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[en] Liquid desiccant dehumidification systems have drawn a great deal of attention in the HVAC industry due to its great energy saving potentials. Numerous studies have been conducted to investigate the relationships between the operating conditions and the system performance. However, it seems that the existing relationships were built improperly since almost all of them were established through the incomplete single-factor tests, rather than the full-factorial tests. This makes the existing work unable to clarify the overall significance of the various operating conditions on affecting the system performance. To address this unexplored issue, an L18 × L8 cross-product orthogonal array together with the statistical analysis method (ANOVA) was adopted in this work to investigate the significance of operating conditions in promoting the system performance (i.e. sensitivity analysis) and stability (i.e. stability analysis). 144 experimental and simulation runs were conducted within the ultrasonic atomization liquid desiccant dehumidification system (UADS) as the example to demonstrate the analysis. It was found that though direct influence on the system can be exerted by all the operating conditions, their significance differed markedly. Based on the analysis, the operating conditions can be classified into four types while the optimal conditions for the UADS were also figured out and validated. - Highlights: • We clarified significance of operating conditions on performance of UADS by ANOVA. • Sensitivity of operating conditions on UADS performance was analyzed and ranked. • Significance of operating conditions on UADS stability was revealed and ranked. • The optimal running conditions for UADS were figured out and validated.
[en] This study presents a model-based optimization strategy for an actual chiller driven dehumidifier of liquid desiccant dehumidification system operating with lithium chloride solution. By analyzing the characteristics of the components, energy predictive models for the components in the dehumidifier are developed. To minimize the energy usage while maintaining the outlet air conditions at the pre-specified set-points, an optimization problem is formulated with an objective function, the constraints of mechanical limitations and components interactions. Model-based optimization strategy using genetic algorithm is proposed to obtain the optimal set-points for desiccant solution temperature and flow rate, to minimize the energy usage in the dehumidifier. Experimental studies on an actual system are carried out to compare energy consumption between the proposed optimization and the conventional strategies. The results demonstrate that energy consumption using the proposed optimization strategy can be reduced by 12.2% in the dehumidifier operation. - Highlights: • Present a model-based optimization strategy for energy saving in LDDS. • Energy predictive models for components in dehumidifier are developed. • The Optimization strategy are applied and tested in an actual LDDS. • Optimization strategy can achieve energy savings by 12% during operation
[en] Owing to the stringent indoor air quality (IAQ) requirements and high cost of desiccants, one of the major concerns in liquid desiccant technology has been the carryover, which can be eliminated through indirect contact between desiccant and air. Membrane contactors using microporous semipermeable hydrophobic membranes have a great potential in this regard. This communication investigates the performance of semipermeable membrane based indirect contactors as dehumidifiers in liquid desiccant cooling applications. Experiments on different types of membrane contactors are carried out using lithium chloride (LiCl) solution as desiccant. The membrane contactors consist of alternate channels of air and liquid desiccant flowing in cross-flow direction. Hydrophobic membranes form a liquid tight, vapor permeable porous barrier between hygroscopic solution and moist air, thus eliminating carryover of desiccant droplets. In order to provide maximum contact area for air–desiccant interaction, a wicking material is sandwiched between two membranes in the liquid channel. It is observed that vapor flux upto 1300 g/m2 h can be achieved in a membrane contactor with polypropylene (PP) membranes, although the dehumidification effectiveness remains low. The effect of key parameters on the transmembrane vapor transport is presented in the paper. - Highlights: • Indirect membrane contactors developed to avoid carryover in liquid desiccant system. • Dehumidification effectiveness and vapor flux reported under varying conditions. • Vapor flux upto 1295 g/m2 h in polypropylene contactor with high area density. • Dehumidification effectiveness with LiCl solution varies within 23% to 45%
[en] In drying processes it is necessary to appropriately control air humidity and temperature in order to enhance water evaporation from product surface. The aim of this work is to investigate several HVAC configurations for product drying based on desiccant wheels, in order to find systems which reach high primary energy savings through the appropriate integration of refrigerating machines, adsorption wheels and cogenerative engines. Simulations are carried out for different values of sensible to latent ambient load ratio and the effect of ambient and outside air conditions is evaluated for each configuration. It is shown that primary energy savings can reach 70–80% compared to the reference technology based on a cooling coil. With respect to works available in literature, the results of this study keep a general approach and they can be used as a simple tool for preliminary assessment in a wide range of applications. -- Highlights: ► Several HVAC systems for product drying based on desiccant wheels are investigated. ► The sensible to latent ambient load ratio influences the choice of the best system. ► Energy savings can reach 80% compared to the technology based on a cooling coil. ► Simulation results can be used for preliminary assessment in many applications.
[en] A novel marine rotary desiccant A/C (air-conditioning) system was developed and studied to improve energy utilization efficiency of ship A/C. The orthogonal experiment was first carried out to investigate the influence of various parameters of the marine rotary desiccant A/C system. During the orthogonal experiment the analysis of variance was used to exclude interference from the secondary influencing factor on system performance. The significant influencing factors of system were studied in great detail using the first and second laws of thermodynamics to find optimal setting parameters for best system performance. It is suggested from the analysis results that as regeneration temperature increases, the COPth (thermal coefficient of performance) and exergy efficiency of system (ηe) decreases by 46.9% and 38.8% respectively. They decrease in proportion to the increase of the temperature. ηe reaches its maximum value of about 23.5% when the inlet humidity ratio of process air is 22 g/kg. Besides, the exergy loss of system concentrates on the regeneration air heater, the desiccant wheel and the regeneration air leaving the desiccant wheel, which account for 68.4%–81% of the total exergy loss. It can be concluded that applying the marine rotary desiccant A/C in high-temperature and high-humidity marine environment is advantageous. - Highlights: • Significant influencing factors of the system are found by the analysis of variance. • The change trends of the COPth and the ηe are nearly proportional with the regeneration temperature. • The ηe reaches its maximum value (about 23.5%) when the inlet humidity ratio of process air is 22 g/kg. • The contribution rate of the dry-bulb temperature of fresh air is up to 73.91% for the COPth. • Applying the marine rotary desiccant A/C in high-temperature and high-humidity marine environment is advantageous
[en] The dehumidification process involves simultaneous heat and mass transfer and reliable transfer coefficients are required in order to analyze the system. This has been proved to be difficult and many assumptions are made to simplify the analysis. The present research proposes the use of ANN based model in order to simulate the relationship between inlet and outlet parameters of the dehumidifier. For the analysis, randomly packed dehumidifier with lithium chloride as the liquid desiccant is chosen. A multilayer ANN is used to investigate the performance of dehumidifier. For training ANN models, data is obtained from analytical equations. Eight parameters are used as inputs to the ANN, namely: air and desiccant flow rates, air and desiccant inlet temperatures, air inlet humidity, desiccant inlet concentration, dimensionless temperature ratio, and inlet temperature of the cooling water. The outputs of the ANN are the water condensation rate and the outlet desiccant concentration as well as its temperature. ANN predictions for these parameters are validated well with experimental values available in the literature with R2 value in the range of 0.9251-0.9660. This study shows that liquid desiccant dehumidification system can be alternatively modeled using ANN with a reasonable degree of accuracy. -- Research highlights: → Artificial neural network (ANN) based model is used to simulate the performance of the liquid desiccant dehumidification process. → Three ANNs each with eight inputs and one output have been trained. → Water condensation rate, outlet desiccant concentration and its temperature are predicted. → ANNs predicted parameters are validated well with the experimental results.
[en] A compression heat pump driven and membrane-based liquid desiccant air dehumidification system is presented. The dehumidifier and the regenerator are made of two hollow fiber membrane bundles packed in two shells. Water vapor can permeate through these membranes effectively, while the liquid desiccant droplets are prevented from cross-over. Simultaneous heating and cooling of the salt solution are realized with a heat pump system to improve energy efficiency. In this research, the system is built up and a complete modeling is performed for the system. Heat and mass transfer processes in the membrane modules, as well as in the evaporator, the condenser, and other key components are modeled in detail. The whole model is validated by experiment. The performances of SDP (specific dehumidification power), dehumidification efficiency, EER (energy efficiency ratio) of heat pump, and the COP (coefficient of performance) of the system are investigated numerically and experimentally. The results show that the model can predict the system accurately. The dehumidification capabilities and the energy efficiencies of the system are high. Further, it performs well even under the harsh hot and humid South China weather conditions. - Highlights: • A membrane-based and heat pump driven air dehumidification system is proposed. • A real experimental set up is built and used to validate the model for the whole system. • Performance under design and varying operation conditions is investigated. • The system performs well even under harsh hot and humid conditions
[en] A novel rotary desiccant cooling cycle is proposed and studied using thermodynamic analysis method. The proposed cycle integrates the technologies of isothermal dehumidification and regenerative evaporative cooling, which are beneficial for irreversibility reduction. Thermodynamic investigation on the basic rotary desiccant cooling cycle shows that the exergy efficiency of the basic cycle is only 8.6%. The processes of desiccant dehumidification and evaporative cooling, which are essentially the basis for rotary desiccant cooling, affect the exergy performance of the cycle greatly and account for about one third of the total exergy destruction. The proposed cycle has potential to improve rotary desiccant cooling technology. It is advantageous in terms of both heat source utilization rate and space cooling capacity. The exergy efficiency of the new cycle is enhanced significantly to 29.1%, which is about three times that of the ventilation cycle, and 60% higher than that of the two-stage rotary desiccant cooling cycle. Furthermore, the regeneration temperature is reduced from 80 °C to about 60 °C. The corresponding specific exergy of the supply air is increased by nearly 30% when compared with the conventional cycles. -- Highlights: ► A novel rotary desiccant cooling cycle is developed using thermodynamic analysis method. ► Isothermal dehumidification and regenerative evaporative cooling have been integrated. ► The cycle is advantageous in terms of both heat source utilization rate and space cooling capacity. ► Cascaded energy utilization is beneficial for cycle performance improvement. ► Upper limits, which will be helpful to practical design and optimization, are obtained.
[en] Liquid desiccant dehumidification is an energy-efficient approach for humid air handling process. Increasing attention has been paid to heat pump driven liquid desiccant (HPLD) systems in China. The current research focuses on a counter-flow HPLD system. The configuration of this HPLD system is introduced and theoretical models of key components are analyzed. Based on the simulation model, operating performances with varying input parameters are obtained. Effects of input NTU_m, NTU_e_v_a_p and required ω_s_a are drawn from the simulated results. NTU_m of the packed tower is a key parameter, which affects Q_f_a/Q_e indicating the heat recovery performance and COP_h_p indicating the energy performance of heat pump. As NTU_m is not sufficient, heat-cold offset resulted from solutions circulating between dehumidifier and regenerator severely affects the system performance. Then adding a solution heat exchanger is regarded as an appropriate approach to improve the performance. Besides, adopting a multi-stage heat pump cycle helps to improve the match properties between solution and refrigerant. It's treated as an approach to improve the energy efficiency of this counter-flow HPLD system to a certain extent. The present study is expected to be beneficial to design an optimized HPLD system. - Highlights: • A simulation model for a counter-flow HPLD system is built. • Effects of NTU_m, NTU_e_v_a_p and required ω_s_a are drawn from simulation results. • NTU_m is a key parameter influencing both Q_f_a/Q_e and COP_h_p. • The solution heat exchanger is efficient with an insufficient NTU_m. • Adopting a multi-stage heat pump cycle helps to improve the system performance.
[en] Solar energy, heat exchange with the ambient & plants and vapor balance are the key variables for modeling the greenhouse. However, due to solar energy, crops continuously produce water vapors through evapotranspiration, which continuously need to be dehumidified to maintain the relative humidity in the required range. Therefore, modeling and simulation of dehumidification system seems to be indispensable, as it will help to investigate the workability and operating performance of the greenhouse. Hence, this work models and simulates the various components of liquid desiccant based dehumidification system for greenhouse cultivation. Each component of an entire stand-alone dehumidification system, namely reference greenhouse, dehumidification & regeneration reactors and solar collector are thoroughly modeled in MATLAB Simulink environment. The overall system can be effectively utilized to analyze the working performance for greenhouse cultivation. The obtained results indicate that proposed modeling is effective in showing the moisture removal, which crops generate inside the greenhouse. Besides, the developed system is very unique, as most of the previous desiccant system are designed for domestic and commercial buildings. It is envisaged that this work is very useful for the researchers and energy professionals to develop efficient integrated systems for stand-alone buildings. - Graphical abstract: Moisture contents in air and liquid desiccant along the packed bed. - Highlights: • Modeling and simulation of liquid desiccant based greenhouse dehumidification system. • Dehumidification system assists to analyze the working performance for greenhouse cultivation. • Model is capable of removing the moisture, which is generated by crops. • Proposed modelling requires lower value of heat for regeneration.