Parameter analysis of Earth-air heat exchangers over multi-layered soils in South Brazil

Abstract

Earth-air heat exchangers (EAHE) represent a sustainable way to explore the soil to heat or cool buildings. Due to the Earth's thermal inertia, its superficial layers can be used as a heat source or sink, respectively, in the winter or summer. Hence, the EAHE employ buried ducts where the air is forced to flow and exchange heat with the soil. Such devices demand little energy, as they can use low powered fans or passive techniques for natural convection. Recurrent issues in the literature of EAHE explore how to improve their performances by analyzing their operational parameters; soil, climate, and global economic prospects; the development and validation of complete or simplified simulation models. This paper presents parameter analysis of EAHE to increase their thermal potentials (which is also related to the efficiency and the prospects for thermal comfort) and the heat exchanges (linked with the economy) considering a Brazilian city in a subtropical climate. The research uses simulations based on a validated 1D model. It is compared with three others, including 3D ones, giving more accurate results. Unlike most works covering 1D models, the methodology proposed here allows the study of multi-layered soils. After assessing the soil temperature variations, the results point out that most of the local thermal potential is attainable at relatively low depths, with magnitude peaks of 5°C. The parameter analyses are focused on this target potential, addressing changes in the airspeed, length, and diameter of the ducts. For all cases, the goal was achieved, allowing to obtain annual efficiencies close to 90%. However, only the tests increasing the length also increased the quantity of heat exchanged (in about 20%) and, consequently, proved to be economically affordable.