Buildings effects on the operation of horizontal ground heat exchangers: comparison between numerical modeling and experimental results

In horizontal ground source heat pumps (GSHPs) systems heat exchangers are located underground horizontally and a heat carrying medium is circulated within the exchanger, transferring the heat from or to the ground via a heat pump. The horizontal ground source heat pumps configuration is usually the most cost-effective when adequate yard space is available and trenches are easy to dig, especially while a building is under construction. The aim of this study is to model trends in temperatures around a real GSHP system under the influence of several weather parameters, including solar radiation, thermal radiation, and convective heat transfer, at the ground surface. The experimental ground source heat pump plant is installed at the Caleffi Research Center (Piedmont Region, NW Italy) and involves a network of pipes buried 1 m below the surface, covering an area of about 210 m2. A numerical simulation model that considered the energy balance at the ground surface is developed using the finite-element Comsol Multiphysics® package; Fig. 1 shows a schematic view of the 3D model geometry. Heat transfer processes at the ground surface included those due to variable wind speed, air temperature, solar radiation, and long-wave radiation. The thermal field is solved with transient conditions by determining the ground temperature distribution. Model results are compared with experimental temperature data, which are collected with a monitoring system comprised of five recording thermometers, positioned about 80 cm below the ground surface. The thermometers are located at the four corners and at the center of the area. Simulations are performed for one year operation. In general, good agreement is obtained between the measured and simulated values of the ground temperatures (Fig. 2). Present work aims to investigate the effects that buildings may have on the operation of horizontal ground heat exchangers. A not negligible effect due to heat interaction with the building is detected. These interactions are further investigated using the numerical model. The results showed that the presence of buildings near the exchangers influenced the temperature trend through the subsoil and, consequently, GSHP performance. Therefore, the modeling assumption of a vertical adiabatic surface may be inappropriate, especially when the application to urban environment is analyzed.