Experimental studies on the dehumidifier and regenerator of a liquid desiccant cooling system

Liquid desiccant cooling systems are being explored as an alternative to the conventional vapor compression systems. The dehumidifier and the regenerator form the heart of these systems. A falling film tubular absorber and a falling film plate regenerator have been studied experimentally. The experimental results have been compared with the predictions from theoretical models. Two wetness factors to account for the improper wetting of exchanger surfaces have been defined and estimated. After taking these factors into account, the theoretical models predict the experimental data within ±30%.     

Investigation of heat and mass transfer in a gauze‐type structured packing liquid desiccant dehumidifier

In this experimental investigation, a packed bed column suitable for 5‐ton hybrid cooling system has been designed to study the absorption of water vapour from moist air by contact with aqueous solutions of calcium chloride. The packing material used in the study was two elements of the BXPEP structured packing and the height of the each element was 17 cm. This packed bed dehumidifier handles desiccant flow rates from 10 to 32 l/min. This paper presents results from a detailed experimental investigation of the heat and mass transfer between a liquid desiccant (calcium chloride) and air in a gauze‐type structured packing dehumidifier. The effects of different independent variables such as air inlet absolute humidity, desiccant inlet temperature, flow rate and its concentration on the performance of the dehumidifier have been investigated. Copyright © 2002 John Wiley & Sons, Ltd.     

Coupled heat and mass transfer characteristic in packed bed dehumidifier/regenerator using liquid desiccant

The dehumidifier and regenerator are two key components in liquid desiccant air conditioning systems. The heat transfer driving force and the mass transfer driving force influence each other, the air and desiccant outlet temperatures or humidity ratio may exceed the air and desiccant inlet parameters in the dehumidifier/regenerator. The uncoupled heat and mass transfer driving forces, enthalpy difference and relative humidity difference between the air and desiccant are derived based on the available heat and mass transfer model and validated by the experimental and numerical results. The air outlet parameter reachable region is composed of the air inlet isenthalpic line, the desiccant inlet equivalent relative humidity line and the linkage of the air and desiccant inlet statuses. Except the mass flow rate ratio and the heat and mass transfer coefficients, the air and desiccant inlet statuses and flow pattern have great effects on the dehumidifier/regenerator performance. The counter flow configuration expresses the best mass transfer performance in the dehumidifier and the hot desiccant driven regenerator, while the parallel flow configuration performs best in the hot air driven regenerator.     

Solar desiccant systems for grain drying

The mathematical model for the Kathabar Spray-Cel System developed by the authors in a previous work was used to predict the temperature and humidity ratio of the air at the drying bin inlet. The unsteady grain cooling model developed by Huzayyin (Ph.D. dissertation, Kansas State Univ. at Manhattan (1972); Ref. [1]) is modified and coupled with the liquid desiccant system model to study the feasibility of using the system desiccated air in drying applications. The model shows the infeasibility of using desiccated air for grain drying. The liquid desiccant system was modified to simulate adiabatic operation and coupled to the drying model. The new system gave much better results, but its use for grain drying is still predicted to be economically unfeasible.     

Analytical solution of combined heat and mass transfer performance in a cross-flow packed bed liquid desiccant air dehumidifier

Heat and mass transfer between air and liquid desiccant in a cross-flow packed bed dehumidifier is investigated. Analytical solutions of air and desiccant parameters as well as enthalpy and moisture efficiencies are given in the present study, based on the analogy between the combined heat and mass transfer process in the cross-flow dehumidifier and the heat transfer process in the cross-flow heat exchanger. The results given by the analytical solution are compared with numerical solutions and experimental findings. Good agreement is shown between the analytical solutions and the numerical or experimental results. The analytical solutions can be used in the optimization of the cross-flow dehumidifier.     

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.     

Prediction of pressure drop in a packed bed dehumidifier operating with liquid desiccant

In this paper a more rigorous model, which is valid for both structured and random packing columns, is used for predicting the irrigated pressure drop in a desiccant–air contact system. Calcium chloride solution is considered as the desiccant. Four different random packing materials and three different structured packing materials are considered in the present study. The effects of random packing shape and the type of structured packing on the hydraulic performance are studied. The model has been validated for a wide range of operating values available in the literature. It is found that the structured packing has the lower pressure drop and higher capacity compared with random packings. Among the random packing materials considered in the present study, Intalox saddles can provide the least irrigated pressure drop and among the structured packing materials the sheet-type Mellapak 250 Y has the lowest pressure drop.     

Experimental and theoretical analysis of heat and mass transfer in a packed column dehumidifier/regenerator with liquid desiccant

This paper presents the experimental tests and the theoretical analysis on the chemical dehumidification of air by a liquid desiccant and desiccant regeneration in an absorption/desorption column with random packing.The experimental set-up is fully described together with measurements, procedures, data reduction and accuracy. The experimental tests include dehumidification and desiccant regeneration runs carried out with the traditional hygroscopic salt solutions H₂O/LiCl and H₂O/LiBr and the new salt solution H₂O/KCOOH in the typical operative ranges of air conditioning applications.A theoretical model of the packed column and the relative simulation computer code was developed to predict the performance of the system and to analyse the system sensitivity to the main operating parameters. A fair agreement was found between the experimental tests and the simulation computer code.The experimental tests and the theoretical analysis show that the chemical dehumidification of air by hygroscopic salt solutions ensures consistent reduction in humidity ratio, which is suitable for applications to air conditioning or drying processes. Moreover, desiccant regeneration requires a temperature level around 40–50 °C which can be easily obtained by using solar energy or heat recovered from an industrial process or from a thermal engine.     

太阳能溶液除湿空调(下)

五太阳能溶液蓄能太阳能是分布广泛、使用清洁的可再生能源,但太阳辐射的不连续性和不稳定性,成为太阳能利用的关键问题。太阳能溶液除湿空调系统可吸收太阳能,并以空调能力的形式储存于除湿剂浓溶液中,是一种有效的蓄能方式。太阳能溶液除湿空调循环中,浓溶液在除湿     

太阳能制冷讲座(8) 太阳能溶液除湿空调(上)

介绍了太阳能溶液除湿空调的工作原理及特点。详述了系统主要部件除湿器、太阳能集热再生器的构成,介绍了太阳能溶液蓄能原理,并对太阳能溶液除湿空调的研究进展和应用前景进行了总结。     

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

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

A simulation study of heat and mass transfer in a honeycombed rotary desiccant dehumidifier

A one-dimensional coupled heat and mass transfer model, which is expected for use in designing and manufacturing of a honeycombed rotary desiccant wheel, is presented in this paper. The mathematical model has been validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed and verified by experimental data. The numerical results indicate that in the regeneration process a hump curve of air humidity ratio along the channel exists all the time. In the regeneration process the hump of air humidity ratio moves from the duct entrance to the duct exit and increases gradually until the hump reaches the duct exit, where the hump will drop subsequently. The effects of velocity of regeneration air V_reg inlet temperature of regeneration air T_reg and velocity of process air V_ad on the hump moving speed are investigated. To improve the performance of desiccant wheel, it is essential to accelerate the hump moving from the duct entrance to the duct exit as soon as possible.     

Model validation and case study on internally cooled/heated dehumidifier/regenerator of liquid desiccant systems

Traditional dehumidifiers and regenerators of liquid desiccant systems often use packed columns supporting adiabatic heat and mass transfer between air and liquid desiccant. As new-style equipment, internally-cooled dehumidifiers can improve dehumidification performance due to restraining temperature increase of the desiccant. Similar to internally-cooled dehumidifiers, an idea of internally heating is imitated to put forward internally-heated regenerators. The uniform mathematical model for an internally cooled dehumidifier and internally heated regenerator was presented and validated by comparison of computation results with experimental data in this study. The case study focused on the parameters distribution comparisons of the internally cooled/heated dehumidifier/regenerator with adiabatic ones and demonstrated coupled heat and mass transfer behavior. The results show that the internally-heated regenerator can produce higher regeneration efficiency than the adiabatic one to produce better energy efficiency and eliminate the dehumidification possibility which would happen in adiabatic regenerators. The internally-cooled dehumidifier can also provide better dehumidification performance comparing with the adiabatic one; however its benefit would be not as good as the internally-heated regenerator. In addition, effect of the width of the air channel on internally cooled/heated dehumidifier/regenerator was discussed and the results can help the optimal design of this kind of dehumidifiers and regenerators.     

Predictions of moisture removal rate and dehumidification effectiveness for structured liquid desiccant air dehumidifier

In hot and humid climates such as in the Sultanate of Oman, the humidity puts extra load on the electric vapor-compression air conditioning (VAC) systems. Liquid and solid desiccants can reduce the moisture content of humid air and thus reduce the latent load imposed on the VAC systems. In the present work, the performance of air dehumidifiers using triethylene glygol (TEG) as desiccant was investigated. Three differently structured packing densities were used (77, 100 and 200 m²/m³). The performance of the dehumidifier was evaluated and expressed in terms of the moisture removal rate (m_cond) and the dehumidifier effectiveness (ε_y). The experimental work was undertaken to study the effects of several influencing design factors on this performance. The design factors covered included the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and the inlet TEG concentration. The desiccant flow rate investigated was much less than that covered in previous studies and the range of the inlet temperatures of air and desiccant was significantly wider. The objective this study was to use the multiple regression method and the principal component analysis to obtain statistical prediction models for the water condensation rate and the dehumidification effectiveness in terms of these design factors. The results of both techniques agree with each other affirmed that the desiccant flow rate, desiccant inlet concentration and air inlet temperature are the most significant variables in predicting m_cond, whereas desiccant flow rate, air inlet temperature and packing density are the most significant variables in predicting ε_y.     

Performance of a proposed complete wetting surface counter flow channel type liquid desiccant air dehumidifier

An idea that improves the wettability over the surfaces of a cylindrical dehumidifier channel was proposed and experimentally proved. Fibrous sheets were attached to the inner surfaces of the channel. The capillary effect of fibers sustains the complete wetting of the heat and mass transfer surfaces. The air to be dehumidified and cooled flows upward in the annulus space between the two layers of fibrous sheets, which are saturated with the downward flowing desiccant solution. The permeability of the fibrous sheet was determined experimentally. It was 2.43 × 10^?10 m². The measured solution flow rate due to the capillary suction of the sheets was Γ_in,min = 1.12 kg/h m. The liquid desiccant tested was H₂O/CaCl₂ with salt concentration ratios ranging from 35 to 40%. The measured distribution of the solution flow rate along the circumference of the sheets at the outlet showed 5% deviation from the average flow rate. This is a good indication for the good wettability of walls inside the dehumidifier.Feeding the solution by this mechanism has many advantages over spray feeding. Beside sustaining complete surface wetting, it also prevents channel blockage with solution, which is a main factor in increasing the air pressure drop. About 95% of the air pressure drop is saved in this study by avoiding these problems. A simple theoretical model for the heat and mass transfer processes inside the dehumidifier was developed and experimentally validated. In general, there is good agreement between the predicted and measured data. The developed model was utilized to study the effect of the different parameters on the dehumidifier performance. For a 1 m height dehumidifier with an inlet specific humidity and air temperature of 0.0234 kg_v/kg_a, and 35 °C, respectively, the predicted outlet air specific humidity was 0.0102 kg_v/kg_k and the corresponding outlet air temperature was 27.4 °C. The inlet solution temperature and salt concentration were 25 °C and 40%, respectively.     

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

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

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

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

Numerical simulation and theoretical analysis of heat and mass transfer in a cross flow liquid desiccant air dehumidifier packed with honeycomb paper

Heat and mass transfer processes in a cross flow liquid desiccant dehumidifier, in which wet durable honeycomb paper constitutes the packing material, is investigated in this paper. The device is expected to be used in hot and humid areas to control the indoor humidity environment. A mathematical model, able to determine the heat and mass transfer between the air and the falling film of liquid desiccant, is developed, and the analysis on Nusselt and Sherwood numbers at the liquid–air interface is performed considering the solution of 40% H₂O/CaCl₂. Also obtained is the theoretical Nusselt number under assumed conditions and the relevant analysis, as well as the comparison between the two results.