Analytical dehumidification performance evaluation of a novel window-type liquid desiccant ventilation system based on CFD simulation

As climate change leads to a transition into a hot and humid subtropical climate in Korea, maintaining a comfortable indoor thermal environment necessitates proper control of indoor temperature and humidity.

Traditional refrigerant dehumidification systems offer easily adjustable dehumidification levels but suffer from energy inefficiency due to overcooling and reheating processes. In contrast, liquid desiccant systems offer the potential for energy conservation through latent cooling for humidity control. However, existing studies mainly focus on individual performance analysis of air conditioning systems or standalone dehumidification units, with limited analyses of dehumidification effects and applicability in residential indoor spaces. The conventional heat and mass transfer analyses in dehumidification are constrained by the assumption of constant properties for incoming moist air and liquid desiccants under steady-state conditions.

To overcome this limitation, extensive research on mass transfer analysis utilizing the penetration theory, which considers time-varying factors including gravity and surface tension, is imperative. In the penetration theory, a fluid element moves from the bulk to the interface, where mass transfer occurs through molecular diffusion in a transient state.

Consequently, this study proposes a novel window-type liquid desiccant ventilation system module employing a LiCl solution applicable to residential buildings, and dehumidification performance was evaluated using a CFD (computational fluid dynamics)-based penetration theory and multiphase flow analysis methodology. Additionally, the validation of the analysis methodology was assessed through experimental evaluations.

The analysis employed two-dimensional unsteady-state governing equations with the RNG k-ε turbulence model. The body force weighted scheme was applied to account for gravity on the liquid desiccant flow. The VOF (Volume of Fluid) model was used to analyze heat and mass transfer caused by vapor pressure differences at the interface between moist air and the liquid desiccant. A UDF (User-Defined Function) was applied to implement heat, mass, and reaction processes during heat and mass transfer, as well as changes in the liquid desiccant. Simulation results showed that the dehumidification rate varied from 32.1% to 90.8%, depending on ventilation conditions and LiCl concentration. The dehumidification rate showed a difference of up to 35.8% and 23.8% with increasing ventilation rates and outdoor relative humidity, respectively.

Across diverse outdoor environments and ventilation conditions, the validation of the analytical model through experimental evaluations showed significant predictive performance.

This study proposes that the window-type liquid desiccant ventilation system can efficiently eliminate moisture from incoming outdoor air in residential buildings and can serve as a fundamental resource for its implementation.