Keith Henson wrote:
[...]
Assuming the radiator and collector mass per square meter is about the
same, then you can see from the graph that the minimum occurs a bit
above 100 deg C, which is far below the 370-650 deg C quoted in an old
paper he
http://contrails.iit.edu/DigitalColl...2article42.pdf
I'd use something like 1,000 K as T_l. High efficiency and high rate
heat radiation in space is problematic unless the temp is high.
Incident radiation on the collector is 1.1 MW/m^2, the mirror (which
weighs 0.005 kg/m^2 excluding support) concentrates sunlight from 1.33
kW/m^2 to 1.1 MW/m^2, approximately 820 times at 80% efficiency.
T_h is 1800 K, Carnot efficiency is 44%, overall efficiency to local
electricity is 29%.
I can't say for sure what the mass per unit area of radiation or
collection are. I need to analyze a canvas tube (like an air
mattress) radiator filled with low pressure gas and air float
charcoal, Bucky balls or BeO. Assuming they are both around a
kg/m2, a kW should come in around 3.2 kg.
I do not understand that. Ignoring the mirror, which I think - actually,
I don't know what you are doing at all - I assumed that that figure is
for heat collectors and radiators??
In my example design, which I have just posted to sci.space.tech but
which is on sci.space.policy (moderation delay?), the single sided
collector has a mass of 5 kg/m^2, and the double sided radiator has a
mass of 1 kg/m^2.
Those figures are for the radiation transfer areas. The gas contact
areas are 15-20 times the collecting or radiating areas. This can be
done in manufacture by strip-bonding two high-surface-area sheets, or by
forming an open-cell foam between the two outer sheets after bonding (my
reference structure), or by other means.
The coefficients of convective heat transfer are 800 and 80 W/m^2 K. The
the gas in the high temperature collector is at twelve times the
pressure of the low temperature radiator - the collector is at 5 MPa,
the radiator is at 0.4 MPa. The fluid is argon gas.
The collector surface is at 1900 K, the radiator surface is at 900 K,
collector gas-out is at 1,800 K and the radiator gas-out is at 1,000 K.
The high temperature collector is at 1900 K, with an incident radiative
energy of 1.1 MW and a blackbody temperature of 2200 K, which means that
it has to be shrouded to prevent losses - but the shroud can be very
light, a few tens of grams per square meter, and the shroud mass is
negligible.
Radiative heat dispersal is about 80 kW/m^2 for the low temp radiator,
at 900 K.
One m^2 of collector produces 400 kWe local, and needs 8 or 10 square
meters of radiator, so 15 kg of collectors and radiators are needed to
produce 400 kWe, or 0.0375 kg/kW.
My numbers might be a little hard to achieve, though they are meant to
be only medium-tech at best, so let's be generous and say 100 grams per kW.
That's still 30 times less than your estimate. I don't know why that is,
I'm designing this on-the-fly - have I made a mistake, or a ridiculous
assumption? I'm no Henry Spencer, and I'm not infallible.
Turbines and generators
are around 0.1 kg/kW based on Boeing 777 engines.
Okay, though I may have more to say on this later.
Transmitters have been analyzed at less than a kg/kW.
I have a 1kW FM transmitter which weighs about 100g (and which would be
totally illegal to use
So giving room for such parts as
power conductors and the joint to the transmitter, it *might* come in
at 5kg/kW.
I think it might come in at less than 1 kg/kW overall, for a very large
system.
If anyone has some spare web space to hang a small xls file, I can
send it to you.
Yes please. Will put it up too. Can put the other one up if you like
too. Link/URL will be ok for few years, but not forever.
-- Peter Fairbrother
Keith
-- Peter Fairbrother