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.
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.
