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Estimation of ground thermal properties for coaxial BHE through distributed thermal response test

Nian, Yong-Le, Wang, Xiang-Yang, Xie, Kun, Cheng, Wen-Long
Renewable energy 2020 v.152 pp. 1209-1219
heat exchangers, heat pumps, heat transfer, models, renewable energy sources, stochastic processes, temperature, thermal conductivity
Ground thermal properties (thermal conductivity and heat capacity) are the key parameters for design of borehole heat exchanger (BHE) in ground source heat pump (GSHP). This study presented a novel sequential estimation for ground thermal properties of coaxial BHE by using distributed thermal response test (DTRT) data. Firstly, a comprehensive heat transfer model for coaxial BHE was built to obtain the vertical temperature files. Then a new sensitivity analysis using Spearman correlation method was performed to obtain the correlation between the thermal properties and fluid vertical temperature, determining the estimation sequence. Lastly, according to the sequence, Monte Carlo stochastic method was used to estimate the thermal properties by applying the DTRT data. In addition, the effect of borehole depth and random samples on estimations was investigated. The simulations revealed that the heat input into BHE was mainly used to heat the circulating fluid during the earlier time. It was found that the estimation steps following the thermal conductivity first, then heat capacity got higher precision than other estimation steps with 0.6% and 4% respectively for the two parameters. In addition, the effect of borehole depth on estimation error could be eliminated by using the DTRT data.