On Jul 21, 10:48 am, Peter Fairbrother wrote:
Sylvia Else wrote:
On 20/07/2011 3:35 AM, Peter Fairbrother wrote:
Sylvia Else wrote:
On 19/07/2011 10:48 PM, Peter Fairbrother wrote:
Bohica Bohica wrote:
On Jul 12, 9:17 pm, Sylvia Else wrote:
The Australian Government has, for reasons that have much to do wi
th
politics, and little to do with the environment, decided to throw
$Au 10
billion into the bottomless pit that is renewable energy.

Lest it all get turned into yet more solar panels and windfarms, I
invite all comers to submit their plans for orbital power
satellites. At
least then we might get some technological advance for our money,
even
though I doubt we'd actually see any orbital power.

Sylvia.

You could make a **** load of parabolic reflectors aimed at the hot
part of a Stirling engine, these things are about 6m wide and produ
ce
about 10Kw
A Spanish comp[any makes them. The main problem is the colour, all
shiny and not a bit og brown or green on them :-)

That's actually close to what the generating part of an orbital powe
r
sat should be - lots of mirrors feeding sunlight to a Brayton cycle
gas
turbine. Forget acres of solar cells, they are too heavy and too
expensive and too fragile.

A Brayton cycle engine in that size range is lighter than a Stirling
engine, no regenerator needed. Not as efficient, but cheaper and
lighter
to launch.

Will it run maintenance free for a couple of decades?

The compressor and turbine, I don't see why not. It's only one moving
part, gas bearings are well-developed technology and there are no
critical rotating seals.

It's much simpler than a Stirling engine, and they reckon they can mak
e
those work for long periods in space.

The generator? Yes, I'd think so too.

The heat exchangers might need some work though, probably multiple
redundant circuits. leak sealing and gas refills or something. Likewis
e
the mirrors and mirror pointing stuff.

But would it need to last 20 years without maintenance? If you are goi
ng
to build it in the first place, you'd need a good launch capability
anyway.

And if it's providing a goodly proportion of your energy, you'd want t
o
be able to fix it if it breaks. no matter what the built-in reliabilit
y
claimed was.

I would assume that there would be enough examples in orbit to provide
redundancy in the case of failure. After all, even if you can perform
in-orbit maintenance, it's unlikely you can do so at the kind of short
notice required for power supply failures that cause blackouts.

If it lasts 20 years on average without intervention, then you can
probably afford to deorbit it when it breaks, and send up a new one. If
you're talking about in-orbit repairs you're almost certainly talking
about manned missions, with all that that entails. One thing the shuttl
e
missions have shown us is that getting up there is just the start.
Fixing things in zero-g while wearing a space suit is not such an easy
task.

Hmmm - suppose a 1 million square meter (just over 1.1 km across) mirror
shining on a 1000 square meter (35 meters across) collector/Brayton
engine/radiator. It collects a little over a gigawatt of solar energy,
and produces maybe 500MW of electricity. It short-range-beams that to
the downlink satellite, which beams it on to Earth, and that provides
300MW on Earth.

Unless you know something I don't, you take a ~50% hit in each
microwave link.

The downlink satellite services a flight of - what, 100 of these
powersats? - giving 30 GW electrical on Earth, equivalent to 20 large
nuclear power stations. Say you have three downlink stations which can
be fed by any of the powersats as required, and 300 powersats, that's
maybe 80 GWe on Earth, or GWeE.

The 300 powersats can be deorbited (or perhaps brought to LEO for
refurbishment in a shirt-sleeve atmosphere?)

It's a good idea. Boeing first thought about it in the 70s and even
did some great artwork of assembling power sats in LEO. Then someone
worked out what would happen to them in the months of going from LEO
to GEO on ion engines. It was not nice. Even in those days they got
hit several times with space junk. :-(

if they fail, so an average
life of twenty years is acceptable. I don't think that would be too hard
to achieve, though they might need a scheduled resupply of maneuvering
fuel. However as they have ample power, ion drives using very little
fuel might be okay.

I've been considering what to make the powersats from, the hard part is
the high temperature solar collector. First, operating fluid. Initial
options are hydrogen, helium, methane, water, neon and argon. Strike
helium for leakyness and neon for lack of availability. Methane, water
and hydrogen are liable to have reactivity problems over 20 years, and
we are left with argon. A little heavy, but in the amounts needed the
extra weight is lost in the noise.

Apart from that, argon is all good. First it's cheap and readily
available. It is monatomic, which is thermodynamically useful, as it
gives better efficiency. And it is almost completely inert chemically,
which means we can use it a high temperatures (with concomitant high
efficiency) without worrying too much about chemical corrosion of the par
ts.

Argon is good, but it turns out that supercricial CO2 is even better.
I favor two cycles because the more heat you can convert to electric
power, the less you have to radiate.

On to the collector. It has an area of 1,000 square meters, and receives
1 MW/m2 of energy. That's a blackbody temperature of 2050 K. We could
make it really thin, say 0.1 kg/m2 - like a thin sheet of paper - so it
would only weigh 100 kg, but a very thin collector would be leaky, and
it would require many tiny channels. We could make it from
carbon-carbon-carbon-carbon or something, but that would be leaky and
fragile.

I suggest zirconium (or a zircalloy) at 0.8 mm thick, which would mass
about 5 tons. That won't melt even if the gas supply fails. The
collector is made from 2 large sheets of 0.4mm zircalloy, one
corrugated, which are roller-welded in strips, so as to leave channels
for the gas between welds. This provides a very low-leak solution.

The waste heat radiator could be made from something less heat resistant
and lighter, possibly titanium. It would need an area of about 6,000
square meters, but unlike the collector it could be double sided,
meaning 3,000 square meters overall. Mass, about 10 tons.

The engine could be about 10 tons - compared to a jet engine which
weighs 4 tons in aircraft form and produces 60 MWe in marinised form,
that's 15 MW/ton, the spools are simpler and do less work and there are
no combustion chambers, so 30 MW/ton should be quite possible using
available technology.

A bit of handwavium now, I'm out of time:

Add 20 tons for the alternator, 5 tons for the beam transmitter, 10 tons
for fuel, 5 for pipework etc, and 5 for the mirror, and we have a mass
of 70 tons. If the mirror support structure can come in under 30 tons,
that's 100 tons for the powersats.

300 of them is 30,000 tons in GEO. Plus you need the downlinks, say
10,000 tons each, or 60,000 tons in total. For 80GWe.

That's kind of optimistic. 60,000 t for 80 GW is 750 t per GW. My
numbers (and Dr. Phil Chapman's) come in at around 5,000 tons per GW,
6.7 times as high. On the other hand, Solare's numbers come out a lot
lighter so you are not that far from various estimates.

Keith

-- Peter Fairbrother