In contrast, a heat pump works by circulating a refrigerant and changing its pressure, and therefore temperature, to pull in thermal energy from the outdoor air in the summer and then vice versa, acting like an air conditioner. Over the years, improvements have been made to the various components and refrigerants, making the units more and more efficient. “It all depends on the refrigerant,” says Katie Davis, vice president of engineering and technology for residential HVAC (heating, ventilation, and air conditioning) at Trane Technologies, which makes heat pumps. “It expands and contracts, so depending on what cycle you’re running in, it can go from liquid to gas, liquid to gas, liquid to gas, or vice versa.”
In very cold winter climates, it’s crucial that the boiling point of the refrigerant is typically between minus 55 and minus 59 degrees Fahrenheit, so even when the air outside is below freezing, “the refrigerant will still boil,” Davis says. “It allows for really good heat transfer.”
Manufacturers are building heat pumps designed specifically for cold climates, capable of continuous operation even when temperatures drop below zero. Trane is developing its own cold-climate heat pump, which it plans to launch in 2025. It uses vapor injection technology, which works like fuel injection in a car engine, except that it injects refrigerant into the closed-loop cycle of the compressor. This increases the heat pump’s ability to extract thermal energy. “The addition of this vapor injection compressor gave us the additional capacity we needed to run the system in these frigid temperatures,” Davis says. In tests, Trane’s prototype operated at minus 23 degrees Fahrenheit.
When scientists figure out the efficiency of various heating technologies, they consider their “coefficient of performance,” or COP, which is the ratio of energy consumed to heat produced. If a technology is 100 percent efficient, its COP is 1: one unit of energy goes in, and one unit of heat gets out. For example, a gas furnace produces heat and blows it into your home, but some of that heat is lost during combustion, so even the most efficient models have a COP of less than 1.
Overall, heat pumps are much more efficient at moving heat than they are at generating it, like gas furnaces. Because heat pumps run on electricity rather than fossil fuels, they can achieve a COP of 3, meaning they can produce three units of heat for every unit of energy, although in extreme cases the COP can be up to 6, depending on the conditions and model.
In a study published last year, Rosenow and his colleagues looked at data to see how heat pump efficiency drops as temperatures drop. They found that even at minus 10 degrees Celsius (14 degrees Fahrenheit), the units maintained a COP of 2, or 200 percent efficiency. The study also looked at cold-climate heat pumps in more extreme environments: At an extreme temperature of minus 30 degrees Celsius (22 degrees Fahrenheit), the Mitsubishi model had a COP of 1.5 to 2, and the Toshiba model had a COP of 1 to 1.5.
