Improved performance in tube-encapsulated phase change thermal energy stores for HVAC applications

Abstract

Thermal energy stores can be used to shift building peak electric loads for cooling to off-peak hours. Compact, high-density thermal storage solutions are required for buildings that may not have the space necessary for typical thermal storage systems, such as chilled water tanks. Previous studies show that latent thermal stores using polymer tube-encapsulated paraffin-based phase change materials (PCM) yield acceptable thermal performance at low to moderate encapsulation tube packing densities, but that thermal performance falls as tube packing densities climb due to increasingly restricted heat transfer fluid (HTF) flow paths. This paper reports the experimental results found when using a highly conductive encapsulation material to redistribute the thermal energy around the PCM, mitigating the effect of the restricted HTF flow paths and enabling maximum encapsulation tube packing densities. These experiments show a more than two-fold increase in the percentage of recoverable thermal energy over a similar polymer tube based system, providing thermal performance comparable to that of a low density polymer tube encapsulation system while increasing PCM capacity by 111%. Test results are provided in a format suitable for estimating design criteria for larger thermal stores, and a design parameter – the resistance ratio – is proposed to assist in optimizing encapsulation tube parameters.