Sizing Principles and Practice of an Earth-Air Heat Exchanger (EAHE) for Thermal Conditioning of Buildings

Earth-air heat exchangers (EAHE) provide a promising low-grade heating and cooling solution for buildings in temperate climates, with extensive experimental and numerical studies. However, rational design procedures for building-EAHE systems capable of fully offsetting thermal loads, serving as the sole active conditioning system, remain underdeveloped. This paper establishes practical limits and a rational sizing procedure from this overlooked perspective. The strategy starts by identifying extreme weather conditions (heating or cooling) to set two goals: the required temperature difference along the EAHE, defining heat exchanger effectiveness, and the total airflow rate to compensate building load. To ensure feasible total airflow and prevent infinite flow, supply air temperature must exceed minimum comfort temperature in heating (or fall below maximum comfort temperature in cooling) by a margin called hysteresis. Applying steady-state heat transfer and fluid flow equations yields three key variables: branch pipe length, diameter, and velocity, determining branch airflow. Total airflow is met using parallel branches fitted to available land. With two degrees of freedom, an optimization via minimum entropy generation (EGM) identifies a theoretical optimum and practical design range, minimizing fan energy. This physically grounded method avoids complex CFD or computational optimization, enhancing accessibility for architects and HVAC practitioners. A worked example demonstrates the approach.