Thermal performance of horizontal closed-loop ground-coupled heat pump systems using flowable-fill

Abstract

This research evaluates the use of flowable-fill containing fly-ash in place of conventional dirt back-fill in horizontal closed-loop heat exchangers used in ground-source heat pump systems. A computer model was developed to simulate the transient heat transfer response in the fill material and native soil surrounding the heat exchangers. The model consists of a two-dimensional transient conduction model based on the finite-volume method. It simulates the energy extraction from the soil during the heating season and energy addition during the cooling season. A daily system load curve was used to approximate the daily heating and cooling load for a nominal two ton residential application in central Virginia. The computer model determined the thermal performance of various configurations when subjected to seasonal weather conditions. The variation in overall system performance was influenced by earth and fill thermal properties as well as geometric design parameters, such as trench length and depth. Ground-coupled heat pump systems with straight pipe and horizontal Slinky/sup TM/ configurations were installed at several residential demonstration-sites in Virginia. Temperature measurements were recorded at selected locations surrounding the ground heat exchanger, allowing evaluation of heat exchanger performance. Electrical measurements including heat pump compressor, air handler and auxiliary electrical resistance energy were used to characterize overall system performance. Experimental measurements and model simulation studies have shown that encasing the ground-loop heat exchangers in flowable-fill rather than in earth backfill improves the overall thermal performance of ground source heat pump systems. Improved performance has been indicated by less extreme fill and soil temperature and by lower annual electrical energy costs.