Desiccant Dehumidification System Research Articles

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.

Experimental study on mass transfer performances of the ultrasonic atomization liquid desiccant dehumidification system

A great deal of attention has been devoted to the liquid desiccant dehumidification system, which presents great potential in energy saving of buildings. Combined with the ultrasonic atomization technology, the desiccant solution can be atomized into tiny droplets with diameter about 50μm, which may greatly facilitate the dehumidification process. In this work, thorough experiments were carried out to investigate the effects of air and desiccant inlet parameters on mass transfer performances of the ultrasonic atomization liquid desiccant dehumidification system (UADS), where dehumidification effectiveness, moisture removal rate and desiccant consumption rate were employed as the performance indicators. Instead of proposing empirical formulations, the Ideal dehumidification model, developed in the authors’ earlier work, was employed to predict the system's performances in comparison with the experimental results. Good agreement between the experimental results and predicted values was achieved while the maximum difference of dehumidification effectiveness was less than 8%. Moreover, compared with the cross-flow packed tower, much less desiccant was consumed in present UADS and the average reduction reached up to 73.94%. The data presented in this work may help in the design of the ultrasonic atomization liquid desiccant dehumidification system and validating the feasibility of the Ideal dehumidification model in UADS.

Feasibility and economic analysis of solid desiccant wheel used for dehumidification and preheating in blast furnace: A case study of steel plant, Nanjing, China

To overcome the shortcomings of huge energy consumption from conventional dehumidification using lithium bromide adsorption refrigerating (LBARD) system, a novel desiccant wheel dehumidification and preheating (DWDP) system using two-stage desiccant wheel for blast furnace is brought forward. The DWDP system was designed for dehumidification and preheating in blast furnace of steel plant. It takes waste heat in the slag flushing water as desiccant regeneration and preheating energy. To validate the feasibility of the new DWDP system, experimental studies were conducted based on a steel plant in Nanjing, China. The experiment was designed to use DWDP system in humid outdoor climates e.g. summer seasons. The experimental results indicate that the moisture removal capacity of DWDP system can reach 8.7 g/kg which will lead to the improvement of steel production by 0.9% and the coal is saved of about 2100 tons per year. With the DWDP system, the energy consumed by cooling tower of slag flushing water can decrease 7.3%. All of these energy saved equates to 10.3 million CNY annually. A comparison of initial investment and operating cost between DWDP system and LBRAD system was then carried out. The results show that the initial investment and operating cost of DWDP system is 37% and 57% of present LBARD system, and the payback period is shortened 66%.

Analysis on the Significance of Effects from Operational Conditions on the Performances of Ultrasonic Atomization Dehumidifier with Liquid Desiccant

In this work, simulations were carried out based on a L18×L8 cross-product orthogonal array to investigate the significance of the effects from inlet operational conditions on the performances of the ultrasonic atomization liquid desiccant dehumidification system (UADS), where dehumidification effectiveness was adopted as the performance indicator. Taguchi method was employed to analyze the results. It was found that though all of the inlet operational parameters revealed direct effects on the performances of UADS, the significance of their effects was quite different among which, the desiccant flow rate was the most sensible factor in improving DE while air humidity ratio exhibited the least significance. The results presented in this work may help in achieving the optimal running of the liquid desiccant dehumidification system.

The application of a desiccant wheel to increase the energetic performances of a transcritical cycle

A desiccant dehumidification system with air can be driven by low grade (<80°C) waste heat. In this paper, which is based on experimental data, the air flow at the outlet of the gas cooler of a trans-critical cycle is forced through a desiccant wheel regenerated by the air flow itself. The hybrid trans-critical refrigerator-desiccant system improves COP by approximately 77% as compared to a classical transcritical. The economical analysis suggests that the investment return-time is acceptable (lower than about 8years) only at ambient temperature exceeding 35°C. The ecological analysis indicates that the TEWI of the classical transcritical cycle exceeds that of the hybrid system, by approximately 60%.

Improvement of the ultrasonic atomization liquid desiccant dehumidification system

Liquid desiccant dehumidification system has received much attention in recent years for its important economic benefits in energy saving of buildings. Furthermore, the specific surface area of desiccant solution can be expanded dramatically by the ultrasonic atomization technology, which generates numerous tiny droplets with diameter about 50μm. However, the tiny atomized droplets are found to be easily captured by the wall of dehumidifier chamber, resulting in the poor performance of this type of system. In this paper, a new criterion called “droplets suspension rate” together with an optimized design method was proposed to improve the performance of ultrasonic atomization liquid desiccant dehumidification system via optimizing its dehumidifier chamber. Experiments were carried out to verify the improvement, where the dehumidification effectiveness, desiccant consumption rate and the air temperature rise were adopted as indicators to evaluate the performance of the system. The results showed that the performance of ultrasonic atomization liquid desiccant dehumidification system was enhanced significantly and the desiccant consumption was reduced markedly after the improvement. Moreover, the improved system was found to be able to utilize lower-grade desiccant solution. The criterion and the method proposed may build a foundation for the better design of ultrasonic atomization liquid desiccant dehumidification system.

Effect of adsorption to desorption air mass ratio on the performance of double bed desiccant dehumidification system

In the present work, experimental investigation of the performance of double bed desiccant dehumidification system has been carried out. The two desiccant beds operate simultaneously. When one of the two beds functions as an adsorber, the second operates as a regenerator. In this investigation, silica gel is applied as the desiccant material. Hot air from an air heater is blown through the regenerated bed using an air blower. System performance at different conditions of flowing air is demonstrated. The experimental tests were carried out at different conditions of inlet air and initial bed parameters. Temperature and humidity of air at the inlet and exit of the bed were measured. The effect of adsorption to desorption air mass ratio is examined. In the present study, it is found that 63% regeneration to total air mass ratio is the optimum value.

Experimentally validated model for atmospheric water generation using a solar assisted desiccant dehumidification system

This paper examines an alternative solution for emergency situations where freshwater and utilities are often interrupted. Generating freshwater from the atmosphere using a small-scale air-cooled desiccant wheel dehumidifier was experimented. Condensed water was collected and systematically recorded against local meteorological data. A synthetic model simulating the actual lay-out of the experiment was built in TRNSYS. The model validated the experimental results and generated approximately 52 litres in 9 days. The validated model is then run for three different climates; Sydney, Abu Dhabi and London to estimate annual water production. Abu Dhabi showed the best results compared with Sydney and London by generating 18.5 kL of water per year. The model is further developed to evaluate thermodynamic benefits of using dehumidified processed air as a feed stream for a proposed small-scale air-conditioning system. The energy required for the wheel regeneration process is met by thermal gain of modified solar PVT panels where the stagnant heat at the back is used to heat up the regeneration air stream. It was found that the dehumidification process can significantly decrease the latent load of the air-conditioner and easily bring indoor humidity level to the human comfort zone. However, dehumidified process air is also increasing the sensible load because of the higher temperature associated with dehumidification process. Rooftop solar PV panels can easily meet the power demand of appropriate lighting, a computer and mini refrigerator for extended hours if an appropriate set of batteries are fitted, but unable to exclusively meet the air-conditioner power demand to maintain indoor temperature within human comfort zone.

Heat and Mass Transfer Model for Desiccant Solution Regeneration Process in Liquid Desiccant Dehumidification System

In this paper, a simple model, but with high accuracy for a packed column liquid desiccant regenerator, to describe the heat and mass transfer process is developed. By lumping fluids’ thermodynamic properties and the geometric specifications as constants, two equations related with seven identified parameters are developed to predict the heat and mass transfer rate for solution regeneration processes in the regenerator. Commissioning information and the Levenberg–Marquardt method are employed to determine the unknown parameters. Compared with previous models, the presented model is simply constructed and accurate and requires no iterative computations while applied in predicting the heat and mass transfer rate once the parameters of the proposed model are determined. Experimental results demonstrate that the current model is effective to predict the performance of desiccant regeneration in the regenerator over wide working conditions. The proposed model promises to have wide application for real-time performance monitoring, optimization, and control for liquid desiccant regeneration.

Energy performance of an innovative liquid desiccant dehumidification system with a counter-flow heat and mass exchanger using potassium formate

Abstract An innovative micro-scale liquid desiccant dehumidification system is numerically investigated. The liquid desiccant dehumidification unit employs a counter-flow low-cost and efficient heat and mass exchange core, improving

Exergy calculation and analysis of a dehumidification system using liquid desiccant

Liquid desiccant dehumidification is an effective method for extracting moisture from humid air and consumes less energy than conventional methods. Exergy is an essential tool to analyze liquid desiccant dehumidification systems. The problem with current exergy analysis of liquid desiccant dehumidification systems is that the exergy calculation for humid air is inconsistent with the exergy calculation for liquid desiccant. The choice of the dead state is the key issue in exergy analysis. In this paper, the saturated air state of ambient temperature is selected as the ultimate dead state. The ultimate dead state of liquid desiccant is defined by limitation of the infinitely diluted solution that will guarantee the uniqueness of the ultimate dead state in the system and address the previously mentioned inconsistency. The exergy flow in the liquid desiccant dehumidification cycle is also calculated (LiBr aqueous solution is used as an example), and exergy expressed per kilogram of dry air and per kilogram of solute can simplify the calculations. Exergy destruction, a measurement of irreversibility in the heat and mass transfer process, indicates the direction of system improvement.

Statistical evaluation of a liquid desiccant dehumidification system using RSM and theoretical study based on the effectiveness NTU model

In the present study the performance of an air dehumidifier using lithium bromide (LiBr) as a desiccant was investigated. Response surface methodology (RSM) was used to assess individual and interactive effects of the six main factors (velocity, temperature and humidity of air, flow rate, temperature and concentration of desiccant) on dehumidification mass rate. A reduced quadratic statistical model was derived to predict dehumidification mass rate. The maximum dehumidification mass rate was obtained 0.154g/s under the optimal conditions of an air velocity of 4.1m/s, desiccant flow rate of 0.035kg/s, air humidity ratio of 0.0185kg/kg, desiccant concentration of 0.48kg/kg, air temperature of 29.5°C, and desiccant temperature of 21.8°C. The effectiveness number of transfer unit (NTU) model was employed to describe the coupled heat and mass transfer. The results of the model and the experimental data show good agreement. Dimensionless mass and heat transfer coefficients correlations are proposed; the average absolute differences between the predicted values and the experimental findings for Sh and Nu numbers were calculated as 2.14% and 5.27%, with the discrepancies mainly within ±9% and ±13%, respectively.

A heat pump driven and hollow fiber membrane-based liquid desiccant air dehumidification system: Modeling and experimental validation

A compression heat pump driven and membrane-based liquid desiccant air dehumidification system is presented. The dehumidifier and the regenerator are made of two hollow fiber membrane bundles packed in two shells. Water vapor can permeate through these membranes effectively, while the liquid desiccant droplets are prevented from cross-over. Simultaneous heating and cooling of the salt solution are realized with a heat pump system to improve energy efficiency. In this research, the system is built up and a complete modeling is performed for the system. Heat and mass transfer processes in the membrane modules, as well as in the evaporator, the condenser, and other key components are modeled in detail. The whole model is validated by experiment. The performances of SDP (specific dehumidification power), dehumidification efficiency, EER (energy efficiency ratio) of heat pump, and the COP (coefficient of performance) of the system are investigated numerically and experimentally. The results show that the model can predict the system accurately. The dehumidification capabilities and the energy efficiencies of the system are high. Further, it performs well even under the harsh hot and humid South China weather conditions.

Effective desiccant dehumidification system with two-stage evaporative cooling for hot and humid climates

This study examines the design and operation of a hybrid air conditioning system that uses 100% fresh air and integrates a solid desiccant dehumidification system with a two-stage evaporative cooling system to optimize the system operation with respect to energy and water consumption while maintaining occupant thermal comfort. The first stage consists of cooling a fraction of the dehumidified air stream using an evaporative cooling pad, mixing the cooled air with the remaining bypassed air fraction and then supplying it to the space in order to minimize water consumption and limit the indoor relative humidity to acceptable levels. The second stage consists of locally cooling the occupant's microclimate using a personalized evaporative cooler (PEC) that will allow for higher room bulk air temperatures. The system was implemented in an office space in Beirut and the optimization was carried using a derivative free genetic algorithm that handled three variables: the regeneration temperature; the air mass flow rate; and the fraction of air entering the evaporative cooler. The two-stage system achieved a 16.15% reduction in energy consumption and a 26.93% reduction in water consumption compared to a single-stage evaporative cooling system at the same thermal comfort level.

Performance analysis of a two-stage desiccant cooling system

Multi-stage desiccant systems are an effective way to improve the performance of desiccant dehumidification systems, which can greatly decrease the required regeneration temperature and make possible the utilization of exhaust heat from the heat pump. The performance of a heat pump-driven two-stage desiccant wheel system is analyzed in this paper. Models of the desiccant wheel and heat pump systems are utilized to predict system performance. The effects on system performance of the compressor power input, the heat exchange area distribution between evaporators and condensers, the wheel’s rotation speed, and the inlet parameters of the processed air are investigated. When the supplied air humidity ratio is 10g/kg, COPt of the desiccant system is 5.5 under Beijing summer condition. The key to improving system performance is to match the cooling capacity and exhaust heat provided by the heat pump with the requirements of dehumidification and regeneration. An improved system utilizing an indirect cooler to recover the cooling capacity from the indoor exhaust air is then proposed, with COPt improving by 15% compared to the original system.

The Optimized Operation of a Solar Hybrid Desiccant/Displacement Ventilation Combined with a Personalized Evaporative Cooler

The study investigates by modeling and experimentation the performance of displacement ventilation (DV) aided with personalized evaporative cooler (PEC) system that operates in humid climate and uses a solid desiccant (SD) dehumidification system regenerated by parabolic solar concentrator thermal source. Predictive component models of the conditioned space, the SD, the solar concentrator system, and the PEC were developed and used for the prediction of associated operational energy consumption while utilizing an optimized control strategy. The control strategy seeks optimal values of supply air flow rate and temperature and the desiccant regeneration temperature for both cases with and without the aid of the PEC while meeting space load, indoor air quality, and thermal comfort requirements. Energy consumption was calculated for the optimized strategy using genetic algorithm optimizer and the integrated DV/SD-PEC models. The results agreed well with experimental data obtained from tests on a DV climatic chamber. In addition, votes of comfort recorded by participants in the experiment using PEC were very similar to predicted comfort. The optimized hybrid system performance was applied to a typical office space of area of 64 m2. The operation of the hybrid system with PECs resulted in a higher supply air temperature. The increment in supply air temperature is 1.1°C when PEC is used compared to case without PEC. This increase in temperature showed that an energy saving of 13.5% is achieved for the PEC aided hybrid system.

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.

Experimental and theoretical investigations on the cyclic operation of TSA cycle for air dehumidification using packed beds of silica gel particles

Dehumidification using desiccant beds provide a good alternative for the conventional vapor compression cooling system. Desiccant material in the desiccant dehumidification system should undergo both adsorption and desorption processes. In the present work, experimental tests have been carried out for thermal swing adsorption (TSA) cycle utilizing two packed beds of silica gel spherical particles. The pseudo gas side controlled (PGC) mathematical model has been presented for predicting the cycle performance. The mathematical model has been validated using the experimental results. The root mean square of errors ranges from 1.15% to 9.03% for the exit air humidity ratio and from 1.08% to 9.68% for exit air temperature. The dynamics of desiccant bed during the cyclic operation has been investigated numerically. In addition, it has been found from the parametric study that the cycle efficiency is maximum for a regeneration temperature of 90–95 °C when the bed length varies from 50 to 300 mm and for desiccant particle diameter ranging from 2 to 5 mm. A sensible cooling for the process air before undergoing the dehumidification period is recommended for increasing the cycle efficiency and the dehumidification time which is an added advantage.

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.

Operating parameter analysis to improve the performance of solar desiccant wheel using a mathematical model: Part 1

A mathematical model for analysis of various operating parameters to improve the performance of desiccant wheel has been used and heat and mass transfer for both moist air and the desiccant material have been considered. An experimental setup is fabricated using the evacuated tube solar air collector with desiccant wheel. The hot air needed for regeneration is produced by evacuated tube solar air collector, which has collector surface area of 4.44 m2. The regeneration can be started from 40°C. The temperature of outlet air is obtained in the range of 40–65°C by this evacuated tube solar air collector. The experimental results are used to validate the mathematical model with good agreement. The effects of velocity of the regeneration air and the process air have been studied and it was found that moisture removal of desiccant wheel increases with increase in regeneration air velocity and decrease with increase in process air velocity. On the basis of obtained results, the following recommendations have been made: velocity of the process air is between 1 m/s and 2.5 m/s and the regeneration air is between 3 m/s and 5 m/s. These results are useful to study and analyse of solid desiccant dehumidification systems.