Modeling of standing column wells in ground source heat pump systems

Scope and methodology of study. The need for alternative low-cost energy sources has given rise to the development of ground source heat pump systems for residential and commercial heating and cooling applications. Standing column wells can be used as highly efficient ground heat exchangers in ground source heat pump systems, where hydrological and geological conditions are suitable. A detailed two-dimensional Finite Volume Model and a simplified one-dimensional Finite Difference Model have been developed in this study. Both models are validated against limited experimental data. The detailed model has been used in a parametric study to identify the most significant design parameters and their effects on the well performance. The simplified model with "enhanced" thermal conductivity is intended for use in hourly simulation programs or design tools. Simulation studies are presented to examine the thermal and economic performance of SCW systems with and without groundwater bleed in different locations. The required design depth and energy cost for SCW systems are compared with those of single U-tube closed loop systems.; Findings and conclusions. The performance of SCW systems is improved with increasing thermal and hydraulic conductivities and decreasing water table depth. The results of this study show that the introduction of bleed flow can dramatically improve the performance of SCW systems. Significant improvements in performance are found with only moderate rates of bleed (5--15% of system flow). SCW systems allow significant reductions in borehole depth. Because of the significant reduction of capital cost due to less required total borehole depth, the 20-year life cycle cost analysis shows cost savings in the standing column well system with bleed control over single vertical U-tube closed-loop system of at least 27%. The average required design depth for SCW systems for a small office building in a few selected climates without bleed is 75 feet per ton, and 53 feet per ton for standing column well systems with 10% deadband bleed control, compared to 141 feet per ton for U-tube closed-loop systems.