powered
by its own PV system.
> As you can see, there are a lot of factors to consider.
Indeed.
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