Analysis of Geo-Temperature Restoration Performance under Intermittent Operation of Borehole Heat Exchanger Fields

Abstract

Intermittent operation can improve the coefficient of performance (COP) of a ground source heat pump (GSHP) system. In this paper, an analytical solution to analyze the geo-temperature restoration performance under intermittent operation of borehole heat exchanger (BHE) fields is established. For this purpose, the moving finite line source model is combined with the g-function and the superposition principle. The model takes into account the heat transfer along the borehole, thermal interference between BHEs, and the influence of groundwater flow. The accuracy of the model is validated through comparison with an experiment carried out under intermittent operation. The model makes it possible to analyze the geo-temperature restoration performance and its influencing factors, such as BHE spacing, heat flow rate, operation mode, and groundwater flow. The main conclusions of this work are as follows. The heat transfer along the borehole should be considered when analyzing the geo-temperature restoration performance. When the BHE spacing increases, the soil temperature change decreases and the heat recovery improves. Therefore, adequate borehole separation distance is essential in the case of a multiple BHE system with unbalanced load. The presence of groundwater flow is associated with interference between the BHEs, which should not be ignored. In the case of long-term operation, the groundwater flow is beneficial to the geo-temperature recovery process, even for downstream BHEs. Finally, a greater groundwater flux leads to a better geo-temperature recovery.

Seasonal Thermal-Energy Storage: A Critical Review on BTES Systems, Modeling, and System Design for Higher System Efficiency

“…This confirms McDaniel's observation that high groundwater flows lead to low BTES efficiency [98]. Li et al establish that this applies only to BTES with solar or other heat injection, with GSHP installations lacking heat injection featuring higher thermal restoration in areas with higher groundwater flow [123]. The effects of convective boundary layers regarding heat transfer from soil to pipe have not been fully explored and should be studied further to fully understand the effect upon BTES efficiency [99].…”

Ground Thermal Diffusivity Calculation by Direct Soil Temperature Measurement. Application to very Low Enthalpy Geothermal Energy Systems

“…As it is well known the coefficient of performance (COP) of a heat pump is inversely proportional to the temperature difference of the heat sources [ 2 , 4 , 7 ]; in the case of the VLEGE it is not big and substantially independent of seasonal variations. Even so the COP of the heat pump of a VLEGE system is around 4, which means that the system captures an average of 3 kWh of thermal energy from the ground per kWh consumed from the mains supply to power the heat pump.…”

Inverse heat conduction to model and optimise a geothermal field

“…We illustrate the main steps for obtaining the analytical solution of problem (8). This procedure uses standard arguments on the theory of partial differential equations, so the reader can find all the details in [26].…”

“…So, it is usually more effective a simplified objective function that can be computed directly from the soil temperature dynamics that is induced by the presence of the exchangers. The soil temperature dynamics is determined by the energy flux between exchangers and soil, which is influenced by a variety of factors [4], [8]- [10]: the number and the geometric characteristics of the devices, the arrangement of the devices, the lithotypes composing the stratigraphy of the soil from which depend both the heat diffusion rate and the presence of aquifers or rainwater infiltrations.…”