Effect of intermittent operation on the thermal efficiency of energy tunnels under varying tunnel air temperature

This paper focusses on hot underground tunnels and studies the effect of ground source heat pump (GSHP) intermittent operation and changing tunnel air temperature profile on energy tunnel thermal efficiency. The effects of heat pump operation on the tunnel surrounding soil and the soil recovery rate when the heat pump is not in operation were also studied. A 3D numerical model was developed to simulate the transient heat transfer intermittent operation of an energy tunnel. The intermittent operation was reproduced by controlling the convective heat flux at the boundary between the absorber pipe and the tunnel lining. Variation in the tunnel air temperature was defined in the model as a periodic sink amplitude. Results show that in energy tunnels, strategic intermittent operation increases thermal efficiency and allows the surrounding soil to thermally recover and prevent any adverse effect on the system. A high daily intermittent operation ratio increases the average thermal output but might lead to higher operating costs. Therefore for a given site, it is important to determine the optimum intermittent ratio. The paper also shows how the variation in tunnel air temperature affects the thermal performance of the energy tunnel. The importance of including this variation (rather than assuming a constant average value) when estimating the geothermal potential of underground tunnels was also highlighted. Insights are also provided on the soil temperature recovery rates after prolonged operation. These would serve as a basis for working out a seasonal intermittent operation strategy to optimise the use of ground source heat pumps in underground tunnels.