Liquid Desiccant Cooling System Research Articles

Investigation of a Wheel Liquid Desiccant Cooling System Performance via ANSYS CFX

A liquid desiccant cooling system is an alternative air-conditioning system with significant energy-saving potential. This system is commonly used in the industrial sector, such as in dehumidifiers. The key component of the liquid desiccant cooling system is its dehumidification performance. Therefore, a study on a liquid desiccant dehumidifier has been conducted to predict its performance using computational fluid dynamics analysis. In these simulations, the effects of the temperature of the liquid desiccant and the velocity of air flow on the absorption process between the liquid desiccant and air were studied. The same applies to the velocity profile, which helps determine the airflow pattern within the dehumidifier. The model was constructed using ANSYS FLUENT and Autodesk Inventor. The model is the rotary desiccant wheel dehumidifier which was used to dehumidified the air.. ANSYS CFX was used for simulating the velocity profile to achieve the airflow pattern in the dehumidifier. The volume of fluid was selected as the multiphase method for the second simulation process. The temperature and mass fraction of water were monitored within the air during the counterflow simulation between the liquid desiccant and the air. The liquid desiccant chosen for this study was LiCl. The simulations were conducted at specific air velocities, and the behavior within the dehumidifier was observed. The analysis results revealed that the air entering the rotary desiccant wheel dehumidifier experienced a drop in velocity after passing through the desiccant wheel in the middle. Additionally, turbulence occurred after the air passed through the wheel and the dehumidifier’s wall. Based on these findings, the design of an optimal dehumidifier and the selection of an appropriate air velocity for cooling can be carried out.

Optimization of the Liquid Desiccant Cooling Systems in Hot and Humid Areas

Air-conditioning systems in hot and humid regions account for over 50% of total energy usage. Integrating an indirect evaporative cooling (IEC) and a liquid desiccant dehumidifier (LDD) as the liquid desiccant cooling system (LDCS) presents an energy-saving and emission-reducing solution to replace traditional mechanical vapor compression refrigeration (MVCR) systems. This integration overcomes the regional limitations of IEC in hot and humid areas. The newly developed LDCS uses exhaust air as the working air source and solar energy as the heat source for desiccant solution regeneration. This study aims to develop an empirical model for the outlet parameters of the LDCS, propose an optimization strategy for its operating parameters, and assess the potential and energy performance through parameter analysis and multifactor optimization. By conducting sensitivity analysis and optimizing six critical parameters based on a response surface model (RSM), the system outlet temperature, relative humidity, and coefficient of performance (COP) are improved as the optimization objectives. The regional capability is demonstrated in three selected hot and humid cities. The results indicate that the LDCS can significantly increase the COP by 57.3%. Additionally, it can meet the dehumidification demand when operating with 25% of the air extracted in the RIEC during months with high humidity and temperature. This study will facilitate the application of IEC and LDD technologies, guide the design and operation scheme of the system, and promote energy-saving and emission-reducing solutions in hot and humid regions.

Research on solar-driven interfacial evaporation regeneration performance of different solutions for liquid desiccant cooling system

Liquid desiccant cooling systems (LDCS) are promising air-conditioning systems for energy conservation in buildings. However, their development is limited by significant energy waste and low regeneration efficiency. To improve, a solar-driven interfacial evaporation (SDIE) regeneration method is proposed: adopting photothermal materials to locate heat at the regeneration interface can prevent heat loss into the bulk solution and enhance efficiency. The thermal efficiency of the new regeneration method is about 2.5 times that of the conventional regeneration method. The type of desiccant greatly influences the performance of SDIE, but its impact has not been fully studied yet. To select the best desiccant for optimization, different solutions have been investigated with theoretical analysis and experiments. Considering both regeneration performance and economic benefits, the influence of some key parameters has been revealed. The results show that LiBr has the best regeneration performance, but MgCl2 and CaCl2 are more economical. Mixed desiccant could be a better substitute, showing a good balance between performance and cost. It was found that mixing CaCl2 and MgCl2 solutions in an appropriate proportion is a very cost-effective selection. These advances could make LDCS based on SDIE a potential competitor for the future air-conditioning system.

Solar interfacial regeneration performance of different solutions for liquid desiccant cooling system

A liquid desiccant cooling system (LDCS) is a promising energy-saving air-conditioning system with the advantages of being driven by low-grade heat and excellent humidity control ability. However, the problems of high energy consumption and low energy utilization efficiency of its regenerator limit its further development. To improve, a novel solar interfacial regeneration (SIR) method is proposed. The method can convert solar energy into heat and locate the heat at the evaporation interface so that the regenerator has high energy utilization efficiency. Experimental studies were carried out on different liquid desiccants. The results show that the thermal regeneration efficiency of this method is 2.6 to 2.9 times that of the conventional thermal regeneration method. LiBr solution has the best regeneration performance, but MgCl2 and CaCl2 are cheaper. Considering energy utilization efficiency and economic cost, the mixed desiccant may be a better choice. These advances could make SIR-based LDCS a potential contender for future air-conditioning systems.

Energy and Thermal Comfort Analysis of Optimised Hollow Fibre Liquid Desiccant Cooling and Dehumidification Systems in Mediterranean Regions: Modelling, Validation and Case Study

Energy and Thermal Comfort Analysis of Optimised Hollow Fibre Liquid Desiccant Cooling and Dehumidification Systems in Mediterranean Regions: Modelling, Validation and Case Study

Performance investigation of the wood-based heat localization regenerator in liquid desiccant cooling system

An energy conservative air-conditioning system is critical to promoting energy conservation and emission reduction. One promising alternative is liquid desiccant cooling system (LDCS), which can be driven by low-grade heat and has superior dehumidification performance. However, energy waste from traditional regeneration process restricts its application from widespread use. To overcome this disadvantage, a wood-based heat localization regenerator is presented: it localizes the thermal energy on the regeneration surface avoiding the unnecessary heating of bulk solution. The real performance of this new regenerator has been investigated theoretically and experimentally in this paper. The regeneration process of desiccant solution has been analyzed and a regeneration performance prediction model has been developed. Some performance indexes have been derived from experimental data. A regeneration performance comparison has been conducted between the heat localization method and the traditional heating method. The results show that the new method improves regeneration thermal efficiency by above 50% and the maximum value can achieve 43.6%. Besides higher performance, the reduction of energy consumption, initial cost and complexity has achieved with the heat localization regenerator. The overall improvement makes LDCS such a competitive alternative to the current air-conditioning system.

Direct contact membrane distillation for liquid desiccant regeneration and fresh water production: Experimental investigation, response surface modeling and optimization

Liquid desiccant cooling (LDC) systems are being widely considered as a promising alternative for energy savings and the regeneration is one of the most significant processes of the LDC systems. This paper presents a direct contact membrane distillation (DCMD) regenerator for liquid desiccants regeneration of LDC systems and fresh water production simultaneously. The novelty of this study is to use the response surface method to establish models to predict the DCMD regeneration performance with highly concentrated lithium chloride solutions based on the experimental investigation. The interactive effects of the operating parameters, i.e. initial feed concentration, feed inlet temperature, distillate inlet temperature, and feed and distillate flow rates were studied and the optimal conditions of the DCMD regenerator to maximize the feed concentration increase and transmembrane water flux were identified. The experimental results showed that stable regeneration performance and water production with the lithium chloride salt rejection rate over 99.99% by the membrane were achieved using the DCMD regenerator. The temperature difference between the feed and distillate sides should be controlled higher than a threshold to ensure continuous liquid desiccant regeneration. Initial feed concentration and flow rate exhibited a significant interaction in improving the DCMD regeneration performance. An optimal flow rate for better regeneration performance can be observed when increasing the initial feed concentration. The optimization results showed that the feed concentration increase and transmembrane water flux can be improved by 39.7% and 41.6% respectively under the optimal conditions as compared to the best case observed in the response surface method designed experiments.

A review of heat and mass transfer mechanisms of dehumidifiers and regenerators for liquid desiccant cooling systems

Liquid desiccant cooling (LDC) has attracted increasing attention as an alternative to conventional vapor compression systems, to address the challenges of increasing electricity consumption and carbon emissions, and ozone depletion. The major advantages of LDC systems are that they can be driven by low-grade thermal energy such as solar energy and waste heat and can deal with latent load effectively with relatively low energy consumption. This paper provides an overview of the mechanisms governing heat and mass transfer (HMT) of direct-contact and indirect-contact dehumidifiers and regenerators (DRs) used in LDC systems. The results from this review showed that the HMT mechanism of direct-contact DRs using packing materials and fins with complex structures has not been extensively investigated and still remains a research gap to be filled. More in-depth investigations based on meso-scale and micro-scale are needed to investigate the HMT process through the membranes of indirect-contact DRs. The findings discovered can be used to guide future research in the development and optimization of LDC systems.

Feasibility of Liquid Desiccant Cooling (LDC) system under Mediterranean climate

Contrary to the conventional air conditioning systems, the liquid desiccant cooling (LDC) systems are considered efficient systems to control the indoor air conditions. In addition, the LDC technologies are more adequate for the hot and humid climates. In this paper, we present an analytical investigation at assessing the feasibility of a LDC technology under Mediterranean climate. The mathematical equations including the sensible and latent heat transfer equations in both air stream and desiccant solution are presented. The impacts of climatic and operating parameters on the supplied air qualities, moisture removal rate (MRR) and sensible heat ratio (SHR) are evaluated. As a consequence, this study provides a solution to investigate the feasibility of this type of air conditioning technologies under hot and humid climate.

Thermal properties study and performance investigation of potassium formate solution in a falling film dehumidifier/regenerator

Compared with the extensive attention drawn by salt solutions, the promising potassium formate (KCOOH) solution which has smaller causticity and lower price receives little attention in the field of liquid desiccant cooling system (LDCS). Consequently, the present paper studied both the thermal properties and mass transfer performance of KCOOH solution for its practical engineering application. The vapor pressure and density of KCOOH solution under different concentrations and temperatures were measured and then fitted in the form of polynomials with the mean absolute relative deviation (MARD) of 0.06% and 0.551% respectively. Referring to the data in the existing literature, the heat capacity of KCOOH solution was also fitted with the MARD of 0.15%. Mathematical models of dehumidification/regeneration with the film shrinkage on the plate dehumidifier/regenerator were developed and validated by the experimental data. Results showed that the wetting ratio predicted by the model matched well with the measured one with the relative difference of 3.4%. Besides, the dehumidification/regeneration performance obtained by the model in terms of absolute moisture change was also in line with the experimental ones with the MARD of 6.05%/4.2%. In summary, the correlations of KCOOH solution thermal properties, mathematical model, and experimental results were valuable for the engineering application of KCOOH solution in LDCS.

Development and investigation on the dehumidification and corrosion resistance performance of a new mixed liquid desiccant

The liquid desiccant cooling system (LDCS) is regarded as a promising alternative for the traditional vapor compression cooling system due to its energy saving potential and accurate control ability of both temperature and humidity. However, metal corrosion of mass exchanger plate caused by the liquid desiccant greatly restricts the application of LDCS. The present study developed a new kind of mixed liquid desiccant to reduce the corrodibility, and the corrodibility of new mixed solution was examined as well as the dehumidification performance. The vapor pressure of the mixed solution was measured by static method firstly, and the corrosion characteristics were tested by electrochemical method and compared with the ones of normal lithium chloride (LiCl) solution. Comparative investigations were also conducted to study the dehumidification performance under various operating conditions. The experimental results indicate that the corrodibility of new mixed solution is far smaller than that of normal LiCl solution, and the dehumidification performance in terms of absolute moisture change and effectiveness has a significant improvement of 8.7–14.3% and 5.8–15.2% for the newly mixed solution compared with LiCl solution because of the excellent wettability of mixed solution on dehumidifier. The wetting area increases from 0.172 m2 for LiCl solution to 0.184 m2 for mixed solution. In addition, a correlation to predict the dehumidification effectiveness of the mixed solution was also proposed with the average mean absolute relative deviation of 4.51%. The newly developed mixed liquid desiccant is a promising alternative for the commercial application of LDCS with small corrodibility and better dehumidification performance.

Investigation on the Regeneration and Corrosion Characteristics of an Anodized Aluminum Plate Regenerator

The traditional vapor compression cooling system (VCS) has been criticized for its heavy reliance on electricity consumption and limited control ability of humidity. Compared with the VCS, the liquid desiccant cooling system (LDCS) is more efficient by handling the sensible and latent load separately. The present study investigated the regeneration and corrosion characteristics of an anodized aluminum plate regenerator for the first time. Comparative tests were conducted to present the regeneration and corrosion characteristics of the normal and anodized regenerator. The influence of internal heating on the regeneration performance was also identified. The results showed that the corrosion rate was reduced from 0.0005218 mm/year for normal aluminum to 0.000011 m/year for anodized one under the same operating conditions. However, pitting corrosion was observed at operating conditions with high solution temperature, as the anodized layer of anodized aluminum was damaged due to the high temperature. Compared with the normal aluminum regenerator, the anodized one had an average enhancement of 24% and 23.7% in terms of regeneration rate and effectiveness, respectively. It was attributed to the increment of surface energy from 26.4 mN/m for normal aluminum to 47.6 mN/m for anodized plate. Besides, it was found that, compared with adiabatic one, the internally-heated regenerator improved the regeneration rate by 6.0%~38% and the regeneration effectiveness by 6.3%~32% depending on the operating conditions. The advantage of present study is that it can guide the design of anodized aluminum regenerator for LDCS.

Enhancing the dehumidification performance of LiCl solution with surfactant PVP-K30

Adding additives into liquid desiccant cooling system (LDCS) can enhance its mass transfer performance. However, the additives proposed by previous researchers are volatile and odorous which is a great threat to the indoor air quality. Therefore, this study introduced a new kind of non-volatile, odorless and nontoxic additive, i.e. polyvinyl pyrrolidone (PVP-K30), into LDCS for better dehumidification performance. Comparative studies were carried out to investigate the dehumidification performance of LiCl solution with and without the additive PVP-K30. The mass fraction PVP-K30 was chosen to be 0.4%. The results show that the dehumidification rate and dehumidification effectiveness have an average relative increment of 22.7% and 19.9% respectively with the addition of surfactant. The enhancement is attributed to the reduction of surface tension of liquid desiccant which is demonstrated by the decrement of contact angle from 58.5° to 28.0°. Correspondingly, the wetting ratio of desiccant falling film on plate has a relative increment of 16.9% from 82.7% to 96.7%, and the film thickness of falling film decreases from 0.681 mm to 0.583 mm. To sum up, the adding of additive PVP-K30 into LDCS provides an effective and practical way to enhance the vapor absorption performance in an absorber and so does the system efficiency.

Experimental investigations on heat and mass transfer performances of a liquid desiccant cooling and dehumidification system

The last decades have witnessed the growth interest in Liquid Desiccant Dehumidification Systems (LDDS). In the conventional LDDS, the high dilution rate of desiccant solution in dehumidifier leads to a high desiccant regeneration frequency, which consequently results in more thermal energy consumed by desiccant regeneration system. Therefore, a more energy efficient Liquid Desiccant Cooling and Dehumidification (LDCD) system is developed in this study, which mainly composes of a cooling coil and dehumidifier. A simple static model is proposed to predict the performances of heat and mass transfer process in this system. The thermal efficiency, moisture effectiveness and desiccant dilution rate are utilized as the performance indicators. The influences of several relevant parameters on the cooling and dehumidification performances of LDCD system are investigated. The model predictions are compared with the experimental data, and the results show that the model predictions are well in line with the experimental data with the maximum errors less than 10%. Moreover, the feasibility of LDCD system in reducing the dilution rate of desiccant solution and the system energy consumption is validated. The results indicate that the dilution rate of desiccant solution and energy consumption of the LDCD system are reduced by 39.64% and 22.3% over the conventional LDDS, respectively.

Investigation on the regeneration performance of liquid desiccant by adding surfactant PVP-K30

The liquid desiccant cooling system (LDCS) is a promising alternative for the conventional vapor compression system due to its high energy efficiency. To enhance the mass transfer between the regeneration air and liquid desiccant in the regenerator, the paper firstly introduced a kind of surfactant called polyvinyl pyrrolidone (PVP-K30) which was added into the LiCl solution for better desiccant regeneration performance. The falling film characteristics and regeneration performance were investigated and compared with and without surfactant. The results indicated that the contact angle of desiccant solution on plate decreased from 58.5° to 28.0° by adding surfactant with 0.4% of concentration. The average wetting area increased from 0.174 m2 to 0.209 m2 with a relative increment of 20.1% with the addition of surfactant. Correspondingly, the film thickness had a reduction of 0.103 mm from 0.696 mm to 0.593 mm averagely. The regeneration rate had an average enhancement of 26.3% under the same working conditions, resulting from the decrement of solution contact angle with the addition of the surfactant. Finally, correlations to predict the mass transfer coefficient were proposed with and without surfactant and the mean absolute relative deviation between the results of correlations and the experiments were kept within 8%. The results and findings of present paper could be applied to guide the design of compact regenerator for higher regeneration performance.

Development and experimental study of a novel plate dehumidifier made of anodized aluminum

The falling film dehumidifier is a key component in the liquid desiccant cooling system (LDCS). However, the most commonly used metals, such as steel and aluminum, can hardly resist the erosion of liquid desiccant. It greatly limits the fabrication of compact dehumidifiers and hinders the promotion of LDCS. The present study introduced a novel falling film dehumidifier which was made by the metal of anodized aluminum. Experiments were carried out to compare the dehumidification performance between ordinary aluminum dehumidifier and anodized one. The influences of air temperature, mass flow rate, inlet humidity and solution temperature, mass flow rate, temperature on dehumidification performance were identified. The results showed that the anodized aluminum could alleviate the erosion significantly. In addition, with the surface treatment by anodizing, the contact angle of lithium chloride solution decreased from 85.2° on an ordinary aluminum plate to 43.1° on an anodized one. Accordingly, the wetting area on the plate dehumidifier increased from 0.143 m2 to 0.178 m2 with a 24.5% increment at certain operating conditions. Both the absolute moisture removal and dehumidification effectiveness increased in various degrees for anodized dehumidifier compared with the ordinary type. The relative increments could reach up to 50.6% and 36.7% under certain conditions respectively. The newly introduced anodized aluminum dehumidifiers can not only alleviate the plate corrosion but also improve the dehumidification capability due to smaller surface contact angles, which can be promisingly applied in LDCS.

Experimental investigations on a solar assisted liquid desiccant cooling system with indirect contact dehumidifier

Challenges on energy and environmental fronts have initiated momentum in increasing the role of renewable energy in air-conditioning industry. Liquid desiccant cooling systems (LDCS), emerging as promising alternative to conventional vapor compression systems, can run on low grade heat which can easily be drawn from solar energy. In the present study a small capacity liquid desiccant evaporative cooling system for small office application is developed. The system is a dedicated outdoor air system (DOAS). Lithium chloride solution is used as a liquid desiccant. The system consists of a dehumidifier, a regenerator, a regenerative evaporative cooler, heat exchangers (solution-solution, air-water, and solution-water) and non-concentrating solar collectors. The dehumidifier in the system is an indirect contact heat and mass exchanger which eliminates the carryover of desiccant. The major energy consumption in the LDCS is for the regeneration process which is tapped from solar energy. The performance of the overall system is presented in terms of its dehumidification effectiveness, moisture removal rate, cooling capacity and thermal COP.

Photovoltaic capacitive deionization regeneration method for liquid desiccant cooling system

Liquid desiccant cooling system (LDCS) is an ideal choice as a green air-conditioning system. However, its performance is not good enough because a lot of energy is wasted in the conventional thermal regeneration process. Moreover, the system is unstable when the environmental humidity is high. To improve, a capacitive deionization regeneration method is proposed: Driven by solar photovoltaic generator, strong desiccant are acquired in an electric field with capacitive deionization units. It avoids the energy waste in the conventional pattern and has a better performance. Theoretical and experimental researches are presented. Mass and energy models have been developed and some important parameters have been investigated. Performance comparison has been made between the new method and solar thermal regeneration method. Preliminary tests have been made on the regeneration process. Through analysis, it exposes the influences of the solution concentration, voltage and energy recovery ratio. The highest COP can attain 6 under certain working conditions. The experimental results show the actual performance is higher in the lower concentration range. Compared to the solar thermal method, the new method could have better performance and is potential for application.

Simulation of potential standalone liquid desiccant cooling cycles

LDCS (Liquid desiccant cooling systems), capable of achieving dehumidification and cooling with low-grade heat input, can be effectively used for treating fresh air in hot and humid regions. These can also be operated using non-concentrating solar collectors. The present study is concerned with the evaluation of various potential liquid desiccant cycles for tropical climatic conditions. Six potential standalone liquid desiccant cycles are identified and analyzed to select the best configuration for achieving thermal comfort. A computer simulation model is developed in EES (Equation Solver) software platform to evaluate the performance of all the cycles at various operating conditions. Aqueous solution of LiCl (lithium chloride) is used as desiccant. Mass and energy balance equations of all the components along with their effectiveness and LiCl property correlation equations are solved simultaneously for given ambient conditions. As the desiccant circuit is a closed loop, no assumptions are made about its concentration and temperature in the algorithm. Supply air conditions, cooling capacity, COP (capacity and coefficient of performance) and CR (circulation rate) per unit cooling capacity and hot water temperature requirement are used as a measure for analyzing the performance of the different cycles. The effect of hot water temperature on the performance of the cycles is evaluated at ARI conditions. The performances of the cycles are also evaluated for cities selected from each of the climatic zone of India that represent typical tropical climates. Although all the cycles are feasible at ARI and hot and dry conditions, only two cycles can achieve the selected indoor conditions in the peak humid conditions. The results would be useful for selecting suitable liquid desiccant cycle for a given climate.

Experimental Study on dehumidification/regeneration of Liquid Desiccant: LiBr Solution

The growing demand for air conditioning has caused a significant increase in demand for energy resources. The traditional commercial, non natural working fluids, like CFC, HCFC and HFC result in both ozone depletion and global warming emission of CO2. The use of hygroscopic salts in direct contact with moist air provides an attractive alternative to conventional cooling systems. The liquid desiccant can substitute the dangerous fluids. The main operations in Liquid Desiccant Cooling System (LDCS) are dehumidification and regeneration. This paper presents an experimental study of dehumidification/regeneration processes using LiBr as liquid desiccant in direct contact with the air at different operating conditions. An analysis of the mass transfer is made in order to prove is this material is the best liquid desiccant for LDCS. Experimental results showed the effect of air conditions on mass transfer. It can be seen that LiBr is able to absorb moisture and can be regenerated at low temperature.