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"The best performing systems show that well-designed and installed heat pumps can operate well in the UK.” Those are not the words of your correspondent, although they could be, but the words of the Energy Saving Trust in a report on its heat pump field trial. The field trial covered 83 heat pumps – 29 air source and 54 ground source – at various locations across the UK. The trial monitored the installation and performance of heat pumps for 12 months from 2008 until 2009. The heat pump industry has learnt from that experience but the fact is that customer experience of heat pumps varies widely — and that variation is based almost entirely on running costs.
‘Coefficient of Performance’ (COP) is a measure of the efficiency of a heat pump. A typical figure would be COP 4 which means that for every 1kW of electricity consumed by the heat pump it outputs 4kW of heat. This is often stated as 400% efficient — which can be misleading. The COP is calculated by the manufacturer on a specific set of criteria, which may or may not include such things as circulation pumps and defrost cycles but will not include the electric heater (immersion or cassette) used to raise the water temperature above space heating temperature. The COP will also relate to specific circumstances. Typically these will be an outside temperature of 0°C or 7°C, and a flow temperature of 35°C or 40°C. Which leaves the question, what is the COP when the circumstances change?
Essentially there are three factors: the amount of heat needed by the house, the efficiency of the heat pump and the temperature of the heat source.
Heat pump efficiency varies between manufacturers but within fairly close limits. Water-source heat pumps can get up to COP 5 and some air source can fall below COP 2.5, but outside those extremes, COP will tend to be in the 3.0 to 4.3 range. The actual efficiency of a heat pump is a factor of the amount of work it has to do — the difference between outside temperature and inside temperature. The closer they are together the less work the heat pump has to do and the more efficient it is. Heat pumps can reach output temperatures of 65°C or more, if we design them to do so, but efficiency will fall and running costs increase. That is true of all heat pumps — it lies in the physics of the technology and there is no way around it.
Let’s consider a two storey house of 200m2 floor area built to 2010 Building Regulations standards. That gives us a known space heating demand of 55kWh/m2/yr, so for 200m2 we need 11,000kWh per year. In addition we need domestic hot water (DHW) and for that, assuming four people are living in the house, we need 4,000kWh.
A fairly new condensing gas boiler operating at 85% efficiency will cost £1,005 per year to meet that demand — £734 for space heating and £271 for DHW. An oil-fired boiler operating at the same efficiency will cost some £1,615 per year.
A good-quality ground-source heat pump delivering space heating only will operate at a COP of 4.3. That means that to produce the 11,000kWh of space heating it will need2,558kWh of electricity at a cost of £349. But we still need DHW. If the heat pump is properly set up, it will raise water temperature to 40°C and the on-board electric heaters raise it to the required storage temperature. Done in that way we maintain the COP at 4.3 for part of the heating cycle and minimise the work the electric heaters do (at effectively COP 1.0). The cost of DHW will be some £395, giving a total running cost of £744 per year — a 26% reduction in annual running cost.
When outside air temperature is above 7°C, a reasonable air-source heat pump will operate at COP 3.2 when delivering space heating to underfloor heating. But Met Office data shows that the average UK temperature from November to March (from 1971 to 2001) is consistently below 7°C – the monthly average varies from 4.2°C to 6.9°C – so the COP of the heat pump when we actually want to use it is likely to be lower than advertised. The actual operating COP will be closer to 2.8, allowing that it will vary as outside air temperature will vary. In this case the heat pump will need 3,928kWh of electricity for space heating at a cost of £510. If the heat pump is set up to produce DHW in the same way as for the ground source then we must add £460 to running costs, giving a total of £970 per year. That is a reduction in running costs of just 3.5% compared to a gas boiler.
Using the heat pump to produce DHW will always increase running costs. As above, if the ground-source heat pump is used to deal with space heating only, the running cost (for that element) reduces by 52%. Add DHW and the reduction is just 26%.
A house with a high heat load (poorly insulated, draughty) needs a higher flow temperature to maintain the desired internal temperature. A higher flow temperature increases work load and reduces efficiency. This is often where air source fails.
But by far the most common reason for high running costs is poor installation or installation in unsuitable properties. The big one here, for ground-source heat pumps, is the ground array: insufficient pipework in the ground or pipes laid too close together. The ground has a finite quantity of heat stored in it (it is solar heat we are using, not geothermal). If we try to extract more than is there or extract it too quickly then theheat pump is working hard to get some heat where there is none. The COP drops significantly and running costs rocket.
The rules for heat pumps remain largely as the EST found: use them in a well insulated house, only use them with underfloor heating or low flow temperature radiators and have a separate heat source for DHW, ideally solar panels. Flaunt the rules and we have an expensive machine with high running costs and CO2 emissions.
The Energy Saving Trust (EST) field trial found that the actual COP over a 12 month period for ground source varied between 1.3 to 3.6 and for air source from 1.2 to 3.3. In most cases the COP was lower than expected and the reasons given were:
The Renewable Heat Incentive has been set up by the Government to provide an annual income to homeowners who install low- or zero-carbon heat sources. It is scheduled to come into operation in autumn 2012 and the amount to be paid for each technology has not yet been set. The amount under consideration for heat pumps is 7p per kWh. The amount paid is for the renewable element of the heating. So in our average self-build*, the heating demand is 15,000kWh (11,000 space heating and 4,000 DHW), less the electricity used to produce it. With the ground-source heat pump we need to buy 5,605kWh so we will be paid for 9,395kWh and receive £657.65 per year. (Assuming that the scheme actually startsand that the tariff is 7p/kWh.)