Desiccant Solution Temperature Research Articles

Experimental investigations on a solar assisted packed bed regeneration system for building air conditioning applications

In the current study, an experimental investigation carried out on a binary liquid desiccant-based adiabatic counter flow regenerator is presented. The heat harvested from solar collector was supplied into the desiccant for desiccant regeneration. The corrosion characteristics of the binary desiccant solution on stainless steel material were studied using potentiodynamic test. Subsequently, the effects of various inlet process parameters on the performance parameters of the regenerator were studied employing structured pads as packing material. The effect of the phase change phenomena involved in a desiccant regeneration system was also assessed experimentally, along with the system performance under the inlet solution temperatures of 53, 55, 60, 65 and 70 °C at a constant desiccant solution of 50 wt%. It was observed that the corrosion rate of lithium bromide (LiBr), calcium chloride (CaCl2) and LiBr + CaCl2 (85:15) was found to be 0.0303 mm/year, 0.0144 mm/year and 0.0161 mm/year, respectively. The results showed that with the increased solution temperature and air mass flux rate, the desorption rate was increased, whereas the regeneration effectiveness was decreased. The highest desorption rate was found to be 6.8 g/m2-s at the desiccant solution temperature of 70 °C, whereas the highest regeneration effectiveness of 0.85 was obtained at the air mass flux rate of 1.1 kg/m2-s.

Liquid Desiccant Air Conditioning System for Flat Plate Solar Collector with Nanofluids Regenerator

This paper deals with liquid desiccant air conditioning using solar flat plate collector. The problem in the use of liquid desiccant air conditioning is energy utilization for regeneration, corrosiveness and carryover. To overcome these developments of a dehumidifier for corrosion- resistant and solar regenerated are advised. A dehumidifier is made of stainless steel tubes of 316L type and stacked in aluminium fins to maintain the desiccant solution temperature. In this experiment, calcium chloride solution (CaCl2 -H2 O) is used. The solar flat plate collector is used for heating water using closed-loop of thermosiphon as regeneration. The flow rate for air is fixed at 10CFM and concentration of calcium chloride is 35% by mass is used. The inlet parameters considered are in the experiments solution volume flow rate, inlet temperature, inlet relative humidity, regeneration temperature, and desiccant solution temperature. The performance parameters are the absolute humidity reduction, outlet temperature and dehumidifier and enthalpy effectiveness. The experiments show that for a fixed Ta, inlet and RH% as solution volume flow rate increases, there is increase in absolute humidity reduction. The temperature of the dehumidified air is reduced compared to that of inlet air, if this air is passed over the pad used for the evaporating cooler. It is seen that the increase in relative humidity from 68.88% to 92.8% for the flow rate of 20 L/min and fixed inlet air temperature, increases absolute humidity reduction from 5.56 to 13.3 g/kg. The increase in solution temperature reduces the absolute humidity reduction due to solution vapor pressure increases and dehumidifier effectiveness. When the solution temperature changes from 31.5 to 34 0 C, there are reductions in the absolute humidity reduction and dehumidifier effectiveness by 34.4% and 13.04% respectively

Experimental study and prediction model of a liquid desiccant unit for humidification during the heating season

Liquid-desiccant-assisted air conditioning systems have attracted considerable interest owing to their potential to save energy and enhance performance through decoupled control of latent and sensible loads. In addition, several studies have suggested using the regenerator of a liquid desiccant system for humidification during the heating season; however, its performance under the winter weather condition has not yet been established. Therefore, the goal of this study is to develop the prediction models of the humidification performance of liquid desiccant systems during the heating season. Experiments were conducted to collect performance data based on the factorial experimental design method. Using the response surface method, two empirical models for the effectiveness of regeneration (R2 = 0.927) and enthalpy exchange (R2 = 0.934) were developed and experimentally validated under the actual conditions during the heating season within 10% error bounds. Significant parameters included the liquid-to-gas ratio and desiccant solution concentration and temperature. Moreover, the prediction model of the Sherwood number was developed which can be used for the numerical simulation. Additionally, the suitable operation settings of the regenerator for humidification were investigated to obtain higher energy and exergy efficiencies based on the developed prediction models. From the results, a higher liquid-to-gas ratio and lower concentration and temperature of the desiccant solution could be recommended considering both energy and energy efficiencies and the practical application of liquid desiccant systems.

Mathematical model and energy efficiency analysis of a vacuum-based liquid desiccant regenerator

Liquid desiccant air conditioning (LDAC) has been considered as a promising solution for efficient air temperature and humidity control in air conditioning fields. However, the high regeneration temperature of desiccant solution impedes its spread and application. In this study, a vacuum-based liquid desiccant regenerator (VLDR) for desiccant solution regeneration in LDAC driven by low-grade energy is studied, meanwhile, the mathematical model considering the variation of heat transfer coefficient under different operating conditions is developed to analyze the system performance and energy efficiency. After the model is validated by the experimental data, the regeneration performance and energy efficiency analysis of the VLDR system is simulated and investigated. Simulation results show that hot water temperature, flowrate are the variables that affect the performance of the generator, and the performance of the condenser is influenced by cooling water temperature and flowrate. Opposite influences for vacuum pressure are found on the performances of generator and condenser, therefore the hot water temperature, vacuum pressure, and cooling water temperature should be designed properly to provide the required regeneration capacity with the pressure balance maintained. Moreover, energy efficiency is analyzed by comparing the ratio of water vapor generation rate to the total electricity consumption of the VLDR under different temperatures and vacuum pressures.

Thermodynamic analysis of internally-cooled membrane-based liquid desiccant dehumidifiers of different flow types

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

A novel technical route based on wet flue gas desulfurization process for flue gas dehumidification, water and heat recovery

To recover the water and heat in industrial flue gas before discharging, a novel dehumidification system was proposed by combining the liquid-desiccant-based dehumidification (LDD) method with the existing wet flue gas desulfurization (WFGD) process in a single spraying tower. Calcium chloride (CaCl2) solution was used as a liquid desiccant. A flow pattern controlling (FPC) device was designed to further enhance the mass transfer in the dehumidification process. The effects of liquid-to-gas ratio (L/G), desiccant solution temperature (Tde), desiccant concentration (ω), and superficial flue gas velocity (vg) on the dehumidification performance were investigated. The water and heat recovery process of the LDD-WFGD system were analyzed theoretically in detail. Furthermore, the application potential of the proposed LDD-WFGD system for water and heat recovery for coal-fired power plants was discussed by conducting case studies. The recovered water from flue gas could supply 47.2% and 44.8% of the water demand for the WFGD systems in 660 MW and 330 MW power plants, respectively.

Dynamic modeling of liquid-desiccant regenerator based on a state–space method

The regenerator is the main energy-consuming component in a liquid-desiccant-based air-conditioning system. Highly efficient control of a regenerator in dynamic operating conditions can improve system performance and reduce system energy consumption. An accurate and reliable dynamic model of a regenerator is fundamental to understand its dynamic operation characteristics and design in a high-performance control system. In this study, a state–space based model of the dynamic operation characteristics of a liquid-desiccant regenerator was developed from the control point of view. In the modeling process, the relationship between the input and output variables was reflected by constructed state–space matrixes. An experimental validation of the proposed dynamic regenerator model showed the dynamic responses of the following three case studies: (1) increase of mass flow rate of inlet desiccant solution, (2) increase in inlet desiccant solution temperature, and (3) increase in inlet air temperature and humidity. The proposed state–space model accurately reflected the dynamic response process of regeneration under different disturbances and initial conditions. Comparison between the modeling and experimental results showed that the relative errors of the outlet air temperature/humidity and outlet desiccant solution temperature were less than 5%, and the mean relative errors of their variations were less than 15%, which validates the accuracy and reliability of the proposed state–space regenerator model. This novel model can be useful for further research on the dynamic operation characteristics of regenerators and the development of high-performance control systems.

Experimental analysis of dehumidification performance of counter and cross-flow liquid desiccant dehumidifiers

In a liquid desiccant dehumidifier, the air-to-solution flow direction plays an important role in the dehumidification performance and the physical size of the dehumidifier tower. In this research, the dehumidification performance of counter-flow and cross-flow liquid desiccant dehumidifiers were compared via a series of experiments carried out in the environmental test chamber. To predict the dehumidification performance of both flow type dehumidifiers in various operating conditions, empirical models returning the dehumidification effectiveness and the enthalpy effectiveness, which were adopted as performance indices, were also derived based on the experimental data. From the sensitivity analysis conducted by using the proposed empirical correlations, it was observed that when the inlet air humidity ratio increased from 10.1 g/kg to 22.7 g/kg, the dehumidification effectiveness and enthalpy effectiveness of the counter-flow liquid desiccant dehumidifier increased from 51.2% to 82.4% and 40.2% to 72.5%, respectively. Meanwhile, both effectiveness values of the cross-flow dehumidifier increased from 62.3% to 63.4% and 46.3% to 55.0%, respectively. Likewise, when inlet solution temperature varied from 15.2 to 31.1 °C, the dehumidification effectiveness and enthalpy effectiveness of the counter-flow type dehumidifier decreased from 71.7 to 45.4% and 64.4 to 47.3%, respectively, while those of a cross-flow type dehumidifier decreased from 65.3 to 54.8% and 55.8 to 45.2%. Consequently, one may conclude that the cross-flow liquid desiccant dehumidifier would provide relatively stable dehumidification performance regardless of the changes in operating parameter, but the cross-flow dehumidifier may have limited dehumidification performance, especially at the lower desiccant solution temperature and in highly humid process air conditions.

Artificial neural network model for performance evaluation of an integrated desiccant air conditioning system activated by solar energy

In this study, the performance of an integrated desiccant air conditioning system (IDACS) activated by solar energy is evaluated by back propagation artificial neural network (BP-ANN). The IDACS consists of a liquid desiccant dehumidification cycle combined with a vapor compression refrigeration cycle. The integrated system performance is assessed utilizing the system coefficient of performance (COP), outlet dry air temperature (Tda-out), and specific moisture removal (SMR). The training of the BP-ANN is accomplished utilizing experimental results previously published. The results of the BP-ANN model revealed the high accuracy in predicting system performance parameters compared with experimental values. The BP-ANN model has shown relative errors in the trained mode for COP, Tda-out, and SMR within ±0.005%, ±0.006%, and ±0.05%, respectively. On the other side, the BP-ANN model is inspected in the predictive mode as well. The relative errors of the model for COP, Tda-out, and SMR in the predictive mode are within ±0.006%, ±0.006%, and ±0.004%, respectively. The influences of some selected parameters, namely regeneration temperature, desiccant solution temperature in the condenser and evaporator, and strong solution concentration on the system performance are examined and discussed as well.

Effect of Desiccant Solution Temperature on Regeneration Performance of a Cross-Flow Regenerator

The purpose of this study is to investigate the effect of the inlet solution temperature on the performance of an adiabatic cross-flow regenerator using a lithium chloride (LiCl) aqueous solution, and to propose the optimal inlet solution temperature when operating this type of regenerator. In the experimental tests, the inlet solution temperature range varied from 50 to 90°C. The tests were carried out at 10°C intervals while the other conditions remained constant. The measurement parameters for the test were the inlet air dry-bulb temperature and humidity ratio, outlet air dry-bulb temperature and humidity ratio, air volume flow, solution density, and inlet and outlet solution temperatures. The regeneration effectiveness and coefficient of performance (COP) were selected to assess the heat and mass transfer performance of the cross-flow regenerator. The most important finding of this research was to determine the optimal solution inlet temperature in the cross-flow regenerator with the LiCl aqueous solution considering both the regenerator performance and energy consumption. The test results show that the recommended inlet solution temperature is 60°C, considering both regeneration effectiveness and COP.

Displacement ventilation with cooled liquid desiccant dehumidification membrane at ceiling; modeling and design charts

A novel transport model is developed for a space conditioned by a cooled liquid desiccant dehumidification membrane ceiling (LDMC-C) and displacement ventilation DV in which the space is divided into three zones: an occupied cool with fresh air zone; an upper recirculation zone; and a ceiling adjacent boundary layer zone where the air is drawn at the exhaust grill. The adjacent air boundary layer at the ceiling membrane predicted the latent and sensible heat transfer to the desiccant solution. The boundary model was validated experimentally in a climatic chamber.The developed model predicted of the LDMC-C/DV system operational parameters such as the supply flow rate and temperature and the liquid desiccant concentration, flow rate, and inlet temperature that meet thermal comfort and air quality requirements. Extensive simulations were performed on a typical office load of 75 W/m2 to generate design charts of the system for three DV supply air temperatures of 18, 20, and 22 °C to identify the appropriate DV flow rate and inlet desiccant (CaCl2) solution temperature that provide comfort and air quality and ensures that condensation is unlikely to occur. The LDMC sensible and latent load removal from the space load is directly read off the charts.

A comparative study and prediction of the liquid desiccant dehumidifiers using intelligent models

Dehumidifier in liquid desiccant systems is affected by different influential parameters and precise prediction of its characteristics is vital for a better overall performance. In this communication, the well-known artificial intelligence based methods such as Least Square Support Vector Machine (LSSVM), Adaptive Neuro Fuzzy Inference System (ANFIS), and Artificial Neural Network (ANN) are developed for prediction of the dehumidification effectiveness (ε) as well as the process outlet temperature and humidity (Tout and ωout). Based on a comparative study, brighter conformity was obtained between the predicted and experimental data for the ANN models presented in the current study. The coefficients of determination and mean square errors for the ANN models during the testing phase were respective values of 0.9993 and 2.9740e-05 for the ε, 0.9997 and 0.0039 for the ωout, and 0.9988 and 0.0192 for the Tout. A sensitivity analysis was conducted and showed higher influence of concentration and temperature of desiccant solution at the absorber inlet on the dehumidification effectiveness and process outlet state conditions, respectively. Further to the above, a mathematical technique on the basis of Leverage algorithm was implemented to assess the quality of the collected data, diagnose the doubtful data samples, and indicate the applicability range of the developed ANN models.

Experimental Study on Effectiveness of Celdek Packed Liquid Desiccant Cooling System

ABSTRACTDehumidifier and regenerator are the most important components in a liquid desiccant cooling system. Present paper is focused on study the effect of inlet process parameters on the effectiveness of dehumidifier and regenerator of liquid desiccant cooling system. Experimental study is performed with varying inlet process parameters; mass flow rate of air, desiccant solution flow rate, inlet air temperature, inlet solution temperature, inlet specific humidity and concentration of desiccant solution. Celdek structured pads as packing material and calcium chloride as liquid desiccant is investigated first time using counter flow of the desiccant solution and air. It is concluded from the results that the effectiveness of dehumidifier increases with solution flow rate, inlet specific humidity while decreases with increasing mass flow rate of air, inlet temperature of air and desiccant, temperature and concentration of desiccant solution. The effectiveness of regenerator increases with increasing solution flow rate and inlet desiccant concentration and it decreases with increasing inlet air temperature, air flow rate and inlet solution temperature. Present paper adds to effect of inlet specific humidity, inlet temperature of the air and solution on the effectiveness of desiccant cooling system on the past research.

Numerical study on dehumidification performance of a cross-flow liquid desiccant air dehumidifier

In this investigation, a mathematical model was developed to study the performance of cross-flow liquid desiccant air dehumidifier (LDAD). The results show that desiccant solution to air mass ratio has the greatest impact on performance of dehumidifier. Inlet air humidity ratio and temperature have significant influence on outlet air humidity ratio and temperature, respectively. The outlet air humidity ratio, outlet air temperature and outlet desiccant solution temperature are increased with inlet desiccant solution temperature increasing. The increase of solution fraction leads to a slight decrease in outlet humidity ratio and dehumidification rate, but a little increase in outlet air temperature. In addition, when the solution fraction is 48%, the outlet solution temperature reaches the maximum value. Numerical results are verified by experimental ones with a maximum deviation of 8.7% in the outlet air humidity ratio.

Performance testing of 2-fluid and 3-fluid liquid-to-air membrane energy exchangers for HVAC applications in cold–dry climates

Liquid-to-air membrane energy exchangers (LAMEEs) can avoid the problem of desiccant droplets carryover by using semi-permeable membranes to separate the air and desiccant solution streams. The aim of this study is to test and compare the heat and moisture transfer performances between the air and desiccant solution inside a novel 3-fluid LAMEE and a 2-fluid LAMEE under air heating and humidifying conditions. The effect of flow maldistribution on the deterioration of the 3-fluid LAMEE’s performance is estimated, and an experimental study of the feasibility of membrane pre-tension to reduce flow maldistribution caused by membrane deflections in flat-plate LAMEEs is presented. Unlike heat exchangers where the lowest possible outlet hot fluid temperature is equal to the inlet cold fluid temperature, experimental data show that the outlet desiccant solution (hot fluid) temperature in the 2-fluid LAMEE may be lower than the inlet air (cold fluid) temperature. This is attributed to the effect of the phase change energy associated with the air humidification process.

Performance testing of a novel 3-fluid liquid-to-air membrane energy exchanger (3-fluid LAMEE) under desiccant solution regeneration operating conditions

Liquid-to-air membrane energy exchangers (LAMEEs) use semi-permeable membranes to transfer heat and moisture between air and desiccant solution, and prevent the transfer of desiccant droplets to the airside. A 2-fluid flat-plate LAMEE is composed of several adjacent air and desiccant solution channels each separated by a semi-permeable membrane. A 3-fluid LAMEE is the new generation of LAMEEs which has a structure similar to a 2-fluid flat-plate LAMEE. The novelty of the 3-fluid LAMEE is that it includes titanium tubes inside the desiccant solution channels to control the temperature of the desiccant solution along the exchanger. In this paper, the performances of a 2-fluid LAMEE and 3-fluid LAMEE are tested and compared under diluted desiccant solution regeneration operating conditions. Also studied are the effects of operating conditions (inlet heating water temperature, heating water mass flow rate, desiccant solution mass flow rate, and inlet desiccant solution temperature) on the performances of the 2-fluid and 3-fluid LAMEEs. For the chosen test conditions, results show that effectiveness and moisture removal rate of the 3-fluid LAMEE are higher than the 2-fluid LAMEE under the entire range of inlet heating water temperature, heating water flow rate, and desiccant solution flow rate studied. The effectiveness and moisture removal rate of the 3-fluid LAMEE increase as the inlet heating water temperature and/or flow rate increases. The sensible, latent, and total effectivenesses, and moisture removal rate of the LAMEE increase by 38%, 40%, 39%, and 6 times when the temperature of desiccant solution is maintained constant along the exchanger. Compared with the 2-fluid LAMEE, the sensible, latent, and total effectivenesses, and moisture removal rate of the 3-fluid LAMEE are improved by up to 104%, 141%, 128%, and 17 times, respectively.

Experimental study on performance of celdek packed liquid desiccant dehumidifier

Dehumidifier is the main component of liquid desiccant dehumidification system. Effect of the inlet parameters on various outlet parameters of the dehumidifier is studied in the present paper with structured pads as packing material and calcium chloride as liquid desiccant to process the air. The outlet parameters are change in specific humidity, mass transfer coefficient, moisture removal rate, air temperature, solution temperature, effectiveness and the corresponding inlet process parameters; mass flow rate of air, temperature of air, temperature and flow rate of desiccant solution. It is observed that mass transfer coefficient and moisture removal rate increase with increasing mass flow rate of the air and desiccant while these parameters decrease with increasing temperature of air and desiccant solution. Dehumidifier effectiveness gets increased with increasing solution flow rate. The present investigations are compared with the results of the researchers in the past.

Model-based optimization strategy of chiller driven liquid desiccant dehumidifier with genetic algorithm

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.

Optimization of Liquid Desiccant Regenerator with Multiobject Particle Swarm Optimization Algorithm

In this paper, a model-based optimization strategy for a liquid desiccant regenerator operating with lithium chloride solution is presented. By analyzing the characteristics of the components, such as electric heater, pump, and fan, energy predictive models for the components in the regenerator are developed. To minimize the energy usage while maintaining the regeneration rate within an accepted level, one multiobjective optimization problem is formulated with two objectives, the constraints of decision variables, components interactions, and the outdoor conditions. A multiobjective optimization strategy based on decreasing inertia weight particle swarm optimization (DIWPSO) is proposed to obtain the optimal nondominated solutions of the optimization problem, and a decision making strategy is introduced to select the final solution, desiccant solution flow rate, desiccant solution temperature, and the regenerating air flow rate, to minimize the energy usage in the regenerator. Experimental studies are carried out on an existing system to compare the energy consumption and regeneration rate between the proposed optimization strategy and conventional strategy to evaluate energy saving performance of the proposed strategy. Experimental results demonstrate that an average of 8.55% energy can be saved by implementing the proposed optimization strategy in liquid desiccant regenerator.

Comparison of experimental data and a model for heat and mass transfer performance of a liquid-to-air membrane energy exchanger (LAMEE) when used for air dehumidification and salt solution regeneration

Liquid-to-air membrane energy exchangers (LAMEEs) allow simultaneous heat and moisture transfer between salt solution flows and airflows. These exchangers can be used as air dehumidifiers for supply air or liquid desiccant regenerators in liquid desiccant air conditioning systems while the membrane eliminates entrainment of liquid desiccant as aerosols in air streams. In this study, the heat and mass transfer performance of a LAMEE used as dehumidifier and regenerator is experimentally and analytically investigated to determine the influences of air and desiccant solution flow rates, inlet air temperature and humidity, and inlet desiccant solution temperature and concentration. Using the experimental data, the analytical model for the counter-cross-flow LAMEEs is quantitatively validated for sensible and latent effectiveness during air dehumidification process for the inlet conditions tested, while agreement is not good for the salt solution regeneration or de-watering process. This discrepancy may be caused by the crystallization of LiCl aqueous solution within the membrane pores near the solution side of the membrane during the regeneration tests. The data implies that both the moisture flux ratios and the proximity of the solution concentration to the saturation concentration play a role for moisture transfer blockage within the membrane when the data for latent effectiveness is significantly below the theoretical values. The influences of inlet air and solution parameters on the mass transfer performances of the LAMEE used as dehumidifier and regenerator are also compared with those of direct-contact liquid desiccant devices available in the literature.