Not for their direct power consumption -- this is generally grid electricity though it need no be. any form of energy will power them.

They do however use this power to extract heat from the surroundings -- this heat is largely solar energy.

The COP (coefficient of performance) generally 3 to 4 means that for each kW of energy put in you get 3 or 4 kW of heat energy out.

This is reason that heat pumps are marketed as renewable technology -- you literally pump heat out of the air, ground or water surrounding you.

So long as these surroundings are large then the heat will be replaced by the sun as fast as it is extracted. You get say 3/4 of your heat for free.

We could say that heat pumps are three quarters renewable could we? does that count?

Maybe the question should not be "_do_ heat pumps use renewable energy?" (of course they do, at least partially) but "_can_ heat pumps use _just_ renewable energy?"

The sensible options for renewable space heating and domestic hot water are actually pretty limited: solar, wood burning or renewably generated electricity. Have I missed anything?

If you think the zero-carbon homes idea is wrong, i.e., that renewable electricity is more efficiently generated off-site, then heat pumps are the only answer that makes sense for houses for which solar or wood are, for one reason or another, not practical as the complete solution.

(Any overstatement of COPs is a separate issue unless it's really thought that COPs are so low that actually just using resistance heating is more sensible. That idea would need some justification, I think.)

If you could make a 1kW low temperature stirling engine with an efficiency of say 50% then in theory a GSHP could be self powered making it almost 100% green.

Say the heat pump had a COP of 4, that means it needs 1KW to pump 4KW out of the ground. 2KW of the extracted heat would be used to run the 1KW stirling engine which powers the heat pump. The remaining 2KW could be used to heat the house. You might even recover some of the heat losses in the stirling engine or heat pump by putting them indoors. A means of starting the system is required but I guess compressed air produced by the stirling engine could be used or it could just be left running 24/7.

However it's not clear such a stirling engine can be built although there are photos of a 750-1kW low temperature SE on the web. It appears to be physically big - about the size of a british red phone box.

This is not a "free energy" claim any more than a photovoltaic panel is.

Posted By: CWattersIf you could make a 1kW low temperature stirling engine with an efficiency of say 50% then in theory a GSHP could be self powered making it almost 100% green.

You can't, sorry. If you look at the temperature differentials used in a heatpump and put those back into the Carnot equations that determine the efficiency of a Stirling engine you'll see that the Stirling engine's efficiency will always be less than that needed to run the heatpump. You could use a solar concentrator to provide enough temperature difference to run the Stirling engine, though the efficiency would be no higher than using a PV array in the first place. That said, perhaps the Stirling engine's torque would be sufficient to run the compressor of the heatpump, avoiding the main problem with running compressors off a PV array: that the starting current is usually much higher than the PV/inverter system can supply.

The Canadian Centre for Housing Technology is evaluating Stirling engines for residential CHP systems: http://www.ccht-cctr.gc.ca/projects/chp_e.html as well as microgeneration with waste heat storage in vertical boreholes: http://www.ccht-cctr.gc.ca/projects/earthenergy_e.html

Lots of food for thought for people who want/need to be off-grid.

Paul in Montreal.

Posted By: Ed Davies
The sensible options for renewable space heating and domestic hot water are actually pretty limited: solar, wood burning or renewably generated electricity. Have I missed anything?

I'm nitpicking, but you're assuming that wind, hydro etc can only be used to make electricity. They can also be used to provide shaft power directly to a heat pump compressor. But good luck on all the engineering to get good performance :)

If you think the zero-carbon homes idea is wrong, i.e., that renewable electricity is more efficiently generated off-site, then heat pumps are the only answer that makes sense for houses for which solar or wood are, for one reason or another, not practical as the complete solution.

I think there are a couple of other things to consider:

(1) Capital and maintenance costs, both economic, resource and ecological. Heat pumps are complicated beasts so if only a small amount of heat is required, it's worth considering whether the extra costs are worthwhile.

(2) For the present anyway, off-site electricity in the UK is NOT renewably (or even low-carbon) generated, so your argument is not true in the UK yet. It's also necessary to consider whether e.g. gas boilers or CHP is a better approach for now.

Posted By: CWattersThe 1kW used to drive your heat pump might have taken 1.5 to produce.

In fact, more like 3kW - approx 3kW of coal etc will have been burnt to produce your 1kW of electricity, which mostly ends up in your heated water and the rest hopefully is emitted within your house's heated enclosure. To that is added 2kW of heat extracted from the local environment. Thus 3kW of coal is burnt to provide your house with 3kW of heated water. In total and in effect a COP3 GSHP is a coal-fired system, in no way renewable powered.

Another consideration is that there is the possibility of carbon capture and storage for fossil fuels burnt in a power station but not much chance of the same for those burnt in a house.