CEEE Study Finds Heat Pump Performance Could Be Improved With Multi-Stage Compression
Brrrr. It’s been cold out there. On frigid days, like we’ve experienced recently in College Park, Maryland, we rely on heating technology to keep us safe and toasty. More and more often that means heat pumps, which have outsold gas furnaces over the past couple years in the United States. While electric heat pumps offer an eco-friendly solution, they sometimes struggle to perform efficiently in cold climates, with compressor performance deteriorating as temperatures dip, and may necessitate backup heat. A new study 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. In the April 1, 2025, issue of Applied Thermal Engineering, now available online, the CEEE team presents a theoretical model for a heat pump with multiple compression stages. “The problem is that the colder the outside temperature, the lower the pressure of the refrigerant used in a heat pump, which makes it harder and harder on the compressor,” explains lead author and former CEEE postdoctoral researcher Andrew Fix, who recently joined the faculty at The University of Texas at Austin. “The way we addressed this was to break the compression process into chunks,” Fix says. “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.” The other authors include graduate research assistants Dana Kang and Md Mehrab Hossen Siam, CEEE Co-Director Yunho Hwang and CEEE Director Reinhard Rademacher, all of the Department of Mechanical Engineering. The team compared a baseline cycle against two-stage and four-stage saturation cycles, modeling their performance at frigid temperatures, −25°C to 10°C. The study found that at −25°C, a two-stage saturation cycle heat pump using R290 (propane) could provide up to 59% more heating capacity, compared to a baseline cycle using the same compression power. 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. “We think natural refrigerants are the future,” Fix says. The question is: How do they compare with synthetic refrigerants? “Natural refrigerants offer a very similar performance . . . and they’re more environmentally friendly,” he says. The study also addressed another challenge facing cold climate heat pumps. When it's frigid outside and the compressor exerts itself, the refrigerant inside the system heats up. Hot refrigerants degrade the oil used to lubricate the compressor, which could lead to damage and equipment failure. The team modeled the impact of injecting refrigerant between each compression stage to help cool the system and demonstrated that it could dramatically reduce compressor discharge temperatures. “By injecting, or spraying, some cool refrigerant into the hot refrigerant, you can bring it back to a reasonable temperature,” Fix explains. This technique combined with the use of multiple stages offers a promising approach to improving the performance of cold-climate heat pumps. The study shows that a two-stage saturation cycle and four-stage saturation cycle, with two-phase refrigerant injected between each cycle,can reduce the compressor discharge superheating by up to 51–57% or 73–80%, respectively, depending on the refrigerant. 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 the extreme cold. Download the paper: “Multi-Stage Heat Pump with Two-Phase Injection for Cold Climate Applications Implementing Natural Refrigerants.” The full paper is available for free download until March 4, 2025.
January 27, 2025 Prev Next |
