On Jul 21, 10:47 am, Peter Fairbrother wrote:
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 Tl. High efficiency and high rate heat
radiation in space is problematic unless the temp is high. Radiative
heat dispersal is about 100 kW/m^2 for the low temp radiator.

That's not what the minimum mass calculation show, at least for the
assumption that collector surface and radiator surface have about the
same mass per unit area. I am assuming about a kg/m^2 for both,
taking into account the supporting structure.

What you want is for the sum of mass for the collector and radiator
per kW, and taking into consideration the Carnot efficiency to be at a
minimum.

Here is the graph. http://www.htyp.org/Space_radiator

The minimum came out 130 C with not much penalty between 75 C and 200
C.

Of course, there could be an error in the spread sheet. If you can
find one, please let me know.

Keith

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.

Th is 1800 K, Carnot efficiency is 44%, assumed 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, Buckey balls or BeO. Assuming they are both around a
kg/m^2, 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 -

In my example design the single sided collector has a mass of 5 kg/m^2,
the double sided radiator 1 kg/m^2.

The gas contact areas are 15 times the collecting or radiating areas.
The coefficients of convective heat transfer are 800 and 80 W/m^2 K (the
gas in the high temperature one is at twelve times the pressure of the
low temperature one). The temperature difference across each is 100 K -
the collector surface is at 1900K, the radiator surface at 900 K.

One m^2 of collector produces 400 kWe at the station, 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 very generous and say 150 grams
per kW. That's still 20 times less.

Turbines and generators

are around 0.1 kg/kW based on Boeing 777 engines. Transmitters have
been analyzed at less than a kg/kW. So giving room for such parts as
power conductors and the joint to the transmitter, it *might* come in
at 5kg/kW.

If anyone has some spare web space to hang a small xls file, I can
send it to you.

Yes please. I seem to be missing something in your argument. Will put it
up too.

-- Peter Fairbrother



Keith

-- Peter Fairbrother