Ground source heat pumps not so green

At first glance, the green credentials of ground source heat pumps (GSHPs) look unquestionable: because you’re harvesting free heat from the ground, you can get up to four times more energy out of the system than you put into it. Sure, it runs on electricity, which is more carbon intensive than gas, but because of this favourable ratio of output-to-input (called the COP for coefficient of performance) the system should still emit less carbon than a gas boiler - in theory.

But the claimed benefits are reliant on incorrect assumptions. A new house will emit about the same carbon using a ground source heat pump as with a new gas boiler. Here’s why:

The key factor is the carbon intensity of electricity from the grid. This is commonly stated as 0.422 or 0.43 kgCO₂/kWh. But these figures were proposed several years ago, when everyone thought the steady decline in grid intensity would continue. And it didn’t. Instead it levelled off at around 0.53 kgCO₂/kWh as shown in the graph below. (Data from BRE available here - pdf). For a realistic carbon calculation, we need to use a realistic grid intensity figure.

In addition, to maintain a high COP, GSHPs need to keep their output temperature low: around 40°C. This is why they work so well with underfloor heating. But domestic hot water should be stored nearer to 60°C, so a backup system is required to heat up the water the rest of the way. This is usually an electric immersion coil, similar in principle to an electric kettle, which requires more grid electricity - but this time without the COP advantage of a ground source heat pump.

For the purposes of comparison here’s a baseline house, using a condensing gas boiler and grid electricity.

Now here’s our house with a ground source heat pump and immersion coil backup for domestic hot water. I’m assuming the GSHP meets 60% of the DHW load.

So our GSHP house saves less than 2% of carbon relative to the baseline. That hurts, especially when you consider that installed costs for a GSHP are around £1000/kW, six to ten times as much as a gas boiler.  In addition, the above figures assume a COP of 4, but in practice the performance isn’t always as good as this. For example, Barratts recently found that the GSHPs at their scheme in Chorley Lancashire are averaging a COP of about 2.6.

So here’s an illustration of how the assumed carbon intensity of the grid affects the carbon savings of GSHP versus the baseline gas boiler system:

From the above graph it looks like the scheme at Chorley would have emitted much less carbon if they’d stuck with condensing gas boilers.

Having said that, the environmental performance of GSHPs does look better if compared against a system based on a higher carbon fuel, such as heating oil. Here’s the same table, this time compared against a baseline using heating oil at a carbon intensity of 0.265 kgCO₂/kWh:

So the GSHPs look better here. At today’s grid intensity, a GSHP with a COP of 2.6 (like the ones at Chorley) save 6.5% carbon vs. a new system based on heating oil. At a COP of 4, they save 17% (though a gas system would save 15.5% anyway since gas is so much less carbon intensive than heating oil).

Where does this leave GSHPs? The use of domestic GSHPs where gas is available is very questionable. If gas is unavailable and you’re replacing a high carbon fuel such as heating oil (and the GSHP performance is able to match manufacturers’ claims) then they do make environmental sense. But you’d need to look carefully at whether you’d get more carbon benefit by spending your money elsewhere (e.g. biomass).

And of course you could always power a GSHP from a turbine or PVs but again, you may want to look carefully at alternative heating systems that would give you more carbon savings for your money.