Heat pumps trial – verdict in
The long awaited, real-world trial of heat pumps has been published by the Energy Saving Trust (EST). It reveals some interesting and for some suppliers, potentially uncomfortable, findings. On paper heat pumps offer huge potential as a low carbon way of extracting solar energy from the ground, air or water to provide domestic space heating and hot water. So knowing just how efficiently they can perform in the UK is important as part of efforts to cut domestic CO2 emissions.
Up until now we have been reliant on performance data supplied by manufacturers from testing under ideal conditions. Not surprisingly, the claims made about heat pumps have often been extravagant. So do these stand up in the real world? On the basis of this trial the answer is, in-part.
The study monitored 83 installations from north east Scotland to south west Cornwall over the course of a year. The ‘mid-range’ system efficiencies* for ground source heat pumps were between 2.3 and 2.5 with the highest above 3.0. For air source heat pumps the mid-range system efficiencies, were around 2.2 with the highest in excess of 3.0. What the EST found was wide variation in performance between installations, and that householders’ behaviour has a bearing on system efficiency. It is also clear that in some cases the systems were poorly designed, had poor controls, and householders had little understanding about how to operate them.
Heat pumps work most efficiency in very well insulated homes with low temperature distribution systems such as underfloor heating or low temperature radiators. In existing properties the trick is to ensure that these are included in the system design and the heat pump is not simply bolted on to replace an existing boiler.
Heat pumps have found a ready market in new homes particularly those built to Code for Sustainable Homes Level 4 and above. But in our view often developers would do better to invest in more insulation, controlled ventilation and create homes which are ‘self-heating’ (passively heated and cooled) eliminating the need for space heating altogether.
When correctly installed, specified and operated heat pumps can provide a low carbon alternative to conventional forms of space heating and hot water. In some cases heat pumps will achieve lower emissions than gas CHP systems. What this study, which continues for another year presents, is the real world performance data lacking up to now, as well as the key issues for specifiers, developers, and householders.
Getting Warmer: A field trial of heat pumps, by the Energy Saving Trust can be downloaded from here.
Heat pumps are categorized as a renewable energy (heat) technology because for every kWh of electricity needed to operate the pump 2 to 5 kWh of heat are produced.
The performance of heat pumps is described in terms of their Coefficient of Performance (CoP) and System Efficiency.
The CoP is defined as the ratio of heat produced by the pump to electrical energy used to operate the pump. So a CoP of 3 means that 3 kWh of heat are produced for each kWh of electricity used to operate the pump.
*System efficiency is the ratio of heat produced by the heat pump to the amount of electrical energy needed to run the entire heating system (including domestic hot water – if applicable, supplementary heating and pumps).
Tags: Renewable energy, heat pumps.