Using a capillary mat as a shallow heat exchanger for a ground source heat pump system

Abstract

The ground heat exchanger is an important part of a ground source heat pump system. The site selection and arrangement affect the thermal performance of the entire system. A horizontal (shallow) ground heat exchanger has lower initial cost than a borehole heat exchanger. However, it has a larger physical footprint and its thermal performance is strongly affected by surface heat flux and rainfall infiltration. The capillary mat heat exchanger comprises small parallel capillary tubes and provides larger effective heat transfer area per unit footprint installation area. Additionally, it is more easily prepared and installed. This paper reports experimental result and numerical validation of ground source heat pump cooling application using shallow horizontal capillary mat ground heat exchangers. From the experiment, it was found that the installed capillary mat heat exchangers provide sufficient heat rejection rate for the heat-pump's thermal load. A numerical model for capillary mat heat exchangers is developed and the numerical result are validated using experimental data, within 1.54% root-mean-square error. A new and simple numerical relationship is also proposed to express the rate of heat transfer from capillary pipes to the surrounding soil. The extended numerical simulation found that currently installed ground heat exchangers can provide higher thermal load. The estimation of the pressure drop suggests that for a given pressure drop and flow rate, a single capillary mat having a surface area of 13.18 m2 is equivalent to a 40-m high-density polyethylene pipe with a standard diameter of 3.2 cm and a surface area of 8.05 m2.