Abstract

This paper investigates the transient heat conduction around the buried spiral coils which could be applied in the ground-coupled heat pump systems with the pile foundation as a geothermal heat exchanger. A transient ring-coil heat source model is developed, and the explicit analytical solutions for the temperature response are derived by means of the Green’s function theory and the image method. The influences of the coil pitch and locations are evaluated and discussed according to the solutions. In addition, comparisons between the ring-coil and cylindrical source models give that the improved finite ring-coil source model can accurately describe the heat transfer process of the pile geothermal heat exchanger (PGHE). The analytical solutions may provide a desirable and better tool for the PGHE simulation/design.

Introduction

The ground-coupled heat pump (GCHP) technology is one of the most energy efficient ways to provide both space heating and cooling for buildings because the underground environment experiences less temperature fluctuation than the ambient air temperature. In a GCHP system, heat is extracted from or rejected to the ground via a geothermal heat exchanger (GHE) through which pure water or anti-freezing solution circulates. The commonly used GHE in closed loop systems typically consists of high-density polyethylene (HDPE) U-tubes installed in vertical boreholes which are called vertical GCHP systems. A number of vertical GCHP systems have been installed around the world including in China due to the increasing awareness of environmental issues and growing energy crisis [1,2]. However, the main restriction against the GCHP application in urban areas is that the system requires a large plot of land for installation of GHEs, and this is usually difficult in cities. Besides, larger GHEs significantly increase the initial cost of the GHE installation. Therefore, the high installation cost of excessively large GHEs and the limited urban land area have restricted to a large extent the wide applications of this technology in densely-populated areas.