Thermal Performance of a Helical Steel Energy Pile Incorporating Latent Thermal Energy Storage for Ground Source Heat Pump Applications

Abstract

Ground source heat pump systems (GSHPs) have the potential to meet building space heating and cooling needs with significantly higher efficiencies. Using the relatively stable deep ground temperatures, GSHPs can reach efficiencies between 400–600% compared to 90% with advanced gas boilers. However, despite these high efficiencies, GSHPs are not widely used, mainly due to the associated high up-front costs. In addition, ground thermal imbalance when heating and cooling loads vary reduces GSHP performance and might result in eventual failure. Coupling ground heat exchangers with building foundations, an already existing structural element of the building, has potential to improve the cost competitiveness of GSHPs. In this study, the performance of a helical steel pile used as a ground heat exchanger for GSHPs is numerically investigated. A pile of 0.1298 m diameter and 20 m depth was considered in the analysis. Furthermore, the potential for improved performance using latent thermal energy storage with phase change material (PCM) was investigated. An organic PCM with a melting temperature of 6.5–8.5°C and a latent heat of 190 kJ/kg was used. A thoroughly validated numerical model of the helical steel pile was developed using a finite volume based computational fluid dynamics tool. With the use of PCM, considerable improvement in performance with an over 57% increase in the heat transfer rate per unit meter of the heat exchanger was achieved.