Pumping heat
Spent the weekend dispensing bon-mots and advice in Harrogate at the Homebuilding & Renovating show, one of six held throughout the UK each year. This year I have been delivering a short lecture on sustainable homebuilding and it has sparked some interesting questions and comments from the audience. However this Sunday it all got a little fiery when someone asked about the difference between air source and ground source heat pumps and whether either made sense for his building project. Rather like the output from these heat pumps, my response was just a little lukewarm.
What I specifically said was that heat pumps don’t make much sense if mains gas is available but that there should be a reasonable payback against oil. “You are doing well if you get a Coefficient of Performance of more than 3.0,” I said. I have been consistently saying this for some time now and at least one heat pump manufacturer, Kensa, seem happy to agree with me.
But up stands this man in the audience who said that heat pumps could now deliver over 6.0 — i.e. twice as much heat output for the power input. Before I could stop myself, I blurted out “That’s rubbish.” It obviously hit a nerve, because he stood up and started getting shirty with me. “What do I know about it” sort of stuff. I have no idea who he was but can only guess he was working for one of the many heat pump suppliers exhibiting at the show.
This made me go all defensive and I started quoting a couple of studies back at him that showed that heat pumps often don’t deliver what manufacturers claim. If only to prove that I do know something about it, if not exactly ranking at world expert status. This of course made matters worse and our man turns around and walks out of the seminar theatre in an act of brazen defiance.
You could have heard a pin drop. Normally, these events pass by without any rancour at all and everything is sweetness and light from start to finish. Here there was a definite feeling that someone thought I that I was being out of order and should be upbraided.
What I think this shows is that the heat pump market is maturing fast, perhaps a little too fast. By all means consider the merits of using a heat pump, but don’t get sucked in by the hype, and beware claims of extraordinary efficiencies achieved.
posted by Mark Brinkley @ 8:10 PM
4 Comments:
Mark, I have been arguing this very point for the past five years - particularly if natural gas is available for space and water heating (which it was in the case of my own self build). And that's from an economic point of view.
However, I've had a growing sense that there is a stronger environmental argument against GSHP, especially if nat. gas is an option. Taking a (generous) COP of 3.0 then there's a lot of electricity required for this operation to work and this electricity will be coming from predominantly "dirty" (i.e. coal fired) power stations. And let us not forget how inefficient the transport of this energy is - up to 70% losses down the grid. However, advocates of GSHP argue that if this electricity is puchased on a green tariff (ie produced from renewables) then that's OK. Well, it's not, because there isn't enough renewable electricity to go round, and there won't be for a long time, even if the industry adheres to the Government's target of an extra 1% p.a.
Electricity is essential for things like lighting and appliances such as fridges, freezers, TV's, computers etc where there is no alternative. To use precious renewable generated electricity for any form of heating (space or water) is sinful but to use it where there is an alternative (i.e. natural gas or solar) is unspeakable, and for the government to be encouraging this through the provision of grants is quite inexplicable.
If we analyse a the heating requirements of a house on purely CO2 emissions (taking the big picture - right back to the source, i.e. the power station or gas terminal) then the picture becomes somewhat different to the economic assessment. And if CO2 emissions is the big bad bogey then why is there even any discussion about using GSHP (if there's a nat. gas alternative)?
Or am I missing something here?
Archie Hunter
I was there with Mark and witnessed the spat he describes. The point is that any COP is achievable, if the manufacturer chooses to bend the figures. A COP of 4 is a reasonable claim and is not uncommon. But as Archie says, COP is not the whole story. Even at a COP of 4 the carbon emissions of your typical house will increase over gas or oil-fired boilers. Grid electricty has a carbon figure of 0.43kg/kWh compared to 0.25 for oil and 0.19 for natural gas. GSHP is a more efficient use of electricty, but is only zero carbon if you are producing your own electricty.
Archie,
I agree with your sentiments, but on a technical pont, I don't think the transmission losses over the grid are usually more than about 8% of the total. The great bulk of the losses occur in the power stations and are released as CO2 there.
In fact, the three big power stations in Yorkshire, which I saw on my way to Harrogate, namely Drax, Egborough and Ferrybridge, are between them responsible for around 20% of the UK's carbon emissions.
Guys. Heat Pumps do produce lower CO2 emissions. The figures you quote are reasonable, but you misapply them. You have to divide the emissions figures (which relate to input) by the efficiency of the heat generator, to obtain the emissions per kWh output. For example, with the GSHP heat pump (300% efficient), the figure will be 0.43 divided by 3, which equals 0.143 kgCO2/kWh output. For a gas boiler (let's assume 90% efficient), the figure is 0.19 divided by 0.9, which equals 0.21 kgCO2/kWh output. You can quickly establish that even an air source heat pump with a typical COP of 2.5 will still produce much less CO2 than a gas fired boiler for the same output.
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