Air-recirculating Earth-to-Air Heat Exchangers for efficient HVAC: Energy and environmental analysis in school and residential buildings

This study investigates the energy and economic performance of a hybrid HVAC system that integrates an Earth-to-Air Heat Exchanger (EAHE) operating in both open loop and semi-closed loop configurations. The system, placed upstream of a conventional Air Handling Unit (AHU), is analyzed as a passive strategy for pre-conditioning intake air in different building scenarios. A detailed numerical model, based on the finite element method and validated against literature data, is used to simulate transient heat and moisture transfer in buried ducts under realistic climatic and occupancy conditions. Two typical applications—a residential and a school building located in Shanghai—are evaluated in both heating and cooling modes. The results show that coupling the EAHE to the HVAC system can yield substantial seasonal energy savings: up to 52% for school buildings and 71% for residential ones during heating. In cooling mode, although savings are lower, the integration of the EAHE into the HVAC system remains beneficial, especially during high ventilation demand. The semi-closed loop configuration, which combines outdoor and recirculated air, improves inlet air stability and reduces peak loads, while maintaining comparable energy savings. From an economic standpoint, the open loop configuration proves more cost-effective, achieving a Simple Payback Period in open-loop of 3.2 years and of 2.0 years respectively in school and residential cases. The findings demonstrate the potential of hybrid EAHE systems—operating in both open and semi-closed loops—as scalable, cost-effective solutions to improve HVAC performance and reduce carbon emissions in buildings. • Hybrid HVAC system with earth-to-air heat exchanger is numerically assessed • School and residential buildings studied in heating and cooling modes • Open and semi-closed loop systems compared in terms of energy performance • Seasonal energy savings up to 71% with earth-to-air heat exchanger • Air recirculation lowers power demand while maintaining similar savings