Numerical Modeling of Slinky-coil Horizontal Ground Heat Exchangers Considering Snow Coverage Effects

The number of Slinky-coil horizontal ground heat exchangers (HGHEs) has been increasing in geothermal heat pump (GHP) systems due to the low initial cost. The performance of the Slinky-coils, however, has not been successfully predicted with good accuracy since the complicated shape of the Slinky-coils is not easy to model either analytically or numerically. To enable this, Fujii et al. (2012) developed a numerical model of Slinky-coils using a simplified grid system with FEM based software. They also demonstrated the validity of the models through the history matching between field test and numerical results. In Fujii et al. (2012), the validity of the model was proved only in a region of mild climate, where snow accumulation is not expected in winter. Hence, to promote the application of the system in colder regions, it is important for the model to be applicable to a HGHE with snow coverage. For this purpose, a long-term heating test of a GHP system using Slinkycoil GHEs as heat source was carried out in Aomori Prefecture of Japan, where snow depth often exceeds 1 meter in winter. During the test, the ground and heat medium temperatures and the operation data of the GHP system were measured for interpretation. The numerical model by Fujii et al. (2012) was modified to calculate the near surface ground temperatures with snow coverage. By setting the ground surface temperature at 0oC, reasonably good history matching results were obtained between measured and calculated temperatures. A sensitivity studies was also carried out to compare the performance of HGHEs with and without snow coverage. The advantages of snow accumulation were clearly shown to achieve better efficiency in the GHP system. INTRODUCTION Horizontal ground heat exchangers (HGHEs) are known to be more cost-effective than vertical heat exchangers in terms of installation cost of groundsource heat pump (GSHP) systems since HGHEs do not require expensive drilling machines to construct. Among the HGHEs, Slinky-coils are considered more efficient than conventional straight HGHEs due to the denser coverage by heat exchange pipes. At this moment, there are far fewer applications of Slinkycoil HGHEs than those of vertical GHEs owing to the large land area required to bury the heat-exchange pipes. Therefore, to promote the use of HGHEs in locations with limited space, the heat exchange rate per unit land area should be improved based on an optimum design of the HGHEs. In recent years, the researches on Slinky-coil HGHEs have been activated. Fujii et al. (2010) presented the results of thermal response tests and long-term cooling and heating tests on Slinky-coil HGHEs with different loop angles and compared their heatexchange capabilities. Wu et al. (2010) developed a 3D numerical model to simulate the performance of Slinky-coil HGHEs and conducted sensitivity studies on coil diameters and pitches. Congedo et al. (2012) developed 3D numerical models for straight, Slinky and spiral HGHEs and compared their performances. Using fine meshes to model the Slinky-coils, however, the numerical models by Wu et al. (2010) or by Congedo et al. (2012) could handle only smallscale models and were not applicable for the modeling of field-scale HGHEs. Fujii et al. (2012) then developed a full-field numerical simulation model of single-layer Slinky-coil HGHEs applying a simplified shape of Slinky-coils using numerical software, FEFLOW, and validated the model using the results of short-term and long-term heat-exchange tests. Li et al. (2012) developed an analytical model of Slinky-coil HGHEs using the moving line source theory and validated the model using laboratory experiments. Through the above researches, the optimum designs of the Slinky-coils have been well studied. In some of the above studies, the authors pointed out the importance of the consideration on surface boundary conditions and coupled the heat balance calculation with the heat transport simulation in the ground successfully (e.g., Fujii et al., 2012). The heat balance calculation, however, has been limited to a land surface without snow coverage. For accurately predicting the behavior of HGHEs in cold regions during winter, researches need to be carried out assuming snow accumulation in winter. In this study, numerical simulation models of Slinkycoil HGHEs with snow coverage are constructed after modifications of the numerical model developed by Fujii et al. (2012). The models are validated using the results of a long-term heating test of double-layer Slinky-coil HGHEs, which was carried out from Nov., 2011 to Feb., 2012 in Aomori Prefecture, northern Japan, where quite heavy snowfalls are experienced during winter every year. Using the developed numerical model, sensitivity studies are then performed to compare the performance of HGHEs with and without snow coverage.