Liquid Desiccant System Research Articles

Experimental study of coupled heat and mass transfer phenomena between air and desiccant in a solar assisted thermal liquid desiccant system

Liquid desiccant dehumidification is a promising energy-extensive process for air dehumidification, which can easily be driven by any waste or renewable heat sources. In the current study, a hybrid method is proposed by combining the solar evacuated tube collectors as a regeneration source to drive liquid desiccant system in a close-loop. Subsequently, an experimental setup has been fabricated to assess the performance of the overall system using a novel desiccant mixture. The overall energy balance between the ambient air and the liquid desiccant was also studied. Effects of independent parameters such as solution to airflow rate, solution concentration and temperature on the dehumidifier-regenerator performance parameters such as latent heat ratio, condensation rate, desiccant mass fraction index, evaporation rate, latent and enthalpy effectiveness were analyzed. The result of present investigation showed that high solution to airflow (S/A) ratio enhanced the dehumidification and low S/A ratio enhanced the liquid desiccant regeneration rate. The maximum latent heat ratio for the dehumidifier at the design condition was 0.92, and the thermal coefficient of performance of the system was found as 1.1. The airside pressure drop in the dehumidifier/regenerator was also estimated at different flow rates of desiccant. Further, adaptive neuro-fuzzy inference system (ANFIS) prediction models are developed to predict the system performance as a function of system-independent parameters. The model results exhibited a good agreement with the experimental outcomes. The latent heat ratio and evaporation rate are predicted within a mean absolute percentage error (MAPE) of 2.7% and 2.3%, respectively.

Optimization of operational parameters of a liquid desiccant system integrated with a heat recovery unit

The liquid desiccant system can make a contribution to a sustainable path for maintaining healthy and comfortable indoor environments. The liquid desiccant system has two major subsystems, dehumidification and regeneration. Cold energy recovery was achieved by passing the indoor exhaust air through an air hose to import and dehumidify the inlet air prior to the precooling stage/phase. Heat energy recovery was effected by a heat exchanger. The system simply recycles the heat energy without any moisture transfer in the air, preheating the inlet air to the regeneration subsystem. Furthermore, based on the dehumidification capacity, regeneration capacity and the personal comfort criteria, the optimal match of air volume was selected. The results show the condition with the dehumidification airflow of 2600 CMH and the return air ratio in the range from 75% to 80% range was suitable for personal comfort. In this case, the matching regeneration airflow was 1600 CMH without heat recovery, so that the air flow rates for dehumidification and regeneration are optimal.

Investigating Solid and Liquid Desiccant Dehumidification Options for Room Air-Conditioning and Drying Applications

This study reports on the investigation of the performance of single and two-stage liquid and solid desiccant dehumidification systems and two-stage combined liquid and solid desiccant dehumidification systems with reference to humid climates. The research focus is on a dehumidification system capacity of 25 kW designed for room air conditioning application using the thermal models reported in the literature. RD-type silica gel and LiCl are used as solid and liquid desiccant materials, respectively. In this study, the application of proposed system for deep drying application is also explored. Condensation rate and moisture removal efficiency are chosen as performance parameters for room air conditioning application, whereas air outlet temperature is chosen as performance parameter for deep drying application. Further, for a given range of operating parameters, influences of air inlet humidity ratio, flow rate, and inlet temperature on performance parameters of the systems are investigated. In humid climatic conditions, it has been observed that a two-stage liquid desiccant dehumidification system is more effective for room air conditioning application, and two-stage solid desiccant dehumidification system is more suitable for deep drying application in the temperature range of 50 to 70 °C, while single-stage solid desiccant and two-stage combined liquid and solid desiccant dehumidification systems are more effective for low temperature, i.e., 30 to 50 °C deep drying application.

Modification of analytical solutions of coupled heat and mass transfer processes in liquid desiccant dehumidifier for deep dehumidification

The dehumidifier is the key component in a liquid desiccant air-conditioning system, and analytical solutions for predicting its performance have been proposed in the literature. The existing analytical solutions cannot be directly used in deep dehumidification circumstances, owing to the significant nonlinearity of the iso-relative humidity (φ) line of humid air under deep dehumidification circumstances (managing air humidity ratio to below 7 g/kg). A concise correction method for modifying the analytical solutions for deep dehumidification is proposed in this study, wherein the nonlinearity of the iso-φ line is taken into consideration. The results reveal that the modified analytical results show good agreement with available experimental data. The average absolute deviations between the modified analytical results and experimental results are within 10%. Moreover, based on the modified analytical solutions, it is illustrated that the enthalpy and moisture efficiencies of the dehumidifier are greater than those of the regenerator under the same solution and air flow rates, NTUm, and change amount of air humidity ratio. This study is beneficial to understand efficiency difference between the conventional dehumidification and deep dehumidification, and helps to illustrate performance difference between the dehumidifier and regenerator.

Experimental investigation on structured packed bed liquid desiccant dehumidifier: An optimal mixture design of experiments strategy

Abstract Maintaining human thermal comfort and extensive power consumption of the conventional heating ventilating and air-conditioning systems drive the researchers to look for an alternative dehumidification system. Among the potential alternatives, the liquid desiccant dehumidification system has evolved as a long-term renewable and less power consuming system due to its effective controlling capability of humidity. In the present study, an experimental investigation is carried out to demonstrate the performance of lithium bromide (LiBr) and potassium formate (HCOOK) desiccant solution mixture in a spray type counterflow-packed bed adiabatic dehumidifier. Subsequently, ternary plots and 3D-contour trade-off paradigms are patterned to assess the performance parameters using an optimal mixture design methodology. Additionally, empirical correlations are developed to pattern the performance parameters (moisture removal rate (MRR), enthalpy effectiveness (ξh) and moisture effectiveness (ξm)) as a function of blend combinations. Further, the present study endeavours to explore the application of offline calibration methods based on a balanced optimum mixture design-centred desirability approach for optimizing the mixture proportion to obtain the optimal performance parameters (i.e., MRR, ξh and ξm). The wettability area is explained based on the static contact angle on a plane glass. The results showed that the contact angle was decreased from 47.3° to 44.1° for the optimum mixture of LiBr solution (37.2%-LiBr +17.8%-HCOOK), which further confirmed the higher MRR. Lastly, the tuning effect of the liquid to gas flow rate on the dehumidifier performance and economic viability of LiBr-HCOOK blends are studied.

Limiting concentration during batch electrodialysis process for concentrating high salinity solutions: A theoretical and experimental study

Electrodialysis (ED) has demonstrated favorable concentration performance and good energy-saving potential in the treatment of high salinity solutions. In this work, the study is focused on the investigation of the limiting concentration (maximum concentration) during a batch ED concentration process. A theoretical model was built to describe the coupling of salt and water transport and the limiting concentration. Effects of initial concentration, volume ratio of the diluted to concentrated solution, and current density on performance were investigated with two commonly used LiCl and NaCl solutions. The results showed that the salt concentration had a significant effect on the salt transport number and salt hydration number while the current density exhibited negligible. The limiting concentration could be increased significantly by increasing the current density. However, the trade-off between the limiting concentration and energy consumption should be balanced. Furthermore, the ideal and system concentration characteristic curves were proposed to provide theoretical guidance for the batch concentration process. According to the system curve, the three-stage continuous operation was required for NaCl solution to reach saturation at an initial mass concentration of 3.5% and a current density of 400 A/m2. The proposed methods in this study would be instructive for future design of relative systems such as the liquid desiccant regeneration system.

Design of quaternary ammonium type-ionic liquids as desiccants for an air-conditioning system

The liquid desiccant air-conditioning system allows reducing energy consumption compared to the conventional compressor-type air conditioners. In order to develop desiccant materials for air conditioners, we have investigated the dehumidification capability of quaternary ammonium Ionic Liquids (ILs) and the equilibrium water vapor pressure of aqueous solutions of these ammonium salts. Among the seven tested types of ILs, 2-hydroxy-N,N,N-trimethylethan-1-aminium dimethylphosphate ([Ch][DMPO4]) displayed the best dehumidification capability and the lowest equilibrium water vapor pressure. Furthermore, the 80% aqueous solution of [Ch][DMPO4] exhibited a less corrosive effect on four types of metals, i.e., steel (hot dip zinc-aluminum alloy plated steel), copper (C1100P), aluminum (A5052), and stainless steel (SUS: SUS304). It should be noted that this [Ch][DMPO4] is not only non-toxic but also exhibits a stable nature; the aqueous solution produced no odor after storing for over 1 year under ambient conditions.

Design of heat pump-driven liquid desiccant air conditioning systems for residential building

In this study, two workable designs of a heat-pump-driven liquid desiccant (HPLD) air-conditioning system were proposed for residential buildings. The proposed HPLD systems were designed to work as a multi-functional air-conditioning system without additional equipment which would solve the low practicality due to the large scale and complexity of the conventional liquid-desiccant-assisted air-conditioning system. The critical difference between the two systems is the sump design for containing the desiccant solutions: Case A has a single sump instantly mixing weak and strong solutions, whereas Case B has dual sumps containing weak and strong solutions separately. The thermal load of target space was estimated using TRNSYS 18, and energy performances of both HPLD systems for summer and winter operation modes were analyzed using EES by integrating several established models and theoretical analyses. Owing to decrease in solution cooling and heating loads, the results show that Case B, compared with Case A, saved 28% of energy consumption and improved mean energy-efficiency by 37.5% for summer operation mode, and saved 12% of energy consumption and improved mean energy-efficiency by 4.95% for winter operation mode. In conclusion, Case B has a better application potential for residences in terms of energy performance.

Optimal flow type in internally-cooled liquid-desiccant system driven by heat pump: Component level vs. System level

The internally-cooled liquid-desiccant (ICLD) air-conditioning system has been increasingly popular, owing to its energy-efficient performance. The air and solution can have different flow types in the ICLD system, i.e., parallel flow, cross flow, and counter flow. Counter flow is believed to be optimal among the flow types for a simple heat exchanger, owing to its competitive heat exchanger effectiveness. However, this no longer applies to the ICLD system circumstance. To reveal the reason difference, the optimal flow type in the heat-pump driven ICLD system is discussed at two levels in this study, i.e., the component (dehumidifier/regenerator) level and the system level. At the component level, the results reveal that the heat and mass transfer driving force may reverse in the counter flow dehumidifier/regenerator, which can deteriorate its performance significantly. Counter flow may lead to the worst dehumidification/regenerator performance. At the system level, the counter flow system shows a benefit in reducing heat-cold offset. The COPsys of the counter flow system is 7.2%–26.4% and 1.4%–5.3% superior to those of the parallel flow and cross flow systems, respectively. Thus, the counter flow is recommended in ICLD systems, because of its low heat-cold offset, rather than because of its competitive dehumidification/regeneration performance.

Performance investigation of a liquid desiccant regenerator with CFD technology

The regenerator is a critical component for a liquid desiccant dehumidification system. Compared with the dehumidifier, the studies of the regenerator are much fewer. Besides, most of the present models only focused on the outlet parameters while usually ignored what happened in its interior, and therefore the underlying heat and mass transfer mechanism was remained to be explored. In the paper, a numerical model was established based on Computational Fluid Dynamics (CFD) technology. It revealed that the simulation results agreed well with the experimental ones within an average deviation of 10.2%. The response time for regeneration time was significantly affected by the velocity of air and solution. A greater velocity led to a shorter response time due to an enhanced mixture in vertical direction. The regeneration performance could be improved at higher solution flow rate as a result of greater interface area ratio and faster solution surface renewal. The influence of inlet air temperature on the regeneration process was relatively smaller compared to other factors. And it was found that increasing the solution temperature could be more beneficial to regeneration performance than reducing the mass fraction of water vapor at air inlet.

On the importance of the heat and mass transfer driving force reversal and heat-cold offset in internally-cooled liquid-desiccant system

The internally-cooled liquid-desiccant (ICLD) air-conditioning system has gained more attention, owing to its energy-efficient performance. In this study, two restriction factors of the conventional heat-pump driven ICLD system are revealed: (i) heat and mass transfer driving force reversal phenomena exist in the dehumidifier (DEH) and regenerator (REG), which deteriorate the moisture efficiencies of the DEH and REG; (ii) considerable heat-cold offset occurs during solution exchanging between the DEH and REG. To avoid the driving force reversal phenomena, the system I with a pre-cooler/heater before the DEH/REG is proposed. The COPsys of the system I is 8.2%–15.0% greater than that of the conventional ICLD system, under variable conditions. To reduce the heat-cold offset to further improve the system performance, the system II is then proposed. In the proposed system II, 20% of the original DEH/REG is changed from the original internally-cooled type into the adiabatic type. The hybrid DEH and REG help to reduce the temperature difference between the outlet solutions of the DEH and REG, and thus, the heat-cold offset in the system is reduced. Consequently, COPsys of the proposed system II is 11.0%–19.5% greater than that of the conventional system, under variable conditions.

Experimental based multi-objective optimisation for structured packed bed liquid desiccant dehumidification systems

In last few decades, a substantial advancement has been observed in dehumidifier modules for their energy-conservative and environment-friendly nature; however, still uncertainty in decision-making is of concern in selection of optimum input parameters to attain an effective performance. In this perspective, an experimental study is performed in an adiabatic packed bed, counter-flow liquid desiccant dehumidifier fuelled with the blend of lithium bromide and calcium chloride solution at various operating conditions with different control settings. It is observed that air mass flux rate (Fa), solution concentration (ζ) and air specific humidity (ωa) have significant impact on condensation rate (CR), whereas, air mass flux rate (Fa) and solution temperature (Ts) are the most effective parameters for moisture effectiveness and latent heat factor, respectively. The experimentally obtained results for multiple response performance characteristics are represented based on inlet process parameters using gene expression programming (GEP). The proficiency of developed GEP meta-models is provided by statistical parameters, Taylor diagram and Theil uncertainty. Subsequently, GEP meta-model based fuzzy logic is developed for optimising dehumidifier inlet process parameters in terms of multi-responsive performance characteristics using Genetic Algorithm. The optimisation results showed that dehumidifier performance characteristics have a tendency to become optimal at Fa = 0.766 kg/m2-s, Ta = 30.745 °C, ωa = 0.023 kgwv/kgda, Fs = 1.812 kg/m2-s, Ts = 24.01 °C, and ζ = 48.1%. Finally, experiments were conducted at optimum conditions and observed proximity between the predicted and experimental outcomes, which further validates the current optimisation approach.

Experimental Research on Regeneration Characteristic of ED Regeneration for Lithium Bromide Desiccant Solution with High Concentration: Operating Condition and Electrode Solution

Electrodialysis is regarded as a novel liquid regeneration method, and the regenerated solution can satisfy the dehumidification requirements even in a hot and humid environment. LiBr solution is an important choice for a liquid desiccant air conditioning system due to its great dehumidifying ability, so it is necessary to conduct experimental exploration of the regeneration characteristics of ED regeneration for LiBr solution. In this paper, the effects of solution concentration, circulation flow rate, current and electrode solution on the performance of the electrodialysis regeneration system were studied by constructing an experimental electrodialysis regeneration system. The results show that growing the starting concentration of the LiBr solution adversely affects the regeneration characteristics of the electrodialyzer and of the air conditioning system dehumidified by the solution. Under test conditions, as the initial concentration of LiBr solution increased from 45% to 55%, the performance coefficient (COP) of the system decreased from 2.12 to 1.05. When the dehumidification requirement is met, the initial concentration of the LiBr solution should be reduced. Increasing the circulating flow rate can improve the regeneration performance of the electrodialyzer and the capability of the air conditioning system dehumidified by the solution, but excessively increasing the circulating flow rate will decrease the regeneration performance of the electrodialyzer and the performance of the air conditioning system dehumidified by the solution. Increasing the current can increase the concentration of the LiBr solution in the regeneration cells in a short time, but it will reduce the regeneration performance of the electrodialyzer and the characteristic of the air conditioning system dehumidified by the solution. The current needs to be minimized when meeting regeneration requirements. With the growth in the flow rate of the electrode solution, the regeneration performance of the electrodialyser decreases continuously.

Performance characterization of a novel membrane-based liquid desiccant air conditioning system

Abstract Membrane-based liquid desiccant air conditioning (MLAC) system is one of the energy-efficient alternatives to the conventional vapor compression based air conditioning systems. However, due to inherent resistance for heat and mass transfer in the membrane and heat of absorption generated during the mass transfer, the desired room temperature and specific humidity are not achieved in the MLAC system. Present study proposes a novel MLAC system with an additional air cooling heat exchanger for the efficient control of both temperature and specific humidity of the supply air. The influences of different liquid desiccants, ambient conditions, and design and operating parameters are studied. The performance of MLAC system is presented in terms of room temperature and specific humidity, latent effectiveness of dehumidifier and regenerator, coefficient of performance and exergy efficiency. The results of the present study prove that the proposed MLAC system is capable of achieving better indoor conditions than the conventional MLAC system. The effects of the mass flow rate of desiccant and the effectiveness of the desiccant to desiccant heat exchanger on the performance of the proposed MLAC system are found to be significant.

Review on the Influence of Various Types Liquid Desiccants on the Performance of Dehumidification System

Air conditioning is one of the large energy consuming sectors, as it comforts the human in closed environment. Especially, in coastal regions where both temperature and humidity are high, it demands for air conditioning. Usually, in such cases, cooling and dehumidification of outside air is done by cooling coils. Such cooling and dehumidification coils requires relatively large amount of energy. In the era of energy saving technologies, Liquid desiccant system (LDS) has proved to be one of the best alternatives for controlling the humidity, economically and efficiently. Different liquid desiccants are available for dehumidification purpose with their own advantages and disadvantages. This paper summarizes the different liquid desiccants used by researchers in their study and the performances analysis of dehumidification process. An attempt is also made to summarize the characteristics of the different hybrid or composite desiccants developed by researchers in order to improve the desiccant properties for the better dehumidification process.

Energy performance analysis on segmented liquid desiccant air-conditioning system for bus spray-paint booths

A paint booth is the dominant energy consumer in a manufacturing plant. Large amount of conditioned air with accurate temperature and humidity is required in the spray-paint booth to guarantee constant product quality, which implies huge energy consumption for air-conditioning. However, there has been limited research on energy-saving design for air-conditioning systems applied in the spray-paint booth. In this study, a novel segmented liquid desiccant air-conditioning system is proposed for energy conservation in the spray-paint booths of a bus manufacturing plant. A mathematical model is built to simulate the performance of the proposed system during the cooling season. The advantages of the proposed system can be summarized in terms of the following two aspects: first, the segmented and cascaded configuration of the proposed system helps to reduce the cooling load from 2844.4 to 1078.0 kW under basic conditions when compared with the separated configuration; second, the adoption of the liquid desiccant dehumidifier helps to avoid reheating, and a reheating capacity of 630.8 kW in the current operating system could be saved by operating the proposed system under basic conditions. When operating the proposed system, a yearly primary energy saving rate of 40.7% can be achieved, while a yearly running cost saving rate of 35.6% can be achieved in this plant as compared to the current operating system. This study would provide valuable reference for energy-efficient air-conditioning system design for applications in spray-paint booths.

Numerical investigation of performance trade-off characteristics of a packed bed dehumidifier using aqueous blends of lithium chloride and calcium chloride

Current research in dehumidification systems is mainly focused on thermally driven liquid desiccant systems. However, the selection of proper desiccant solution has a greater significance on the performance of a dehumidifier. In this perspective, current work is presented with governing equations of energy, mass and species balance to study the performance of a dehumidifier. The governing equations are solved using finite difference method coupled with Jacobi’s model for an air–desiccant contact system of a cross-flow packed bed adiabatic dehumidifier operating with different desiccant blends of lithium chloride (LiCl) and calcium chloride (CaCl2) solution. The effect of variation in blend proportion on outlet parameters of a packed bed dehumidifier is patterned under various desiccant to air (L/G) mass flow rate ratios. It is observed that the addition of CaCl2 desiccant solution into LiCl changes the nature of thermo-physical properties of blends‚ which are predicted using non-random two liquid (NRTL) equation. Further, a trade-off analysis among the performance parameters is presented. It demonstrates that dehumidifier performance paradox such as enthalpy effectiveness (ɛh), moisture removal rate (MRR) and moisture effectiveness (ɛm) have a tendency to be optimal for 35% LiCl with 5% CaCl2 blend (BL1) among all blends at L/G ratio of 2. The variation of thermodynamic properties (air temperature, humidity, desiccant temperature and desiccant concentration) inside the dehumidifier module is visualized in terms of contour plots. Furthermore, the irreversibility in heat and mass transfer operation for the optimal blend is investigated in terms of physical and chemical exergy destruction paradigm. The result of such investigation elucidated that the maximum chemical exergy destruction is 0.88 kW, whereas the physical exergy destruction is 0.06 kW.

Optimization of solution concentration in liquid desiccant air-conditioning system driven by heat pump

The heat pump driven liquid-desiccant air-conditioning (HPLDAC) system becomes increasingly popular in buildings, because of its compact size and energy-efficient performance. Although numerous efforts have been made in previous studies to save energy in the HPLDAC system, it has not been fully revealed from the perspective of desiccant solution concentration. In this study, two crucial issues concerning about the solution concentration are discussed for performance optimization of the HPLDAC system: (i) for only dehumidification, the optimal solution concentration for achieving the minimum energy consumption of the compressor; (ii) for both cooling and dehumidification, an energy-efficient strategy for realizing a required solution concentration. First, the simulation results reveal that the optimal desiccant solution concentration occurs when all of the condensing heat is used to heat the desiccant solution, with no condensing heat exhausted to outdoors. Second, two strategies for achieving the required solution concentration are compared, i.e., replenishing water to the solution (strategy I) and adding an auxiliary condenser to exhaust the redundant condensing heat to outdoors (strategy II). By model simulation, it is revealed that COP of the system in strategy II is 23.5% higher than that in strategy I under basic conditions. During the entire cooling season in Beijing, the running cost of the system in strategy II is 19.3% lower than that in strategy I.

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.

Parametric and Performance Investigations on Novel Multipurpose Liquid Desiccant Drying/Desalination System

In humid climates, due to high relative humidity present in the ambient air, traditional air drying system is becoming inefficient for drying the agricultural/food products. On the other hand, there is a scarcity of useful drinking water in many parts of the world. In order to meet the aforementioned requirements, a novel multipurpose liquid desiccant drying/desalination system has been proposed in the present study by replacing water as a working fluid instead of air in the liquid desiccant regenerator and by adding an energy efficient heat exchanger to the conventional liquid desiccant air conditioning system. A thermal model has been developed for assessing the performance of the novel system and for comparing the performance of the novel system with conventional system. Using the developed model, parametric studies on novel and conventional systems are carried out by choosing the system efficiency as a performance criterion and Lithium chloride as a liquid desiccant. From the parametric studies and performance comparison analysis, it is observed that novel system efficiency is 20.8% higher than the conventional system and the amount of pure water extracted from the novel system is observed to be 4.3 l/min as a byproduct.