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Some utilities and fossil-fuel interests like to say heat pumps don’t work in the cold. A new study provides yet more evidence to debunk that misrepresentation.
Alison F. Takemura
Not only do heat pumps function in freezing temperatures — they work far more efficiently than fossil-fuel heating systems in the cold.
That’s according to a team of researchers in Europe affiliated with the independent nonprofit Regulatory Assistance Project. They published a study in Joule this week that provides yet more evidence to debunk the myth that heat pumps can’t handle cold climates.
Electric heat pumps both heat and cool indoor spaces by moving heat into or out of them as needed. And while global sales grew by 11 percent in 2022, according to the International Energy Agency, heat pumps still only account for about a tenth of the world’s building heating. To achieve the Paris Agreement’s target of net-zero emissions by 2050, heat pumps will need to replace far more fossil-fuel boilers and furnaces — including in places with frigid winters.
Extreme cold historically has been a barrier for the technology, with major utilities and fossil fuel interests asserting that heat pumps don’t work below freezing and pointing to drops in efficiency as evidence. But as the new study and examples from places including Norway and Maine have shown, modern heat pumps are reliable and outperform fossil-fuel heating in the cold.
To find out how well air-source heat pumps work as temperatures plummet, the team analyzed data from seven field studies across three continents, drawing on observations of different heat-pump models from Canada, China, Germany, Switzerland, the United Kingdom and the U.S.
These studies reported on heat-pump performance in the depths of winter — January — using a metric called the “coefficient of performance.” COP measures how much thermal energy you can get out of a heating system for every unit of energy you put in. Heating technologies that burn fossil fuels or use electric resistance convert one form of energy into another, so they hit a thermodynamic limit at 100 percent efficiency, or a COP of 1.
But heat pumps cleverly move heat around using refrigerant. They can routinely achieve COPs of 3 to 4, though higher values are possible, according to Duncan Gibb, senior advisor at the Regulatory Assistance Project and co-author of the new study. For instantaneous measurements (as opposed to those averaged out over a day), “I’ve seen some data where it gets up to 7.”
Efficiency declines when temperatures drop, however, as the gap between outdoor and desired indoor temperature widens. (This is the major reason why geothermal heat pumps, which draw heat from the earth and are much more insulated from the temperature swings in the ambient air, are more efficient.)
Even though the COP of air-source heat pumps declines as temperatures fall, the team found that heat pumps surpass fossil and electric-resistance systems in efficiency — including at temperatures that define, for many Europeans, “the coldest days of the year,” Gibb said.
Ninety-five percent of European households dwell in countries where mean January temperatures are higher than -5°C (23°F), according to Gibb. In those conditions — and indeed even at lower temperatures — heat pumps still hum along with a COP of 2 to 3, as shown in the chart.
Specially designed cold-climate heat pumps can produce heat in even more extreme weather, the team found, although these results aren’t shown in the chart above. Heat pumps tested in temperatures as low as -30°C (-22°F) in Finland ran with a COP of 1.5 or higher, Gibb said.
These observations are critical for countries figuring out how to get off fossil fuels, including France, Germany, the Netherlands and the U.K., Gibb noted. For Europe, “there are very few — if any — technical conditions where a heat pump is not suitable based on the climate.”
That helps explain why heat pumps are taking off in some colder parts of the continent. The Scandinavian countries of Norway, Sweden and Finland have some of the frostiest winters in Europe, but they are also some of the most enthusiastic heat-pump adopters. These countries experienced the highest per capita heat-pump sales in Europe in 2022, according to the authors.
As for the U.S., in 2022, Americans bought more heat pumps than gas furnaces. That includes success in Maine, which despite its bone-chilling winters has already blown past its goal to install 100,000 new heat pumps by 2025. In July, it upped its pledge to deploy 175,000 more by 2027. Customers in both Maine and Colorado have attested that their heat pumps kept them cozy during extreme cold snaps in the last couple of years.
Still, the myth of heat-pump insufficiency persists. Ironically, a nation with milder winters, the U.K., is one of the places Gibb has seen the misconception most. In media stories and social posts, people have reported being dissatisfied with their heat pumps. It can happen, Gibb acknowledged, but it isn’t necessarily a reflection of the tech’s capabilities, he said. “In many cases, they just got a bad installation.”
These types of stories represent “a pretty dangerous line of misinformation,” Gibb said, “because it erodes public trust in [heat-pump] technology when it can achieve very high performance.” The U.K. installed less than a tenth of the number of heat pumps that France did last year.
What’s more, companies in the fossil-fuel business have pounced on even slight declines in heat-pump efficiency as a valid reason to delay a transition to clean heat. For example, in July, U.S. utility Xcel Energy argued that heat pumps should play a limited role alongside the continued use of fossil gas as part of efforts to decarbonize Colorado, citing a recent report the utility had funded. Preliminary results found a modest drop in heat-pump efficiency, 5 to 12 percent, at higher altitudes — the testing facility was about a mile above sea level.
Xcel also stated that while heat pumps do well above 40°F, “their performance degrades at lower levels.”
For those reasons, “most homes using heat pumps for heat would require additional backup heating, either gas-fired or resistance electric heating,” Xcel concluded.
“It’s true that the performance does decline when it gets colder. That’s a fact,” Gibb said. But when “entrenched interests” use that to write off heat pumps, they’re ignoring the fact that the technology still handles cold weather reliably — and more efficiently — than fossil-based alternatives.
Alison F. Takemura is a staff writer at Canary Media.
Reprinted with permission from Canary Media’s blog on September 13, 2023 found at https://www.canarymedia.com/articles/heat-pumps/heat-pumps-outperform-boilers-and-furnaces-even-in-the-cold
Air-source heat pumps keep churning out heat in cold temperatures. Each dot is either a heat pump’s instantaneously measured COP at a specific temperature or averaged across a day’s range of COP and temperature values. Data is from seven field studies. (Joule)
The outdoor unit of an air source heat pump operating in freezing conditions. (Wikipedia)