How is the seasonal performance factor of my heat pump calculated?
Posted by Graham Hazell on 26 September 2013 at 9:09 am
Q: I am still unclear how the seasonal performance factor (SPF) of my heat pump is calculated for the purposes of making tariff payments under forthcoming domestic renewable heat incentive (RHI). Is it simply based on the manufacturers' claims or is there some way of calculating it from real data?
A: The answer is neither!
The seasonal performance factor (SPF) of a heat pump is the pump's average output of heat over a full heating season. The SPF is the figure which department of energy and climate change (DECC) will use to calculate whether you are eligible for renewable heat incentive (RHI) payments when the domestic scheme starts in spring 2014.
On July 12, 2013 DECC announced that the derivation of the SPF will be purely based on the operating temperature of the system. Currently this means all equipment will be assumed to have the same efficiency at given flow temperatures, with the only differentiation being between whether your heat pump is ground or air source.
The Domestic Heat Emitter Guide (DHEG) will be used to determine the operating temperature of your system. Published by the Microgeneration Certification Scheme (MCS), the guide states that any air source heat pump system with a peak design flow temperature of greater than 53degC will be ineligible for RHI, as will any ground source heat pump systems with a peak design flow temperature of greater than 65degC. This is because these are the points at which the DHEG determines that the SPF falls below the EU minimum SPF threshold of 2.5.
Whilst this makes the analysis very simple it does raise a number of issues which trade associations, such as the Heat Pump Association, are attempting to resolve with DECC.
For instance, Manufacturer A may have a very efficient heat pump which is capable of achieving an SPF greater than 2.5 at say 55degC but which would not qualify if operated with 55degC as its maximum design flow temperature. Conversely, manufacturer B may have a heat pump which is inferior at higher flow temperatures. As these rise it may even have a poorer SPF at 50degC than Manufacturer A at 55 degC but that heat pump will still qualify!
In addition, the DHEG does not differentiate between deep vertical borehole ground source heat pump and a shallow horizontal array model.
The DHEG also assumes that systems utilise weather compensation energy saving flow temperature control. While all MCS equipment must be capable of this type of control, there is no requirement on the part of the user to actually engage it! This is a surveillance rather than DHEG issue.
These issues are currently being worked on by the heat pump certification body the MCS, whose working group are in the process of adopting ownership of the DHEG.
The trade associations are currently making a case to allow systems to be voluntarily metered in order to determine the renewable heat quantity where it is felt they can easily achieve the greater than 2.5 SPF threshold even though they may work at peak flow greater than 53degC or 65degC (air source and ground source, respectively). This would mean the electrical input must also be metered.
The question remains that what happens if, for some reason, the system does NOT provide an SPF of 2.5 or greater in which case the owner does not receive any RHI payments? Who is responsible? Was it due to inclement weather (in which case deemed systems would still receive assumed payments!)? Degree day correction could be used (assuming local data was available) but then it becomes very difficult for the non-expert heat pump owner to understand and predict performance.
There are some exceptions to note, in particular for second homes and bi-valent or hybrid systems, where the renewable heat output IS measured and hence directly rewarded up to a predetermined maximum kWh (kW rating x 1314 hours). This is basically to discourage excessive production of heat to receive payments which exceed the cost of the generating fuel (only over the first 7 years). In this case, it will be necessary to meter energy input in order to determine that the minimum 2.5 threshold has also been achieved.
More detail should be published in the next few months in preparation for April 2014 launch of the RHI.
Watch this space!
Picture: An extract from the Domestic Heat Emitter Guide, which is supported by DECC and was produced by trade associations representing heat pumps and heat distribution technologies.
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About the author: Graham currently works as a freelance specialist consultant and as a consultant to the Heat Pump Association.
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