Membrane-based quasi-isothermal humidifier: Performance characteristics and energy–exergy analysis for HVAC applications

Abstract

Maintaining appropriate indoor humidity during winter is critical for occupant health and comfort. However, conventional heating, ventilation, and air-conditioning (HVAC) humidifiers, such as steam injection, ultrasonic atomization, and direct evaporative systems, often involve either high-grade heat consumption, aerosol carryover concerns, or additional reheating requirements. In this study, a membrane-based quasi-isothermal humidifier (QIMH) was investigated as an alternative humidification approach for HVAC applications. A two-dimensional steady-state heat- and mass-transfer model for a cross-flow hollow-fiber membrane module was developed and validated against laboratory-scale experimental data. Parametric simulations were then conducted by varying inlet water temperature and the air and water flow rates to identify the operating region satisfying the study-defined quasi-isothermal criterion of |ΔTa| ≤ 0.5 °C. The results showed that quasi-isothermal operation is achieved when the inlet water temperature is maintained close to the inlet air temperature and the water flow rate is sufficiently high to supply latent heat without causing appreciable sensible heating or cooling of the air. Under these conditions, the membrane humidifier delivered humidification with near-theoretical thermal efficiency while avoiding the downstream reheating penalty associated with evaporative humidification. An HVAC-oriented exergy comparison under identical outlet-air conditions further showed that the membrane humidifier achieved the highest average exergy efficiency (10.2%), followed by evaporative humidification with reheat (8.9%) and steam injection (5.4%). These results demonstrate the thermodynamic potential of membrane-based quasi-isothermal humidification using low-grade heat in winter HVAC applications.