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Comparative Thermal Performance of Novel and Conventional Borehole Heat Exchangers for Geothermal Heat Pump Applications
Sajjan POKHREL, Xiaobing LIU, Lingshi WANG, Helia ZANDI
[Oak Ridge National Lab, USA]
This paper presents a comparative thermal performance analysis of four distinct borehole heat exchangers (BHEs) installed at the University of Illinois Urbana Champaign Energy Farm for application in geothermal heat pump system. Among the four heat exchangers, one is a novel borehole heat exchanger known as the Underground Thermal Battery (UTB), which has a depth of only 20 feet. The UTB is installed alongside three conventional configurations: a 120-ft single U-tube, a 240-ft single U-tube, and a 240-ft double U-tube. Different combinations of heat exchangers are operated at various times of the year to enable comparative evaluation. This study presents two performance evaluations: (i) the performance of single and double U-tube heat exchangers, both 240 ft in depth, and (ii) the combined output of the UTB with a 120-ft borehole heat exchanger compared with that of a 240-ft single U-tube borehole heat exchanger. Results indicate that a double U-tube heat exchanger of the same depth delivers approximately 30% higher thermal output than a single U-tube heat. Furthermore, the findings suggest the potential to substitute a single deep borehole heat exchanger with a combination of the UTB and a shallow borehole heat exchanger. In addition, the performance of UTB resulting from phase change of water in UTB during winter heating operations will also be presented. A preliminary cost analysis of UTB indicates potential cost savings compared with conventional BHEs for meeting the same thermal loads.
Topic: Low Temperature