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.     

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。     

An experimental study on multi‐purpose desiccant integrated vapor‐compression air‐conditioning system

In this paper, a multi‐purpose hybrid desiccant integrated vapor‐compression air‐conditioning system of a small capacity is experimentally investigated. The system, referred as hybrid desiccant‐assisted air conditioner (HDAC), is designed to meet the cooling load of spaces having large latent heat portions and at the same time to extract water from atmospheric air. The system is mainly consisted of a liquid‐desiccant dehumidification unit integrated with a vapor‐compression system (VCS). The dehumidification unit uses lithium chloride (LiCl) solution as the working material. The effect of different parameters, such as desiccant solution flow rate, process airflow rate, evaporator and condenser temperatures, strong solution concentration and regeneration temperature on the performance of the system, is studied. This system has a water recovery rate of 6.7 l/h TR (1.91 l/h kW) of pure water at typical north Egyptian climate (20–30°C dry bulb and 35–45% relative humidity). The HDAC system has a COP as high as 3.8 (an improvement of about 68% over the conventional VCS). The system offers a total cooling capacity of about 1.75 TR (6.15 kW) using a 0.75 TR (2.6 kW) VCS unit. Finally, the proposed system is found to have a payback time of about 10 months without any considerable extra capital cost compared with the known split air‐conditioning system. The results emphasize the potential benefits of the HDAC system. Copyright © 2010 John Wiley & Sons, Ltd.     

Energy and economic performance analysis of an open cycle solar desiccant dehumidification air-conditioning system for application in Hong Kong

In this article, a transient simulation model and the EnergyPlus were used to study the energy performance and economical feasibility for integrating a solar liquid desiccant dehumidification system with a conventional vapor compression air-conditioning system for the weather condition of Hong Kong. The vapor compression system capacity in the solar assisted air-conditioning system can be reduced to 19 kW from original 28 kW of a conventional air-conditioning system as a case study due to the solar desiccant cooling. The economical performance of the solar desiccant dehumidification system is compared with that of the conventional air-conditioning system. The results show that the energy saving potentials due to incorporation of the solar desiccant dehumidification system in a traditional air-conditioning system is significant for the hot wet weather in Hong Kong due to higher COP resulted from higher supply chilled water temperature from chiller plants. The annual operation energy savings for the hybrid system is 6760 kWh and the payback period of the hybrid system is around 7 years. The study shows that the solar assisted air-conditioning is a viable technology for utilizations in subtropical areas.     

Study of a solar powered solid adsorption–desiccant cooling system used for grain storage

A hybrid solar cooling system, which combines the technologies of rotary desiccant dehumidification and solid adsorption refrigeration, has been proposed for cooling grain. The key components of the system are a rotary desiccant wheel and a solar adsorption collector. The former is used for dehumidification and the later acts as both an adsorption unit and a solar collector. The heating load from sunshine can thus be reduced to a greater extent since the solar adsorption collector is placed on the roof of the grain depot. Compared with the solid adsorption refrigeration system alone, the new hybrid system performs better. Under typical conditions, the coefficient of performance of the system is >0.4 and the outlet temperature is <20°C. It is believed that the system can be used widely in the regions with abundant solar resources due to such advantages as environmental protection, energy saving and low operation costs. Additionally, some parameters, for example, ambient conditions, the effectiveness of the heat exchanger and evaporative cooler, mass air-flow rate, etc., which affect system performance, are also analyzed.     

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.     

Thermodynamic analysis of a counter flow adiabatic dehumidifier with different liquid desiccant materials

Liquid desiccant systems have been proposed as energy saving alternatives to the conventional vapor compression systems for handling the latent load. This paper presents the results from a study of the performance of a counter flow liquid desiccant dehumidifier. A heat and mass transfer theoretical model of an adiabatic packed column has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Good agreement was found between experimental tests and the theoretical model, with the maximum deviation being ±2.9% in air outlet temperature, ±15.9% in air outlet humidity ratio and ±2.8% in solution outlet temperature. Following the model validation, the rate and the efficiency of the dehumidification process were assessed under the effects of variables, such as air temperature and humidity, desiccant temperature and humidity and air and desiccant flow rates. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl₂ were evaluated against each other. The results show that high absorber efficiency and system efficiency could be achieved under humid conditions, low air mass flow rates and LiCl as the desiccant solution.     

Investigation on a two-stage solar liquid-desiccant (LiBr) dehumidification system assisted by CaCl2 solution

A two-stage solar powered liquid-desiccant dehumidification system, for which two kinds of desiccant solution (lithium chloride and calcium bromide) are fed to the two dehumidification stages separately, has been studied. In the studied system air moisture (latent) load is separately removed by a pre-dehumidifier using cheap calcium chloride (CaCl₂) and a main dehumidifier using stable lithium bromide (LiBr). Side-effect of mixing heat rejected during dehumidification process is considerably alleviated by an indirect evaporative cooling unit added between the two dehumidification stages. The feasibility of high-desiccant concentration difference achieved by reusing desiccant solution to dehumidify air and regenerating desiccant repeatedly is analyzed. By increasing desiccant concentration difference, desiccant storage capacity is effectively explored. It is found that the pre-dehumidification effect of CaCl₂ solution is significant in high ambient humidity condition. Also seen is that the desiccant investment can be decreased by 53%, though the cost of equipments is somewhat increased, and the Tcop and COP of the proposed system can reach 0.97 and 2.13, respectively.     

Effectiveness of heat and mass transfer in packed beds of liquid desiccant system

A liquid desiccant system (using CaCl₂) is presented for air dehumidification using solar energy or any other low grade energy to power the system. The system utilizes two packed beds of counterflow between an air stream and a solution of liquid desiccant for the processes of air dehumidification and solution regeneration. To simplify the prediction of the performance of the system an effectiveness of heat transfer and an effectiveness of mass transfer in the packed beds are defined. A finite difference model is developed to model the heat and mass transfer in packed beds during the air dehumidification mode and the solution regeneration mode. This finite difference model is used to calculate the effectiveness of heat and mass transfer in the packed beds at various bed heights, various air and solution flow rates, various inlet temperatures of air and solution to the bed, and various concentrations of CaCl₂ solution at the bed entrance. Charts of the effectiveness of heat and mass transfer are presented in a convenient form. A designer of a liquid desiccant system may use the charts in predicting the performance of these systems without having to use the finite difference model for this purpose.     

Solar hybrid cooling system for high-tech offices in subtropical climate – Radiant cooling by absorption refrigeration and desiccant dehumidification

A solar hybrid cooling design is proposed for high cooling load demand in hot and humid climate. For the typical building cooling load, the system can handle the zone cooling load (mainly sensible) by radiant cooling with the chilled water from absorption refrigeration, while the ventilation load (largely latent) by desiccant dehumidification. This hybrid system utilizes solar energy for driving the absorption chiller and regenerating the desiccant wheel. Since a high chilled water temperature generated from the absorption chiller is not effective to handle the required latent load, desiccant dehumidification is therefore involved. It is an integration of radiant cooling, absorption refrigeration and desiccant dehumidification, which are powered up by solar energy. In this study, the application potential of the solar hybrid cooling system was evaluated for the high-tech offices in the subtropical climate through dynamic simulation. The high-tech offices are featured with relatively high internal sensible heat gains due to the intensive office electric equipment. The key performance indicators included the solar fraction and the primary energy consumption. Comparative study was also carried out for the solar hybrid cooling system using two common types of chilled ceilings, the passive chilled beams and active chilled beams. It was found that the solar hybrid cooling system was technically feasible for the applications of relatively higher cooling load demand. The annual primary energy consumption of the solar hybrid cooling system was lower than that of the conventional vapour compression refrigeration system up to 36.5%. Between the two options of chilled ceilings, the passive chilled beams were more energy-efficient to work with the solar hybrid cooling system in the hot and humid climate. Harnessing solar energy for driving air-conditioning would help in reducing the carbon emission, hence alleviating the climate change.     

Solar hybrid air-conditioning system for high temperature cooling in subtropical city

Although solar energy is able to power the heat-driven refrigeration, its contribution is quite limited due to the conventional cooling requirement. In building air-conditioning, it is common to supply low temperature chilled water, usually in 5–7 °C. If this temperature can be elevated, it would enhance the effectiveness to harness solar energy and minimize auxiliary heating. Solar refrigeration would then be more effective through high temperature cooling, by providing 15–18 °C chilled water instead. In such provision, radiant ceiling cooling can be coupled to handle the space cooling load, particularly space sensible load. And the space latent load and ventilation load are handled by a separate dehumidification provision, like the heat-driven desiccant dehumidification. Therefore, a solar hybrid air-conditioning system is formulated, using adsorption refrigeration, chilled ceilings and desiccant dehumidification. In this study, the year-round performances of the proposed solar hybrid air-conditioning systems were evaluated for two typical office types. The performance metrics include the solar fraction, coefficient of performance, solar thermal gain, primary energy consumption and indoor conditions. Comparative study was conducted for the hybrid air-conditioning system worked with the three common types of chilled ceilings, namely the chilled panels, passive chilled beams and active chilled beams. The solar hybrid air-conditioning system was also benchmarked with the conventional vapour compression refrigeration for office use. It is found that the proposed solar hybrid air-conditioning system is technically feasible through high temperature cooling. Among the three types of chilled ceilings, the passive chilled beams is the most energy-efficient option to work with the solar adsorption refrigeration for space conditioning in the subtropical city.     

Energy consumption analysis on a dedicated outdoor air system with rotary desiccant wheel

A dedicated outdoor air system (DOAS) with rotary desiccant wheel is the combination of a desiccant dehumidification system and a vapor compression refrigeration system. An energy consumption model of this hybrid DOAS is established for its analysis. Coefficient of performance, COP, is appropriately defined for evaluation on performance of the hybrid DOAS. The results indicate that, compared with a conventional DOAS, energy savings are possible for the suggested DOAS, when solar energy or natural gas is used for regeneration. Ventilation air flow rate, temperature or humidity of outdoor air, as well as regeneration-to-process air ratio, influence the energy consumption and the COP of the hybrid DOAS, greatly.     

Modeling and performance of a forced flow solar collector/regenerator using liquid desiccant

A forced flow solar collector/regenerator is one of the effective ways of regenerating the weak liquid solution in an open cycle liquid desiccant air conditioner using solar energy. In this system, the weak solution flows over the absorber plate of a tilted collector/regenerator as a thin liquid film. The forced air stream, which flows parallel or counter to the solution film, removes the moisture which is evaporated from the liquid solution due to absorption of solar energy. The absorber plate of the collector/regenerator is blackened and glazed to enhance the solar energy absorption and protect it from the environment. To evaluate the thermal performance of the solar collector/regenerator, a computer model has been developed using calcium chloride as the desiccant. A parametric analysis of the system has been performed to calculate the rate of evaporation of water from the solution as a function of the system variables and the climatic conditions.     

An experimental study of structured packing dehumidifier/regenerator operating with liquid desiccant

The present work presents an experimental investigation on the performance of the structured packing cross flow desiccant dehumidification system (DDS). This system is referred as DDS; its heart is the dehumidifier/regenerator. It is used to meet a latent heat load by air dehumidification. Calcium chloride (CaCl₂) solution is used as the working desiccant material in this system. The structured packing has a density (specific surface area) of 390 m²/m³, corrugation angle of 60° and void fraction of 0.88. The effect of relevant parameters such as air flow rate, desiccant solution flow rate, desiccant solution temperature and concentration and packing thickness on the performance of the system is studied. The performance of the system is evaluated using the mass transfer coefficient, moisture removal rate (MMR), effectiveness and the coefficient of performance (COP). The remarkable increase of mass transfer coefficient and MRR for both deh/reg is observed by increasing both air and solution flow rates. Eventually, the payback period (PP) of the DDS is 11 months with annual running cost savings (ΔC_RC) of about 31.24% compared with vapor compression system (VCS) dehumidification. The overall environmental impacts of DDS are nearly 0.63 of VCS. This may emphasize the need of incorporating a desiccant system along with air conditioning applications.     

Solar cooling systems: Mass transfer studies for a liquid desiccant dehumidifier

Liquid desiccant cooling systems have the advantage over conventional compression systems of being able to operate with largely solar thermal energy sources, and of efficiently handling the latent load. The solar energy is used to regenerate the liquid desiccant by removing the water absorbed from air in the dehumidifier. A packed-bed liquiddesiccant (LiBr) dehumidification unit has been operated with varying air conditions and liquid streams and with three levels of packing (0, 28 and 40 cm). Number of transfer units (NTU) values of 2–2.5 were obtained in the best performing configuration; the corresponding height of transfer unit (HTU) values were 0.25–0.31 m. Overall, gas-side mass transfer coefficients calculated for the dehumidifier are made up of contributions from the packed bed and spray sections of the tower. With full packing and a higher solution flow rate, the overall K_ya was 151.3 g mol/sm³ contact-volume log mean concentration driving force. Spray-only contacting at the higher solution flow rate gave a K_ya of 15.7 g mol/sm³ contact-volume log mean concentration driving force. The individual mass transfer coefficients for the two sections have been separated; to the authors' knowledge, this is the first time the separate contributions of spray and packing have been quantified in a composite dehumidifier tower. Spray contributions were found to contribute from 10 to 70% of the mass transfer occurring in the dehumidifier, the higher percentages being found for a very inefficient deep bed and low liquid flow conditions.     

Optimized solar-powered liquid desiccant system to supply building fresh water and cooling needs

This paper studies the feasibility of using a solar-powered liquid desiccant system to meet both building cooling and fresh water needs in Beirut humid climate using parabolic solar concentrators as a heat source for regenerating the liquid desiccant. The water condensate is captured from the air leaving the regenerator. An integrated model of solar-powered calcium chloride liquid desiccant system for air dehumidification/humidification is developed. The LDS model predicted the amount of condensate obtained from the humid air leaving the regenerator bed when directed through a coil submerged in cold sea water. An optimization problem is formulated for selection and operation of a LDS to meet fresh water requirement and air conditioning load at minimal energy cost for a typical residential space in the Lebanon coastal climate with conditioned area of 80 m² with the objective of producing 15 l of fresh drinking water a day and meet air conditioning need of residence at minimum energy cost. The optimal regeneration temperature increases with decreased heat sink temperature with values of 50.5 °C and 52 °C corresponding to sink temperatures of 19 °C and 16 °C.     

An experimental study of a forced flow solar collector/regenerator using liquid desiccant

One of the main components of a liquid desiccant cooling system is the regenerator. In a liquid desiccant air conditioner, outside air is dehumidified by liquid desiccant and cooled within the absorber. The diluted desiccant solution thus obtained has to be concentrated for reuse, by passing through the regenerator and the cycle is, consequently, repeated. The regenerator used in this application is a forced parallel flow type solar collector/regenerator. The regenerator has been designed and optimized and the prototype of the solar collector/regenerator has been built and tested. Calcium chloride has been used as the absorbent solution. The results of the tests conducted as a parametric analysis indicate that the air and solution mass flow-rates and the climatic conditions affect the regenerator performance. Furthermore, a comparison between the experimental data obtained and a previously developed model for a forced parallel flow solar collector/regenerator reveals that the experiments are in good agreement with the model predictions. Finally, it was concluded that the proposed solar collector/regenerator performs satisfactorily under the summer conditions of Adelaide, Australia.     

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.     

太阳能驱动的中空纤维膜液体除湿系统分析

建立太阳能驱动的中空纤维膜液体除湿系统的分析模型,对系统各部件进行分析,分析空气、冷水进口参数和溶液流量对系统性能的影响.结果显示:1)影响系统效率较大的因素是空气进口含湿量、空气流量、冷水温度和冷水流量;2)整个系统中太阳能集热器部件的损最大,可见提高太阳能集热器的性能是提高系统效率的关键.     

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.