In ground-source heat pump (GSHP) systems, the application of horizontal ground heat exchangers (HGHEs) greatly reduces the initial costs for the system construction in comparison with vertical GHEs since the HGHEs can be installed using commonly-used excavation machines. Though HGHEs have been widely used in the United States and Canada, where abundant land space is available for HGHEs, the distribution of the system is still slow in Japan due to the limited availability of land space.

In this paper, data analysis and numerical modeling were carried out on a GSHP heating and cooling system using HGHEs installed in the basement of a railway tunnel in Tokyo, Japan. Through the interpretation of operation data for 2 years, the decline of system performance by the change of ground temperature was not observed though the amount of heat release and extraction were not in a good balance.

A numerical simulation model of the HGHE was then developed based on the ground soil properties and HGHE designs. The model was validated using the recorded data during the heating and cooling periods. Sensitivity studies were then carried out using the validated model for the heating and cooling operations of 10 years. The predicted inlet temperature of heat pumps showed that the GSHP system could maintain the initial capacity for 10 years in case the annual heating and cooling periods were set at 4 months and 2-4 month, respectively. Also, the effect of burial depth of HGHE on system performance was examined using the numerical model. The simulations showed that deeper installation is more preferable when the heat exchange rate per HGHE length is large, while the difference is negligible when the heat exchange is relatively small.