Coming in from the Cold: Heat Pump Efficiency at Low Temperatures

Duncan Gibb is a senior advisor at the Regulatory Assistance Project (RAP), a global team of highly skilled energy experts with a focus on heat decarbonization policy.Dr. Jan Rosenow is a principal and director of European programs at RAP, as well as an honorary research associate at Oxford University’s Environmental Change Institute.Dr. Richard Lowes is a senior associate at RAP.Professor Neil Hewitt is the Head of School at the Belfast School of Architecture and Built Environment at Ulster University. Duncan Gibb is a senior advisor at the Regulatory Assistance Project (RAP), a global team of highly skilled energy experts with a focus on heat decarbonization policy. Dr. Jan Rosenow is a principal and director of European programs at RAP, as well as an honorary research associate at Oxford University’s Environmental Change Institute. Dr. Richard Lowes is a senior associate at RAP. Professor Neil Hewitt is the Head of School at the Belfast School of Architecture and Built Environment at Ulster University. Heat pumps have emerged as a key tool in the global transition toward clean and reliable energy and have been identified in multiple net-zero scenarios as the most important future heating technology.1Rosenow J. Gibb D. Nowak T. Lowes R. Heating up the global heat pump market.Nat. Energy. 2022; 7: 901-904https://doi.org/10.1038/s41560-022-01104-8Crossref Scopus (19) Google Scholar A question frequently raised is how well these devices perform when temperatures drop below freezing, as some commentators and the media have repeatedly suggested that heat pumps cannot deliver useful efficiencies at lower temperatures. This commentary responds to this question by analyzing field studies with real-world performance data of air-source heat pumps. It finds that well below 0°C, heat pump efficiency is still significantly higher than fossil fuel and electric resistive heating systems at an appliance level. The standard heat pumps investigated in this commentary demonstrate suitable coefficients of performance for providing efficient heating during cold winters where temperatures rarely fall below −10°C, i.e., most of Europe. In extreme cold climates, such as where the lowest temperatures approach −30°C, performance data have shown that heat pumps can provide heat at efficiencies up to double that of resistive heating; however, more analysis is required. Even though heat pump efficiency declines during the extreme cold and back-up heating may be required, air-source heat pumps can still provide significant energy system efficiency benefits on an instantaneous and annual basis compared with alternatives. Air-source heat pumps typically use electricity to drive a refrigeration cycle that moves heat from a colder source to a warmer destination. One important aspect of measuring a heat pump’s performance is its efficiency. Other technical attributes relevant to performance, such as heating capacity, are not covered in this commentary. Heat pump efficiency is measured by the device’s coefficient of performance (COP), the ratio of the useful heat outputted to energy consumed. Typical COP values for heat pumps lie in the range of 3–6, indicating that 3 to 6 units of heat are created from each unit of electricity used. A year-round average COP of 3–4 is common for household applications. The temperature difference between a heat pump’s source (the outside air) and sink (heating supply location) plays a determining role in the COP and, therefore, its overall performance. If the source temperature dips and the sink temperature is maintained, the COP falls. Around freezing temperatures, air-source heat pumps also can experience a reduction in COP due to the defrosting of external components. Ground-source heat pumps typically provide a very high level of efficiency, even during cold weather. The reason is that soil temperature does not change significantly between seasons, resulting in a higher—and more constan—COP. In addition, ground-source heat pumps do not need to expend energy on defrosting. This commentary focuses on the performance of air-source heat pumps in mild European winters with average January temperatures above −10°C. We refer to these heating conditions as “mild cold climates”, whereas those with average temperatures below −10°C in the coldest month are designated “extreme cold climates”. Heat pumps have seen increasing deployment in many countries. Intriguingly, in Europe their use is most concentrated in countries with colder climates. These countries have installed heat pumps for decades and see the highest heat pump penetration both in terms of existing fleet and new sales, as shown in Figure 1. As of 2021, Norway had just over 60 heat pumps installed per 100 households, followed by Sweden and Finland (around 45 each) and Estonia (35), respectively.1Rosenow J. Gibb D. Nowak T. Lowes R. Heating up the global heat pump market.Nat. Energy. 2022; 7: 901-904https://doi.org/10.1038/s41560-022-01104-8Crossref Scopus (19) Google Scholar These countries also experienced the highest per capita sales in Europe during 2022. The data do not provide insights about the achieved efficiency of those heat pumps, but the large share of household installations suggests that heat pumps can effectively provide heating in colder climates. Many countries in Europe experience relatively mild winters. From 1990 to 2020, mean January temperatures across the European Union, the United Kingdom, and Norway ranged from 9.1°C in Portugal to −9.2°C in Finland. Around 80% of European households are in countries where mean January temperatures do not fall below 0°C and 95% of households are in countries where mean January temperatures are higher than −5°C. Such climate zones are not just restricted to Europe, as the data we have analyzed for this paper highlights. Our research collected raw performance data from seven different field studies, focusing on heat pump efficiency in mild cold climates. The datasets represent a range of climatic zones, heat pump models, and heat pump configurations from Switzerland (CH), Germany (DE), the United Kingdom (UK), the United States (US1), Canada (CA), China (CN) and an additional lab-testing sample from the United States (US2).2The Cadmus GroupResidential ccASHP Building Electrification Study. 2022https://e4thefuture.org/deep-dive-research-heat-pump-building-electrification/Google Scholar,3Lämmle M. Bongs C. Wapler J. Günther D. Hess S. Kropp M. Herkel S. Performance of air and ground source heat pumps retrofitted to radiator heating systems and measures to reduce space heating temperatures in existing buildings.Energy. 2022; 242122952https://doi.org/10.1016/j.energy.2021.122952Crossref Scopus (21) Google Scholar,4Prinzing M. Berthold M. Bertsch S. Eschmann M. Feldmessungen von Wärmepumpen-Anlagen 2020/21 (EnergieSchweiz). 2021.https://www.ost.ch/fileadmin/dateiliste/3_forschung_dienstleistung/institute/ies/wpz/sonstige_wichtige_dokumente/2021_bericht_feldmessungen.pdfGoogle Scholar,5Energy Systems CatapultElectrification of Heat - Interim Heat Pump Performance Data Analysis Report. 2023https://es.catapult.org.uk/report/electrification-of-heat-interim-heat-pump-performance-data-analysis-report/Google Scholar,6Safa A.A. Fung A.S. Kumar R. Performance of two-stage variable capacity air source heat pump: Field performance results and TRNSYS simulation.Energy Build. 2015; 94: 80-90https://doi.org/10.1016/j.enbuild.2015.02.041Crossref Scopus (74) Google Scholar,7Wu C. Liu F. Li X. Wang Z. Xu Z. Zhao W. Yang Y. Wu P. Xu C. Wang Y. Low-temperature air source heat pump system for heating in severely cold area: Long-term applicability evaluation.Build. Environ. 2022; 208108594https://doi.org/10.1016/j.buildenv.2021.108594Crossref PubMed Scopus (19) Google Scholar,8Johnson R.K. Measured Performance of a Low Temperature Air Source Heat Pump.US Dept. Energy. 2013; (https://doi.org/10.2172/1260317. http://www.osti.gov/servlets/purl/1260317/)Google Scholar These datasets are plotted with the average COP in relation to the average outside temperature (°C) (Figure 2). Each dot represents an observation of average COP for space heating and temperature measurements that are either instantaneous (as in CA, CH, CN, DE, and US2) or daily averages (UK and US1). The number of heat pump systems represented in Figure 2 is around 550, and there are 2,760 total measurements. The heat pumps are a mix of air-to-water (CH, DE, UK) and air-to-air (CA, CN, US1, and US2) systems. More information on the system configurations can be found in the supplemental information. When the outside temperature was between 5°C and −10°C, the mean COP across all systems was 2.74 and the median was 2.62, sufficient to meet heating loads at much higher efficiency than fossil heating and electric resistance heat alternatives. Field studies also have been conducted in extreme cold climates, which we consider to be below −10°C and approaching −30°C. In these temperature ranges, specially engineered “cold-climate heat pumps” are typically deployed. We analyze their performance results in extreme cold climate conditions. Some of the market-leading cold-climate air-source heat pumps were tested in Finland at very low temperatures.9SCANOFFICEVTT:n Testiraportit | Ilmalämpöpumppuvertailu. Scanoffice.2022https://scanoffice.fi/vttn-testiraportit-ilmalampopumppuvertailu/Google Scholar Models from Mitsubishi and Toshiba both provided COPs above 2 even at temperatures as low as −20°C. At −30°C, COPs were still between 1.5 and 2 for the Mitsubishi model and 1 and 1.5 for the Toshiba model. In field testing carried out in Minnesota (US3), the performance of central-ducted cold-climate air-source heat pumps was measured at four different sites.10Schoenbauer B. Bohac D. Kushler M. Cold Climate Air Source Heat Pump Field Assessment. Minnesota Department of Commerce, Division of Energy Resources, 2017https://www.mncee.org/cold-climate-air-source-heat-pump-field-assessmentGoogle Scholar Three of the sites returned COPs between 1 and 2 during heat-pump-only operation below −12°C. Field testing was also conducted in Alaska by the Oak Ridge National Laboratory (US4) using a cold-climate air-source heat pump.11Shen B. Baxter V. Abdelaziz O. Rice K. CCHP – Finalize Field Testing of Cold Climate Heat Pump (CCHP) Based on Tandem Vapor Injection Compressors.2017Crossref Google Scholar These tests found that the COP remained relatively high, achieving 2.0 at −25°C and 1.8 at −35°C. Figure 3 below depicts the range of COPs during mild cold climate conditions, from above 5 to just under 2. The minimum temperature point is the lowest measured temperature (albeit above −10°C) in each study. The width of the vertical “violin” indicates the number of measurements taken at that horizontal COP. The relationship between temperature and average COP is illustrated by the lower COPs seen in studies with colder temperatures. Importantly, all studies see an average COP above 2.4 between −10°C and 5°C. A summary of the heat pump performance in mild cold climate conditions (−10°C to 5°C) is shown in Table S1 in the supplemental information. A summary of the heat pump performance in extreme cold climates is shown in Table S2 in the supplemental information. It includes the average COP at the study’s minimum temperature, highlighting the performance of heat pumps in these extreme conditions. The studies analyzed for this commentary reveal two key findings: (1) standard air-source heat pumps can maintain average COPs between 2 and 3 in mild cold climates and (2) cold-climate air-source heat pumps can see COPs above 1.5 in extreme low temperatures, even at −30°C. Both ranges are relevant, as policies regarding heat pumps need to consider two distinct performance goals: heat pump effectiveness in meeting heating loads during both average conditions as well as short-term minimum temperatures. The results from the field testing suggest that heat pumps are an efficient heating solution across mild cold climates. Below 0°C, the COP maintains a level well above 2 in all cases, meaning that an air-source heat pump would operate at more than twice the efficiency of combustion or resistive electric heating technology. Strategies exist to improve heat pump performance in buildings. A key technique is reducing the heat delivery temperature. Many legacy and inefficient heating systems have relatively high water supply temperatures, in the range of 60°C–70°C.12Cantor J. Harper G.D.J. Heat Pumps for the Home.2nd edition. Crowood Press, 2020Google Scholar Lowering these can improve heat pump performance, as the difference between source and output temperatures decreases, increasing the COP.12Cantor J. Harper G.D.J. Heat Pumps for the Home.2nd edition. Crowood Press, 2020Google Scholar In hydronic systems, replacing just a small number of radiators to lower the required water supply temperature can greatly improve heat pump efficiency.3Lämmle M. Bongs C. Wapler J. Günther D. Hess S. Kropp M. Herkel S. Performance of air and ground source heat pumps retrofitted to radiator heating systems and measures to reduce space heating temperatures in existing buildings.Energy. 2022; 242122952https://doi.org/10.1016/j.energy.2021.122952Crossref Scopus (21) Google Scholar The question whether back-up heating is needed for extreme conditions is often raised. Several of the studies included in this commentary, such as (US3), used back-up resistive or combustive heating—or at least had it available in case it was needed. However, back-up heating was typically only engaged when the outside temperature dropped past −10°C or lower. Above −10°C, heat pumps were able to provide the required heat at relatively high efficiency. From a heat provision standpoint, this suggests that concerns over the need for back-up heating during mild cold climate conditions may be unfounded and the role for hybrid systems may be limited. There is an outstanding question over the role of hybrid systems in the coldest climates, not necessarily because of efficiency performance but because of the high output capacity of heat pumps needed at very low temperatures. Recognizing the limits of focusing only on efficiency, we suggest a valuable route for further research would be to explore the specific value of hybrid-type heating systems. In any case, to mitigate the impact of peak heating loads on energy systems, efforts can be made toward improving the performance of the building stock to minimize load and level off heating demand peaks, as well as encourage demand response. Some heat pumps are specifically designed for extreme cold. Though installing cold-climate heat pumps may de-risk performance in the coldest weather, there are potential trade-offs. For one, performance may suffer during milder temperatures. This is because the cold-climate systems are designed more specifically for frigid temperatures and higher heating demands. Physical components such as the expansion valve and compressor may struggle to operate at lower outputs. Strategies to address these issues can be found in the supplemental information. Heat pumps are increasingly used in various types of climates to provide space and water heating. Measured performance data show that heat pumps can provide the most efficient heating in many cold climates around the world. As most European countries experience milder winters with minimum temperatures above −10°C, our analysis suggests that heat pumps can be successfully installed in these conditions without concerns over performance or the need for back-up heating capacity. This is subject to thorough heating system design and a high-quality installation in a building.