The coupled two-step parameter estimation procedure for borehole thermal resistance in thermal response test

Changxing Zhang *, Hang Xu, Jianhua Fan, Pengkun Sun, Shicai Sun, Xiangqiang Kong

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

The ground thermal properties and borehole thermal resistance are the essential parameters for the design of borehole heat exchanger (BHE) field, and they are usually estimated using the experimental inlet/outlet fluid temperatures of BHE in thermal response test (TRT). This paper proposes the coupled two-step parameter estimation procedure (TSPEP) for estimating ground thermal conductivity and borehole thermal resistance of BHE by evaluating the actual averaged - over-the -depth mean fluid temperature (MFT) using the quasi-three-dimensional model inside the borehole. The simulated annealing algorithm (SAA) is used to iteratively find the minimum values of the two objective functions to obtain the optimal estimated results. In TSPEP, the estimated ground volumetric heat capacity and weighted factor f in the 1st step are transferred to calculate MFT using the experimental data in the 2nd step, which guarantees the direct approach based on the infinite line source model (ILSM)applied to improve the accuracy of the estimated borehole thermal resistance. For 50 m depth BHE, the estimated borehole thermal resistance is increased by 12.1% using TSPEP than the effective borehole thermal resistance evaluated by the arithmetic average fluid temperature (AFT). The estimated ground thermal conductivity in TSPEP is almost same with that from the direct approach based on ILSM, and the maximum relative error between them is only 0.91% even though borehole depth of BHE changes from 50 m to 200 m.
Original languageEnglish
JournalRenewable Energy
Volume154
Pages (from-to)672-683
ISSN0960-1481
DOIs
Publication statusPublished - 2020

Keywords

  • Mean fluid temperature
  • The weighted factor
  • Ground thermal conductivity
  • Borehole thermal resistance
  • Simulated annealing algorithm

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