# Efficiency of a heat engine in space

• phyzguy
In summary, the conversation discusses the potential for building efficient heat engines in outer space, utilizing a hot reservoir heated by the sun and a cold reservoir that could potentially be cooled to very low temperatures. However, the practicality and feasibility of this idea is questioned due to the potential size of the required radiator and the limitations of heat absorption in the vacuum of space. The conversation also touches on the difference between heat and temperature and how it affects the efficiency of such a heat engine.
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?

What is the heat capacity of vacuum?

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.

What is the heat capacity of vacuum?
Roughly infinite.
phyzguy said:
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.

phyzguy said:

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.

## 1. How does the efficiency of a heat engine in space differ from one on Earth?

The efficiency of a heat engine in space is typically lower than one on Earth due to the lack of an external heat source. In space, the engine must rely on a closed system and the heat generated from the engine itself.

## 2. What factors affect the efficiency of a heat engine in space?

The efficiency of a heat engine in space is affected by several factors, including the temperature difference between the hot and cold reservoirs, the type of working fluid used, and the design of the engine itself.

## 3. Can a heat engine operate at 100% efficiency in space?

No, a heat engine cannot operate at 100% efficiency in space due to the laws of thermodynamics. The maximum theoretical efficiency for a heat engine is limited by the temperature difference between the hot and cold reservoirs.

## 4. How can the efficiency of a heat engine in space be improved?

The efficiency of a heat engine in space can be improved by using a more efficient working fluid, optimizing the design of the engine, and minimizing heat loss through insulation and other techniques.

## 5. Are there any unique challenges in designing a heat engine for use in space?

Yes, there are several unique challenges in designing a heat engine for use in space. These include the lack of an external heat source, the extreme temperatures and vacuum environment of space, and the need for a closed system to prevent the loss of working fluid.

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