Estimation of Ground Thermal Properties of Shallow Coaxial Borehole Heat Exchanger Using an Improved Parameter Estimation Method

Abstract

Ground thermal properties are prerequisites for designing the size of borehole heat exchanger. In this study, a 3D heat transfer model is developed to simulate the thermal response test (TRT) of shallow coaxial borehole heat exchanger (SCBHE), and effects of ground thermal properties on the slope of the mean value of inlet and outlet fluid temperatures are studied. The results show that the slope is strongly affected by ground thermal conductivity and is slightly affected by ground thermal capacity, and that ground thermal capacity only has a small effect on the slope. Then, by using the difference between the experimental slope and calculated slope as the objective function to estimate ground thermal conductivity, an improved parameter estimation method (PEM) is proposed to estimate ground thermal properties of SCBHE using the simulated TRT data, and it is compared with the direct method. The results show that ground thermal conductivity and thermal capacity estimated by the improved PEM are accurate for different ground thermal properties, and that ground thermal conductivity estimated by the direct method probably has some errors especially for small ground thermal conductivity or thermal capacity, indicating that the improved PEM has much higher precision than the direct method and can be applied for estimating the ground thermal properties of SCBHE.

Thermal Response Measurement and Performance Evaluation of Borehole Heat Exchangers: A Case Study in Kazakhstan

“…Mogensen [22] reported values λ g = 2.69 W/m K and R b = 0.1266 K/W m, while Reuss [19] reported that λ g = 2.2 W/m K and ρc = 2.8 MJ/m 3 K. Gehlin [20] indicated that the thermal conductivity of the ground when a borehole is filled with grout is 3.45 W/m K, while it is 3.62 W/m K when filled with water. In recent years, studies in [23][24][25][26][27][28][29][30][31] showed improvements and applications of the TRT procedure for evaluating BHE efficiency. Luo et al [23] studied the influence of an unstable power input on the unacceptable errors of the TRT measured data.…”