## The Problem With Heat

For a more in-depth analysis of the impact of thermodynamics on your science-fictional universe, obtain and read Ken Burnside's The Hot Equations: Thermodynamics and Military SF.

Now in some subsequent pages in this website, there will be unpalatable scientific truths. But of them all, there are two in particular that give me more hate mail than all the rest put together. The fact that there ain't no stealth in space, and the heat radiator "Achilles Heel".

Both problems are caused by heat. In one case, it is the waste heat of a spacecraft that becomes a stealth-destroying beacon. In the other, it is the fragile nature of heat radiators.

About once a month, I get an email from some eager fan who believes they have the miracle solution to both problems. "Eureka! I've got it! All you have to do is use the Peltier-Seebeck effect to convert the heat into electricity! Then you can store it or use it as is convenient. A simple thermocouple and the problem is gone!"

This solution is elegant, marvelous — and totally wrong. People who come up with this are to be congratulated on their brainpower, but they need to study their physics a bit closer.

What's the problem? Well, the general problem is that pesky Second Law of Thermodynamics. In this context, it tells you that it is impossible to destroy heat, the best you can do is move it around. So using a thermocouple to convert heat into electricity is impossible.

The specific problem is that a thermocouple does NOT convert heat into electricity. It converts a heat gradient into electricity. The original heat is still there. In fact, the conversion process adds even more waste heat to the original total.

As an analogy, think about a hydroelectric dam. The water in the reservoir is at a higher gravity gradient than the water downstream. The hydroelectric dam converts the gravity gradient into electricity. But the water is still there after passing through. The dam does not convert water into electricity, if it did the water would disappear. In the same way a thermocouple does not convert heat into electricity, the heat is still there.

But don't take just my word for it:

## Joke Laws

The joke version of the Three Laws of Thermodynamics are:

1. You can't win, you can only break even.
2. You can only break even at absolute zero.
3. You can never reach absolute zero.

## Atomic Rockets notices

This week's featured addition is SPIN POLARIZATION FOR FUSION PROPULSION

This week's featured addition is INsTAR

This week's featured addition is NTR ALTERNATIVES TO LIQUID HYDROGEN