Liquid Desiccant Air Conditioning Research Articles

Performance investigation of a novel hybrid deep-dehumidification process using liquid desiccant

As one of the essential dehumidification ways, liquid desiccant air conditioning system has developed quickly in recent years, especially in the deep dehumidification field. A novel hybrid deep- dehumidification system using liquid desiccant driven by heat pump is proposed in this study to simplify the arrangement of air ducts and guarantee a competitive energy efficiency. In the proposed process, the regenerator, condensing dehumidifier and liquid dehumidifier are cascaded, and the process air flow through them sequentially. Heat pump cycle is utilized in the system, where the cooling capacity of evaporator is used to cool the liquid desiccant in dehumidifier, and the heating capacity of condenser is used for desiccant regeneration. The humidity ratio of the supplied air is as low as 2.6 g/kg. No extra regeneration air or corresponding air duct are needed in the proposed process, which obviously simplifies the layout of the system. Performance of the proposed system is then analysed by numerical results. It’s indicated 17.5~29.2% energy is saved compared with the conventional deep dehumidification process with two-stage heat pump, with the outlet humidity ratio of 2.6 g/kg.

Study on desiccant solution control strategies for efficient liquid desiccant air conditioning system control performance

Frequent change in ambient conditions requires precise control of the desiccant solution operating parameters to suit the required conditioned space load. Either of the two control strategies shall be adopted for this, which are the dehumidifier solution inlet temperature (DSIT) control strategy and the dehumidifier solution inlet mass flow rate (DSIM) control strategy. This article aims to study the efficient control strategy selection approach for a specific air conditioning application since it leads to energy savings as well as thermal comfort. Initially, the optimum desiccant mass flow rate will be found for the liquid desiccant air conditioning system at peak ambient conditions. And the effect of the control strategies on system performance has been analytically investigated at different ambient conditions. A significant reduction in the solution heat addition rate has been observed with both control strategies with the addition of an air-solution heat exchanger. It has also been seen that the required chiller load is nearly same with both control strategies but a little drop (3%–14.5% variation) was observed with the DSIT strategy at low ambient conditions. However, a considerable drop in solution pumping power due to the significant reduction in solution mass flow rate and its pressure drop with the DSIM strategy suggests that it is an energy-efficient control strategy.

透湿膜式液体デシカント空調機の開発 （第２報）試作した透湿膜式液体デシカントシステムの運転結果

透湿膜式液体デシカント空調機の開発 （第２報）試作した透湿膜式液体デシカントシステムの運転結果

Brain activity in face-name memory

Liquid desiccant air conditioning system has unique advantages and prospects in both humidity control and low-grade heat driven cooling. The liquid film wettability is one of the significant factors governing the efficiency of dehumidification/regeneration processes in the system. In this paper, the effect of the LiCl solution properties on falling-film plate wettability characteristic is studied using a VOF method. This paper analyzes the dehumidification and regeneration processes respectively and gets the recommended amount of spray, which was 0.619 m3/(m·h) for dehumidification and 0.662 m3/(m·h) for regeneration. The recommended amount of spray can not only ensure the full-film flow, but also avoid the excessive solution flow rate which may increase the possibility of droplets entrainment. Since the wettability is determined by the coupled effects of the solution density, viscosity and surface tension, the effects of dynamic properties of liquid desiccant on the wettability are studied to explain the influence of the solution temperature and concentration on the wettability. Finally, least squares method is used to fit a formula to describe the effect of solution properties on falling-film plate wettability, which can guide the control of the amount of spray in actual operation.

Development and application of a dynamic model for a solar assisted liquid desiccant air conditioning system

The investigation showed that there were few research works about the real dynamic performance of solar-assisted liquid desiccant air conditioning systems with an unsteady term in the governing equations. Regarding the unsteady properties of the components, a novel dynamic model was developed for a solar-assisted internally cooled liquid desiccant air conditioning system. In the model, the variable t (time) was involved in the governing equations of each component to take into consideration the influence of the previous condition on the following state of the system. The accuracy of the above mathematical model was verified partly with experimental results of some key components. Then the model was applied to a real solar-assisted internally cooled liquid desiccant air conditioning system for an office building with the capacity for eight people in Hong Kong. The transient performance of the system on a summer design day in Hong Kong was investigated. The results reflected the necessity of considering the influence of the former state on the following running of the system. In conclusion, the novel model could be a reliable tool to predicate the system operation and provide useful information for better control of the system.

Performance analysis of solar vacuum membrane distillation regeneration

This paper proposes a novel solar vacuum membrane distillation (VMD) regeneration for liquid desiccant air conditioning (LDAC) system, and establishes a mathematical model of the VMD regeneration process and traditional thermal (TH) regeneration process based on the heat and mass balance. The corresponding simulation model was established by using Matlab software, and the model was verified by previous experiments. The results show that: the model results are consistent with the previous experimental results, and the maximum error is less than 6%. Model simulations are carried out to study the regeneration performance under various operating parameters and climatic conditions, meanwhile the comparison the solar power consumption for regenerating one kilogram desiccant between the VMD regeneration and TH regeneration. Results demonstrate that when the air temperature is less than 32 °C, the humidity ratio is higher than 19 g/kg, and the air flow rate is less than 0.13 kg/s, the VMD regenerator solution regeneration effect is better than the conventional thermal regenerator. VMD regeneration is more suitable for solution regeneration in wet areas than TH regeneration. Furthermore, when the solution temperature is lower than 60 °C, VMD regeneration outlet concentration is higher than TH regeneration. VMD regeneration is more suitable for low-grade energy. Analyze the solar power consumption for regenerating one kilogram desiccant found that the VMD regeneration saves 10–37% energy than TH regeneration. Through economic analysis, it is found that the VMD regenerator can save 582.4 kWh of electricity per year compared to the TH regenerator. According to the cost recovery calculation formula, the cost recovery period can be calculated as 13.8 years.

Investigation of falling-film plate wettability characteristics under dehumidification and regeneration conditions using LiCl-H2O

Liquid desiccant air conditioning system has unique advantages and prospects in both humidity control and low-grade heat driven cooling. The liquid film wettability is one of the significant factors governing the efficiency of dehumidification/regeneration processes in the system. In this paper, the effect of the LiCl solution properties on falling-film plate wettability characteristic is studied using a VOF method. This paper analyzes the dehumidification and regeneration processes respectively and gets the recommended amount of spray, which was 0.619 m3/(m·h) for dehumidification and 0.662 m3/(m·h) for regeneration. The recommended amount of spray can not only ensure the full-film flow, but also avoid the excessive solution flow rate which may increase the possibility of droplets entrainment. Since the wettability is determined by the coupled effects of the solution density, viscosity and surface tension, the effects of dynamic properties of liquid desiccant on the wettability are studied to explain the influence of the solution temperature and concentration on the wettability. Finally, least squares method is used to fit a formula to describe the effect of solution properties on falling-film plate wettability, which can guide the control of the amount of spray in actual operation.

Theoretical investigation on energy storage characteristics of a solar liquid desiccant air conditioning system in Egypt

This work presents theoretical study of a standalone outdoor liquid desiccant air conditioning (LDAC) system assisted by solar energy. The liquid desiccant dehumidification cycle was integrated with a double indirect evaporative air cooler and a regenerative air heat recovery. The double indirect evaporative air cooler was used to recover heat from the return air, while air-air heat exchanger was used to recover heat from air leaving the regenerator. A water-in-glass evacuated tube solar collector was used to supply the system with solar heat. The remaining required heat is provided by natural gas fired boiler. The system was simulated by transient model under thermal energy storage, thermochemical energy storage and combined thermal and thermochemical energy storage modes.The theoretical results show that the mean daily thermal coefficient of performance (TCOP) is a strong function of the difference between absolute humidity ratio of outdoor air and saturated air. Results also show that, each tube of solar collector can deliver 30-Watt to operate the LDAC system under thermal energy storage mode for 16 h daily. While in the LDAC system under thermochemical energy storage mode, each tube would deliver 38.5-Watt for 16 h operation.It has been found that, to rise the solar fraction from 90% to 100% in the thermal energy storage mode, the required size of tubes array will be increased by 20%. Also, an analysis was carried out on the TCOP and required storage volume of the different energy storage modes. It shows that the combined energy storage method is the most energy efficient method.Finally, life cycle cost analysis (LCCA) was carried out for the LDAC system using various types of energy storage modes. The comparison indicated that 90% solar fraction LDAC system is the most economic system related to unit prices of solar collector and gas prices in Egypt. The total annual cost for the 90% solar assisted LDAC system is found to be lower than that of a vapor compression cycle (VCC) by about 50.5%. At the second order, the thermal energy storage mode by 49%. On the other side, the thermochemical energy storage mode is the least cost saving energy storage mode.

Study on model of household split air conditioning solution dehumidifier

Liquid desiccant technology in central air conditioning system is very promising, however, rare researchers proposed to carry out it on household liquid desiccant air conditioner. In this paper, a two-dimensional heat and mass transfer theoretical model of cross flow dehumidifier for household split air conditioner has been established, and it is shown that the model established has a good accuracy and the relative error is less than 3.72% by comparing the simulation results with the experimental data in the literature. Based on the model, a series of experiments are conducted and the experiment data is adopted to fit the correlation formula of NTUm and Le. The results show that the model established has a good reaction to the actual situation. Furthermore, the influence of air, solution inlet conditions on the dehumidification performance of the system has been discussed in the paper.

An Innovative Smart Liquid Desiccant Air Conditioning System for Indoor and Outdoor Cooling using Seawater Bittern

The design and construction of buildings in hot-humid climates requires high energy consumption typically for air conditioning due to higher thermal loads. In hot-humid climates, there is a rising concern on the current rate of energy consumption due to air conditioning. Considering the wider impacts of carbon emissions on our climate, and the need to reduce these emissions, effective energy efficiency solutions are necessary to achieve the overall goal of reducing carbon emissions. Conventional energy resources continue depleting and the energy demands of a growing global population continue to increase. Ventilation rates to buildings must be increased to ensure occupants healthy indoors environment. This more outdoors air gave the designers a much bigger load to be removed especially in humid climates. Outdoor cooling remains a challenge but will have lots of social and economic benefits. This paper presents the design and performance of a fully integrated liquid desiccant air conditioning system that could be used for indoor and outdoor cooling. The superefficient packed liquid desiccant evaporative air conditioning system can provide 22000 m3/h treated fresh air at supply temperature of 16°C with 50% reduction in energy consumption compared to conventional systems.

Experimental studies on the performance of dehumidifier using calcium chloride as a liquid desiccant

The performance of liquid desiccant air conditioning system (LDAS) mainly depends on dehumidifier of the system. In this study, the effects of inlet parameters on the performance of liquid desiccant dehumidifiers were studied with and without indirect evaporative cooler or intercooler (IDEC) between the dehumidifier for single stage and double stage. The results show that: i) the dehumidification effectiveness decreases by 8.6% for the all types considered here, with increase in air flow rate from 0.05 kg/s to 0.11 kg/s; ii) the dehumidifier effectiveness decreases due to decrease of moisture removal rate with increasing desiccant temperature in all stages; iii) the moisture condensation rate and dehumidifier effectiveness increases by 30% and 22.67% respectively with increase in desiccant flow rate; iv) the moisture condensation rate is increased by 13% in increasing desiccant concentration. It is concluded that double stage desiccant dehumidifier with indirect evaporative cooler performs better.

Dynamic behaviour simulation of a liquid desiccant dehumidification system

Among heat driven air conditioning systems, liquid desiccant air conditioning system can be assisted by low water temperature solar collectors. This can reduce the electricity demand of the air conditioning systems. Furthermore, these systems can provide possibility of energy storage in the form of chemical energy in the liquid desiccant solution or thermal energy in the solar collector hot water. This article simulates a liquid desiccant dehumidification cycle by two dynamic models: transient and discrete-steady state model. The transient model takes into consider the unsteady term in the governing equations for both the dehumidifier/regenerator model and also liquid-liquid heat exchanger model. While, in the other dynamic model, the steady state terms are used only. Correspondingly, a liquid desiccant dehumidification system was constructed, troubleshoot and field tested at summer conditions. According to the simulation results, the dynamic models are compatible with the variances in operating conditions and supply heating water. Also, the deviation between the humidity ratio results from simulation and field test effects on the simulated solution concentration with less than 4% deviation. The maximum deviation between the moisture removal rate results from simulation and test are −0.21 g/s in the dehumidifier and −0.5 g/s in the regenerator. Thus, the models of liquid desiccant system have reliability to predict the performance with solution storage mode.The comparison between simulation results shows that the disagreement between the two employed dynamic models is insignificant. However, the transient model reduces the computational time by 21% less than the discrete-steady state model. This can speed up the processing time.

透湿膜式液体デシカント空調機の開発 （第１報）試作した透湿膜式液体デシカント装置の吸放湿特性に関する実験結果

透湿膜式液体デシカント空調機の開発 （第１報）試作した透湿膜式液体デシカント装置の吸放湿特性に関する実験結果

Natural convection liquid desiccant loop as an auxiliary air conditioning system: investigating the operational parameters

Liquid desiccant air conditioning system with natural convection was presented previously as a new generation of AC systems. The system consists of two three-fluid energy exchangers namely absorber and regenerator in which the action of air dehumidifying and desiccant regeneration is done, respectively. The influence of working parameters on system performance including the heat source and heat sink temperature, concentration of desiccant solution fills the system initially and humidity content of inlet air to regenerator is investigated experimentally. The heat source temperatures of 50 °C and 60 °C, heat sink temperatures of 15 °C and 20 °C and desiccant concentrations of 30% and 34%, are examined here. The inlet air to regenerator has temperature of 38.5 °C and three relative humidity of 14%, 38% and 44%. In all experiments, the inlet air to absorber has temperature of 31 °C and relative humidity of 75%. By inspecting evaluation indexes of system, it is revealed that higher startup desiccant concentration solution is more beneficial for all study cases. It is also observed although the highest/lowest temperature heat source/heat sink is most suitable for best system operation, increasing the heat source temperature should be accompanied with decreasing heat sink temperature. Using drier air stream for regenerator inlet does not necessarily improve system performance; and the air stream with proper value of humidity content should be employed. Finally after running the system in its best working condition, the coefficient of performance (COP) reached 4.66 which verified to be higher than when the same air conditioning task done by a conventional vapor compression system, in which case the COP was 3.38.

Modeling and parametrical analysis on internally-heated liquid desiccant regenerator in liquid desiccant air conditioning

Abstract In this paper, the mathematical models of pre-heated and internally-heated liquid desiccant regenerator are established including parallel flow, counter flow and cross flow with two kinds of different flow directions of heated water respectively. Simulation results reflect that regeneration performances of the internally-heated are about 2–4 times that of the pre-heated in most conditions, greatly depending on the ratios of flow-rate of solution to air (ms/ma) and water to air (mw/ma) respectively. The regeneration performances, when the heated-water flows counter to solution, are superior to that of heated water parallel to solution. And the regeneration performances generally increase with increases in the NTU1 and NTU2 while a fitted curve combining NTU1 with NTU2 occurs to indicate the fastest increase in regeneration performance. Although the concentration difference increases with the ms/ma reducing, the evaporation rate gets its maximum value when the ratio of mass flow-rate of solution to air is equal to 0.2. The regeneration performances increase with increasing air temperature and decreasing its moisture content as well as linearly increase with higher inlet temperature of heated-water, but decrease with increasing solution concentration. The proposed regeneration mode and flow pattern will be helpful in the design and optimization of the regenerators.

Research on melting and solidification processes for enhanced double tubes with constant wall temperature/wall heat flux

AbstractLatent heat thermal energy storage (LHTES) improves the energy utilization efficiency between energy supply and energy demand of heating storage in buildings and liquid desiccant air conditioning systems. The present work is focused on validated numerical investigation of the thermal performances of LHTES inside enhanced double tubes. The effects of the number of fins ranging from 2 to 10 and boundary conditions of the inner tube wall on the melting and solidification processes are examined. The results indicate that number of fins and wall boundary conditions play an important role in the thermal performances of LHTES. It is noted that recirculation flow in the liquid phase change material region is formed remarkably. The enhancement ratio for constant wall temperature is more significant than that of constant wall heat flux during the melting process. However, the discrepancy of the enhancement ratio for different inner wall temperatures is limited during the solidification process.

Factor analysis and optimization of operational parameters in a liquid desiccant air-conditioning system

Liquid desiccant dehumidification systems are widely used in many industries including HVAC. Special attention has been paid to the liquid desiccant air conditioning (LDAC) system performance analyses. However, the studies, such as factor analysis and performance optimization of LDAC system, have not received enough attention. This paper focuses on the factor analysis and optimization of the operational parameters of a LDAC system that has been described in previous work. Firstly, through the process of factor analysis, the most important factors and pair-wise interactions can be selected, when Excd is used as the target variable. Secondly, the optimum values for these factors that optimize Excd are found by using the method of orthogonal design. Thirdly, a sectioned polynomial regression model is developed to describe the relationship between the Excd and independent variables inclusive of the factors in the optimal combinations and the outdoor air parameters. With the variation of outdoor air parameters, the optimal values of tgen and ma7/md could also be predicated on the regression model. The accuracy of the regression model is also analyzed. Finally, according to the same analytical method, the operational parameters are analyzed and their levels are optimized when the COP is used as the target variable.

A regulation strategy of working concentration in the dehumidifier of liquid desiccant air conditioner

This study presents a new desiccant concentration regulation strategy for the dehumidifier of a liquid desiccant air conditioner (LDAC). Starting from the mass balance principle, the proposed strategy supplies the strong solution in an intermittent manner to maintain the required working concentration, and the dehumidification rate is monitored as the feedback to improve the regulation performance. Compared with the conventional method, this new solution transfer strategy avoids the continuously solution exchanging, while allows multiple dehumidifiers to work simultaneously with only one regenerator. A mathematical model emphasizing on the concentration profiles is developed to determine the parameters of the proposed strategy. Simulations and experiments are conducted to evaluate the effectiveness of this strategy in different working conditions. The results prove that the proposed strategy can keep the working concentration at the pre-specified range with less energy consumption during the solution transfer. With this strategy, conventional LDAC can be extended to a mode with multiple dehumidifiers and one regenerator for large-scale applications in buildings.

Aqueous solution of [EMIM][OAc]: Property formulations for use in air conditioning equipment design

Liquid desiccant air conditioning (LDAC) is a promising alternative to vapor compression refrigeration based space cooling and dehumidification for the reduction of energy consumption and the improvement of indoor air quality. However, its use in the air conditioning market is still very limited due to its high installation cost, intensive carryover, and high cost of operation and maintenance associated to the corrosion of the current desiccant liquids. Substitutes for the traditional desiccant liquids with better properties and no corrosion are highly needed. Ionic liquids, which are salts comprised of organic cations and inorganic anions or organic anions, have high thermal stability, negligible or no vapor pressure, varied solubility in water, low or no corrosion to metals, and low driving temperatures to achieve dew point temperatures. All of those characteristics make them as perfect substitutes for traditional desiccant liquids. Up to date, a very few research groups investigated ILs as potential alternatives to traditional desiccant liquids for LDAC. The study in the paper aimed to identify the ionic liquids as ideal desiccant liquids to achieve better cost-effectiveness and higher system performance of LDAC. There were 13 different ionic liquids (IL) identified and screened for the most promising candidate. 1-Ethyl-3-methylimidazolium acetate, [EMIM][OAc], was selected as an ideal candidate for the capable of the most adsorption and desorption. The paper was mainly focused on the development of calculation models for the thermo-physical properties of the aqueous solution of [EMIM][OAc] to represent the property data needed for use as the desiccant liquid in sorption-based air conditioning equipment. It aims to provide convenient methods for use in the design and model of the sorption-based air conditioning. The calculation models for the following thermo-physical properties of the aqueous solution of [EMIM][OAc] were proposed and presented in the paper: the vapor pressures, specific heat capacity, density, and dynamic viscosity at various temperatures and concentrations.

Thermodynamic analysis of an innovative liquid desiccant air conditioning system to supply potable water

Liquid desiccant air conditioning systems are cost-effective, environmentally friendly and energy efficient techniques, especially in coastal areas. In the conventional liquid desiccant air conditioning system, the scavenging air is expelled into the atmosphere carrying a considerable amount of energy and water vapor. Thus, there is plenty of room to improve the system performance by recovering these losses. The proposed system consists of a conventional liquid desiccant air conditioning system plus a condenser. The aim of this study is to reduce the energy consumption by recovering the heat from the scavenging air using the condenser while also producing freshwater in addition to space cooling. Lithium chloride (LiCl) is used as the liquid desiccant for this study. The mathematical formulation for simultaneous heat and mass transfer between the condenser and the regenerator was developed to establish a comparison between the performance of the conventional and modified systems. Using the generated model, it is found that the modified system performance is 11.25% better than the conventional system and that it produces 86.4kg of freshwater per hour as a by-product under the given conditions.