Study Investigates How to Optimize PCM-Integrated HVAC Systems

As global energy demands rise, the UMD Center for Environmental Energy Engineering (CEEE) and its collaborators are working to develop smarter ways to heat and cool buildings. A new study by CEEE and Oak Ridge National Laboratory investigates how phase change materials (PCMs) — substances that absorb and release heat as they change physical state — can be used more effectively to improve the performance and reliability of heating and cooling systems.

PCM-integrated thermal energy storage (TES) systems offer a promising approach to improving HVAC efficiency, because they store energy during low-demand times and draw on it during peak times. However, key questions remain about how to optimize PCM-based TES; the CEEE-Oak Ridge study addresses one of the core research gaps.

The study, published in Applied Thermal Engineering, offers guidance on how to best manage refrigerant subcooling in PCM-based systems. In heat pumps and air conditioners, performance depends partly on how much the refrigerant is cooled below its condensation temperature in the condenser — a process called subcooling.

“While many studies have explored subcooling optimization in conventional vapor compression cycle systems, little research has been done on subcooling in PCM-based systems,” says co-author Research Professor Yunho Hwang, director of CEEE’s Consortium for Energy Efficiency and Heat Pumps, in the Department of Mechanical Engineering. Other co-authors are CEEE alum Yiyuan Qiao, Ph.D. ’23, and Yifeng Hu, both on the R&D staff at Oak Ridge National Laboratory.

The researchers developed a parameter called nominal subcooling degree (NSD) to evaluate subcooling effects in a PCM-integrated condenser, enabling consistent comparison of condenser behavior across different operating conditions. They studied the effects of different NSDs on PCM-based heat pump performance in a controlled environmental chamber at CEEE’s Daikin Energy Innovation Laboratory.

The results indicate that there is no single “best” subcooling level for all situations. Instead, NSD selection should be application-specific. A moderate NSD of approximately 5 K is preferable for short-duration operation near the PCM melting range, while a smaller NSD of approximately 0.8 K is more suitable for extended operation requiring sustained performance. The study also provides the first systematic experimental evidence linking refrigerant subcooling to dynamic PCM melting behavior.

“These findings offer a valuable framework for improving PCM-based thermal energy storage, which could help improve the overall efficiency and reliability of HVAC systems,” Hwang says.

Download the paper: “Optimal operation of phase change material integrated condenser under various subcooling conditions.”

Published February 2, 2026