• Energy Research

Abstract It is well known that internally-cooled membrane-based liquid desiccant dehumidifier (IMLDD) has better dehumidification efficiency than adiabatic membrane-based liquid desiccant dehumidifier (AMLDD). The flow directions of air, solution, and cooling water in the IMLDD can dramatically affect dehumidification and cooling performance. In this paper, the mathematical models for the IMLDD of ten flow types were developed and validated experimentally. Based on the validated model, the differences of air temperature and humidity among various flow types of the IMLDD were investigated, and the effects of operating conditions on the performance of different flow types were also studied thoroughly. The main conclusions of this paper include the following: The flow types of IMLDD have different average temperature and humidity differences between air and solution. Under different operating conditions, the IMLDD of flow type b4 always has a maximum total cooling capacity. The moisture removal rate of flow type b4 is up to 8.2% greater than flow type a2 of worst performance, and for the sensible cooling capacity, the flow type b4 is up to 5 times flow type a2. This paper can help researchers and engineers to choose the appropriate form of the IMLDD according to inlet conditions.

Abstract Internally-cooled membrane-based liquid desiccant dehumidifier (IMLDD) can well alleviate the dehumidification deterioration due to the increase of desiccant solution temperature. In the IMLDD, air channels and desiccant solution channels are separated on both sides of semi-permeable membrane. Cooling media flows through tubes inside solution channels to reduce desiccant solution temperature. There exist many flow patterns of fluids (air, desiccant solution and cooling water), which have been proved to have significant effects on the performance of IMLDD. Therefore, to find the optimal flow type of IMLDD and operating conditions, the mathematical model for IMLDD was developed and validated experimentally, and the exergy and entransy theories were first adopted to analyze the effects of inlet parameters on the performance of IMLDD of various flow types. The results showed that counter flow arrangement between air and solution and parallel flow arrangement between solution and cooling water had the best dehumidification performance. The trend of dehumidification efficiency was opposite to exergy efficiency with the increase of inlet air humidity ratio and solution concentration. The increase of inlet cooling water temperature improved exergy efficiency but decreased dehumidification efficiency accordingly. The research of this paper is helpful for the design and optimization of IMLDD.

Abstract In this paper, a state-space model for a membrane-based liquid desiccant dehumidifier with counter-flow configuration was developed and validated experimentally. Compared with experimental data, the maximum relative error of model results is less than 2%, and the mean relative error is about 1%, which indicates that the state-space model can well predict the transient characteristics of the membrane-based liquid desiccant dehumidifier. Further, the transient responses of the dehumidifier to the step changes of inlet parameters (air temperature and humidity, air flow rate, solution temperature and flow rate) were investigated in detail. This dynamic model can clearly reflect the transient relationships among input, state and output variables. It was found that the effect of perturbation of the inlet air temperature on the moisture transfer in the membrane-based dehumidifier was too small to be neglected, and the variation of the inlet air humidity had little effect on the heat transfer. The response time constant of the outlet solution temperature to the inlet perturbations was obviously larger than that of the outlet air temperature and humidity. The air and the solution flow rate can be taken as the controllable parameters for the outlet air humidity control of membrane-based liquid desiccant dehumidifier. With the advantages of short calculation time and simple solving process, the state-space model is helpful for the control design and the optimization of the membrane-based liquid desiccant dehumidifier.

Abstract As the key components in a liquid desiccant dehumidification system, liquid desiccant regenerator produces great effect on the whole system’s performance. This paper studies the counter-flow ultrasonic atomization liquid desiccant regenerator. Firstly, a mathematical model for the regenerator has been established and experimentally validated with finite difference method. Two performance indicators, i.e., the moisture removal rate and the regeneration thermal efficiency, were suggested to evaluate the performance of the counter-flow regenerator, and the influences of inlet parameters on the regeneration performance of the counter-flow regenerator have been investigated by the established model in this study. Decreasing temperature difference and increasing humidity difference between air and solution can improve regeneration performance. With the temperature of air and solution increasing, the regeneration efficiency increases, but the energy loss also increases. The regeneration efficiency decreases with the increase of solution concentration. The regeneration efficiency increases nonlinearly with the increase of gas-liquid ratio. Finally, there exists an optimum droplet size for the best regeneration performance. Compared with the parallel-flow configuration, the counter-flow has better regeneration performance, which is attributed to the more uniform distribution of moisture transfer driving force and the longer residence time of droplets in the regenerator. The study contributes to the development and the better applications of the counter-flow ultrasonic atomization liquid desiccant regenerator.

Abstract Desiccant solution regeneration is an important task of the desiccant solution air conditioning system. The regeneration temperature of conventional packed-bed regenerator is high, which consumes a lot of thermal energy to regenerate the desiccant solution and prohibits the potential use of some low-grade and renewable energy as heat source. The ultrasonic atomization technology may be an effective way to reduce the regeneration temperature. In this paper, an ultrasonic atomization desiccant solution regenerator (UADR) was designed and studied. A theoretical model was developed to predict the heat and mass transfer characteristics of the UADR. The model was experimentally validated and used to investigate the influence of inlet parameters (e.g., air temperature, air humidity, solution concentration, solution temperature, gas-liquid ratio, droplet diameter and droplet jet velocity) on the outlet parameters and regeneration performance of the UADR. The results manifest that the contact area between the air and the desiccant solution droplets has great effect on the regeneration performance, and there exists an optimum droplet diameter for the best regeneration performance. In comparison with packed-bed regenerator, the regeneration temperature of UADR can drop from 3.1 to 6.6 °C.