# Wind Turbines in Space

Solar Sails are a great, low-impact way of driving your spacecraft. Whilst wind turbines are a low impact way of collecting power from the wind, however, they only work when the wind is blowing.

My thought is, the solar wind is always blowing in one way. Would it be feasible to use Solar Sail Wind Turbines,

• How big would they need to be?
• What speed would they spin at?
• How much power would they generate?
• Would they be blown out of their orbits?
• Reaction mass is very expensive to get into space, and is finite once you are there. Solar sails are good for propulsion as they reduce the need for reaction mass and provide very slow but steady acceleration. Neither provide electrical power to the spaceship. – Jeremy French Oct 17 '16 at 14:12

# No, they are not feasible

There is one way you could possibly do it, and that is to have counter-rotating turbines that provide counter-torque to each other. You can then orbit this satellite at a speed that is slightly too slow for the orbit altitude, and the force of the solar wind will provide the "missing" part of the centrifugal force.

However...

According to Wikipedia, the radiation force on a solar sail that is...

1. 800 meters by 800 meters in size
2. in a co-orbit with the Earth around the Sun

...is approximately 5 Newton, which is about 1 lb of force. Even if you could convert this from a normal force to one that acts radially instead, it would be a next to infinitesimally small force compared to the photoelectric energy that is hitting the turbine.

That same solar sail of 800 meters by 800 meters would be receiving $800m \cdot 800m \cdot 1400 {W}/{m^2}$, which is approximately 900 MW... which is the power of a decently sized nuclear reactor.

In short: it is extremely doubtful that the radiation pressure of the solar wind could even overcome the friction forces of this counter-rotating turbine. And just making it into a static solar panel is many magnitudes more efficient.

But it would make for one heck of an impressive space novelty toy...

Picture two of these, stacked on top of each other, about 1 km wide, in space. :D

• Comments are not for extended discussion; this conversation has been moved to chat. – Serban Tanasa Oct 20 '16 at 12:54

# Yes, it is possible

• But it is not what sane solution should be

## How big would they need to be?

• Depends on how much energy you wish to have, but bigger than solar panels.

## What speed would they spin at?

• Limit are defined by strength of material you have build it from and size of construction. To be efficient, fast. IDK more.

## How much power would they generate?

• Depends on how big they are, but less than solar cells and other solutions.

Also depends on mass of construction, how close to star they are, how deep they are in gravity well(deeper they are, better and more efficient per kg they work)

## Would they be blown out of their orbits?

• Whole point of such system is based on the fact that they do not have natural orbit, and they hover using force of solar wind near star, which counteracts attraction from star, which(attraction) allows to actually generate energy.

They are called Statite, I'm not very familiar with them, but you may probably even find something similar to your idea. It is perfectly valid idea, it is just not mass efficient and have lots of other problems, and only positive thing I may say about it, is that it is perfectly capable to work.

## As note

That fact that solar wind blows in one direction(from star), does not mean Solar Sail Wind Turbines should rotate in one direction, and direction can be regulated by angle of blades. And system of 2 such Propellers is perfectly capable to keep momentum. Theoretically you even do not have to have 2 propellers for this system to work, just change rotation periodically, and use massive core (I fail to describe it in English, so just use 2 Propellers it is simpler to describe).

### As note2, efficiency

Efficiency depend on speed of rotating blades. Faster this thing rotates, more energy it will generate, more efficient it will be. Work is equal force multiplied by distance, Power is equal force multiplied by velocity.

If we take same sail 800x800m as in Michael Karnerfors answer and same force 5N, then to get 900MW with this "blade" it have to have velocity 180'000'000m/s it is almost 2/3 of speed of light. Centrifugal force in this situation will be a problem, would be construction bigger(light years big), this would be less a problem. But "blade" not necessary have to be thin foil, it can be a plasma, and with magnetic field this plasma can be contained at this velocity, in similar size construction(km sizes or less). Although with small size(km's) and not dense plasma we will have other issues(plasma will be to transparent, so we should have ticker layer of that plasma, or something like that)

It still can have exterior of big Solar Sail Fan, where blades used to change angle of light on which it hits the plasma rotor. Faster it(plasma fan) will rotate, bigger red shift of reflected light will be.

But eventually it will be just particle accelerator, which uses light to accelerate particles(plasma) inside it.

So as simple construction SolarSailTurbine may be not the most efficient solution, but more sophisticated design may improve situation.

The reason is that a turbine works by spinning something around a non-moving, anchored hub. The power is generated by the speed difference. The spinning causes a wire coil to break the magnetic lines of a magnet; which induces an electric current. The process of drawing power from the spin induces drag on the hub. The only reason the hub doesn't start spinning is because it is anchored.

In space, the hub has nothing to keep it from spinning. Even on a large ship, the ship would eventually start to spin with the solar turbine unless you used the power that you generate to keep the ship from spinning. That would be counter productive.

• because of the torque reaction effect wouldn't the base spin the other way creating a larger speed difference? – Sievert Apr 20 '18 at 18:48
• @Sievert, no. The only reason a turbine on Earth doesn't match the spin of the blades is because it is anchored to the Earth. The turbine creates power through physical friction or electrical resistance (which is close enough to friction). It tries to pull the turbine to rotate with it. The lowest energy state (entropy here) for the system is for them to rotate together and not produce friction. – ShadoCat Apr 20 '18 at 23:37
• Thank you. Does the angle of the blades affect which direction it would spin? (I am not denying that it wouldn't generate energy. I am just asking about the way it would rotate.) – Sievert Apr 22 '18 at 4:45
• @Sievert, the angle does affect the direction of spin. It will spin toward the blade edge closest to the source of the wind. You could set up counter rotating blades but the set in front will spin faster since they will use up a lot of the energy of the wind. It will also cause the whole station to wobble and slew to the side. Force-wise, the situation gets messy. – ShadoCat Apr 23 '18 at 19:26