Multi-Objective Evaluation of Recirculating Air-Conditioning Operation in University Dormitories: Thermal Comfort, CO2 Exposure, and Energy Use

Previous studies have often focused on individual supply parameters or limited operating conditions, leaving the combined effects of supply air temperature, velocity, and angle insufficiently characterized in compact, high-density dormitory environments. To address this gap, this study evaluated the combined effects of air conditioner set point temperature, supply air velocity, and supply air angle on thermal conditions, local carbon dioxide (CO2) exposure, and electricity use in a four-person university dormitory in Wuhan, China. A three-factor full-factorial field experiment covered 36 operating conditions, and seven representative conditions were further analyzed using transient computational fluid dynamics (CFD). Increasing the set point temperature from 18 to 26 °C reduced mean measured electricity use from 0.751 to 0.552 kW. At the marginal-mean level, increasing the supply air velocity from 2 to 5 m/s was associated with a decrease in measured CO2 concentration at the monitoring points from 764.17 to 603.92 ppm, while mean electrical power increased from 0.547 to 0.729 kW. Factorial analysis further identified significant interaction effects among the operating parameters, particularly for CO2 concentration, indicating that these marginal trends varied across parameter combinations. PMV-PPD evaluation identified nine conditions satisfying the thermal-comfort criteria, while low set points and high local air speeds were associated with cool discomfort. Normalized CFD fields showed that velocity mainly governed jet strength and mixing intensity, whereas angle redirected cold-air and CO2 transport paths. By combining full-factorial field measurements with mechanism-oriented CFD analysis, this study provides an integrated evaluation of thermal comfort, local CO2 exposure, and electrical power demand and identifies a feasible operating window within the investigated dormitory configuration and operating range.