The great heat pump mystery: where’s the COP?

Of all possible barriers that might impede the adoption of heat pumps in the United Kingdom, performance receives less attention across the literature than might otherwise be expected. If one was to review a decade’s worth of UK heat pump studies, the following issues are listed as the most probable barriers to adoption: high upfront costs, limited consumer awareness and understanding, complex installation process, limited availability of installers, building and retrofitting challenges, planning and policy barriers, and financing and incentive issues. When performance is mentioned, it is often in the context of performance ‘concerns’. I.e., the studies highlight the consumers’ perception of heat pump performance rather than an investigation into the technical delivery and function of the heat pumps in situ. This research gap is intriguing, given one of the heat pump industry’s persistent mysteries has been the distance between the performance of heat pumps in UK field trials and those conducted on the European continent.

As far back as 2010, the Energy Savings Trust conducted a field trial of 83 homes installed with either a ground-source heat pump (GSHP) or an air-source heat pump (ASHP). Simultaneously with collecting performance, energy, and heating data, the Open University surveyed these 83 households to ascertain their satisfaction and user experience. The findings were less than optimal:

“…some UK installations performed as well as average heat pumps in German and Swiss field trials. However, many did not (especially the GSHPs) and a number performed very poorly indeed, and only a few reached the minimum system efficiency (about 2.9 for the UK electricity supply mix) to count as a renewable energy technology under the EU Renewable Energy Directive.”

The magic number for heat pump efficiency is 3.5. If the heat pump delivers a coefficient of performance (COP) of 3.5, one unit of electricity produces three and a half units of heat. The heat pump leverages 350% of the electricity it draws, making it cheaper and less carbon-intensive to run than natural gas. And this is precisely the point. The UK intends to transition from natural gas as its primary heating fuel to electricity to meet its net-zero targets. However, this only makes sense because electricity is being utilised at an efficiency of 350%. In practice – outside the controlled laboratory setting – ground source and air source heat pumps generate an average COP of 2.2 and 2.0, respectively. Therefore, the results of the Energy Savings Trust trial pose something of a problem in need of a solution. Why do heat pumps not perform in the United Kingdom as in European trials? Why can’t we achieve a COP of 3.5?

The Open University attempted to answer this question by correlating variations in heat pump performance with their survey data. However, since their questionnaire dealt principally with consumer sentiment, the researchers acknowledged that “reasons for the underperformance of many of the UK field trial systems are not known”. They reported that “likely reasons uncovered in the user study include the lack of understanding among UK consumers of heat pumps and their operation… poor quality installations… [and] inappropriate design and sizing of some systems.” Over the past decade, the suggestion that the ‘size’ and ‘suitability’ of the unit for the home is the most likely reason for UK heat pumps’ missing COP has become popular. One study suggests that since the “heat pumps currently available in the UK are designed for these larger homes, the minimum capacity offered is 5 kW, which is much more than necessary to meet the winter load of a small well-insulated home.” This statement invites more questions than, perhaps, the researchers intended: “of a small well-insulated home.” Well insulated… What is the relationship between the insulation of a home and the COP of a heat pump? How well insulated are UK homes on average? How does the insulation of UK homes compare with those in Germany or Scandinavia, for example?

When further field trials have been conducted in England and Scotland to test the findings of the Energy Savings Trust trial, the insulation situation has been conspicuous. West Lothian District Council in Scotland ran a trial of ten homes installed with air-source heat pumps. Critically, the study notes the “building fabric was upgraded prior to installation of the heat pumps,” including 120 mm insulation for the cavity walls and 200mm insulation in the ceiling. With the ‘fabric first’ retrofit, the annual COP for the ASHPs was 2.7 – significantly higher than the Energy Savings Trust trial but still below the German standard of 3.5. Harrogate Borough Council in North Yorkshire also ran a trial. This time monitoring ten social housing properties retrofitted with ground source heat pumps. Once again, before the heat pump installation, the homes had been “upgraded with cavity-wall insulation, double-glazing and additional loft insulation.” Despite this focus on insulation, the research found that the “thermal time constant of the building is identified as a critical factor that needs to be considered in retrofit projects incorporating heat pumps.” This is a technical way of saying that the houses lost heat too quickly when the outside temperature dropped, undermining the efficiency the heat pumps were intended to deliver.

One issue in studies of heat pump performance across the UK is the “disparity between the COP values published by manufacturers and the… values experienced in real-world testing.” Heat pump manufacturers consistently report COPs of 3.5 or higher – the magic number – based on laboratory tests. These controlled environments, however, are often tested at “7°C external temperature (dry bulb) and 20°C indoor temperature.” The laboratory setting, therefore, differs from a typical English winter, where the average temperature is between 2 and 7°C and often just below 0°C – to say nothing of the heat loss that may be experienced by houses that are not suitably insulated or sealed.

There needs to be more attention paid to the question of whether heat pumps can generate a COP of 3.5 in all homes across the United Kingdom. If not all, then we must try to identify those houses which can be assured of a COP of 3.5 and those which cannot. Homes that can generate a heat pump COP of 3.5 (or higher) ought to be informed, incentivised, and motivated to adopt heat pumps in the near future. Householders that fall into this category (e.g., newbuilds, more modern constructions, and fully sealed and insulated homes) will reduce their carbon footprint and save money on heating in perpetuity. Unfortunately, these economic and environmental benefits do not accrue to householders whose homes cannot generate this order of COP. For these homes – those where COPs fall below 2.5 – perhaps a strategic retrofit, fabric-first approach to heat decarbonisation should be a priority rather than the heat pumps themselves. Householders have little incentive to swap gas heating for a heat pump if the heat pump produces just as much (or more) carbon and is costlier to run. To this end, a comprehensive mapping of UK housing that identifies where COPs of 3.5 or higher can be achieved and where they cannot would be accommodating for both researchers and policymakers alike.


Date: May 22, 2023 5:42 pm


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