Liquid Desiccant Solution Research Articles

An innovative closed-air closed-desiccant HDH system to extract water from the air: A case for zero-brine discharge system

In a conventional HDH system, a constant feed of seawater is required to run the system. As a result of extracting freshwater from the saline water, it is rejected at higher salinity, thus it cannot be recirculated as a feed again. The aim of this study is to come up with a zero-brine discharge system that eliminates the continuous need for the supply of source and sink in the conventional system. Based on the fact that earth's atmosphere holds a vast amount of water that can be considered as a reliable freshwater resource, especially in coastal areas. A water-from-air system is proposed in this study. The proposed system configuration comprises of two parts, a basic HDH system that uses a liquid desiccant solution instead of saline water and a desiccant-based air dryer. The liquid desiccant leaves the HDH part at a higher concentration due to the extraction of freshwater. It then enters the air dryer where it is diluted by absorbing water vapor from the ambient air. This diluted desiccant is directed to the HDH part to close the desiccant loop. In this study, Lithium chloride is used as the working fluid of the proposed system. Mathematical modeling of heat and mass transfer processes between the air and desiccant in the humidifier and dehumidifier is developed to predict the performance of the desalination system. The proposed system produces about 8 kg of freshwater per hour.

Interfacial heat and mass transfer analysis in solar-powered, packed-bed adiabatic liquid desiccant regeneration for air conditioning

In this article, a hybrid photovoltaic–thermal (PV/T) module generating both electrical and thermal energy simultaneously has been used in a closed-cycle system to provide regeneration heat via a dynamic solar radiation model as well as electrical power relative to location, time of the day and day of the month. Electrical power generated drives the air fan, water and solution pumps, while the thermal component is used for desiccant’s regeneration. This combination enhances energy efficiency of the air conditioning system. A simplified analytical model of the complex occurrence of coupled heat and mass transmission phenomenon in liquid desiccant regeneration system powered by a hybrid PV/T module is developed. The interfacial air–desiccant interaction in a structured packing vertical column using lithium bromide solution and Mellapak was analysed. The resulting differential equations are solved simultaneously using separative evaluation and step-by-step iterative procedure. The system’s performance was projected with regeneration effectiveness, subject to varying temperatures of air and desiccant solution, moisture content and mass flow rate. It was established that subject to the prevailing local weather conditions, the PV/T module significantly raised desiccant temperature to a high of 67.22°C good enough for the regeneration process. The regeneration rate and effectiveness improved with upsurge in mass flow rate but reduces with a rise in humidity ratio. The optimum flow mix for effective regeneration was therefore established to be 0.847 and 0.00331 kg/min for air and desiccant solution, respectively, for maximum effectiveness of 69.3%. The liquid desiccant solution concentration increased by 30% during when solar radiation peak hours. The obtained theoretical outcomes matched with experimental results from the available literature show a permissible discrepancy of within ±20%, largely due to the fact that the simulation parameters were not the same as the prevailing experimental conditions.

Regeneration of liquid desiccant for solar air-conditioning and desalination using hybrid solar still

In a global era, continuous increment in energy requisition with its associated cost and relevant climate problems is causing accentuation in exploring more efficient ways to provide cooling/air-conditioning in enclosed space without degrading the environment. In the hot and humid area, major part of conventionally produced electrical energy is consumed by air-conditioning. Also in the rapidly growing world, scarcity of clean water is dilemma as equal as green-house effect and ozone layer depletion. Thus, there is emergence of dehumidified air for human comfort with potable clean water at a same time. In the present paper, performance of the hybrid solar still has been evaluated for the regeneration of liquid desiccant and distillation of saline water for the three days in the month of March and April 2017 at the location 20.61°N, 72.91°E. For the experimentation, weak liquid desiccant (40% concentration) solution of Calcium Chloride (CaCl2) was selected whose stream flows through the upper basin and constant depth (10 mm) of saline water with 0.10% concentration of Zinc Oxide (ZnO) nanofluid, to improve the energy transfer, was maintained in the lower basin of hybrid solar still. The results show that the daily water desorption of 1417.22 ml/m2, 1253.88 ml/m2 and 1368.74 ml/m2 was found from the weak solution of CaCl2 in upper basin with yield of 757.22 ml/m2, 728.33 ml/m2 and 727.78 ml/m2 in lower basin for three days respectively. Considering still as dehumidifier/regenerator, daily average capacity obtained to absorb the latent heat from the humid air by the regenerated liquid desiccant was 0.168 kW, 0.148 kW and 0.143 kW respectively for the three days. The overall thermal efficiency of 16.74%, 25.39% and 26.19% was achieved for the three days respectively.

Membrane distillation to regenerate different liquid desiccant solutions for air conditioning

The capacity of membrane distillation (MD) to regenerate three commonly used liquid desiccant solutions (i.e. CaCl2, LiCl, and a mixture of CaCl2/LiCl) for liquid desiccant air-conditioners (LDAC) was evaluated. The results demonstrate considerable impact of the concentration polarisation effect on the process water flux during MD regeneration of these three desiccant solutions. For each of these liquid desiccant solutions, the experimentally measured water flux of the MD process was about half of the calculated value using the process mass transfer coefficient (Km) obtained during the process characterisation without taking into account the concentration polarisation effect. The observed deviation between the experimentally measured and calculated process water flux indicates the need to include the concentration polarisation effect in the model for calculating water flux. Although Ca2+ concentration in the CaCl2 and CaCl2/LiCl liquid desiccant solutions exceeded the solubility limit for CaCO3, membrane scaling was not observed. Nevertheless, there was evidence that membrane fouling might occur during extended MD regeneration of liquid desiccant solutions containing CaCl2.

Experimental analysis of a novel internally-cooled dehumidifier with self-cooled liquid desiccant

With respect to air dehumidification, an internally-cooled dehumidifier is more efficient than an adiabatic dehumidifier. In the present study, a new method to control the increase in the temperature of the liquid dehumidifier solution was presented. A self-cooled liquid desiccant solution was defined and prepared. Anhydrous ethanol was added into the lithium chloride (LiCl) aqueous solution as the vaporizable coolant to control the increase in the temperature of the solution. A working hypothesis for the self-cooled solution dehumidification was established with a main feature wherein the heat exchangers in conventional internally-cooled dehumidifier were replaced by using self-cooled liquid desiccant solution. Its performance was compared and experimentally investigated. An environmental chamber and a visible experimental setup were established. Experimental results under different operating conditions were obtained. The performance indices including moisture removal rate and dehumidification efficiency were analysed. When compared with conventional LiCl liquid desiccant solution, the current analysis indicated that the self-cooled solution is superior with respect to the above performance indices.

Design optimizations of the thermal performance and cost in an air-gap hollow fiber membrane contactor (AHFMC)

An air–gap hollow fiber membrane contactor (AHFMC) has been proposed and applied for fluid heating in the HVAC applications. The water and the liquid desiccant (solution) streams flow inside the neighboring hollow fiber membranes in a cross-flow arrangement. The solution is heated due to the absorption and mixing heats by absorbing the water vapor from the water stream. A mathematical model of the coupled heat and mass transports in the AHFMC is established by converting it into a parallel-plate one. The optimal design of the AHFMC is studied by using the multi-objective particle swarm optimization (MOPSO) algorithm to obtain the maximum thermal performance and minimum total annual cost together with the coupled heat and mass transfer model. A set of multiple optimum solutions, which are named as “Pareto optimal solutions”, are then obtained. Influences of geometric variables of the contactor length, width, height, fiber outer diameter and fiber distances in different directions on the heating performance and total annual cost are studied. The optimally geometrical parameters of the AHFMC are obtained using the MOPSO algorithm. These results are useful for the design of the AHFMC based on the desired temperature lift, the contactor constraint, and the available investment in the real applications.

Analysis and optimization of total heat recovery devices by using liquid desiccant as a coupling fluid in air-conditioning systems

Total heat recovery in an air-conditioning system can recycle both heat and mass from return air to preprocess fresh air. This study analyzes liquid desiccant total heat recovery (LDTHR) devices with a counter-flow path or a cross-flow path. The inlet states of fresh air and return air are divided into two types for analysis, namely Type I: on the same iso-relative humidity line (φin); and Type II: on different iso-relative humidity lines (φin). With respect to Type I, the single-stage LDTHR with a counter-flow path is investigated by using an analogy with a sensible heat recovery device in which the performance is optimized under matched heat and mass transfer ability ((NTUm)F = (NTUm)R) and matched flow rates (mRcp.e = mFcp.e = mscp.s). With respect to type II, the liquid desiccant solution of a single-stage device works at a fixed iso-relative humidity line, and this limits the mass transfer between air and solution. Increases in the stage number alleviates the problem and improves humidity efficiency (ηω). For certain heat and mass transfer abilities (NTUm), the single-stage counter-flow LDTHR device exhibits the highest enthalpy efficiency (ηh). The application of the counter-flow device is restricted by the height and complex arrangement of air and solution flow. The enthalpy and humidity efficiency of a double-stage cross-flow device is close to that of the counter-flow device, and this is a practical method that is an alternative to the counter-flow device.

Mathematical modeling and parametric study of liquid desiccant system with internally-cooled dehumidifier

In this study, liquid desiccant system equipped with an internal cooling part has been modeled mathematically, and its efficiency has been investigated in different geometrical, physical and environmental conditions. The system uses a plate heat exchanger with co-current flow pattern, and liquid desiccant solution exchanges heat with cool water in adjacent channels. The results show that, if the effect of air humidity on changes in process air temperature is taken into account, the results of mathematical modeling error reduces in comparison with experimental results. If the height and width of the heat exchanger increases, system efficiency is improved, and at the best condition, the humidity ratio of intake air reduces up to 50%. In addition, although by increasing the air and desiccant flow rates entering the heat exchanger, humidity ratio of outlet air increases, the amount of humidity absorbsion increases as the water flow rate rises. When the temperature and relative humidity of ambient air increases, the amount of humidity absorbed will increase. Liquid desiccant system decreases ambient air humidity at 40 °C in both relative humidity of 100% and 40% in the same conditions specified by 43.4% and 10.1%, respectively. It is found that an increase in the temperature of the water flow and liquid desiccant solution at system input reduces the amount of moisture absorbed, and the effect of prior on humidity absorbtion is higher than the latter. When water and desiccant inlet temperatures rise 10 °C, the humidity ratio of outlet air increases 10.0% and 40.0%, respectively.

Dynamic behaviour simulation of a liquid desiccant dehumidification system

Among heat driven air conditioning systems, liquid desiccant air conditioning system can be assisted by low water temperature solar collectors. This can reduce the electricity demand of the air conditioning systems. Furthermore, these systems can provide possibility of energy storage in the form of chemical energy in the liquid desiccant solution or thermal energy in the solar collector hot water. This article simulates a liquid desiccant dehumidification cycle by two dynamic models: transient and discrete-steady state model. The transient model takes into consider the unsteady term in the governing equations for both the dehumidifier/regenerator model and also liquid-liquid heat exchanger model. While, in the other dynamic model, the steady state terms are used only. Correspondingly, a liquid desiccant dehumidification system was constructed, troubleshoot and field tested at summer conditions. According to the simulation results, the dynamic models are compatible with the variances in operating conditions and supply heating water. Also, the deviation between the humidity ratio results from simulation and field test effects on the simulated solution concentration with less than 4% deviation. The maximum deviation between the moisture removal rate results from simulation and test are −0.21 g/s in the dehumidifier and −0.5 g/s in the regenerator. Thus, the models of liquid desiccant system have reliability to predict the performance with solution storage mode.The comparison between simulation results shows that the disagreement between the two employed dynamic models is insignificant. However, the transient model reduces the computational time by 21% less than the discrete-steady state model. This can speed up the processing time.

Effects of the Random Distributions on the Laminar Flow and Heat Transfer in a Quasi-Counter Flow Parallel-Plate Membrane Channel

ABSTRACTAn internally cooled parallel-plate membrane contactor has been employed for liquid desiccant air dehumidification. The contactor is comprised of a series of quasi-counter flow parallel-plate membrane channels (QCPMC). The processing air and the liquid desiccant (solution) streams are separated by the membranes. Cooling tubes are installed in the solution channel to take away the absorption heat. The laminar flow and heat transfer in the QCPMC with the cooling tubes in the solution side (QCPMCC) are studied based on a unit cell containing the sandwiched domain outside the cooling tubes between two neighboring membranes. This paper is focused on the effects of the random distributions in the height direction on the transport phenomena in the QCPMCC. The governing equations of the momentum and thermal transports are built up together with a uniform wall temperature boundary condition. The product of the mean friction factor and Reynolds number and the mean Nusselt number are then calculated. Effects of the various random distributions in the height direction on the product of the mean friction factor and Reynolds number and the mean Nusselt number are analyzed. Compared to the regular arrangement, the mean Nusselt number for the random distribution is larger than that for the regular one when the Reynolds number is less than 68.5. However when the Reynolds number is larger than 68.5, the mean Nusselt number for the random distribution is smaller than that for the regular one.

Thermodynamic analysis of a compression-absorption refrigeration air-conditioning system coupled with liquid desiccant dehumidification

The vapor compression system driven liquid desiccant dehumidification to realize temperature and humidity independence control (THIC) is a very good idea for the utilization of condensation heat. Absorption refrigeration system (ABS) is widely applied all over the world, due to its energy saving and environment protection; meanwhile, large amount of condensation heat is discharged to the environment. In this study, a hybrid compression-absorption refrigeration air-conditioning (AC) system combined with liquid desiccant dehumidification is proposed. All of the condensation heat from the condenser of the ABS can be recovered to regenerate the diluted liquid desiccant solution. Meanwhile the evaporation temperature in the ABS rises since the latent load is handled by liquid desiccant, which improves the performance of the ABS. The PEE of the proposed system is 34.97% higher than that of traditional absorption refrigeration AC system at the same operation condition. And compared with the traditional absorption refrigeration AC system, the generation temperature of the proposed system can decrease from 100°C to 60°C due to the existence of the compressor.

Coupled Heat and Mass Transfer Analysis of an Adiabatic Dehumidifier – Unique Approach

Abstract Liquid desiccant dehumidifier is one of the most desirable systems for dehumidification of air when compared to conventional refrigeration system and solid desiccant based dehumidifier. In the present study, a finite difference model is developed to simulate the coupled heat and mass transfer processes occur in a counter flow adiabatic dehumidifier employing lithium chloride (LiCl) as a desiccant solution. Mat-lab R13 is used as a simulation package for solving the aforementioned model. This model has taken into the consideration of thermal effectiveness, effective height and moisture effectiveness as variable parameters for obtaining the correlation of the desired operating parameters. The predicted numerical parameters for absorption process have good agreement with the experimental data reported in the literature. This model is unique and can be extended to any type of liquid desiccant solution by changing the solution properties. The performance analysis of adiabatic counter flow dehumidifier is also carried out by varying the operating parameters and the detailed results are presented in the full manuscript.

Two-Dimensional Numerical Analysis of Membrane-Based Heat and Mass Cross-Flow Exchanger

ABSTRACTA two-dimensional numerical simulation model for a membrane-based heat and mass exchanger was developed. The system model equations were used to determine the coupled heat and moisture transfer from the humid air to the high concentrated liquid desiccant solution (LiCl, lithium chloride) by means of a parallel stack hydrophobic permeable membrane. The two streams of air and liquid desiccant solution were arranged in cross-flow directions. The fourth-order Runge–Kutta method was employed to solve these system model equations in a steady-state condition. This model enables one to predict the latent effectiveness of a membrane-based parallel cross-flow exchanger for dehumidification purpose in response to air to liquid mass flow ratio and the mass transfer unit number.

Laminar flow and heat transfer in a quasi-counter flow parallel-plate membrane channel in the solution side with cooling tubes

An internally-cooled parallel-plate membrane contactor has been proposed, designed, and used for liquid desiccant air dehumidification. It is comprised of a series of side in and out parallel-plate membrane channels (QCPMC). The processing air and the liquid desiccant (solution) streams are separated by the membranes. Cooling tubes are installed in the solution channel to take away the absorption heat. The friction factors and heat transfer coefficients in the complex QCPMC with the cooling tubes in the solution side (QCPMCC) are necessary for the structural design and energy analysis. Therefore the laminar flow and heat transfer in the solution channel are studied based on a unit cell containing the sandwiched domain outside the cooling tubes between two neighboring membranes. The momentum and thermal transport governing equations are built up together with a uniform wall temperature boundary condition and solved by a finite volume approach. The mean (fRe) and Nusselt numbers (Num) are then calculated. Influences of the Reynolds numbers (Re), tube number (Ntube), tube outer diameters (douter), tube arrangements, and connection types of tubes on the (fRe)m and Num are analyzed. It can be found that when douter=0.003m, the (fRe)m rises with an increase in the Ntube. However the Num decrease with the Ntube increasing. When Ntube=4, the (fRe)m rise with an increase in the douter. However the larger the douter are, the smaller the Num are.

Performance investigation and exergy analysis of air-handling processes using liquid desiccant and a desiccant wheel

Air-dehumidification systems using liquid desiccant and a desiccant wheel are common approaches using desiccant for air humidity control. This article focuses on the performance of air-dehumidification systems using liquid desiccant and a desiccant wheel. Based on basic air-handling processes using desiccants, the characteristics of the driving forces are investigated with the help of an unmatched coefficient ξ and exergy analysis. The performace of liquid desiccant and desiccant wheel systems is investigated, and approaches for improvement are proposed. It is indicated that the temperature difference ΔT or humidity ratio difference Δω in the liquid desiccant process is influenced by the inlet state of the air and solution. The liquid desiccant process heating solution, rather than air, for regeneration is optimal from the perspective of lowering ξ and reducing unmatched destruction. The ξ is approaching 1 between air and solid desiccant, i.e., the driving force is relatively uniform with the same dehumidification and regeneration regions. A multi-stage process is supposed to be effective for optimizing the desiccant wheel system, helping to reduce the exergy destruction arising from the unmatched coefficient. The present study is beneficial to cast light on the relationship between air-dehumidification systems using liquid desiccant and a desiccant wheel.

Empirical model for predicting the dehumidification effectiveness of a liquid desiccant system

The main purpose of this research was to propose a practical correlation for predicting the dehumidification effectiveness of a liquid desiccant (LD) system. The experimental data were collected during the operation of a typical liquid desiccant dehumidification unit. A lithium chloride (LiCl) solution was used as the liquid desiccant solution. The 2000m3/h LD unit was installed in a real building, and the variation of the dehumidification effectiveness values was monitored under various operating conditions. The data were statistically analyzed by the response surface methodology (RSM), and the impact of each system operation parameter on the dehumidification effectiveness was estimated quantitatively. Six operating parameters – the mass flow rate of process air, the mass flow rate of the desiccant solution, the dry-bulb temperature and the humidity ratio of the inlet air, and the inlet temperature and concentration of the desiccant solution – significantly affected the dehumidification performance of the LD unit. Consequently, a simplified linear equation model was derived as a function of the operation parameters that significantly impacted the dehumidification effectiveness. The proposed model was verified by comparison with existing models.

Nu and Sh correlations for LiCl solution and moist air in plate type dehumidifier

To cope with environmental pollution caused by rapid industrial development, international regulations such as the Montreal Protocol and the United Nations Framework Convention on Climate Change are becoming materialized day by day. As a result, interest in energy saving devices is increasing and studies are being concentrated on highly efficient refrigeration air conditioning systems. In this study, a desiccant cooling technology using a liquid desiccant of LiCl solution was researched. The plate was treated with hydrophilic coating and the wettability was improved by giving a groove shape. Consequently, a thin liquid film was maintained even with a small amount of desiccant solution, thereby increasing the stability of the liquid film and solving the scattering problem of the desiccant solution. In this study, it is found that the absorption rate increases with the increases of the liquid desiccant and air velocity and the air velocity has more significant effect on the absorption rate than the liquid desiccant solution flow. It is found that the absorption rate becomes higher with increasing the higher relative humidity and the concentration of liquid desiccant. The dehumidification effectiveness decreases greatly as the air velocity increases. Furthermore, it is concluded that the air velocity has the greatest effect on the improvement of the absorption rate and heat and mass transfer. In this experiment, Sh and Nu correlations of the air side and desiccant solution side with an error of ±25% were developed, respectively, and the absorption rate, dehumidification effectiveness and the experimental correlations were compared with other studies.

Using electrodialysis for regeneration of aqueous lithium chloride solution in liquid desiccant air conditioning systems

This study evaluates the potential of using electrodialysis (ED) technology to regenerate the aqueous lithium chloride (LiCl) solution, a commonly used liquid desiccant in liquid desiccant air conditioning (LDAC) systems. Experiments were performed using an ED system with ten cell pairs of ion-exchange membranes. A range of tests were carried out to examine the effects of the circulation flow rate, supplied current density, solution initial concentration and the concentration difference between the regenerated and spent solutions on the performance of ED for regenerating LiCl liquid desiccant solutions. The results showed that the regeneration capability of the ED stack decreased with the increase of the circulation flow rate. Regeneration performance in terms of the concentration enrichment increased as the supplied current density increased and the solution initial concentration decreased. It is also shown that the concentration difference between the regenerated and spent solutions is critical for the regeneration performance of ED. The ED stack can continuously increase the concentration of the regenerated solution when the concentration difference between the regenerated and spent solutions is below 5.86% (wt/wt), under the supplied current density of 57.1mA/cm2, circulation flow rate of 100L/h, and the initial concentrations of the solutions in the regenerated and spent tanks of 28.77% and 23.96% (wt/wt), respectively. The current efficiency of the ED in two hours running for all experiments was in the range of 55.17–73.54%. The results obtained from this study would be useful for the ED regenerator design and system integration.

COUNTER FLOW ADIABATIC REGENERATOR AND COMPARATIVE STUDY

Liquid desiccant air conditioning systems have recently been attracting attention due to their capability of handling the latent load without super-cooling and then reheating the air, as happens in a conventional compression-type air conditioning system. This paper presents the results from a performance study of the simultaneous heat and mass transfer between air and liquid desiccant in an adiabatic, counter flow, structured packed regenerator. A heat and mass transfer numerical model has been developed, based on the Runge-Kutta non-stiff fixed step method, to predict the performance of the device under various operating conditions. Good agreement was found between the theoretical model and experimental tests from previous studies, with the deviation range being ±6.2% in air outlet temperature, ±8.1% in air outlet humidity ratio and ±1.4% in solution outlet temperature. Important design variables are also defined. The effects of air and desiccant flow rates, air humidity ratio, desiccant temperature and concentration have been reported on the regeneration rate and regeneration effectiveness. A detailed sensitivity analysis has been implemented to indicate which input variables mostly affect the output conditions. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl2 were evaluated against each other. The results show that high regeneration efficiency could be achieved under high desiccant mass flow rates, high air mass flow rate, high desiccant inlet temperature, low desiccant inlet concentration and CaCl2 as the desiccant solution.

Study on LiBr-Liquid Desiccant System Driven by Heat Pump

Humidity is one of the most important factors for human thermal comfort, and the liquid desiccant system is an effective dehumidification mode, especially for renewable energy such as solar energy or waste heat acting as the driving heat. The whole system performance is depended on high absorption (or desorption) of water vapor in absorption (or desorption) process. Moreover, the cooling and heating energy should apply to absorption and desorption process respectively since the latent heat of water vapor transferred between air and liquid desiccant solution. However, in a real system, cooling and heating energy are always independent and matching imperfectly. In this study, heat pump is applied in a liquid desiccant system as the cooling and heating energy which can use thoroughly. Moreover, lithium bromide is acted as the working solution which is focused on experimentally test. Furthermore, different operating conditions including moist air temperature, air humidity, air flow et al in absorber and in generator are measured in the experiment test. Consequently, the experimental results is given and analyzed.