Numerical Modeling of the Effects of Micro-Encapsulated Phase Change Materials Intermixed with Grout in Vertical Borehole Heat Exchangers

Date of Award

2021

Degree Name

Ph.D. in Mechanical and Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Andrew Chiasson

Abstract

One way to reduce conventional energy consumption is through the use of a vertical ground-coupled heat pump (GCHP) systems where heat is charged/discharged to/from the ground by an array of grouted vertical borehole heat exchangers. Although this technology is promising to increase the efficiency of heat-pumps, the main obstacle is the high initial cost. This work examines the viability of one possibility means to overcome the first cost challenge, which is to add micro-encapsulated, paraffin-based phase-change material (PCM) to the borehole grout to dampen the borehole heat exchanger (BHE) peak fluid temperatures. 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 numerical analysis of the heat transfer characteristics of a GCHP systems is performed to investigate the effects of adding micro-encapsulated PCM into the borehole grout. The numerical model was completed in COMSOL, where the apparent heat capacity method is used, and validated against experimental data. 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. Also, there is an insignificant increase in the heat-pump energy consumption because EFT is more favorable in PCM cases around the melt temperature, but less favorable after melting completely.Placement of the grout within the borehole was also examined, and it was found that mixing the grout with the micro-encapsulated PCM is the best location among three other different locations. When the micro-encapsulated PCM and grout mixture was added into the bottom half-depth of the borehole, and the top part was filled with regular grout, (the PCM cost was reduced by 50%), the benefit was reduced by about only 19%. This is crucial since the extra high cost of the micro-encapsulated PCM can be minimized, not just by looking over the best PCM melt temperature and the quantity but also by choosing the best location of the micro-encapsulated PCM within the borehole.

Keywords

Mechanical Engineering, Energy, Geophysics, GCHP, Phase Change Materials, Geothermal, Heat pump

Rights Statement

Copyright © 2021, author

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