Sensitivity analysis of a vertical geothermal heat pump system

Abstract

This paper conducts sensitivity analyses on factors affecting the performance of vertical geothermal heat pump system, aiming to formulate design and operation strategies to improve its performance. It firstly describes the development of a 3D coupled Finite Element Model (FEM), which is utilized to simulate the steady state and transient behaviors of geothermal heat exchanger (GHE). The model holistically couples the heat exchange processes between pipe fluid flow, grouting backfill material, and adjacent ground associated with GHE. The model is firstly validated by comparison with the experimental data from an in-service GHE. Base on the calibrated model, a series of sensitivity analyses are conducted on the influence of geological, design, and operational factors intermittent operation mode of GHE achieves higher such as the initial ground temperature profile, GHE pipe installation depth, circulation fluid flow velocity, inlet temperature, subsurface water flow velocity, and material thermal properties. It also assess the behaviors of GHE under continuous operation versus intermittent operation modes. The results show that both design parameter (i.e., GHE pipe installation depth) and operational parameters (i.e., circulation fluid flow velocity) have major influence on the GHE performance. For a certain design length of GHE, the GHE performance improves with higher circulation fluid flow velocity until beyond a critical velocity. For GHE working in the heating mode, the heat extraction by GHE increases with decreasing fluid temperature at the inlet. In the geological factor aspect, the thermal conductivity of the ground material plays a very important role on the GHE performance operating in the continuous operation mode, while its specific heat capacity exerts no appreciable influence. However, for intermittent operation mode, both thermal conductivity and specific heat capacity of the ground, particularly the grouting materials, affect the ground thermal energy extraction. The results also showed that the presence of subsurface ground water flow improves the heat exchange of GHE. Operation wise, the GHE achieves higher performance and Coefficient of Performance (COP) under intermittent operation mode than under continuous operation mode. These observations point to ways to improve the performance of GHE from both design and operation aspects.