Long Term Simulation of Horizontal Ground Heat Exchanger for Ground Source Heat Pump

Abstract

The earth is an energy resource which has more suitable and stable temperatures than air. Ground Source Heat Pumps (GSHPs) were developed to use ground energy for residential heating. The most important part of a GSHP is the Ground Heat Exchanger (GHE) that consists of pipes buried in the soil and is used for transferring heat between the soil and the heat exchanger of the GSHP. Soil composition, density, moisture and burial depth of pipes affect the size of a GHE. There are plenty of works on ground source heat pumps and ground heat exchangers in the literature. Most of the works on ground heat exchangers are based on the heat transfer in the soil and temperature distribution around the coil. Some of the works for thermo-economic optimization of thermal systems are based on thermodynamic cycles. GHEs is commonly sized according to short time (one year or less) simulation algorithms. Variation of soil temperature in long time period is more important and, therefore, long term simulation is required to be assure the performance of the GSHP system. In this study, long time (10 years) simulation for parallel pipe GHE of a GSHP system was performed numerically with dynamical boundary conditions. In the numerical study ANSYS CFD package was used. This package uses a technique based on control volume theory to convert the governing equations to algebraic equations so they can be solved numerically. The control volume technique works by performing the integration of the governing equations about each control volume, and then generates discretization of the equations which conserve each quantity based on control volume. Thermal boundary conditions can be defined in four different types in ANSYS Fluent: Constant heat flux, constant temperature, convection-radiation and convection. In this study, periodic variation of air temperature boundary at upper surface condition is applied, the lateral and bottom surface of the solution domain are defined as adiabatic wall type boundary condition; the pipe inner surface is taken as wall with a constant heat flux. In order to provide the periodic variation of air temperature boundary at upper surface condition a User Defined Function (UDF) was written and interpreted in ANSYS Fluent. Likewise, a UDF was also written to give constant heat flux intermittently for the pipe inner surface. Constant heat flux of 10, 20, 30 W per unit length of pipe used for calculations. Effects of distance between pipes and thermal conductivity on temperature distribution in the soil were investigated. Heat transfer in the soil is time dependent three dimensional heat conduction with dynamical boundary conditions. Temperature distribution in soil were obtained and storage effect of the soil has also been investigated. An optimization methodology based on long term simulation of GHE was suggested.