Even at low temperatures, heat pumps still work

 From Joule/Science Direct

Main text

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.1 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.

Background

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”.

Penetration of heat pumps in 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.1 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.

Heat pump efficiency in mild cold climates 

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).2,3,4,5,6,7,8 

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.

 

Heat pump efficiency in extreme cold climates

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.9 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.10 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.11 These tests found that the COP remained relatively high, achieving 2.0 at −25°C and 1.8 at −35°C.

 In the past, when I have suggested that heat pumps are the way to go for heating, many have commented that heat pumps don't work at very low temperatures.  It seems that this was true for early heat pumps; but the newer ones, which use a different refrigerant (a kind of propane) are much better.  It may be that at the very coldest places (-30 C) , heat pumps won't work, and we'll still have to use more conventional heating.  But everywhere else, it's an option.  Heating buildings creates 10-12% of emissions.  So it's important that we find better ways to do it.