# Compressing a gas; Where does the energy come from?

1. Oct 22, 2012

### smjpl

Hi all,

Question about compressing a gas. When you compress a gas it heats up. Does the compression supply a certain amount of energy to raise the temperature or does the compression merely raise the temperature using the energy that was already present in the gas?

What I am getting at is the compression cycle in a heat pump. How much heat energy is supplied by the source and how much is supplied by the energy input?

I was under the impression that the heat pump took all of the heat energy it needed from its source but needed electrical energy to compress the gas to initiate a phase change allowing it to release the heat.

Last edited: Oct 22, 2012
2. Oct 22, 2012

### Staff: Mentor

Welcome to PF!

The compression step and the heat addition step are completely different/separate processes, so just keep them separate: when a gas comes out of a compressor, it is hotter and at a higher temp than when it came in. You might say the compressor squeezed some sensible heat out of the gas, but regardless of how it is worded, the energy to do that came from the compressor.

3. Oct 22, 2012

### smjpl

This is what I was getting at but I have since found the answer:

If a 4 kW heat pump (for heating) has a COP of 4 (COP => 1 unit in/4 units out = .25), do you size the collector for 3 kW or 4 kW? Does 100% of the heat get taken from the source and you need 25% electrical energy to convert it into usable heat or does the ground supply 75% of the heat and the compressor supplies the other 25%.

From doing more research I believe you do size it for 3 kW. That the compressor apparently does supply 25% of the heat required for the system. I am still a bit unsure on the (physics) reason why but I don't really need to know why. If any one can explain it i would still like to know.

Cheers for your response.

4. Oct 22, 2012

### Khashishi

Compression supplies external energy into the system. Work is force over distance, so it takes work to compress a gas. The force on a piston comes from gas molecules randomly bouncing off the piston. If you push the piston in, the gas molecules will reflect with greater energy than they started with.