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Recharging Phase-Change Material Storage for Personal Conditioning Systems

Recharging Phase-Change Material Storage for Personal Conditioning Systems

From eSociety, January 2020

Personal conditioning systems (PCS) are gaining tremendous momentum in disrupting both residential and commercial air-conditioning markets. 

One PCS is vapor compression-cycle-based and stores its condenser waste heat into a phase-change material (PCM)-based thermal storage. When the PCM is completely melted, the PCM needs to be recharged before the next cooling operation. 

A recent Science and Technology for the Built Environment article analyzes the merits and demerits of two recharge operations. One researcher, Jiazhen Ling, Ph.D., with the University of Maryland, discussed his research further.

1. What is the significance of this research?

Personal conditioning systems (PCS) provide occupants with personalized thermal comfort using very low power. This research focuses on a personal conditioning technology that integrates mini heat pumps with a phase-change material (PCM). The mini heat pump cools/heats a particular person and stores the waste heat in the PCM. The authors compare various PCM charge/recharge strategies in order to achieve a minimum time for recharging the PCS between cooling/heating service periods and, more importantly, minimize the electricity required to charge the PCM. Compared with the state-of-the-art research on personal cooling, the paper is among the first to establish physics-based and experimentally validated models for graphite-enhanced PCM discs and thermosiphon systems, both of which were further incorporated into a fully transient HVAC component library. This library is capable of simulating the charge and recharge cycles of the mini heat pumps. Such a simulation methodology exemplifies the next generation of design and evaluation capabilities that capture the real-time operating characteristics of energy systems.

2. Why is it important to explore this topic now?

PCS are gaining tremendous momentum in disrupting both residential and commercial air-conditioning markets. These devices operate in many ways that are distinct from traditional air-conditioning equipment. Waste heat storage and periodic cooling/heating service periods are among the new features of PCS. Creating modeling tools that capture those distinct features is essential. The paper serves as a prompt response to such a demand, expanding the knowledge base and facilitating the expedient deployment of PCS in many fields.

3. What lessons, facts and/or guidance can an engineer working in the field take away from this research?

To understand the inherent complexity of PCS, the interaction among its components and its operating environment are the most important takeaways. To that end, a robust transient model of PCS is a useful tool that can effectively expose and clarify such complexity for the design engineer and improve overall understanding of the system.  It also serves as a virtual testbed for any innovative ideas one wishes to explore and a basis for any further optimization of the system. This virtual testbed can help engineers get the first prototype 90% right.

4. How can this research further the industry’s knowledge on this topic?

At a minimum, the research showcases a dynamic model for the design and evaluation of a state-of-the-art PCS. It provides the industry with a tool to further explore various high-performance components, say enhanced PCM, and/or control strategies that can minimize the power input and improve cooling/heating service-time. Overall, it can significantly reduce the time-to-market for such systems.

5. Were there any surprises or unforeseen challenges for you when preparing this research?

The concepts described in the paper significantly advance the development of PCS, and there were no new insights to be added from the authors.