A dynamic model of a vertical direct expansion ground heat exchanger

Abstract

The paper presents a general distributed model of a vertical U-tube direct expansion heat exchanger coupled with the ground. This model is developed for studying the dynamic thermal behavior of a buried heat exchanger (the evaporator or condenser) which is an integral part of the so-called direct expansion heat pump. The transient conservative equations of mass and energy considering the two-phase flow of refrigerant are derived and presented. The heat exchange with the ground is treated using an analytical approach to treat short time response of vertical boreholes based on an imposed heat flux. The thermal interference between the two pipes of the heat exchanger is also considered. The mathematical equations of the model are numerically presented using a finite difference scheme and the solution of the system of equations is obtained using a built-in iterative solver function of MatLab®. The dynamic behavior of a condenser and an evaporator are first obtained by imposing a change of the inlet conditions. Experimental data from a test rig are used to demonstrate the validity of the dynamic model