On Jul 19, 5:48 am, 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 with
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 produce
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 power
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.
And it would be hard to scale up to the number needed for 100 GW/year
of new construction.
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.
The turbines themselves are around 1/10th of a kg/kW. The
concentrating reflectors, radiators and heat absorbers seem to make up
the bulk of the satellite.
I have offered a spreadsheet before to anyone interested.
It's partly a refutation of an influential paper published
in 1962 and never revisited as far as I can tell.
What I did was very simple. In the radiation spread sheet, the first
column is absolute temperature, column B is deg C. Col C is
radiation per square meter at 0.95, D is at 0.1. E is how many square
meters per kW based on C (both sides radiate). D isn't further used.
Column E is the area to radiate on kW. F is the Carnot efficiency
from 1400 K down to the radiation temperature, G is the 75% of F based
on the typical real turbines. H is the square meters required to
collect one kW out at 100% of Carnot efficiency based on
1.366kW/meter^2. I is how much area it would take to collect sunlight
based on .75 of Carnot efficiency. I is the area it would take to
radiate heat from ideal Carnot, K is the area for real (.75) of
Carnot. L sums the areas for ideal Carnot cycle, M sums the radiator
area plus collector area at the temperature required to get rid of the
heat rejected by a real (75%) Carnot cycle.
This doesn't take into account the reflector (concentrator) loss or
the re-radiation loss from the working fluid heater, but I have
reasons to think both will be small.
Of course I could have expressed area as a function of the sum of the
two areas computed from radiation and Carnot efficiency as a function
of T and solved it analytically by setting the derivative to zero. I
find spreadsheets give me more insight though.
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 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. 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.
Keith
-- Peter Fairbrother