Numerical Modeling of Slinky-coil Horizontal Ground Heat Exchangers with Snow-covered Ground Surface

Abstract

In geothermal heat pump systems, the use of Slinky-coil horizontal ground heat exchangers (HGHEs) remarkably reduces the initial cost of the systems since the HGHEs can be constructed without using drilling machines. Hence, the HGHEs have been widely used mainly in northern America for heating and cooling purposes. The performance of the Slinky-coils, however, had not been successfully predicted with good accuracy since the complicated shape of the Slinky-coils made the modeling quite troublesome. To enable this, Fujii et al. (2012) developed numerical models of Slinky-coils using simplified grid systems with an FEM based software, FEFLOW. Then, they demonstrated the validity of the models through the history matching between field test and numerical results. In the numerical modeling, the consideration to energy balance at ground surface was found to be important since the horizontal GHEs are buried in shallow ground, less than 2 m deep. In Fujii et al. (2012), the accuracy of the modeling approach was proved for GHEs located in a warm region of Japan, where no snow accumulation is observed in winter. To promote the application of the system, it is important to accommodate the model to be used under any type of climate. For this target, we carried out field tests of Slinky-coil GHEs in Aomori Prefecture of Japan, where snow depth could exceed 1 meter in winter, and obtained ground temperature and heat medium temperatures. The numerical model was then modified to be able to evaluate the near surface temperatures with snow coverage. Reasonably good history matching results were obtained between measured and numerical temperatures demonstrating the validity of the model in cold regions.