Study Suggests Design Change to Make Heat Pumps Work Better on Frigid Days

Eco-friendly heat pumps have outsold gas furnaces in recent years in the United States, but they can struggle to function efficiently in biting cold temperatures like the DMV experienced in January.

Performance of a key element of a heat pump—the compressor—deteriorates as the mercury falls, and may necessitate backup heat. A new study in Applied Thermal Engineering by researchers at the UMD Center for Environmental Energy Engineering (CEEE) finds that heat pump performance could be significantly improved by breaking the compression process into multiple stages.

“Instead of just going from low pressure to high pressure, you go up a step at a time, and that makes the process much more efficient,” with a two-stage heat pump providing up to 59% more heating capacity, said lead author and former CEEE postdoctoral researcher Andrew Fix, who recently joined the faculty at the University of Texas at Austin.

The other authors include mechanical engineering graduate research assistants Dana Kang and Md Mehrab Hossen Siam, CEEE Co-director Yunho Hwang, and CEEE Director Reinhard Rademacher, who passed away last month. The researchers compared the impact of nine different refrigerants, the majority of which are natural refrigerants, like propane, with low or negligible impact on climate change, and found that natural refrigerants offered similar performance to more commonly used synthetic refrigerants.

As a next step, the research team is developing a physical prototype of a rooftop cold-climate two-stage heat pump using propane, with the ultimate goal of building an eco-friendly, energy-efficient heat pump that performs well in extreme cold.