Dynamic characterisation of coupled heat and mass transfer in the packed-type liquid desiccant dehumidifier

Abstract

The liquid desiccant dehumidification is a promising alternative method for removing moisture from the process air, which can achieve more effective independent humidity control in the built environment at lower energy cost. In recent years, a diversity of liquid desiccant hybrid air conditioning systems have been developed. Control issues are becoming critical for maintaining normal operation and improving the performance of the systems under dynamic working conditions. The dehumidifier is a critical component where the coupled heat and mass transfer between the desiccant solution and the air occurs. Understanding the dynamic characteristics of the dehumidifier is essential to design and tune controllers, and to develop operation strategies for the hybrid systems in the real operation. However, almost all existing researches on the liquid desiccant dehumidifier focus on steady state conditions. Few research efforts have been made on its dynamic characteristics. For better understanding the dynamic characteristics of the dehumidifier and for more realistic dynamic simulation of hybrid systems involving real-time control, the experimental study and a computationally efficient yet reasonably accurate dynamic model of the dehumidifier are needed. In addition, the liquid desiccant dehumidifier usually occupies large installation space. The potential drawback in big dimension of the liquid desiccant system hinders its wide applications. There is a lack of research on the dehumidification performance of a compact liquid desiccant dehumidifier. Therefore, this research aims to study the dehumidification performance of a compact liquid desiccant dehumidifier and the dynamic characteristics of the liquid desiccant dehumidifier using both experimental study and numerical modelling. The main works of this research are shown as follows. Firstly, an experimental test rig has been built to study the dehumidification performance of a liquid desiccant dehumidifier. The dehumidifier adopts counter flow configuration and structured packing. In order to examine the measurement accuracy and the adiabatic condition of the experiments, uncertainty analysis for the measured parameters, dynamic responses of the temperature and humidity sensors to step changes, and energy and mass balance analysis are conducted, respectively. Secondly, experimental studies on the dehumidification performance of a compact liquid desiccant dehumidifier using structured packing with a high specific surface area (650 m2/m3) are conducted. New empirical equations correlating the moisture effectiveness and the enthalpy effectiveness as well as the heat and mass transfer coefficients with critical inlet parameters are developed. Lewis number during the dehumidification process is also discussed to analyze the coupled heat and mass transfer characteristics. The performance of the present type of packing is compared with other types of packing studied in literature. Besides, the influences of the inlet conditions of the air and the desiccant as well as the packing height on the dehumidification performance are also investigated and compared with the results reported in previous studies, which is valuable for the design of compact liquid desiccant dehumidifiers. Thirdly, the dynamic heat and mass transfer characteristics of a counter-flow packed-type liquid desiccant dehumidifier are studied experimentally. Two types of transient working conditions are tested in experiments: at the beginning of a dehumidification process (by two different scenarios) and the step changes of inlet fluids parameters (inlet air humidity ratio and the solution flow rate). Dynamic responses of the outlet air and desiccant solution as well as dynamic moisture removal rate are displayed and discussed. Fourthly, a simplified one-dimensional dynamic model is also developed for simulating the dynamic characteristics of the counter flow packed-type liquid desiccant dehumidifier. Convenient approaches to quantifying the thermal mass of the packing and the desiccant solution held in the dehumidifier are proposed, which are essential parameters of the dynamic model. Numerical solution for the proposed model is developed. The model is tested and validated experimentally under different transient operating conditions. Finally, the dynamic characteristics of the dehumidifier are investigated by parameter analysis method. Time constants of the dynamic humidity and temperature of outlet air are introduced to evaluate the dynamic response of the dehumidifier. The influences of two operation parameters (i.e. the air flow rate and the solution flow rate) and one configuration parameter (i.e. the height of the packing) on the time constants are analyzed experimentally. Besides, the influences of thermal mass of the dehumidifier on the time constants are also analyzed using the proposed dynamic model.