Experimental and numerical assessment of a hybrid EAHE–HVAC system under sliding mode control for indoor temperature and CO2 regulation

This study investigates the performance of an Earth-to-Air Heat Exchanger (EAHE) coupled with an Heating-Ventilation-Air-Conditioning (HVAC) unit in a small experimental building. A thermal and CO 2 model was developed and validated with three days of winter measurements from Sal´e, Morocco. A Sliding Mode Control (SMC) strategy was implemented to regulate indoor temperature and CO 2 simultaneously. The model reproduces the measured temperature within about ±0.9 °C and the CO 2 levels within roughly 10 ppm, which is inside the sensor accuracy. Three operating modes were analysed: EAHE alone, HVAC alone under SMC, and a hybrid EAHE + HVAC configuration. Compared to the HVAC-only baseline, the hybrid mode reduces heating demand by about 17% and peak power by around 22%, while maintaining temperature and CO 2 within comfort limits. These results show that combining a passive EAHE with a robust nonlinear controller can improve indoor comfort and reduce heating energy use in small laboratory buildings.