Liquid Desiccant System Research Articles

An Analysis of Packed Bed Liquid Desiccant System

In this paper a new type of open absorption the liquid desiccant, air conditioning system will be introduced. The dehumidifier and regenerator play the most important role in this system.For liquid-gas contact, packed towers with low pressure drop provide good heat and mass transfer characteristics for compact designs. Thus, this analysis considers the packed tower liquid desiccant systems. The experimental data have been obtained from a built prototype of liquid desiccant system in a packed bed unit with a surface area per unit volume ratio of 125m2/m3, the liquid desiccant, viz lithium chloride. The result showed that the mean mass transfer coefficient of the packing dehumidifier was 0.02kg/m2s. Also the absorber characteristic parameter, the packing size or number of transfer units (NTU), and air-to-desiccant solution mass flow rate ratio (ASMR) are crucial parameters. These parameters affect humidity and enthalpy effectiveness and will be introduced and defined in this paper.

Performance analysis of a new desiccant pre-treatment electrodialysis regeneration system for liquid desiccant

Liquid desiccant air-conditioning system (LDAS) is a novel air-conditioner with good energy saving potential. As a renewable energy, solar thermal energy (TH) can be used to regenerate desiccant for LDAS. One problem of the solar regeneration system is that solar energy will depend on weather conditions, which means that the solar regeneration system cannot meet the dehumidification requirements all the time. In this paper, a new desiccant pre-treatment electrodialysis (ED) regeneration system is proposed in order to improve the reliability and the performance of solar desiccant regeneration system for LDAS. The new system makes comprehensively use of solar energy and can work reliably under the variable conditions. Moreover, the new system can be used in the deep dehumidification field, which is required in industrial air-conditioning. Analysis of the new system, the solar TH regeneration system and the PV-ED regeneration system is made and the results reveal that the new system will be more energy efficient than others under the ideal operational condition. Among all factors, the most influential one is the concentration of desiccant at the entrance of concentrate cells in the ED regenerator, and the new system should be running under the ideal operational condition as possible.

Performance comparison of liquid desiccant air handling processes from the perspective of match properties

The energy consumption of air-conditioning systems represents a large part of the entire building energy consumption in non-residential buildings. Heat pump-driven liquid desiccant (HPLD) systems have become more and more popular because of their high energy efficiency, and there are many different ways to utilize the condensing heat in this type of air handling device. In the present study, the match properties of the heat and mass transfer processes in the air–desiccant system varying with the inlet air states are investigated, and the unmatched coefficient based on entransy dissipation is adopted to evaluate the performance of HPLD systems. It is demonstrated that the unmatched coefficient of the process along the iso-concentration line is much lower than that along the isenthalpic line. A higher unmatched coefficient means a higher heat or mass transfer resistance caused by the unmatched flow rates or parameters. The HPLD system in which solution is heated for regeneration (Basic Type II) performs much better than the system in which regeneration air is heated (Basic Type I). The unmatched coefficients of the condenser and regenerator decrease from 2.70 and 1.34, respectively, in Basic Type I, to 1.07 and 1.05, respectively, in Basic Type II. Moreover, COPsys is significantly improved, rising from 1.83 to 4.17.

Investigation on wetted area and film thickness for falling film liquid desiccant regeneration system

The falling film liquid desiccant air conditioning system is promising to achieve a low pressure drop and low possibility of solution droplets carried by air. However, the insufficient wetted area was found by previous researchers, and its value is difficult to be determined. With a single channel internally heated regenerator, this paper experimentally investigated the influencing factors affecting the wetted area and film thickness, by obtaining the flow size with a thermal camera. The increasing wetted area could improve the mass transfer significantly while the performance decreased with the increasing film thickness of liquid desiccant. The thickness of solution distributor impacted the initial film width most greatly, and it also significantly influenced the film thickness and flow velocity. Under incomplete wetting conditions, the film width increased with the solution mass flow rate proportionally, and the change of the distribution of film thickness was found simultaneously. However, solution temperature and concentration contribute insignificantly, and the direct impact of air and water on wetted area was minor. A 3-D model was developed, for predicting the wetting factor, and film thickness and flow velocity of the falling film. The calculation results were compared with experiment ones with average errors of 8.74% and 9.76%. The newly developed model could significantly improve the accuracy of performance evaluation and simulation of liquid desiccant air conditioning systems.

Influence of plate surface temperature on the wetted area and system performance for falling film liquid desiccant regeneration system

The falling film liquid desiccant air conditioning system is a promising technology. The wetted area, as a key factor affecting the system performance, is significantly influenced by the plate surface temperature. With a single channel internally heated regenerator, this paper experimentally investigated the impact of plate temperature on the wetted area and system performance, by obtaining the film area and temperature with a thermal camera. LiCl was chosen as the desiccant. By reducing the film contraction in the transverse direction, the increase of plate surface temperature could enhance the wetted area significantly, especially for the low solution mass flow rate. The growth rate of area increased from 0.0017 to 0.0022m2/°C when the flow rate reduced from 0.062 to 0.034kg/s. The regeneration performance also increased with the temperature. A theoretical model with an analytical solution was developed to obtain the wetted area by describing the film contraction caused by the Marangoni effect. A good match was observed for the prediction and experimental results, with an average error of 10.8%. Furthermore, the wetted width was numerically found to also increase with the decrease of film thickness and contact angle, and its change with the solution concentration was slight. This research helps researchers and engineers to accurately predict the wetted area of falling film and to improve the heat and mass transfer performance of liquid desiccant regeneration system.

Artificial neural network analysis of liquid desiccant dehumidifier performance in a solar hybrid air-conditioning system

A new solar hybrid liquid desiccant air conditioning system has been tested and simulated to investigate the technical feasibility of cooling systems for greenhouse applications using weather data for Malaysia. In this paper, experimental tests are carried out to investigate the performance of a counter flow dehumidifier using lithium chloride (LiCl) solution as the desiccant. A single and multilayer artificial neural network is used to predict the performance of the dehumidifier. Five parameters are used as inputs to the ANN, namely: air and desiccant flow rates, air inlet humidity ratio, and air and desiccant inlet temperatures. The outputs of the ANN are the temperature, humidity ratio, moisture removal rate, and the effectiveness. ANN predictions for these parameters are compared with the experimental values. The results show that the optimum testing model for moisture removal rate in the dehumidifier was the 5-5-5-1 structure with R2 = 0.91, whereas the optimum testing model for effectiveness was the 5-11-11-1 structure with R2 = 0.79. The maximum temperature and humidity ratio difference between the ANN model and experimental are 1.2 °C and 1.9 g/kg, respectively.

A hybrid dehumidifier model for real-time performance monitoring, control and optimization in liquid desiccant dehumidification system

In this paper, a simplified, yet accurate hybrid model to predict the heat and mass transfer processes in a packed column liquid desiccant dehumidifier is developed. Starting from energy and mass balance principles, and by lumping the geometric parameters and fluids’ thermodynamic coefficients as constants, the derived model only requires two equations together with total seven parameters for predicting the heat and mass transfer status in the dehumidifier. Commissioning information together with Levenberg–Marquardt method can be used to identify these parameters. Compared with the existing liquid desiccant dehumidification system dehumidifier models, the proposed model is very simple, accurate and does not require iterative computations. Experimental results demonstrate their effectiveness in predicting heat and transfer performances over a wide operating range. The model is expected to be applied in operational optimization, performance assessment, fault detection and diagnosis in liquid desiccant dehumidification system.

Combined heat and mass transfer analysis during the dehumidification process within a plate type heat exchanger

Liquid desiccant systems have been paid attention because of its advantages in energy saving and an environmental friendliness. The use of liquid desiccant systems offers design and performance advantages over the solid desiccant systems, especially when solar energy is used for regeneration. The objective of this paper is to analyze the simultaneous heat and mass transfer characteristics of lithium chloride aqueous solution for the plate type dehumidification system. The effects of process air and solution inlet conditions on the dehumidification performance are studied in this study. It is found that the heat transfer coefficient of the air side gives much more significant effect on the absorption rate and dehumidification effectiveness than those of the solution and the coolant sides while the mass transfer coefficient of the solution side gives more significant effect than that of the air side. It is also found that the solution concentration is the most important factor for absorption performance improvement during the dehumidification process.

リキッドデシカント空調システム用除湿器の熱・物質移動特性の究明 : 第1報 : 実験による検討

リキッドデシカント空調システム用除湿器の熱・物質移動特性の究明 : 第1報 : 実験による検討

A simple analytical method to estimate all exit parameters of a cross-flow air dehumidifier using liquid desiccant

The dehumidifier is a key component in liquid desiccant air-conditioning systems. Analytical solutions have more advantages than numerical solutions in studying the dehumidifier performance parameters. This paper presents the performance results of exit parameters from an analytical model of an adiabatic cross-flow liquid desiccant air dehumidifier. Calcium chloride is used as desiccant material in this investigation. A program performing the analytical solution is developed using the engineering equation solver software. Good accuracy has been found between analytical solution and reliable experimental results with a maximum deviation of +6.63% and −5.65% in the moisture removal rate. The method developed here can be used in the quick prediction of the dehumidifier performance. The exit parameters from the dehumidifier are evaluated under the effects of variables such as air temperature and humidity, desiccant temperature and concentration, and air to desiccant flow rates. The results show that hot humid air and desiccant concentration have the greatest impact on the performance of the dehumidifier. The moisture removal rate is decreased with increasing both air inlet temperature and desiccant temperature while increases with increasing air to solution mass ratio, inlet desiccant concentration, and inlet air humidity ratio.

Performance analysis of a membrane liquid desiccant air-conditioning system

A new membrane liquid desiccant air-conditioning (LDAC) system is proposed and investigated in this paper. Liquid-to-air membrane energy exchangers (LAMEEs) are used as a dehumidifier and a regenerator in the proposed membrane LDAC system, which can eliminate the desiccant droplets carryover problem occurring in most direct-contact LDAC systems. A parametric study on steady-state performance of the membrane LDAC system is performed using the TRNSYS energy simulation platform. The impacts of various climatic conditions and key system parameters on the system performance are evaluated. Results show that the proposed membrane LDAC system is capable of achieving recommended supply air conditions for productive, comfort and healthy environments if the key system parameters are effectively controlled. The system coefficient of performance (COP) at the design condition is 0.68, and the sensible heat ratio (SHR) for the dehumidifier lies in the range between 0.3 and 0.5 under different climatic, operating and design conditions. The proposed membrane LDAC system is able to effectively remove latent load in applications that require efficient humidity control.

Analytical model based performance evaluation, sizing and coupling flow optimization of liquid desiccant run-around membrane energy exchanger systems

A run-around membrane energy exchanger (RAMEE) is a novel energy recovery system which can avoid carryover of desiccant solution while transferring both heat and moisture between non-adjacent supply and exhaust air streams. In this study, an analytical model for a flat-plate counter-cross-flow liquid-to-air membrane energy exchanger (LAMEE) is compared with experimental test results and numerical simulations for a single LAMEE and a RAMEE system under various operating conditions. Agreements are obtained among the analytical, experimental and numerical results. The effectiveness of a RAMEE system under balanced and unbalanced airflow conditions is evaluated by using the analytical model. Optimization studies for exchanger size and solution flow rate in RAMEE systems are investigated. Results show that RAMEE systems with equal-sized supply and exhaust exchangers can achieve the highest effectiveness in most conditions for both balanced and unbalanced airflow. Optimization control of the solution flow rate can enhance annual energy recovery rate up to 7%.

Experimental study on partially internally cooled dehumidification in liquid desiccant air conditioning system

The dehumidifier is one of the essential components in air-conditioning systems with liquid desiccant. Compared with adiabatic dehumidifiers, internally cooled dehumidifiers can achieve better dehumidification performance. In this study, dehumidification effectiveness and moisture removal rate were adopted to evaluate and compare the performance of these two kinds of dehumidifiers. The results indicated that internally cooled dehumidifiers can significantly improve performance compared with adiabatic dehumidifiers, especially when the solution is at a high temperature and/or low concentration in the downstream section of the liquid desiccant flow. Performance improvement and energy saving can be taken into consideration simultaneously by only supplying cooling water to the latter part of the dehumidifier. Solution temperatures at 20–25°C for adiabatic dehumidifiers are optimal and those at 25–30°C for internal cooled types. Lower temperatures of the cooling water can result in higher values for dehumidification effectiveness and moisture removal rate in an internally cooled dehumidifier; the limited temperature difference between cooling water and solution might seriously weaken the effect of the cooling water. Dimensionless Sherwood number Sh was calculated, and discrepancies between predicted values and experimental data are almost within ±20%.

Experimental study of PCM melting in triplex tube thermal energy storage for liquid desiccant air conditioning system

Thermal energy storage such as latent heat thermal energy storage can reduce the energy supply–demand discrepancy and improve the efficiency of solar energy systems because of its high thermal energy density (per unit mass and per unit volume). The present work investigates the application of a triplex tube heat exchanger with a phase-change material (PCM) (paraffin wax RT 82) in the middle tube to power a liquid-desiccant air-conditioning system. The experiment examined the effects of mass flow rates and heating methods selected on PCM melting. Three techniques were investigated: heating the inside tube, heating the outside tube, and heating both sides tube. The PCM temperature gradients in the radial, angular, and axial directions were analyzed. Results indicated that the third case (i.e., heating both sides) achieved complete PCM melting within a short time and used a lower inlet heat transfer fluid temperature compared with the other cases. No significant temperature variation in the axial direction was indicated. The heat transfer phenomena in the radial and angular directions can be predicted in melted PCM.

Development of simplified prediction model for internally cooled/heated liquid desiccant dehumidification system

Currently, evaluating the performance of internally cooled/heated liquid desiccant dehumidification system (ICHLD) is complicated, requiring a large number of iterative calculations. This study developed a simplified numerical model, for obtaining the outlet parameters of all fluids (air, solution and heating/cooling fluid) and system performance of dehumidifiers and regenerators in ICHLD accurately and directly. The model was built by defining three kinds of effectiveness, i.e. enthalpy effectiveness, moisture effectiveness and temperature effectiveness. Based on the parameter study under different design and operation conditions, the correlations of effectiveness were obtained by multiple linear regressions, using heat and mass transfer gradients and other related parameters as variables. Comparisons between our results and those by existing mathematical models show that the composite errors are acceptable, with an average difference of 14.5% for dehumidifiers and 6.83% for regenerators. Similar trends were also observed in comparisons with previous experimental data, with average errors about 20%. The inlet solution temperature has a greatest impact on the enthalpy effectiveness and moisture effectiveness. The temperature effectiveness is mainly determined by the temperature difference between cooling/heating fluid and solution. This simplified model is very useful for researchers and engineers, especially those interested in the dynamic performance of ICHLD systems.

Quick performance prediction for internally cooled/heated liquid desiccant dehumidification system

Predicting the performance of internally cooled/heated liquid desiccant dehumidification system is complicated, which requires a large number of iterative calculations. This article numerically developed a quick prediction method for falling film dehumidifiers and regenerators in internally cooled/heated liquid desiccant dehumidification system by non-linear regressions, to predict the mass transfer performance directly with design variables, including inlet parameters of fluids (air, desiccant, and heating/cooling fluid) and system configuration (size, air channel thickness wetting factor, and flow pattern). The LiCl and water were chosen as the solution and cooling/heating fluid. Comparison shows that differences between our results and those by existing mathematical models are acceptable, with an average absolute error of 8.0% for dehumidifiers and 5.2% for regenerators. The prediction results also appear similar with previous experiment data, showing errors of less than 10%. The solution temperature has the greatest impact on moisture effectiveness and removal rate of internally cooled/heated liquid desiccant dehumidification system. Air humidity, air flow rate, solution concentration, and solution flow rate also significantly influence the system performance. Considering the limitations of practical projects, two more simplified equations with lower but acceptable accuracy were developed. This prediction method is very useful for researches and practical applications concerning the dynamic system performance.

Investigation on feasibility of ionic liquids used in solar liquid desiccant air conditioning system

The corrosion problem of traditional liquid desiccants hinders the development of solar liquid desiccant air conditioning system. Ionic liquids (ILs) could be a possible substitute because of their low corrosion to metals. According to the characteristic of cation and anion, two ILs 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) and 1,3-dimethylimidazolium acetate ([Dmim]OAc) were selected for possibility investigation. The experimental results demonstrated that the surface vapor pressure of [Bmim]BF4 aqueous solution was so high that it was not suitable for dehumidification. With comparing the structure of [Bmim]BF4 and 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4), it was found that for imidazole salts with the same anion, as the alkyl chain of the cation becomes shorter, the surface vapor pressure of its aqueous solution would be lower. In addition, simulation model was used to compare the dehumidification performance of [Dmim]OAc and [Emim]BF4 with that of the traditional desiccants lithium Chloride (LiCl) and lithium bromide (LiBr). The results showed that in the same condition, [Dmim]OAc with an inlet mass concentration of 81.7% and [Emim]BF4 of 85.5% could reach the same dehumidification rate as LiCl of 40.9% and LiBr of 45.0% mass concentration. Better dehumidification performance of [Dmim]OAc than that of [Emim]BF4 was achieved, thus, like [Emim]BF4, [Dmim]OAc can also be a promising substitute for traditional desiccant.

Thermodynamic analysis of a counter flow adiabatic dehumidifier with different liquid desiccant materials

Liquid desiccant systems have been proposed as energy saving alternatives to the conventional vapor compression systems for handling the latent load. This paper presents the results from a study of the performance of a counter flow liquid desiccant dehumidifier. A heat and mass transfer theoretical model of an adiabatic packed column has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Good agreement was found between experimental tests and the theoretical model, with the maximum deviation being ±2.9% in air outlet temperature, ±15.9% in air outlet humidity ratio and ±2.8% in solution outlet temperature. Following the model validation, the rate and the efficiency of the dehumidification process were assessed under the effects of variables, such as air temperature and humidity, desiccant temperature and humidity and air and desiccant flow rates. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl2 were evaluated against each other. The results show that high absorber efficiency and system efficiency could be achieved under humid conditions, low air mass flow rates and LiCl as the desiccant solution.

Performance optimization of heat pump driven liquid desiccant dehumidification systems

Heat pump driven liquid desiccant dehumidification (HPLD) devices, in which both the cooling capacity from an evaporator and heat from a condenser are utilized, have been developing rapidly in recent years. Because the amount of heat from the condenser is usually more than required for desiccant regeneration, the key to improve the performance of these systems involves effectively exhausting the extra heat. In this paper, two different methods for removing the extra heat are analyzed: adding an air-cooled assistant condenser in either the inlet or outlet of the regeneration air duct, and utilizing a water-cooled assistant condenser that uses cooled water from evaporation of the exhaust air. These handling processes were compared in terms of their HPLD performance under various operating conditions. Devices using an air-cooled or water-cooled assistant condenser showed better performance than a basic system without an assistant condenser. For systems with an air-cooled assistant condenser, the location of the condenser rarely affected the performance: COPsys was an average of about 18% higher than that of the basic system. The system with the water-cooled condenser performed the best out of all the investigated systems: COPsys was about 35% higher than that of the basic system.

Experimental tests of a small-scale microturbine with a liquid desiccant cooling system

SUMMARY In a trigeneration plant, the thermal energy recovered from the prime mover is exploited to produce a cooling effect. Although this possibility allows the working hours of the plant to be extended over the heating period, providing summer air conditioning through thermally activated technologies, it is rather difficult to find in the literature experimental data on trigeneration plants operation, and the availability of performance characteristics at off-design conditions is anyway limited. The paper has the aim of showing the experimental data of a real trigeneration system installed at the Politecnico di Torino (Turin, Italy), composed of a natural gas 100 kWel microturbine coupled to a liquid desiccant system. The data are presented for both cogeneration and trigeneration configurations, and for full and partial load operations. An energetic and economic performance assessment at rated power operation is presented, and compared with the partial load operation strategy. The primary energy savings are calculated through a widely accepted methodology, proposed by the European Union, and through another methodology, reported in literature, which seems to the Authors more suitable to describe the energetic performances of trigeneration plants. Copyright © 2012 John Wiley & Sons, Ltd.