# Thoughts on Stirling engines as source of electrical power in space

1. Jul 24, 2011

### Alfred2011

Curious your thoughts about the use of a Stirling engine to power a space station, auxiliary power for space ships inside the inner solar system, and power for space colonies in the inner solar system.

I think a Stirling engine would be great to generate an electrical field, and thus provide power. The vacuum of space is the perfect place for such an engine. As the Stirling requires only a confined gas and a difference in temperature to operate, and the vacuum of space is a great heat sink, the Stirling would require only a heat source, like the sun (using a parabolic mirror to focus the suns rays on the rod end of a Stirling engine), and the cooling fin section around the cylinder would radiate heat away in the cooling section.

I submitted this idea to NASA decades ago, and they still use batteries or radioactive decay as a method to power spacecraft. This technique could easily work for the space station, or other human occupied settlement or craft while within the inner solar system. It only requires a computerized control system to control the orientation of the parabolic mirror so that the heated end stays facing the sun, and the cooled end would be in the shadow of the parabolic mirror focusing its rays on the heated end.

http://auto.howstuffworks.com/stirling-engine1.htm

2. Jul 24, 2011

### Staff: Mentor

Stirling engines have an extremely low power to weight ratio.

3. Jul 24, 2011

### Drakkith

Staff Emeritus
While a stirling engine is usable in space, it has drawbacks. Like Russ said is has a very low power to weight ratio and unlike solar panels and batteries it (usually) has mutiple moving parts which add to complexity and bring down reliability. It also requires strong sunlight, unlike RTG's, which limit it mostly to the inner solar system. I'm also not sure how well the cool side would get rid of heat, since it would only be lost to radiative transfer. On the up side, it is relatively inexpensive to produce and requires no complex or exotic materials. (Though advanced alloys might help in the power-weight ratio department.)

4. Jul 26, 2011

### Alfred2011

Russ, I agree, however, compared to batteries or nuclear type energy generation, the Stirling would continue to operate (barring mechanical issues or evaporation of the confined gas), while the batteries would not last.

As an engineer, I definitely understand that cost to get something into space depends upon weight...as I understand it, the probes utilize very little energy.

A friend of mine has a stirling that runs off of the waste heat from his coffee mug...albeit it won't produce a lot of torque.

5. Jul 27, 2011

### Alfred2011

I agree about the power to weight...and also it does have moving parts. Those parts are subject to wear but not oxidation like here on Earth. As with any engineering solution, there are plusses and minuses to any design. What I proposed was that the pluses of not needing fuel, and a potential to run longer than batteries or other methods. There are plenty of mechanisms here on earth that work for decades that are much more complex than a Stirling.

The strong sunlight would essentially be provided by a solar concentrator (parabolic dish). As I understand it, the efficiency is increased as the temperature differential is increased. I think I read somewhere that the temperature in space is around -270 C. From what I've read, radiation of heat in space is effective as a means to remove heat from a system. A spacecraft, without any concentration of heat "roasts" on the side facing the sun. I couldn't find a reference as to what "roasting" means as far as a specific temperature. But, concentrating the suns rays with a mirror would make sense for travel in the inner solar system, and potential colonies.

The idea is to use the parabolic mirror to do two things at once:
1. Concentrate the suns energy to a single point and heat the "rod end" of the Stirling very hot, to be conservative, we'll say 500 F.
2. The shade provided by the parabolic mirror puts the opposite end of the Stirling in a shadow.
3. The mounting of the mirror could use an insulation ring around the outside of the housing to provide an insulating effect between the hot rod end and the cold cooling fin end.

6. Jul 27, 2011

### cephron

What advantages does the Stirling engine have over the solar panels we see on nearly every satellite/space station? Just the lack of exotic materials?

7. Jul 27, 2011

### willem2

The temperature of the cold side depends on the surface area of the radiator and the amount of power you need to radiate away.
For a black body (best case) you get:

$$P = \sigma A T^4$$

where sigma is the stephan-boltzmann constant.

To radiate 1kWw from 1 meter squared, the temperature needs to be at least 365K = 92C
If you wanted half the absolute temperature, you need 16 times the surface area, so the temperature of your radiator won't get much lower than this.

8. Jul 27, 2011

### cjl

If you're using solar power to heat the hot side, I imagine you would end up with lower efficiency and more weight than solar panels for the same power output. If you aren't using solar power, there are other alternatives that have fewer moving parts and are more reliable (RTGs use thermopiles for example). I really don't see any benefit to a stirling engine, and I see several disadvantages.

9. Aug 4, 2011