Solar Power Satellites

--- "Combs, Mike" wrote:

> From: Bill
>
> > The average PV array is
> > less than 10% efficient, while a rankine cycle can
> be up to 40%
> > efficient with most running at 35% efficiency.

There are also Brayton cycle and Carnot cycle heat
engines; if using a Carnot cycle (with some working
fluid such as Helium, which can stay fluid even at
supercold temperatures) then a 300K input and 3K
(cosmic background) output gives a theoretical 99%
efficiency.

> > 2) is the mass of PV array significantly (>300%)
> less than an
> > equivilent rankine cycle system?

The mass of a PV array would be significantly more
than a heat engine system. For a heat engine, the
heavy parts are your boiler, turbine, generator, and
radiator, and of course the working fluid. That
represents only a very small fraction of the area of
the system, the remainder of which is a simple
reflector, perhaps Aluminum foil, with its shape held
by inflatable ribs. Assuming a thickness of typical
household Al foil (which at 0.018mm thick is probably
much thicker than is needed), the mass of the
collector would be about 48 grams per square meter. I
don't think that there are any PV arrays out there
that can match that mass to area ratio. Beyond some
size (which I can't calculate without knowing the
masses of the power-gernating section of a heat engine
nor the mass per unit area of PV arrays), a heat
engine will be less massive than a PV array. I
strongly suspect that this size would be on the order
of the size of a practical SPS.

The chief disadvantage of a heat-engine-type would be
that it has moving parts. All of those moving parts
are concentrated in one small section, which is where
I would think the bulk of all repairs would occur.

Ed