A study on the thermal performance of geothermal energy tunnels.

Shallow geothermal energy is a renewable and sustainable form of energy with the potential to help alleviate the growing climate and energy crisis. The most common system used for extracting shallow geothermal energy is the ground source heat pump system (GSHP). The concept of using existing underground structures in harnessing shallow geothermal energy for space heating and cooling is beginning to attract considerable interest. This study focused on the use of tunnels (so-called energy tunnels) as a means to extract shallow geothermal energy. There is limited research on the effect of design and ground parameters on the thermal output of energy tunnels. In this study, a novel experiment (laboratory-scale) energy tunnel model was designed and built to allow the assessment of some of the energy tunnels design parameters and ground parameters (fluid flowrate, tunnel air temperature, continuous versus intermittent operation and pipe spacing) on its thermal output. A numerical model was developed and validated to assess and highlight critical parameters associated with the operation of energy tunnels. The numerical model together with the Taguchi method was used to further investigate the influence of design parameters (absorber fluid diffusivity, concrete diffusivity, pipe thermal conductivity, pipe diameter, length of pipe, pipe spacing and heat exchanger pipe location) on energy tunnels thermal efficiencies. The results show that the overall thermal output of an energy tunnel depends largely on the available area for the heat exchange and the thermal properties of a tunnel lining, while the choice of pipe spacing does not have a significant impact on the thermal performance of energy tunnels. This study highlights the importance of including the effect of variation in tunnel air temperature (rather than assuming a constant average value) when estimating the geothermal potential of underground tunnels. Insights are also provided on the soil temperature recovery rates after prolonged operation. These could serve as a basis for working out a seasonal intermittent operation strategies. This study has also provided optimisation guidance for engineers looking to improve the efficiency of energy tunnels. It further validates that a carefully designed system could operate at a relatively low cost to conventional fossil fuel-based heating systems