INDUSTRIAL & COMMERCIAL HEATING HEAT PUMPS
UK* Air source
➤ 60% of installations: SPFs 1.7 – 2.2
➤ 30% of installations: SPFs 2.3 – 3.2
Ground source
➤ Range of SPFs <2.0 – 3.2
➤ 30% of installations: SPFs 2.3 – 3.2
Germany ➤ Range of SPFs 2.3 – 3.4
➤ Cluster of SPFs around 2.5 – 2.6 (retrofit) and 2.9 (new build)
➤ Range of SPFs 2.6 – 5.0
➤ Nearly all of which above 3.0
Switzerland ➤ Range of SPFs 2.2 – 3.0
➤ Half the units in the range 2.5 – 2.8
➤ Range of SPFs 2.7 – 4.0 ➤ Average SPF 3.4
Figure 2: Seasonal performance of heat pump systems recorded in field trials Sources: Figure courtesy of Delta Energy & Environment (
www.delta-ee.com) based upon data from Energy Saving Trust, Fraunhofer Institute ISE and Swiss Federal Office of Energy. *The Energy Saving Trust’s figures for the UK are not a direct measure of the seasonal performance factor because they added losses from the hot water system to the load, so the actual SPFs (the standard comparison measure) might in practice have been a little more favourable (see main text).
some circumstances impact very badly. There are some very poor systems out there. The good news is that, in many cases, it
would be relatively easy to improve things significantly – and there are also some excellent installations that take on board the special requirements that heat pumps call for.
That being so, what could ASHPs really
deliver in the UK? The short answer is, we cannot yet know, until we start setting up and using them properly, and monitor and report on the performance. However, combining the data available
In other words, a heat pump on its
own can’t give a predictable, guaranteed performance (whatever it might say in the specification or sales blurb). A heat pump only has POTENTIAL efficiency. The ACTUAL performance is that of the entire system, of which the heat pump is only one element. A ‘system’ would include heat pump,
space heating system (heat emitter system), hot water system (tank, pipe-runs), integral electric back-up heaters (if used), heating and hot water controls, building fabric – and also the building users. The weather also plays a role. Any one of these system elements can impact on the overall performance – and, in the authors’ experience, they often do, and they can in
from the UK and abroad, plus some practical experience, we could take as a starting point the proposition that an ASHP correctly installed into an existing UK home, and supplying both space heat and hot water, might achieve on average an SPF of around
A heat pump only has potential efficiency. The actual performance is that of the entire system, of which the heat pump is only one element
2.6. In a new-build dwelling with a lower heat load, and more freedom to specify a heat pump-friendly system, it might be reasonable to expect SPFs of around 3.0. The SPF of any system has a relation
Definitions How is efficiency expressed – and can we trust the figures?
COP: Coefficient of performance is a measure of energy efficiency. It’s the useful heat output divided by the electricity input. However, the COP is an instantaneous reading. Since the COP will vary dramatically depending on the operating conditions, it should always be quoted for a fixed set of conditions. (For air-source heat pumps these are the outside temperature, and temperature of the heated water produced by the system.) SPF: Because the efficiency of a heat pump varies depending on various conditions, we need to consider the average
54 CIBSE Journal October 2011
over the year. This is given by the ‘seasonal performance factor’, which is the total useful annual heat produced, divided by the total electrical input. The electrical input should include any direct electric ‘top-up’ heaters that might be necessary to cover any shortfall in the heat output, or to elevate the hot water cylinder temperature, in the case of pasteurisation for legionella protection, for example. It will also include any energy used to defrost the heat exchanger in cold weather. The SPF would be measured
by placing a heat meter in the system. This measures the liquid flow rate (liquid
being either water or a glycol mix) and also measures the temperature rise (or fall) of the liquid flowing through the system. The heat output is automatically calculated and recorded in kWh. The electrical consumption is also recorded. The ratio of these two values gives the SPF. Since this figure is derived from four variables, and each has its own inaccuracy, the end result can have a considerable error. Heat meters must be calibrated, and the accuracy should be known. Some skill may be required to position the sensors correctly so as to capture the actual useful heat delivered accurately.
with the installation cost. If money were no object, systems could be better, and SPFs could be much higher. Systems tend to be a compromise between capital cost and operating efficiency – this applies not only to heat pump installations, of course, but many other aspects of building design.
Are they green? So, if an SPF of 3.0 would make an ASHP worthwhile to at least some users in some circumstances, how does this level of efficiency measure up in terms of carbon emissions? The graph in Figure 3 shows the carbon content of mains gas and oil, and compares it to electric heat pumps with SPF up to 4.1. We can see by the blue line that we would currently need an SPF of 1.7 to achieve break-even CO2 emissions compared with oil, and an SPF of about 2.2 to achieve comparability to gas. However, for the green claims to hold
true, break-even is not enough. To achieve a worthwhile benefit, the COP must be
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