Assessing the thermal efficiency of energy tunnels using numerical methods and Taguchi statistical approach

The use of ground source heat pump systems (GSHPs) with tunnels (so-called energy tunnels) to provide space heating and cooling is one of the latest concepts that has recently raised research interest but has not yet been commercially established. This study represents the first attempt to investigate the influence of design parameters on the energy efficiency of a GSHP using an underground tunnel as the energy geostructure. Seven important design parameters, namely absorber fluid diffusivity, concrete diffusivity, pipe thermal conductivity, pipe diameter, length of pipe, pipe spacing and absorber pipe location were considered. The influence of these design parameters on the tunnel thermal efficiency was studied by using an experimentally validated 3-D numerical model and then deploying the Taguchi method to efficiently explore parameters space. The results show that concrete diffusivity and pipe total length are the most influential parameters, followed by the pipe location and diameter, while spacing was found to be the least influential factor. Hence the overall thermal output of an energy tunnel depends largely on the available area for heat exchange and the thermal properties of the tunnel lining. Results also show that within the range of pipe diameter considered, using a larger pipe diameter in energy tunnels is more efficient from the thermal output and pump power requirements. These results can be used as thermal efficiency optimisation guidance for both researchers and practitioners.