Multi absorber stand alone liquid desiccant air-conditioning systems for higher performance

Achieving comfortable environment with the use of renewable energy or waste heat without creating the hazardous effects over the earth atmosphere are major challenges in the field of air-conditioning. Liquid desiccant technology is a promising option. For the past few decades research is going on worldwide to commercialize such systems. Hybrid liquid desiccant systems (combination of vapor compression (V-C) and liquid desiccant system) have got more attention probably due to higher COPs and lower regeneration temperature for such systems.In the present communication the steady-state performance of stand alone liquid desiccant systems has been simulated and analyzed. Falling film designs of absorber and regenerator have been selected for the study due to their lower pressure drops. The simulation of these components has been carried out by solving the basic mass and energy balance equations. These are nonlinear coupled first order differential equations, which have been solved by using fourth order finite difference Runge-Kutta method. The overall system has been simulated using Warner’s technique. Two new stand alone liquid desiccant cycles utilizing the potential of desiccant fully through multiple absorbers have been proposed. The proposed new cycles improve the COP of stand alone systems significantly. A parametric study has also been carried out on these liquid desiccant cycles to identify the key design parameters affecting the performance of the system.     

Performance characterisation of liquid desiccant columns for a hybrid air-conditioner

Use of liquid desiccant-vapor compression hybrid system is encouraged for low humidity applications. The liquid desiccant is primarily used to further dehumidify the supply air. In the present study, by using psychrometric equations and liquid desiccant property data, heat and mass transfer analysis for the dehumidifier and regenerator columns in counter flow configuration has been carried out. The simulation of the columns corresponds to low solution to air (S/A) flow ratio where precooled air gets dehumidified in the absorber while preheated air is used for regeneration of the solution. A detailed study of the performance characteristics for the absorber and regenerator columns confirms the requirement of the desiccant loop for additional dehumidification of the conditioned air. This need develops the main motive towards the concept of hybrid air conditioning.     

Sensitivity analysis and component modelling of a packed-type liquid desiccant system at partial load operating conditions

Prediction of annual energy consumption requires the partial load performance evaluation of each component in an air conditioning system. A packed-type or a coil-type liquid desiccant apparatus is an essential component of a hybrid liquid desiccant system. Therefore, predicting the hour by hour performance of a packed-type hybrid liquid desiccant system to estimate its annual energy consumption requires that the performance of a packed-type system be known at partial load conditions. For this purpose a detailed sensitivity analysis of heat and mass transfer performance of a packed-type liquid desiccant system is carried out to identify important performance parameters. Two output parameters — enthalpy effectiveness and humidity effectiveness — are defined to present the results. Based on the conclusions drawn from the detailed sensitivity analysis, a simple model is developed to predict the performance of an absorber and a regenerator at partial load operating conditions. Performance predictions using the simple model are compared with the predictions from the detailed analysis along with another existing model which is available in the literature. The simple model is in excellent agreement with the predictions of other two models.     

A simulation model for the performance of a hybrid liquid desiccant system during cooling and dehumidification

A finite difference model describing simultaneous heat and mass transfer in a hybrid liquid desiccant cooling system is presented in this paper. This type of system provides cooled and dehumidified air by a combination of a packed absorber tower and a conventional vapour compression system. The model will allow the prediction of the rate of condensation in the dehumidification tower and the evaporator of the vapour compression system, the conditions of the air and liquid desiccant leaving the tower, the conditions of the air leaving the evaporator, and other important parameters. The mathematical model results were validated with existing experimental data for a similar system with an uncertainty of 10.5% for the total rate of condensation and 0.8°C for the evaporator air exit temperature. Copyright © 2005 John Wiley & Sons, Ltd.     

Control performance of a dedicated outdoor air system adopting liquid desiccant dehumidification

Liquid desiccant is energy efficient for dehumidification in air-conditioning systems. In this study, a novel dedicated outdoor air system (DOAS) adopting lithium chloride solution as liquid desiccant is proposed to process supply air. The DOAS mainly consists of a membrane-based total heat exchanger, a liquid dehumidifier, a regenerator and a dry cooling coil. It can realize independent temperature and humidity controls for supply air. Control strategies for the supply air dehumidification and cooling process as well as the desiccant solution regeneration process in the DOAS are developed and verified. The control performances of the proposed dedicated outdoor air system are investigated at different operation conditions by simulation tests. The results show that the DOAS is more suitable for hot and humid climates. The effects of the total heat exchanger on the performance of the DOAS are also evaluated. It can improve the system energy performance by 19.9–34.8%.     

Photovoltaic–electrodialysis regeneration method for liquid desiccant cooling system

Liquid desiccant cooling system (LDCS) is an (a novel) air-conditioning system with good energy saving potential. Regenerator is the power centre for LDCS. Currently, the regeneration process is always fuelled by thermal energy. Nevertheless, this regeneration pattern has some disadvantages in that its performance will become poor when the surrounding atmosphere is of high humidity, and the heat provided for regeneration will be unfavourable to the following dehumidification process. To ameliorate that, a new regeneration method is proposed in this paper: a membrane regenerator is employed to regenerate the liquid desiccant in an electrodialysis way; while solar photovoltaic generator is adopted to supply electric power for this process. Analysis has been made about this new regeneration method and the result reveals: this new manner achieves good stability with the immunity against the adverse impact from the outside high humidity; its performance is much higher than that of the thermal regeneration manner while putting aside the low efficiency of the photovoltaic system. Besides, purified water can be obtained in company with the regeneration process.     

An experimental study of structured packing dehumidifier/regenerator operating with liquid desiccant

The present work presents an experimental investigation on the performance of the structured packing cross flow desiccant dehumidification system (DDS). This system is referred as DDS; its heart is the dehumidifier/regenerator. It is used to meet a latent heat load by air dehumidification. Calcium chloride (CaCl₂) solution is used as the working desiccant material in this system. The structured packing has a density (specific surface area) of 390 m²/m³, corrugation angle of 60° and void fraction of 0.88. The effect of relevant parameters such as air flow rate, desiccant solution flow rate, desiccant solution temperature and concentration and packing thickness on the performance of the system is studied. The performance of the system is evaluated using the mass transfer coefficient, moisture removal rate (MMR), effectiveness and the coefficient of performance (COP). The remarkable increase of mass transfer coefficient and MRR for both deh/reg is observed by increasing both air and solution flow rates. Eventually, the payback period (PP) of the DDS is 11 months with annual running cost savings (ΔC_RC) of about 31.24% compared with vapor compression system (VCS) dehumidification. The overall environmental impacts of DDS are nearly 0.63 of VCS. This may emphasize the need of incorporating a desiccant system along with air conditioning applications.     

Artificial neural network analysis of liquid desiccant dehumidification system

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.     

Single-stage and double-stage photovoltaic driven regeneration for liquid desiccant cooling system

Liquid desiccant cooling system (LDCS) is a novel air-conditioning system with good energy saving potential. However, the present LDCS has a poor performance, mainly because the conventional thermal regeneration method wastes too much energy during the regeneration process. To improve that, photovoltaic-electrodialysis (PV-ED) regeneration method is introduced: it has a higher performance by using solar photovoltaic panels to drive an electrodialysis regeneration process. To further explore the PV-ED method, both single-stage and double-stage photovoltaic-electrodialysis regeneration systems are presented in this paper. Analysis is made on these two systems and some influential factors are investigated. It reveals that the concentration difference between the desiccant solution before and after regeneration has a strong impact on system performance. Moreover, comparison is conducted between the single-stage and the double-stage systems, the results show that the double-stage system is more energy-efficient and it can save more than 50% energy under optimized working conditions.     

An experimental study of a solar-regenerated liquid desiccant ventilation pre-conditioning system

A solar-regenerated liquid desiccant ventilation pre-conditioning system has been installed and experiments were carried out for a period of nine months covering rainy, cold, and hot seasons in a hot and humid climate (Thailand). A heat exchanger was used to cool the dehumidified air instead of typical evaporative cooling to maintain the dryness of the air. The use of solar energy at the regeneration process and cooling water from a cooling tower makes the system more passive. The evaporation rate at the regeneration process was always greater than the moisture removal rate at the dehumidification process indicating that the concentration of the desiccant in the system would not decrease and so the performance would not drop during continuous operation. The system could reduce the temperature of the delivered air by about 1.2 °C while the humidity ratio was reduced by 0.0042 kg_w/kg_da equivalent to 11.1% relative humidity reduction. The experimental results were also compared with models in literature.     

Experimental investigation of a liquid desiccant system for solar cooling and dehumidification

Growing demand for air conditioning in recent years has caused a significant increase in demand for primary energy resources. Solar-powered cooling is one of the environmentally-friendly techniques which may help alleviate the problem. A promising solar cooling method is through the use of a liquid desiccant system, where humidity is absorbed directly from the process air by direct contact with the desiccant. The desiccant is then regenerated, again in direct contact with an external air stream, by solar heat at relatively low temperatures. The liquid desiccant system has many potential advantages over other solar air conditioning systems and can provide a promising alternative to absorption or to solid desiccant systems.Earlier work by the authors included theoretical simulations and preliminary experiments on the key components of the liquid desiccant system. The objective of the present study has been to construct a prototype system based on the knowledge gained, to monitor its performance, identify problems and carry out preliminary design optimization. A 16 kWt system was installed at the Energy Engineering Center at the Technion, in the Mediterranean city of Haifa. The system comprises a dehumidifier and a regenerator with their associated components operating together to dehumidify the fresh (ambient) air supply to a group of offices on the top floor of the building. LiCl-water is employed as the working fluid. The system is coupled to a solar collector field and employs two methods of storage – hot water and desiccant solution in the regenerated state. The performance of the system was monitored for five summer months under varying operating conditions. The paper describes the operation of the experimental system and presents the measured data and the calculated performance parameters.     

空调系统溶液除湿基本问题的研究进展

分析溶液除湿的研究现状,总结溶液除湿的相关基本问题,其中包括除湿/再生原理、蓄能机理、对空气品质的影响和除湿剂及除湿器的选择。提出了溶液除湿的存在问题和发展前景。     

Development of a novel two-stage liquid desiccant dehumidification system assisted by CaCl2 solution using exergy analysis method

Air conditioning system based on liquid desiccant has been recognized as an efficient independent air humidity control HVAC system. To improve thermal coefficient of performance, a novel two-stage liquid desiccant dehumidification system assisted by calcium chloride (CaCl₂) solution is developed through exergy analysis based on the second thermodynamic law. Compared with the basic liquid desiccant dehumidification system, the proposed system is improved by two ways, i.e. increasing the concentration variance and the pre-dehumidification of CaCl₂. The exergy loss in the desiccant–desiccant heat recovery process can be significantly reduced by increasing desiccant concentration variance between strong desiccant solution after regeneration and weak desiccant solution after dehumidification. Meanwhile, the pre-dehumidification of CaCl₂ solution can reduce the irreversibility in the regeneration/dehumidification process. Compared to the basic system, the thermal coefficient performance and exergy efficiency of the proposed system are increased from 0.24 to 0.73 and from 6.8% to 23.0%, respectively, under the given conditions. Useful energy storage capacity of CaCl₂ solution and LiCl solution at concentration of 40% reach 237.8 and 395.1 MJ/m³, respectively. The effects of desiccant regeneration temperature, air mass flux, desiccant mass flux, etc., on the performance of the proposed system are also analyzed.     

Dehumidification of air with a newly suggested liquid desiccant

Calcium chloride solution is a cheap desiccant. It is unstable at certain solution concentrations and dehumidified air temperatures. The aim of this research is to stabilize it by mixing with calcium nitrate in different weight combinations. The physical properties of a proposed liquid desiccant such as density, viscosity, and vapor pressure were obtained. Heat and mass transfer analysis between a thin liquid layer of the proposed desiccant and the air flowing through rectangular channel has been studied. The different factors affecting the dehumidification process of air were studied.     

Empirical correlations to predict the performance of the dehumidifier using liquid desiccant in heat and mass transfer

Dehumidifier is one of the essential components in liquid desiccant air-conditioning system, whose hourly performance is required to predict the annual energy consumption of the system. Model complexity and the required amount of computer time usually do not permit the use of models based on a differential element. In order to estimate the hour-by-hour performance of dehumidifier, a simplified approach is proposed in the present study, in which enthalpy and moisture effectiveness are adopted as indexes to describe the heat and mass transfer performances of the dehumidifier. Empirical correlations of enthalpy and moisture effectiveness are expressed by enthalpy difference, moisture difference, air and desiccant flow rate, based on corresponding experimental results. Empirical correlations are given for a cross-flow packed dehumidifier, the average absolute differences between the calculated values and the experimental findings are 6.3% and 6.0% for enthalpy effectiveness and moisture effectiveness, respectively, with discrepancies mainly within ±20%. Good agreements are also shown for counter-flow dehumidifiers available in literatures.     

An analytical solution for air dehumidification by liquid desiccant in a packed column

This paper presents a new analytical solution of heat and mass transfer processes in a packed bed liquid desiccant dehumidifier based on the equilibrium humidity on the interface is assumed to be constant. In order to maintain the partial pressure difference on the interface, a high liquid desiccant flow rate is often applied in the practical absorber. Therefore, for a narrow range of operating conditions for practical dehumidification process, we can assume that the equilibrium humidity ratio on the interface is constant. The assumption of constant humidity ratio is applied in this paper for derivation of the analytical solution. The model and the analytical solution predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. According to the Lewis definition in this present study, the Lewis number obtains 0.9. The effects of variables such as air and liquid desiccant flow rate, air temperature and humidity, desiccant temperature and concentration have been investigated on the condensation rate. The results present that design variables such as desiccant concentration, desiccant temperature, air flow rate, and air humidity ratio have the greatest impact on the performance of the dehumidifier. The liquid flow rate and the air temperature have not a significant effect. Furthermore, the effects of air and liquid desiccant flow rate have been reported on the humidity effectiveness of the column.     

Suggested solution concentration for an energy-efficient refrigeration system combined with condensation heat-driven liquid desiccant cycle

This paper presents a hybrid energy-efficient refrigeration system enhanced by liquid desiccant evaporative cooling technology for subcooling the refrigerant, where the liquid desiccant cycle is driven by the exhausted heat from the condenser and three commonly used liquid desiccants: LiCl, LiBr and CaCl₂ aqueous solutions are considered here. The solution concentration for the proposed hybrid energy-efficient refrigeration system should be determined and optimized carefully for better performance. Sensitive study of solution concentration involved in the hybrid system is conducted at different condensation temperature. The results indicates that under standard working condition (i.e., condensing temperature is 50 °C), the optimum solution concentration is 0.31 for LiCl aqueous solution, 0.45 for LiBr aqueous solution and 0.42 for CaCl₂ aqueous solution, while the maximum COPs are nearly same. When the condensing temperature is 45 °C, the optimum solution concentration should be set at 0.27 for LiCl aqueous solution, and 0.41 for LiBr aqueous solution and 0.37 for CaCl₂ aqueous solution, while condensing temperature is 55 °C, it is 0.35 for LiCl aqueous solution, 0.49 for LiBr aqueous solution and 0.45 for CaCl₂ aqueous solution. The simple fitting formulas are obtained, and performance improvement potential is discussed.     

Performance study on a structured packed liquid desiccant regenerator

The solution carryover in traditional desiccant regeneration towers is of serious concern in real applications especially when using triethylene glycol (TEG) as a desiccant. In this study, the cellulose rigid media pads are used as the structured packing. The packing arrangements have provided minimum carryover of TEG. A performance study of the simultaneous heat and mass transfer between air and desiccant in a structured packed-stripping tower is conducted. Through the study of the regenerator, important design variables are defined and the regenerator performance is compared with the previous studies. The effects of air and liquid flow rates, air humidity, desiccant temperature and desiccant concentration have been reported on the evaporation rate and humidity effectiveness of the column. It is found that high liquid flow rates do not have a significant effect on the performance variables.     

太阳能液体除湿空调性能的实验研究

在对液体除湿机理研究的基础上，建立了太阳能液体除湿空调系统实验台，采用氯化钙溶液作为除湿剂，对系统的除湿性能进行了实验研究，对影响除湿的各主要因素进行了分析。     

Model-based optimal control of a dedicated outdoor air-chilled ceiling system using liquid desiccant and membrane-based total heat recovery

This study presents a model-based control strategy for a novel dedicated outdoor air-chilled ceiling (DOAS-CC) system with the aim of optimizing the overall system performance. The DOAS-CC system incorporates liquid desiccant dehumidification and membrane-based total heat recovery technologies. Simplified but reliable models of major components in the DOAS-CC system are firstly developed to predict the system performance. A cost function is then constructed to minimize total energy consumption while properly maintaining thermal comfort reflected by indoor air temperature and relative humidity. Genetic algorithm is used to search for optimal set-points of the supply air temperature and humidity ratio of the dedicated outdoor air subsystem as well as the supply water temperature. The performance of this strategy is tested and evaluated with different control settings in a simulated multi-zone space served by the DOAS-CC system under various weather conditions. The results show that optimized control variables produced by the optimal strategy can improve the system energy performance and maintain indoor thermal comfort.