# GSHP - What percentage of heat energy is taken from the ground?

1. Oct 22, 2012

### smjpl

Hi all,

I am wondering about heat pumps, not necessarily ground source heat pump.

What percentage of heat is taken from the ground?

My understanding was that 100% of the heat is taken from the ground but it costs 25% (if COP of 4) in electrical energy (running the compressor) to convert the heat from low temperature heat into high temperature heat (albeit a smaller amount).

From reading some forums, some people think that if I have a 4kW heat pump and achieve a COP of 4, that the ground collector only needs to absorb 3kW of heat energy and the other 1 kW is supplied from the electricity, i.e. the compressor.

Is this correct?

2. Oct 22, 2012

### sophiecentaur

It tells you all you need to know - and agrees with your understading of it. But other factors will determine what sort of value you, personally, will get if you instal it in your home. That is much harder to estimate because it will depend very much on local conditions (wind, air and soil temperatures and soil type etc.).

3. Oct 22, 2012

### smjpl

Cheers for the reply. It answered my question but it appears I am wrong.

http://en.wikipedia.org/wiki/Coefficient_of_performance

"A geothermal heat pump operating at 3.5 provides 3.5 units of heat for each unit of energy consumed (i.e. 1 kWh consumed would provide 3.5 kWh of output heat). The output heat comes from both the heat source and 1 kWh of input energy, so the heat-source is cooled by 2.5 kWh, not 3.5 kWh."

4. Oct 22, 2012

### sophiecentaur

You seem to be right to me. The heat source heat plus the electrical input appear as heat in your house. Isn't that what you are both saying?

5. Oct 22, 2012

### smjpl

No, I was thinking 100% of heat comes from ground but it costs you 25% in electrical energy to convert it into usable energy. I thought the conversion process didn't add any additional heat but it does.

i.e. if you had a 4 kW heat pump, you would cool the ground by 4 kW not 3 kW.

But just to clarify for anyone else, it does only cool the ground by 3 kW granted you have a COP of 4.

6. Oct 22, 2012

### sophiecentaur

Yep - and that's how I read what you wrote.