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UMD Design Could Boost Cold-Climate Heat Pump Performance

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Researcher Jangho Yang adjusts the electrical controls on a cold-climate heat pump test rig that CEEE is developing.

Inside a UMD laboratory chamber, researcher Jangho Yang steps onto a stool and then climbs into a partially disassembled heat pump —  literally immersing himself in his work. Wrench in hand, the graduate research assistant with the Center for Environmental Energy Engineering, crouches among the unit’s internal components as he tackles a persistent challenge in cold-climate HVAC systems: keeping heat pumps efficient when temperatures plunge.

Heat pumps can provide highly efficient heating and cooling, and offer a climate-friendly alternative to fossil-fuel furnaces. “But when outdoor temperatures drop and frost builds up on the outdoor coil, performance decreases, and defrost cycles can interrupt heating,” Yang says. During the extreme cold, many heat pumps must rely on auxiliary heat as a backup. 

As a solution, Yang and his CEEE colleagues have developed an innovative design for improved cold-climate operations, and they are modifying an air-source heat pump to test their approach. The hybrid design combines two proven strategies: refrigerant injection and thermal energy storage (TES).

Refrigerant injection supports heating capacity and lowers discharge temperatures, which can spike on frigid days when the compressor works harder, accelerating oil degradation and reducing performance. TES stores thermal energy, which can be used during defrosting and even during subsequent heating, if enough energy remains.

In a recent paper in Applied Thermal Engineering, the CEEE team presented a theoretical analysis indicating that the hybrid approach has strong potential. The researchers developed a thermodynamic cycle model of an 18-kW air source heat pump, operating at a temperature range of 8°C to -25°C. They evaluated heating and defrosting modes with and without the refrigerant injection and TES integration. Analysis revealed that the hybrid approach could offer significant improvements in heating and defrosting operations.

“We know it’s going to be hard, but it’s worth it because this could eventually provide a way to improve heat pump operation in cold climates.”

CEEE researcher Jangho Yang

“Most importantly, this concept could allow defrosting while still supplying heat indoors, without relying on an auxiliary heater,” says Yang, the paper’s first author. The publication is co-authored by Research Professor Yunho Hwang, director of CEEE’s Consortium for Energy Efficiency and Heat Pumps.

For the theoretical study, the researchers screened four refrigerants. Based on discharge temperature and injection quality screening, R-290 was found to be the most suitable candidate, followed by R-410A and R-454B.  

R-290 showed a combined coefficient of performance of 2.22 for heating and defrosting, about a 60% increase over the theoretical single-stage baseline. However, the use of R-290, also known as propane, would require careful safety engineering and cost-benefit analysis because of its flammability and related requirements for sensors and other safety measures.

Back in the chamber at UMD’s Daikin Energy Innovation Laboratory, Yang is moving the research from theory to reality by converting a heat pump into a test rig for this hybrid strategy. “This is still a futuristic concept, but this setup will allow us to experimentally investigate the approach and consider some of the practical issues,” he says. “We know it’s going to be hard, but it’s worth it because this could eventually provide a way to improve heat pump operation in cold climates.” 

Download the paper: “Theoretical analysis of a refrigerant injection-enhanced air source heat pump with thermal energy storage for extended cold-climate and defrosting operations.”


Published June 25, 2026