Effects of Soil Hygrothermal Properties on the Performance of an Earth-Air Heat Exchanger

Buildings are responsible for a large part of energy demand worldwide. To collaborate to reduce this demand, the use of passive air conditioning has proven to be beneficial for energy savings. In order to improve thermal comfort in built environments, Earth-Air Heat Exchangers (EAHE) can be installed as a low-cost option. Using low energy consumption, in this system, the ambient air circulates through a pipe buried at a certain depth in the ground, causing it to heat up or cool down, depending on weather conditions. These effects are possible due to the high thermal inertia of the soil. In this context, the climatization performance of EAHE depends directly on the hygrothermal properties of the soil, and the effects of heat, moisture, and air transport on the operation of an EAHE are barely explored in the literature due to many difficulties, such as modeling complexity, computer run time, numerical convergence and highly moisture-dependent properties. Therefore, to analyze the effects of hygrothermal properties of soil, a 2D model has been developed to calculate the coupled heat, air, and moisture transfer. The linearized set of discretized governing equations was obtained using the finite-volume method and solved via the MultiTriDiagonal-Matrix Algorithm, substantially improving the numerical stability and reducing the computer run time. To validate the temperature distribution along EAHE, a prototype was built on the Federal University of Technology of Paraná (UTFPR) - Campus Ponta Grossa, which includes 100 mm diameter Polyvinyl Chloride (PVC) ducts, a fan for airflow control, where a series of k-type thermocouples were inserted along the EAHE. Data were recorded hourly using a data acquisition system during the summer period.