The new SAP has a revised carbon intensity for grid electricity (set in the consultation at 0.591 kgCO2/kWh, up from 0.422). This has a big impact on the resulting carbon emissions from heat pumps, in most cases making them higher than emissions from the worst boiler you can legally install. This goes for both air source and ground source.
You can see from the graph above that at a grid carbon intensity of 0.591 even a GSHP with a COP of 4 is struggling to outperform an 86% efficient gas boiler. The real world COPs seen at Barratt’s Chorley scheme (2.6 for GSHP) and recent field trials by Mitsubishi (3.0 – 3.4 for ASHP according to a letter from Mitsubishi in the latest CIBSE mag) mean that heat pumps would emit significantly more carbon than the boiler.
And yet in the low carbon transition strategy, DECC state that heat pumps will be eligible for the Renewable Heat Incentive (pdf – see para 1.22), rewarding them for being a renewable energy source! What the hell are they thinking?
Here’s how I did the numbers:
Here’s the baseline using an 86% efficient gas boiler:
|Primary energy||Assumed efficiency||Useful energy||CO2 kg/kWh||kgCO2/yr|
For the heat pump scenarios I’ve assumed that all space heating comes from the heat pump, along with 60% of the domestic hot water load. The remainder of the DHW load is met from an immersion coil. Here are the numbers for a COP of 4:
|Useful energydemand (kWh)||Demand met (kWh)||Primary energy||CO2 kg/kWh||kgCO2/yr||Demand met (kWh)||Primary energy||CO2 kg/kWh||kgCO2/yr|
|Subtotal HP CO2||714||Subtotal backup CO2||2317|
Electric heating is getting a lot of attention, particularly off the back of David MacKay’s book. But until the grid is significantly decarbonised (a long way off) we shouldn’t be putting any incentives in place for heat pumps.
Note that my numbers assume a heat pump is only being used for heating (which is typical of domestic installations). If you use a GSHP for heating and cooling the picture changes and you can save significant carbon depending on the site.