Using the Sun as a Space-Based Power Source

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jbanes
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Joined: Thu Aug 05, 2004 12:49 pm
First Name: Jerason
Last Name: Banes

Using the Sun as a Space-Based Power Source

Post by jbanes »

Recently, I've been discussing the issue of building a power-collecting space station
with my colleagues. Based on the figure that the Earth receives about 1.3kw/m^2 of
sunlight, we've calculated that a power output of 130kw/m^2 would be received at
0.1au, and that the temperature would rise from 300K to 900K.

My suggestion was to put a space-station at 0.1, which is solar panel surface of
109.73x48.78 meters (the exact dimensions of a football field). Such a surface area
would receive nearly 700MW of power. Unfortunately, the panels would only be about
20% efficient, and would suffer from the heat. Also, the station would have to rotate
fairly quickly to prevent too much heat buildup on its outer surface.

How realistic do you guys think a large Stirling space-station would be? One side
would have a black football-field sized surface that absorbs heat, and the other side
would bleed the heat off. In between, our Stirling engine(s) would be cooling the front
surface, heating the back surface, and generating power in between. Using a
guesstimate that the rear surface would experience about 100K temperatures or
lower, I came up with an 88% efficiency rating.

Other than building and transporting such a station (that's another discussion), what
problems do you see in making something like this work? Would one large engine or
an array of smaller Stirling engines be about to absorb and generate electricity from
the 700MW of power heating the surface? How difficult would the engineering be?

Thank you for your opinions.
busch
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First Name: Ray
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Response to Using the Sun as a Space-Based Power Source

Post by busch »

Question is, can you maintain 100K while at the same time
beaming several thermal gigawatts into vacuum off your
surface ?
Remember, radiation is proportional to T^4 and appears
your only mechanism to get rid of the heat since you do
not have a cooling agent, i.e. matter to turn it loose on.
My physics is waaaay back in time there but I have strong
doubts. Can someone with fresher knowledge throw some
numbers on paper for us ?

Ray
william.delysle
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Joined: Tue Sep 07, 2004 2:01 pm
First Name: William
Last Name: DeLysle

Using the Sun as a Space-Based Power Source

Post by william.delysle »

I think the answer is not as simple as "how much heat could you dump into a vaccuum" as it would actually depend on how efficient your engine was to begin with. As the efficiency depends on the temperature difference, and in this case the cold side temperature depends on how much energy you can radiate, it would seem like the problem becomes a second order differential.

The reality of it would likely be that the cold side would "warm up" until the heat radiated matched the heat transferred to the cold side, which would depend on how much energy was extracted by the engine. I know that sounds like I have repeated the point to some extent, but without specific items such as surface area, radiative ability of the material into a vaccuum etc it would be impossible to even guess.

Also, it occurs to me, that a second engine that used the cold side of the first as its hot side, and others daisy chained in a simiar fashion would be worth considering. The problem here then becomes the mass of the satellite and the cost of putting into orbit.

From various things I have read on the net, it does seem like one big engine is more efficient that several smaller ones, but no one seems to have produced a moderate power output for a sensible price yet.

Another thought - Once your satellite was generating significant quantities of power, how would you ship the energy back earthside..?

I have seen ideas that placed solar reflectors in geo-stationary orbit with collectors on the ground. Yes, there is atmospheric loss, but it is comparatively small (so they suggest), and its far easier to build solar furnaces to power whatever on the the ground. Again though, no one seems to have done this for the purposes of power generation.

There is a large solar furnace in the south of france, and another in California, and they seem to have power outputs in the region of 4 MW, but they are doing high energy particle research, not power, with them.

Sorry if I rambled on a bit...
max___399
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First Name: Max
Last Name: Robinson

Response to Using the Sun as a Space-Based Power Source

Post by max___399 »

Two issues I can think of are:

1. A football field-size collector absorbing as much solar energy as possible will feel a significant push away from the sun. How hard would it be to keep this "satellite" where you want it? If I understand you correctly, you want it to be 1/10 as far from the Sun as the Earth is (0.1 au). This problem is only going to get worse the closer you get to the sun. In addition to light, there are also charged particles streaming out of the sun, and tiny bits of rock that burn in our atmosphere but would be a serious hazard to a power station. I think having the station in space brings up lots of issues that aren't about the thermodynamics and aren't of concern at all for planet-based stations.

2. How does one efficiently "beam" megawatts of energy across a distance of about 1 amu, then down into our atmosphere for use? Until there's a nice answer to that, I don't see any reason to consider building a power plant at such a distance from Earth.
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