Study of an aqueous lithium chloride desiccant system: air dehumidification and desiccant regeneration

Desiccant systems have been proposed as energy saving alternatives to vapor compression air conditioning for handling the latent load. Use of liquid desiccants offers several design and performance advantages over solid desiccants, especially when solar energy is used for regeneration. For liquid–gas contact, packed towers with low pressure drop provide good heat and mass transfer characteristics for compact designs. This paper presents the results from a study of the performance of a packed tower absorber and regenerator for an aqueous lithium chloride desiccant dehumidification system. The rates of dehumidification and regeneration, as well as the effectiveness of the dehumidification and regeneration processes were assessed under the effects of variables such as air and desiccant flow rates, air temperature and humidity, and desiccant temperature and concentration. A variation of the Öberg and Goswami mathematical model was used to predict the experimental findings giving satisfactory results.     

The minimum work required for air conditioning process

This paper presents a theoretical analysis based on the second law of thermodynamics to estimate the minimum work required for the air conditioning process. The air conditioning process for hot and humid climates involves reducing air temperature and humidity. In the present analysis the inlet state is the state of the environment which has also been chosen as the dead state. The final state is the human thermal comfort fixed at 20 °C dry bulb temperature and 60% relative humidity. The general air conditioning process is represented by an equivalent path consisting of an isothermal dehumidification followed by a sensible cooling. An exergy analysis is performed on each process separately. Dehumidification is analyzed as a separation process of an ideal mixture of air and water vapor. The variations of the minimum work required for the air conditioning process with the ambient conditions is estimated and the ratio of the work needed for dehumidification to the total work needed to perform the entire process is presented. The effect of small variations in the final conditions on the minimum required work is evaluated. Tolerating a warmer or more humid final condition can be an easy solution to reduce the energy consumptions during critical load periods.     

A review on the application of horizontal heat pipe heat exchangers in air conditioning systems in the tropics

A literature review on the application of horizontal heat pipe heat exchangers for air conditioning in tropical climates was conducted. This paper focused on the energy saving and dehumidification enhancement aspects of horizontal heat pipe heat exchangers. The related papers were grouped into three main categories and a summary of experimental and theoretical studies was made. It was revealed that although there are a number of valuable researches on the impact of heat pipe heat exchangers on the energy consumption and dehumidification enhancement of air conditioning systems in the tropics, but only limited research work on the application of horizontal configuration heat pipe heat exchangers in air conditioning systems has been carried out in these regions. Therefore, it needs more research to deepen the understanding of the benefits of this heat recovery device in the air conditioning systems. On the basis of results obtained from the reviewed research studies, the application of horizontal heat pipe heat exchangers in terms of energy saving, dehumidification enhancement and condensate drainage is recommended for the tropics.     

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

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

蓄能型液体除湿蒸发冷却系统中除湿性能的实验研究

提出了一种新型空调系统——液体除湿冷却空调系统的设计方案并搭建一功率为3kW的实验台，考虑到除湿过程和再生过程是该系统性能优良的决定性环节，设计加工了水冷型波纹板降膜式结构的除湿器和以丝网填料作为内部填料的再生器。在此实验装置上对系统的除湿过程以及其蓄能能力特性进行了实验研究，得出影响该系统除湿能力、蓄能能力等方面的主要因素，为系统的优化设计和运行提供依据。     

Introduction of a new definition for effectiveness of desiccant wheels

In common air conditioning methods, both sensible and latent loads are handled by cooling systems. Utilizing desiccant, individual cooling and dehumidification processes are possible. In this paper, desiccant wheel operation has been investigated by experimental study. Experimental conditions include different climates (hot dry and hot humid) at different operating parameters (regeneration temperature and wheel speed). Due to the temperature and humidity measurements of inlet and outlet streams, the desiccant wheel efficiency in each definition was calculated. All experiments show that enthalpy of the outlet process air is notably higher than that of the inlet air. This event leads to a novel efficiency definition which presents the deviation of the outlet process air enthalpy from the inlet air enthalpy. By increase in the dehumidification efficiency, the adiabatic efficiency decreases, whereas it increases by the regeneration efficiency. Hence in some situations the adiabatic efficiency will have an optimum value. According to the adiabatic efficiency concept, it seems to be related to the coefficient of performance of the desiccant cooling systems.     

Performance evaluation of a solar-powered flat-plate multichannel liquid desiccant air conditioning system

There has been a limited application of liquid desiccant (LD) dehumidification systems in space air conditioning until now. The key elements responsible for this restricted implementation are leakage of desiccant solution, corrosion of components, and solution carryover along with the processed air to the space to be conditioned. To remove these problems, an evacuated tube solar heat collector-driven multichannel liquid desiccant air conditioning system has been proposed and experimentally investigated. In this study, dehumidification and regeneration rate, their effectiveness, cooling effect of the dehumidifier, and indirect evaporative cooling unit have been analyzed. The results obtained indicate that the process air has been dehumidified and cooled by 6.32 g kg⁻¹ and 5.26°C, respectively. The regeneration rate and effectiveness have been obtained to be 0.26 g s⁻¹ and 0.31, respectively. In terms of the cooling effect, the system output of 0.703 and 0.130 kW has been obtained from the dehumidifier and indirect evaporative cooling unit of the system, respectively. The proposed system validates the possibility of the novel solar-powered liquid desiccant air conditioning system concept and provides growth and development of the LD air conditioning technology for space air conditioning.     

Technical development of rotary desiccant dehumidification and air conditioning: A review

Rotary desiccant air conditioning system, which combines the technologies of desiccant dehumidification and evaporative cooling, is advantageous in being free from CFCs, using low grade thermal energy and controlling humidity and temperature separately. Compared with conventional vapor compression air conditioning system, it preserves the merits of environment-friendly, energy saving, healthy, comfortable, etc. Ongoing research and development works suggest that new desiccant materials and novel system configurations have significant potential for improving the performance and reliability and reducing the cost and size of rotary desiccant dehumidification and air conditioning system, thereby increasing its market competitiveness and breaking out the current fairly small niche market. For the purpose of providing an overview of recent efforts on these issues and showing how rotary desiccant air conditioning systems can be designed and coupled to available thermal energy, this paper presents and analyzes the status of rotary desiccant dehumidification and air conditioning in the following three aspects: the development of advanced desiccant materials, the optimization of system configuration and the utilization of solar energy and other low grade heat sources, such as solar energy, district heating, waste heat and bioenergy. Some key problems to further push forward the research and development of this technology are also summarized.     

Use of liquid desiccant cooling to improve the performance of vapor compression air conditioning

A hybrid air conditioning system, which consists of sections of desiccant dehumidification, evaporative cooling and vapor compression air conditioning, has been established in this paper. Experimental investigation demonstrates that cooling production and COP of the new hybrid system can be increased significantly, if they are compared with those of vapor compression system (VCS) alone. Assuming that the outlet temperature and humidity of the system are constant, psychrometric analysis at ARI conditions has been conducted under three different cases. The benefits are represented by lower electricity consumption of the compressor, higher COP of the system, less flow rate of condensation air, and reduced size of VCS, etc. The reason that the hybrid system is superior in performance to conventional systems lies in that desiccant dehumidification and evaporative cooling changes the inlet states of the air entering into VCS. Furthermore, the effects of dehumidification and evaporative cooling are analyzed in the paper.     

Analysis and Modeling of a Novel Hybrid Solid–Liquid Dehumidifier System

A novel waste-heat-driven hybrid solid–liquid dehumidifier (HSLD) system is proposed and its performance modeling is carried out. This new type of dehumidification system can be efficiently driven by low-temperature heat sources such as solar energy, while achieving high dehumidification performance due to its unique serial dehumidification and regeneration processes. The process airflow is first dehumidified by a liquid dehumidifier and then by a solid one; at the same time, the regeneration air first regenerates the solid dehumidifier and then the liquid one. The reason is because the liquid dehumidifier is more efficient to dehumidify humid air with low-temperature regeneration heat, while the solid dehumidifier can effectively dehumidify drier air with higher temperature regeneration heat. Theoretical analysis and performance analysis of the HSLD system are carried out under varied process air conditions and regeneration temperatures. The results show that this new HSLD system is promising for dehumidification performance improvement with low-temperature regeneration heat. It can be efficiently driven by waste heat as low as 60～70°C with a satisfactory dehumidification performance. For the HSLD system, the dehumidification ratio between the liquid and the solid dehumidifiers varies according to the regeneration temperature and process air inlet conditions.     

Analysis of heat and mass transfer between air and falling film in a cross flow configuration

Heat and mass transfer between air and falling solution film in a cross flow configuration is investigated. Effects of addition of Cu-ultrafine particles in enhancing heat and mass transfer process are also examined. A parametric study is employed to investigate the effects of pertinent controlling parameters on dehumidification and cooling processes and their subsequent optimization. It is found that low air Reynolds number enhances the dehumidification and cooling processes. An increase in the height and length of the channel and a decrease in the channel width enhance dehumidification and cooling processes. It is also found that an increase in the Cu-volume fraction increases dehumidification and cooling capabilities and produces more stable Cu-solutions.     

基于溶液除湿的风冷热泵系统性能分析

为了克服利用冷却除湿的风冷热泵空调系统机器露点过低、需要再冷和过热、难以适应显热潜热比例的变化、不能蓄能等缺点,提出基于集热再生器溶液除湿的热泵空调系统。通过济南某工程实例研究表明,与冷却除湿空调系统相比较耗电量减少12.3%,利用太阳能加热溶液除湿具有降低空调除湿能耗、利用可再生能源、减少高品位能源消耗等优势。证明太阳能溶液除湿在空调系统中是处理潜热负荷的理想选择,具有较好的节能性。     

太阳能再生式除湿换热器动态除湿性能研究

提出了一种新型再生式除湿换热器,建立了物理和数学模型。通过实验得到了该除湿换热器的实际动态除湿性能;将除湿器除湿性能的模拟结果与实验结果进行比较,验证了数学模型的可靠性。研究结果表明:该文研制的再生式除湿换热器具有良好的除湿性能,在给定工况(温度为24.7℃,含湿量为12.41g/kg)下除湿率可达到43.8%;还分析了处理风速、再生温度以及除湿换热器厚度对除湿性能和压降的影响,获得了使除湿换热器性能最佳的管排、翅片间距和迎面风速参数。     

Research on a Refrigeration Dehumidification System with Membrane-Based Total Heat Recovery

An independent air dehumidification system is helpful to improve indoor air quality and decrease energy consumption by heating, ventilation, and air conditioning (HVAC). A refrigeration dehumidification system with membrane-based total heat recovery is the key equipment to realize this goal. The system comprises two subsystems: a membrane total heat recovery and a direct expansion refrigeration system. The total heat exchanger has a membrane core where the incoming fresh air exchanges moisture and temperature simultaneously with the exhaust air. In this manner, the total heat or enthalpy from the exhaust air is recovered. Then the fresh air flows through a cooling coil where it is dehumidified below the dewpoint. Finally, the cold and dry air is supplied to indoors. A prototype of practical application is designed and fabricated. Experiments are conducted under variable operating conditions in the psychrometric calorimeter chamber. The effects of varying operating conditions like temperature and air humidity on the air dehumidification rate, cooling power, coefficient of performance, and compressor power are evaluated with indoor exhaust air dry bulb 27°C, wet bulb 19°C, and fresh air flow rate 200 m³/h. In comparison with a conventional refrigeration dehumidification system, the coefficient of performance and air dehumidification rate of the prototype are 2.3 times and 3 times higher, respectively. The performance of the prototype is rather robust under a hot and humid environment.     

Modeling cooling coils

Finned-tube heat exchangers commonly used as cooling coils in air conditioning systems undergo complex heat transfer and dehumidification. Due to the presence of water film on the outside surface of the coils, the general approach for an analysis of dry surface is not adequate to predict the performance of such coils. This paper presents a modeling procedure for cooling coils with dehumidification based on the approach of Threlkeld. In order to verify the calculational results of the model, experiments were conducted with an aim to determine the outlet air conditions as well as some other parameters required as the inputs to the model. Comparison between the simulation and experimental results reveals that the model is accurate and suitable for predicting the performance of cooling coils with dehumidification.     

复合式太阳能空调优化方案

出了融除湿、蒸发冷却、压缩制冷及太阳能热水为一体的复合式太阳能空调的优化方案。分析比较了三种空气处理方案。结果表明 ,与常规空调相比 ,优化方案节电近 4 7%。     

Performance characterisation of liquid desiccant columns for a hybrid air-conditioner

Use of liquid desiccant-vapor compression hybrid system is encouraged for low humidity applications. The liquid desiccant is primarily used to further dehumidify the supply air. In the present study, by using psychrometric equations and liquid desiccant property data, heat and mass transfer analysis for the dehumidifier and regenerator columns in counter flow configuration has been carried out. The simulation of the columns corresponds to low solution to air (S/A) flow ratio where precooled air gets dehumidified in the absorber while preheated air is used for regeneration of the solution. A detailed study of the performance characteristics for the absorber and regenerator columns confirms the requirement of the desiccant loop for additional dehumidification of the conditioned air. This need develops the main motive towards the concept of hybrid air conditioning.     

Experiments in a solar simulator on solid desiccant regeneration and air dehumidification for air conditioning in a tropical humid climate

This paper presents an indoor and analytical study to evaluate the performance of a desiccant cooling system that uses silica gel as desiccant, electric light bulbs to simulate solar radiation, and forced flow of air through an IDC (integrated Desiccant/Collector). In the regeneration process, the rate at which water is removed from the desiccant increases with irradiation and decreases with air flowrate. In the air dehumidification process, the adsorption rate decreases with irradiation and increases slightly with flowrate. Comparisons between analytical calculations and experimental data show good agreement, and the calculations show that it should be possible to operate this system in tropical humid climates using the regeneration process in the day and the air dehumidification in the night time.     

Dehumidification and humidification process of desiccant solution by air injection

Liquid desiccant dehumidification was proved to be an effective method to extract the moisture from air with a relatively less energy. An experimental study was carried out to evaluate the liquid desiccant system performance during dehumidification and humidification processes using an injected air through the liquid desiccant solution (calcium chloride). A different air mass flow rates though the desiccant solution was considered during the experimental work. The desiccant system was studied at different operating conditions like different temperatures, different humidity ratios and different solution levels. The effectiveness for both the dehumidification and humidification processes was calculated through this work. It was found that, the system effectiveness reached to 0.87 in the dehumidification and about 0.92 in the humidification process. Also; the experimental results showed a mass transfer coefficient of 28 kg s⁻¹ m² mm Hg at an air mass flow rate of 0.022 kg s⁻¹ in the dehumidification process. The cooling effect factor was also studied and analyzed during that work.     

空调中分布型主动式热回收系统的分析

利用水或乙二醇对空调系统中的余热进行回收,加热空调系统进口新风,使除湿后的新风达到升温的目的。通过余热利用降低加热新风所需要的燃料,减少了二氧化碳的排放。分析讨论了瑞士Konvekta分布型主动式热回收产品的工作原理、使用效果及发展前景。