Numerical simulation of slinky-coil ground heat exchangers installed in railway tunnels

Abstract

The use of horizontal ground heat exchangers (HGHEs) greatly reduces the initial cost of GSHP systems in comparison with vertical GHEs. Though the HGHEs are commonly used in North American countries, the installation number of the HGHEs is still few in European and Asian countries due to the limited land availability. The use of tunnels is one of the measures to promote the HGHEs because the tunnel walls are suitable for laying the HGHEs. In this research, analysis of operation data and numerical modeling were carried out in a GSHP system using Slinky-coil HGHEs installed in the basement of a railway tunnel in Tokyo. Through the interpretation of operation data, the stable energy efficiency of the system was confirmed, though the amount of heat disposal and extraction were not well-balanced in the GSHP system. A numerical simulation model of the HGHE was then developed based on the physical properties of ground and the design of the HGHEs. The simulation model was well-validated with the operation data and showed that 1) the GSHP system could maintain the initial heating and cooling capacity for 10 years and 2) the deeper installation of the HGHE is favorable to obtain higher COP.