Experimental study on the thermal performance of tunnel lining ground heat exchangers

Innovative tunnel lining ground heat exchangers (GHEs) were introduced in China for the first time and applied to Linchang Tunnel in Inner Mongolia. This paper is dedicated to studying this new technology based on a field experiment at Linchang Tunnel. The experiment investigated the heat transfer performance of tunnel lining GHEs and its impact factors, including the flow rate, inlet water temperature and pipe distance. The test results show that (i) the ground temperature at a pipe distance of 50 cm cannot completely recover when the intermittent ratio is 1.1, but it can completely recover at a pipe distance of 100 cm; (ii) the heat exchange rate presents a linear variation with the inlet temperature of the heat carrier liquid, and on average, the heat exchange rate increases 2.494 W/m for every 1 °C increase in the inlet temperature; (iii) the heat exchange rate increases exponentially as the flow rate increases; (iv) the heat exchange rate shows an exponential downward trend as the running time increases, and the heat exchange rate for a pipe distance of 50 cm is always higher than that for a pipe distance of 100 cm during the testing.