CFD simulation and experiment of heat exchanger characteristic for single well cycling underground heat pump

Abstract: Ground source heat pump (GSHP) systems are considered as an ideal approach to heat and cool building, due to their attractive advantages of high efficiency, low carbon emission and using renewable energy instead of electricity for heating and cooling. Many of projects have been applied in residential and commercial buildings successfully. Single well groundwater heat pump (SWGWHP) systems are new member of GSHP system, which become increasingly popular for use because of their economic advantages. In general, SWGWHP systems included three different variations, i.e. standing column well (SCW) system, pumping & recharging well (PRW) system, and pumping & recharging well filled with gravel (PRWFG) system. Although there are some theoretical and experimental researches on SWGWHP systems, there are not many researches on Computational Fluid Dynamics (CFD) simulation about flow and thermal performance in these systems. CFD are well known for their capability to carry out in-depth analysis of fluid flow, heat transfer, mass transfer and several other related issues. They provide numerical solutions of partial differential equations governing fluid flow and heat transfer in a discretized form. CFD employs a very simple principle of resolving the entire system in small cells or grids and applying governing equations on these discrete elements to find numerical solutions regarding pressure distribution, temperature gradients and flow parameters in a shorter time at a lower cost. Meanwhile, CFD simulation has been widely used in research field of Heating, Ventilating and Air Conditioning (HVAC), such as indoor climate, horizontal and vertical ground heat exchanger, earth-to-air heat exchanger and so on. To examine the complicated characteristics of groundwater flow and heat transfer in thermal well and aquifer, CFD software FLUENT 14.5 was used in this study. In this analysis, the CFD simulation results were compared with experimental measurements. A good agreement was obtained between predicted outlet water temperature and aquifer characteristics temperature, thus confirming that the CFD model was successful in reproducing the flow and heat transfer processes in aquifer. The relative errors of three thermal wells in 25 min heat accumulation were 12.1%, 3.0% and 8.2%, respectively. Therefore, the establishment of these three thermal well CFD model can be used to analyze and predict the actual SWGWHP performance with the variation of groundwater flow field and temperature field, which can also provide accurate simulation data. This study suggested that the developed multi zone coupling CFD model can be useful tool in evaluating the flow and heat transfer performance. However, the established CFD simulation model can explain the complex flowed and heat transferred in porous media, CFD simulation in determining realistic initial conditions when attempting to model field conditions still remained a challenge. Further exploration of thermal well structure variations throughout the full range of climatic conditions is also needed.