On the contrary, a heat pump works by circulating refrigerants and changing their pressure and, therefore, their temperature, both to capture thermal energy from the outside air and then do the opposite in summer to act as an air conditioner. Over the years, home appliances have become increasingly more efficient as their various components and refrigerants have improved. “It’s really all about the refrigerant,” says Katie Davis, vice president of engineering and technology for residential HVAC at Trane Technologies, which makes heat pumps. “We are expanding and contracting, so we go from liquid to gas, from liquid to gas, from liquid to gas, or vice versa, depending on the cycle we are in.”
Particularly for climates with very cold winters, the boiling point of coolant is typically between -55 degrees and -59 degrees F. So, even if the outside air is below freezing, “the coolant will still boil,” says Davis. “You’re going to transfer heat very, very well.”
Manufacturers make Heat pumps designed specifically for cold climates., which can operate continuously when temperatures drop to negative levels. Trane is developing its own cold-climate heat pump that it hopes to launch in 2025, which uses steam injection technology. This works like fuel injection in car engines, only it injects coolant in a closed loop into the compressor. This increases the heat pump’s ability to extract thermal energy. “With the addition of this vapor injection compressor,” Davis says, “we now have the additional capacity we need to run our systems at these really cold temperatures.” In testing, the Trane prototype operated at -23 degrees F.
When scientists calculate the efficiency of different heating techniques, they consider the “coefficient of performance” or COP, which is the ratio of energy consumed to heat produced. If a technique is 100 percent efficient, it has a COP of 1, that is, one unit of energy in and one unit of heat out. A gas boiler, for example, produces heat that blows into a house, but some of that heat is also lost during combustion, so even the most efficient models have a COP of less than 1.
In general, it is much more efficient for a heat pump to move heat than to generate it, as a gas boiler does. By running on electricity instead of fossil fuels, a heat pump can manage a COP of 3, that is, three units of heat for each unit of energy, but in extreme cases they can reach a COP of 6, depending on the conditions. and the model.
in a study Published last year, Rosenow and his colleagues analyzed the data to see how a heat pump’s efficiency could decline as temperatures drop. They found that even at -10 degrees Celsius or 14 degrees Fahrenheit, appliances still achieve a COP of 2, or 200 percent efficiency. The study also looked at cold climate heat pumps in more extreme environments: at a temperature of -30 degrees C (-22 degrees F), a Mitsubishi model produced COP between 1.5 and 2, and a Toshiba model between 1 and 1.5.