Efficiency of a heat engine in space

[phyzguy]

TL;DR Summary

Could we build a very efficient heat engine in outer space because of the avaialability of a very cold reservoir.

I have a question about building efficient heat engines in outer space. In theory you could have a hot reservoir heated by the sun that was several hundred degrees C, and a cold reservoir that was very cold - maybe 50K - 100K or even colder. Thus, theoretically at least, a heat engine could be very efficient, much more efficent than photovoltaics. Is this practical, or would the radiator required to cool the cold reservoir be prohibitively large? If you did try to build something like this, what working fluid would be best?

 

[Vanadium 50]

What is the heat capacity of vacuum?

 

[phyzguy]

What is the heat capacity of vacuum?

I don't understand your comment. You can radiate away heat, so I don't think heat capacity is relevant.

 

[russ_watters]

What is the heat capacity of vacuum?

Roughly infinite.

Is this practical, or would the radiator required to cool the cold reservoir be prohibitively large?

It could be done, but probably isn't going to be worthwhile due to launch costs/heat exchanger size. But I'd start by looking at a the specs for an existing solar thermal plant and calculating how much the efficiency changes with a drop in reservoir temp.

 

[Vanadium 50]

I don't understand your comment.

It's the difference between heat and temperature. The deep vacuum of space can be at low temperature, but it is difficult to absorb much heat - as Russ says, it's entirely radiation. You max out at something like half a horsepower per square meter. So while you can get a very high efficiency, you are limited to very low power compared to heat engines that you are used to.