Understanding the variables affecting ground source heat exchangers

Abstract

Ground Source Heat Exchangers (GSHEs) are an emergent energy technology aimed at enhancing the energy efficiency of heating and cooling systems in a building. This system in conjunction with Ground Source Heat Pumps (GSHPs) have been extensively improved and studied worldwide since the beginning of the twentieth century. Malta is no different, with investigation particularly concerned towards the appropriate category of the earth sub-system and thermal materials already conducted to achieve optimum thermal performance. These prompted further analysis with numerous refined parameters, especially with the utilisation of a numerical analysis since greater variety of trials are facilitated. The research presented in this study evaluated and built on research already conducted to analyse the performance of GSHEs with particular interest in variations of materials pertaining to the thermal grout, carrier fluid and pipework. Four main parameters were analysed in this study, namely: the thermal conductivity, density, specific heat capacity and viscosity (where applicable). A Computational Fluid Dynamics (CFD) analysis was performed to model and simulate an existing physical experiment conducted by Borg (2011). The in-situ experiment was digitally modelled as built including the fluid section to provide more detail on the overall behaviour of the system. The grouting material when related to a real cement-hased mixture reduced the inflow temperature by around 1.06 to l .26°C but, under extreme parameters the deduction reached a maximum temperature variance of 5.99°C. Metal and plastic pipework when tested in combination with a 6.5 W/mK thermal grout, reached an outflow temperature of31 l.63 and 31 l.73°C respectively. The introduction of an anti-freeze concentration to water improved the thermal performance in some trials, with propylene glycol achieving the highest temperature variation of 1.30°C. These results highlighted the impact on the thermal performance when varying the material thermophysical properties.