Desiccant Dehumidification System Research Articles

Experimental investigation of a novel multi-channel flat plate liquid desiccant dehumidification system

ABSTRACT The continuously growing energy demands for space air conditioning and depletion of conventional energy resources at a fast rate have propagated the need for producing a new and wide range of renewable and sustainable energy technologies. Liquid desiccant dehumidification technologies are the most optimistic approach due to their lower regeneration temperature required, a higher coefficient of performance, and the ability to be used during night hours. The factors responsible for the limited application of these systems are desiccant leakage, corrosion to the fabricating material, and carryover of desiccant droplets with the process air. To overcome these issues, a new multi-channel flat plate liquid desiccant air conditioning system has been developed and experimentally investigated. The performance of the system has been analyzed for the dehumidification and regeneration process. Calcium chloride has been used as a desiccant material with 40% and 35% by wt. concentration. Three sets of velocities of process air have been utilized, i.e., 0.9, 0.7, 0.5 m/s to investigate the system performance. Performance parameters, such as dehumidification rate, regeneration rate, dehumidification, and regeneration effectiveness, have been investigated in the present study. The liquid desiccant dehumidification system investigated in the present study includes a flat plate energy exchanger for heat and mass transfer between ambient moist air and liquid desiccant solution. This system conveys a large interfacial surface area between liquid desiccant solution and process air. Experimental results indicate that, in the dehumidification process, the ambient moist air has been dehumidified and cooled by 5.90 g kg−1 and 1.4°C, respectively, and dehumidification effectiveness of 0.239 has been achieved for the air velocity of 5 m/s at 40% concentration of the desiccant solution. Regeneration rate and effectiveness have been observed to be 0.014 g s−1 and 0.323, respectively, for the air velocity of 5 m/s at 35% concentration. The work presented in this study demonstrates the feasibility of the novel multi-layer flat plat liquid desiccant concept and provide growth and development to the liquid desiccant technologies for space air conditioning. Certain issues regarding the mass imbalance in the dehumidification and regeneration processes have been identified to be considered as future scope.

Feasibility study on improving the performance of atomization liquid desiccant dehumidifier with standing-wave ultrasound

Abstract Liquid desiccant dehumidification systems are promising to increase building energy efficiency. Furthermore, the dehumidification performance can be improved using ultrasound capable of atomizing liquid desiccant into numerous droplets. However, the droplets fall rapidly by gravity and converge into liquid films at the dehumidifier’s bottom, reducing the effective transfer time, which is another crucial factor for dehumidification, and the energy efficiency of the ultrasonic system. This study investigated a new and improved method using a standing-wave ultrasound and its levitation effects; using this method, desiccant droplets can be levitated and vibrated in humid air, and thereby, the mass transfer can be increased. The feasibility of the method was carefully verified through comparative experiments within an improved ultrasonic liquid desiccant system (ULDS), which was switched between the conventional traveling-wave and the standing-wave ultrasound. Performance criteria related to the moisture removal rate and specific energy consumption of the ultrasonic system were adopted. We found that the standing-wave ultrasound significantly improved the dehumidification performance of ULDS (by 56% on average) at a negligible additional energy cost. Moreover, low-concentration desiccant solutions become useable, and the ultrasonic device is more energy-efficient in the standing-wave ULDS. This study can aid in improving the dehumidification performance of ULDSs.

An online model for predicting dynamic response of a packed liquid desiccant dehumidifier

Liquid desiccant dehumidification systems show significant energy-saving potential in HVAC applications. The dynamic characteristics of a liquid desiccant dehumidifier attract increasing attentions in recent years, which are essential for designing appropriate controllers for it. However, due to the complicate coupling of heat and mass transfer inside the dehumidifier, there are few models have been developed for predicting the dynamic characteristics of a packed liquid desiccant dehumidifier. In this paper, an online model was proposed, which predicted the dynamic response of a packed dehumidifier mainly based on the air outlet humidity ratio rather than the inlet parameters of the dehumidifier. The online model was derived from the heat and mass transfer principle and can be described by an exponential function. The dynamic response can be divided into two stages taking the maximum air outlet humidity ratio changing rate as a dividing point. The two unknown parameters (the maximum air outlet humidity ratio changing rate and the time coefficient) in the online model were determined by the first stage of dynamic response, and then the online model predicted the dynamic response of the packed dehumidifier at the second stage. The online model agreed well with the experimental data, casting a new light on learning the internal relation of the dynamic processes of a packed dehumidifier. Furthermore, the relation between the variation of inlets and dynamic response of air outlet humidity ratio could be predicted quickly using the proposed model, contributing to control algorithm development. And a preliminary idea about how to construct an appropriate controller based on the proposed model is presented, which provides a reference for further study.

Detailed experimental analysis of the energy performance of a desiccant wheel activated at low temperature

Desiccant dehumidification systems, such as those using desiccant wheels (DW), are an interesting alternative to conventional dehumidification systems based on vapor-compression cycles. The aim of this work was to obtain experimentally the performance of a DW activated at low temperature along the heat and mass transfer surface under different working conditions. The air temperature and humidity were measured at 32 different points, in order to analyse the performance of the DW at different circular sectors of the rotor.The results showed that the surface temperatures of the desiccant material on the process side decreased when the rotation speed decreased. Conversely, the surface temperatures of the desiccant material on the regeneration side increased when the rotation speed decreased.The minimum outlet process air temperatures and the maximum process air dehumidification were obtained in the circular sector closest to the regeneration side. In this circular sector the highest efficiency values were obtained. The number of circular sectors where the highest efficiency values were obtained increased when the rotation speed or air flow rates increased. Using these data, the wheel control strategy could be designed to increase the dehumidification capacity.

A review of coupled heat and mass transfer in adiabatic liquid desiccant dehumidification and regeneration systems; advances and opportunities

AbstractIn this paper, a comprehensive technical review of liquid desiccant (LD) dehumidification and regeneration techniques is presented. The operational features, processes and performance indices of various flow configurations of adiabatic dehumidifier and regenerator are extensively covered. The heat and mass transfer assessment is presented in terms of past experimental and modelling evaluations and procedures. The existing adiabatic dehumidifier/regenerator heat and mass transfer models are categorized into finite difference, effectiveness-number of transfer units and simple empirical correlation models. The respective performance prediction models are critically analysed in details and compared in terms of assumptions, iterative procedures, solution methods, accuracy, computation time, output variables and applications. The solar regenerator models are also highlighted with a focus on the collector module. The ideal settings, formulation procedures, current state-of-the-art and opportunities for improvements are outlined. The review provides meaningful insight into the research status and available opportunities in the LD adiabatic dehumidifier and regenerator modelling and optimization as well as conceptualization of the applicable models. Finally, some very impactful suggestions for improvement and further research are outlined.

Performance optimization of a heat pump driven liquid desiccant dehumidification system using exergy analysis

Liquid desiccant dehumidification system driven by heat pump is recognized as an efficient approach for humidity control in the air-conditioning system. The liquid desiccant dehumidification system is optimized by exergy destruction analysis method in the present research. According to the exergy destruction theory, the exergy destruction of a liquid desiccant dehumidification system is divided into exergy destructions arising from heat pump cycle, evaporators, condensers, heat and mass transfer modules and various mixing processes, respectively. The heat and mass transfer process uniformity coefficients γt and γω are proposed to describe the uniformity of the heat and mass transfer driving forces along the whole system. It’s indicated γt and γω of the basic cross-flow system are as high as 1.4 and 1.2. Then reducing the exergy destruction is chosen as the guideline to optimize the system. On the basis of the basic cross-flow system, the improved cross-flow system and the improved counter-flow system are further proposed. γt (γω) of the improved systems are reduced to 1.12(1.12) and 1.04(1.01), respectively. Owing to the system optimization, the exergy efficiency increases from 20.1% of the basic cross-flow system to 21% and 25% respectively, COPsys increases from 5.7 to 6.0 and 7.4 respectively. The exergy analysis method is effective for an optimized scenario of a heat-pump driven liquid desiccant process.

Energy and Exergy Analysis of LiBr-aq and LiCl-aq Liquid Desiccant Dehumidification System

In this study, energy and exergy analysis of experimental results obtained from a dehumidification system using LiBr-aq (lithium bromide-water) and LiCl-aq (lithium chloridewater) as desiccant was made. In dehumidifier and regenerator columns polycarbonate sheets, which have not been used before, were used as packing material to increase contact area in purposed liquid desiccant dehumidification system. In the analysis, variation of electrical coefficient of performance and exergy efficiency with airflow rate for different solution mass flow rates were investigated. Because of investigation, maximum values of electrical coefficient of performance and exergy efficiency were calculated approximately as 2.8 and 18%, respectively.

Performance characteristics of a novel internally-cooled plate membrane liquid desiccant air dehumidification system

An air dehumidification system with internally-cooled plate membrane-based liquid desiccant dehumidifier is presented. The novel system can effectively dehumidify the ambient air by the internally-cooled dehumidifier. On the other hand, the dehumidifier and regenerator are in independent operation due to the separate solution passage, which makes the system more flexible and energy saving. In this research, heat and mass transfer processes in the dehumidifier, the regenerator, the air-cooled water chiller, and other key components are modeled in detail. Based on these sub-models, the whole system model is established and solved. An experiment is carried out to validate the theoretical model and the calculated results agree with the experimental data well. The impacts of main operating parameters on the system performance (e.g. specific dehumidification power, coefficient of performance and dehumidification) are investigated numerically. The results show that the specific dehumidification power of the system at design condition reaches 263 g h−1 m−2, which indicates that the system gets strong dehumidification capacity. The COP of the whole system is in the range of 0.46–0.62 and energy consumption for regeneration occupies above 60% of total energy input.

Building thermal load management through integration of solar assisted absorption and desiccant air conditioning systems: A model-based simulation-optimization approach

Performance of standalone air-conditioning systems is affected while simultaneously handling both sensible and latent building loads. Model-based simulation and optimization is one of the best ways to analyze system's performance under load fluctuations at the initial design stages. Therefore, in the current study, seasonal transient simulations of an Integrated Absorption Desiccant System are carried out using TRNSYS to handle sensible (10.94 kW peak) and latent (4.44 kW peak) cooling load separately. Radiant cooling with chilled water from an absorption chiller mainly handles sensible cooling load, while the latent load is achieved by a solid desiccant dehumidification system. Initially, key components of the system are modeled in TRNSYS including a flat plate solar collector system, desiccant wheel, heat recovery wheel, and absorption chiller. Afterwards, the integrated model is coupled with GenOpt to optimize the solar fraction and thermal coefficient of performance by varying collector area, flow rate in the collector loop, flow rate in load side loop, and volume of storage tank. The resulted optimized value of solar fraction is 57.50%, and COPth is 0.55 for standalone absorption system, whereas the solar fraction of 56.20%, COPth of 1.52 are achieved for IADS. In addition, the effects of load variation in terms of sensible to latent load ratio on regeneration heat required for both IADS and conventional desiccant system are also analyzed. A critical value of load ratio of 0.75 is also observed in terms of heat regeneration requirements to compare the both systems. The proposed approach and analysis will be very helpful for HVAC designers for optimal system performance through separate load handling, especially at the initial design stage.

Effect of Substrate Conductivity on the Transient Thermal Transport of Hygroscopic Droplets during Vapor Absorption.

In all kinds of liquid desiccant dehumidification systems, the temperature increase of the desiccant solution due to the effect of absorptive heating is one of the main reasons of performance deterioration. In this study, we look into the thermal effects during vapor absorption into single hygroscopic liquid desiccant droplets. Specifically, the effect of substrate conductivity on the transient heat and mass transfer process is analyzed in detail. The relative strength of the thermal effect and the solutal effect on the rate of vapor absorption is investigated and compared to the thermal effect by evaporative cooling taking place in pure water droplets. In the case of liquid desiccants, results indicate that the high thermal conductivity of copper substrates ensures more efficient heat removal, and the temperature at the droplet surface decreases more rapidly than that on Polytetrafluoroethylene (PTFE) substrates. As a result, the initial rate of vapor absorption on copper substrates slightly outweighs that on PTFE substrates. Further analysis by decomposing the vapor pressure difference indicates that the variation of vapor pressure caused by the temperature change during vapor absorption is much weaker than that induced by the concentration change. The conclusions demonstrate that a simplified isothermal model can be applied to capture the main mechanisms during vapor absorption into hygroscopic droplets even though it is evidenced to be unreliable for droplet evaporation.

Influence of Different Desiccants, Flow Type and Packings on the Liquid Desiccant Dehumidification System: A Review

Desiccant cooling mechanism is one of the alternate methods to control the humidity of air and temperature, compared to the conventional vapor compression method of air conditioning. Desiccant cooling doesn’t use harmful chemicals which effect the ozone layer and it saves lots of energy. Summer air condition system can use this technology since it removes the latent heat load from the room effectively and this process is economical. Selection of the appropriate liquid desiccant and packing material is very vital to obtain maximum dehumidification. This paper focuses on the different desiccants and packings used by different researchers to enhance the dehumidification. Simple and hybrid systems are also reviewed, and their comparison are presented based on the construction and dehumidification performance.

Influence of Phase Change Phenomena on the Performance of a Desiccant Dehumidification System

Demands of standalone dehumidification systems have been increasing in order to realize energy savings in air-conditioning processes. In a desiccant dehumidification system, the water vapor from the moist air undergoes a phase change phenomenon, this being from vapor to adsorbed phase, a process analogous to latent heat exchange. The energy exchange involved in such a process is often significant—up to 80% of the total energy exchange. In this study, the influence of the phase change phenomena involved in a desiccant dehumidification system was evaluated experimentally, along with the performance investigation under low desorption air temperatures of 308, 318, 328, 338, and 345 K. The system was driven by a constant adsorption temperature of 293 K. The dehumidification ability, latent heat ratio, and latent effectiveness were employed as key performance indexes. The results showed that with the increased desorption temperature, the latent heat ratio decreased, whereas the dehumidification ability and latent effectiveness increased. The highest latent heat ratio was found to be 0.61 at the desorption temperature of 308 K, whereas the highest latent effectiveness was obtained at the desorption temperature of 345 K. A suitable temperature for the effective and efficient dehumidification was observed to be 318 K for the current system.

Performance of heat pump driven internally cooled liquid desiccant dehumidification system

Internally cooled liquid desiccant dehumidification systems are considered to be compact and efficient dehumidification systems in the air-conditioning field. A theoretical method is proposed to investigate the ideal maximum coefficient of performance and current gaps between actual and ideal working conditions of the heat pump driven internally cooled liquid desiccant dehumidification system. It is found that the maximum coefficient of performance is determined by the inlet humidity ratio of process air and regeneration air, and the required humidity ratio difference. It has been proved to be not significantly affected by the solution concentration. Furthermore, three perfectness are identified and deduced to explain gaps between the actual and ideal working condition. They include heat–cold offset perfectness due to heat–cold offset of the circulating solution between dehumidifier and regenerator, transfer capacity perfectness due to the limited transfer capacity of the internally cooled/heated dehumidifier/regenerator, and heat pump thermodynamic perfectness. The results indicate that actual coefficient of performance generally only corresponds to 10%–35% of the maximum coefficient of performance. With aid of the aforementioned perfectness, reasons for limited coefficient of performance of the current system are analyzed to illustrate future improvement direction and potential.

Presentation of new approach for energy consumption reduction with use of solar system

In hot and humid areas with high solar radiation intensity, the use of air conditioning systems on the basis of desiccant materials such as solar desiccant cooling systems with a view to reducing energy consumption and economic saving can be a good alternative to conventional air conditioning units. In this paper, in order to offer a cooling load of a sample space in the city of Bandar Abbas, which has a high latent load and high radiation intensity, solar absorption cooling cycle has been considered as the base cycle, and two absorption cooling cycles were designed for comparison. Then the three cycles were modeled in the TRNSYS software, studied technically and economically, and at the end the best cycle has been selected. In terms of current energy prices, application of solar energy in Iran is not economically feasible, so the solar absorption refrigeration cycle has been considered as the basic system for the cycle’s comparison. Modeling of the expressed cycles was done in TRNSYS software. The basic cycle has been composed of two main parts of cooling by absorption chillers and providing the required heat supply by the solar collector, and also two desiccant dehumidification cycles have been added by a desiccant dehumidification system. In the economic analysis of the three systems, the costs estimation during lifetime (LCC) method also has been used. As a result of this modeling and due to the Act to amend energy prices, the cooling system with the help of desiccant wheel and using returned water of chiller’s generator as the regeneration source, with an annual energy saving of about 29.1%, 29.1% (1,371,980 m3) of natural gas consumption has been saved and the amount of 18/15% (156,667$) cost savings over 20 years compared to the base period, has been selected as the most efficient cooling system.

Experimental evaluation of desiccant dehumidification and air-conditioning system for energy-efficient storage of dried fruits

The ideal storage zones of dried fruits in comparison with dried foods and feeds are established on the psychrometric chart. The study investigates the combined effect of the desiccant dehumidification and Maisotsenko-Cycle evaporative cooling for the dried fruits storage. Two configurations of desiccant air-conditioning systems (S-I and S-II) are proposed, and their performance is evaluated under the ambient conditions of Fukuoka, Japan. It is established on the basis of parametric and thermodynamic analysis of both the system configurations (S-I and S-II) that S-II could yield better system performance. The psychrometric evaluation of S-II revealed that it can provide the optimal conditions for the storage of dried fruits at low regeneration temperature. However, modification in the proposed system configuration (S-II) is suggested for the storage of dried foods and feeds.Practical application: Conventional dehumidification and air-conditioning options are either expensive or inefficient for dried fruit storage buildings. The study provides a way forward of using thermally driven desiccant systems which are energy-efficient and environment-friendly. Worldwide, the building industries are actively involved in the development of such systems but their application for dried fruit storage buildings are not explored in detail. Therefore, the study provides thermodynamic details of dried fruits storage in relation with experimental evaluation of desiccant and evaporative cooling systems. Provision of such systems to the storage buildings will provide distinct control of humidity and temperature for optimal storage.

Falling film liquid desiccant air dehumidification

Falling film liquid desiccant dehumidification technology is attracting more and more attention due to lower energy consumption, less pollution, and more flexible humidity control in recent years. This paper conducts a comprehensive review on falling film liquid desiccant dehumidification systems. Firstly, the working principles and features of the liquid desiccant dehumidification are introduced to describe the dehumidification process. The existing liquid desiccants including organic and inorganic desiccants are reviewed. Then, the structures of falling film dehumidifiers including both adiabatic dehumidifiers and internally-cooled dehumidifiers are described. Besides, the simulation models of falling film dehumidifiers, such as finite difference models, effectiveness NTU (e-NTU) models and simplified simulation models are summarized. Finally, the exiting performance enhancing methods of falling film dehumidifiers are collected, which provide valuable guidance to researchers and engineers to improve the dehumidification performance.

Steady-state performance evaluation and energy assessment of a complete membrane-based liquid desiccant dehumidification system

A complete membrane-based liquid desiccant dehumidification system is investigated under the steady operating condition, which mainly consists of a dehumidifier, a regenerator, three heat exchangers, a cold and a hot water supply units. A finite difference mathematical model is developed for the complete system to investigate the system dehumidification performance and energy requirement, and validated by experimental data. The dehumidification performance is evaluated by the system sensible and latent effectiveness and moisture flux rate, while its energy performance is assessed by the total cooling capacity and coefficient of performance. It is found that the number of heat transfer units in the dehumidifier side and solution to air mass flow rate ratio have the most considerable impact on the system performance, while the number of heat transfer units in the regenerator side and solution inlet concentration in the dehumidifier have comparatively weak influences. The system sensible and latent effectiveness can be improved by increasing the dehumidifier side number of heat transfer units before reaching its critical value of 6. However, the amount of moisture being absorbed, total cooling capacity and coefficient of performance decrease with the dehumidifier side number of heat transfer units at the low air flow rate. The critical value of solution to air mass flow rate ratio varies with number of heat transfer units, and it is preferable to keep the flow rate ratio at or below its critical value as further increasing solution flow rate would reduce the system coefficient of performance.

Experimental investigation of a liquid desiccant dehumidification system integrated with shallow geothermal energy

This study proposes a liquid desiccant dehumidification system combined with shallow geothermal energy. Calcium chloride solution is adopted as a desiccant that is sprayed directly on outdoor air for dehumidification purposes. Shallow geothermal energy is a clean renewable energy that is located beneath the ground surface approximately 3–50 m deep. In this study, the shallow geothermal energy water is utilized as a substitute for chilled water or cooling water. Shallow geothermal energy water, at about 20–22 °C, could precool outdoor air and cool the desiccant solution. Precooling outdoor air before it enters the dehumidifier can remove a sensible cooling load in advance and increase the performance of the dehumidifier. Moreover, the lower temperature solution is beneficial for enhancing sensible, latent, and total heat transfer rates. Based on the calculated results, under the same operating conditions, compared to the condensation system and the traditional liquid desiccant dehumidification system, the proposed system saves 86% and 82%, respectively, on power consumption. Result shows that a liquid desiccant dehumidification system integrated with shallow geothermal energy is an effective approach to enhance dehumidification performance and cooling capacity.

Feasibility Study of Various Organic Fibers as Potential Desiccant Materials

Feasibility of using cellulose rich organic fiber materials like, coconut (Cocos nucifera) coir, jute (Corchorus capsularis) fibers and banana (Musaacuminate) pseudostem as desiccant was studied. The selected fibers were first tested for maximum water absorbing capacity with and without acid and alkali pre-treatments. A test set-up with controlled temperature and relative humidity conditions was developed to study the adsorption isotherms of the desiccants. Among the three selected organic fibers, untreated jute fibers had the highest water absorption capacity. The adsorption characteristic of the jute fibers was found to be superior as compared to other two organic fibers. All the three selected organic fibers had the potential to be a good desiccant material in the solid desiccant dehumidification system.

Modeling and performance investigation on the counter-flow ultrasonic atomization liquid desiccant regenerator

As the key components in a liquid desiccant dehumidification system, liquid desiccant regenerator produces great effect on the whole system’s performance. This paper studies the counter-flow ultrasonic atomization liquid desiccant regenerator. Firstly, a mathematical model for the regenerator has been established and experimentally validated with finite difference method. Two performance indicators, i.e., the moisture removal rate and the regeneration thermal efficiency, were suggested to evaluate the performance of the counter-flow regenerator, and the influences of inlet parameters on the regeneration performance of the counter-flow regenerator have been investigated by the established model in this study. Decreasing temperature difference and increasing humidity difference between air and solution can improve regeneration performance. With the temperature of air and solution increasing, the regeneration efficiency increases, but the energy loss also increases. The regeneration efficiency decreases with the increase of solution concentration. The regeneration efficiency increases nonlinearly with the increase of gas-liquid ratio. Finally, there exists an optimum droplet size for the best regeneration performance. Compared with the parallel-flow configuration, the counter-flow has better regeneration performance, which is attributed to the more uniform distribution of moisture transfer driving force and the longer residence time of droplets in the regenerator. The study contributes to the development and the better applications of the counter-flow ultrasonic atomization liquid desiccant regenerator.