Vertical and Horizontal Ground Heat Exchanger Modeling

Abstract

As the cost of energy continues to rise, it becomes essential to save energy and improve overall energy efficiency of air-conditioning systems. In this light, the heat pump becomes a key component in energy saving with great potential. Heat pumps (HP) attain higher efficiencies when they are coupled to ground heat exchangers (GHEs). This is due to the fact that the ground is always cooler in summer and hotter in winter than the ambient air with which the common HPs exchange heat. The basic types of GHE configuration are horizontal or vertical. In the horizontal type the heat exchangers are usually buried in the ground at a depth of about 1–2 m with a number of tubes connected together. These exchangers are affected by the seasonal weather conditions because they are shallowly placed in the ground. Vertical GHE or borehole heat exchangers are widely used when there is a need to install sufficient heat exchange capacity under a confined surface area. Vertical loops are generally more expensive to install and usually have a depth of 20–300 m. This paper describes the modeling of vertical and horizontal GHEs and compares their efficiency. Simulations with the vertical GHE show, as expected, that when the initial ground temperature rises the mean temperature of the heat exchanger fluid increases as well in a straight line relation. Comparisons between the horizontal and vertical GHEs reveal that under the same operating conditions and centre to centre distances of the tubes, the vertical heat exchanger keeps a much lower mean temperature because the initial ground temperature at the buried tube depth is always higher than that of the vertical GHE. Because of this observation one would assume that the vertical GHE is more efficient than the horizontal. Instead, in a proper design, one could increase the distance between the tube centres and in this way decrease the mean temperature of the tube fluid.