CEEE Proposes Hybrid Solution to Improve Heat Pump Defrosting

In a newly published paper, a UMD Center for Environmental Energy Engineering (CEEE) research team examines two methods for addressing frost buildup on heat pump outdoor coils and suggests that combining them may offer a promising path to enhancing heat pump performance in cold climates.

The paper, published in Energy, offers a comprehensive literature review on two advanced defrosting techniques for air-source heat pumps: vapor injection and thermal energy storage (TES). After examining the strengths and drawbacks of each technology, the CEEE team proposes that a hybrid approach could deliver the best of both and would offset each strategy’s limitations.

Electric heat pumps have gained popularity in recent years because they offer an eco-friendly cooling and heating option, maintaining high efficiency across most climates. However, capacity and efficiency lag in cold climates, partly due to frost buildup on outdoor coils.

“Frost accumulation acts as thermal insulation and obstructs airflow, resulting in reduced heating capacity and decreased efficiency,” explains lead author Jangho Yang, a graduate research assistant with CEEE in the Department of Mechanical Engineering. Other authors include former CEEE postdoctoral researcher Cheng-Yi Lee, Senior Faculty Specialist Jan Muehlbauer and Research Professor Yunho Hwang.

The team found that while both vapor injection and TES defrosting strategies have been studied extensively on their own, no studies to date have explored their combined use. The researchers developed a preliminary theoretical hybrid model, which suggests that the combined coefficient of performance could improve by up to 62% under an extreme cold climate condition, compared to a single-stage system.

As a follow-up, the team will present a detailed account of the model in a future publication that will further analyze its potential for defrosting. That paper will include a comprehensive parametric analysis covering low-GWP refrigerants, outdoor climate variations, TES operating temperatures and compressor speeds.

Download the paper: "Advanced defrosting techniques in air source heat pumps: A review of vapor injection, thermal energy storage, and experimental frost accumulation data."

Published December 2, 2025