# Would a solar windmill in space be practical?

From this question we know that solar cells be problematic the farther you get from the sun. We know that solar sails provide viable propulsion far from the sun.

Would it be practical to use a solar windmill to generate enough energy to power a space station?

Given that at least two mills turning on opposite directions would be required for stability.

Not really. The pressure due to solar wind is approximately 1-6 nPa (See Wikipedia). The pressure due to just the light from the Sun at Earth is actually higher, at around 9 nPa (Source). The mass makes somewhat of a difference, but let's just assume somehow you can get a mass of a 1 square meter windmill at 1 gram. The effective power would be very small, far less than a Watt.

Assuming a 35% efficiency with solar panels, which is achievable, the amount of power that 1 square meter of solar cells could generate would be about 450 W.

And then there's the issues that the pressure would tend to push your device further away from the Sun with each passing moment, and that would be very difficult to correct for. Not to mention that this would take away from the effectivity even more, as you would essentially be moving with the solar wind, thus not allowing it to power your station. Bottom line, it's an interesting idea, but just won't work.

• What I do not understand from your answer is how you can assume the effectiveness of 'less than a Watt'. Windmills on Earth are fixed to Earth. Earth is just 'heavy', so you can neglect any force, which is applied to Earth through the windmill. However, in space, how would you 'fix' the windmill to gain any effectiveness in the first place? I mean, it would eventually be 'blown away' ... You need a massive counterweight to make this work. – s-m-e Jul 24 '13 at 20:40
• @ernestopheles: That's the last paragraph of my statement, saying that if you weren't careful, the thing would just blow away. I do believe you are correct, that would diminish the effectivity of the system as well. Hmmm... – PearsonArtPhoto Jul 24 '13 at 20:43
• @ernestopheles If you're getting a lot of radiation pressure, you can hold the station in a somewhat artificial orbit, such that the momentum is ultimately transferred to the Earth. Also, for a general metric of how efficient the windmill will be relative to solar panels, just look at what fraction of the speed of light the windmill tips are traveling at. If it's going less than 0.01 c, then it should (ballpark) be less than 1% the efficiency of a normal solar panel. – AlanSE Jul 24 '13 at 22:16
• @AlanSE Yes, it is the (transfer of) momentum (and the related orbital mechanics), which makes me wonder how this could work. – s-m-e Jul 24 '13 at 22:32

Any design based on physical "sails" will inevitably consist of a lot of large thin membranes anchored to some structure. If you use the same membranes as a solar concentrator to capture sunlight from a large area and focus it on a much smaller area of solar cells, you will always get more power. (the solar wind pressure is nano-pascals, to the force per square meter is nanoNewtons. Even if the blaes are moving at kilometers per second that is still only microWatts of power. On the other hand there are a few Watts/square meter of solar power even out as far as Neptune.

If you could make your "sails" purely magnetic (essentially loops of superconducting wire whose magentic field reacted against the solar wind, then you might get soemwhere, since a very small mass of wire could enclose an area of many square kilometers and experience significant force.

Any method of solar energy production is problematic the farther you get from the sun. If you increase the distance from 1 AU (Astronomical Unit, the distance from sun to earth) to 5 AU, the solar power density decreases to 1/25. Solar cells would need a much larger area and all other methods of solar energy generation too. A solar windmill would require moving mechanical parts, that is a problem for long time relaibility. As long as the efficency of a solar windmill is much smaller than that of solar panels, the necessary area is much larger.

The Rosetta mission sucessfully demonstrated the use of solar panels with an area of 64 square meters in a distance of 3.4 and 5.25 AU.

• This doesn't answer the question – Antzi Jul 17 '18 at 4:44