Abstract
A numerical analysis of the heat transfer characteristics of a vertical borehole heat exchanger as used in ground-coupled heat pump (GCHP) systems is performed to investigate the effects of adding micro-encapsulated, paraffin-based phase-change material (PCM) into the borehole grout. As with any thermal energy storage scheme, its purpose is to reduce the size of equipment and devices required to meet peak loads, and thus the purpose of PCM in this study is to dampen peak temperature response of the borehole, and potentially allow for reduction in design borehole length, and therefore cost, of the borehole array. A parametric study of the PCM thermal properties was conducted to establish design recommendations for the vertical heat exchange borehole grout. Results of this study show that adding PCM into the borehole does not always improve the overall performance of the GCHP system; rather, it could deteriorate the system performance if the PCM thermal properties and melt temperature are not correctly chosen. An optimum mass of PCM exists for borehole grout due to the competing factors of PCM thermal conductivity and its latent heat capacity, but to be effective, the PCM thermal conductivity should be approximately equivalent to that of the grout material. Further, the optimal melt temperature of the PCM was found to be that which results in almost all of the PCM mass to change phase at the time of peak load, and that temperature was found to be about midway between the undisturbed ground temperature and the peak design heat pump entering fluid temperature. The potential reduction in the required BHE-length due to optimal addition of paraffin-based, micro-encapsulated PCM in borehole grout was found to be up to 7% in this study, but this length reduction does not guarantee a reduction in the overall cost of the system owing to the current relatively high cost of the PCM. Thus, from an economic standpoint, the results of this work suggest that enhancing the grout thermal conductivity is currently more cost-effective than adding PCM to the grout.
•Adding phase change material could deteriorate the geothermal heat pump performance.•Optimal phase-change-material melt temperature must be chosen correctly.•An optimum mass of phase change material exists for borehole grout due to its low thermal conductivity.•Enhancing grout thermal conductivity is currently more cost-effective than using phase change material.