Liquid Desiccant System Research Articles

Experimental study on the performance of the internally-heated ultrasonic atomization liquid desiccant regeneration system

Liquid desiccant dehumidification system has been attracting increasing research interests due to its great energy saving potentials. Moreover, the desiccant solution can be regenerated with thermal energy for recycling use. However, due to the limited contact area and significant temperature drops within the regenerator, the regeneration performance has been restrained, and the necessary desiccant temperature tends to be exorbitant. To this end, this work proposed a new internally-heated ultrasonic atomization liquid desiccant regeneration system (IH-UARS) to improve the desiccant regeneration performance. The proposed IH-UARS was thoroughly studied, in comparison with the adiabatic UARS and the conventional internally-heated flat-plate regenerator (IH-FPR), via extensive experiments and a prediction model developed from the conservation laws of mass and energy. It was found that the regeneration performance, together with the thermal efficiency, was substantially improved by 45.2% and 34.6%, respectively, in the UARS after adding internal-heating. Besides, the necessary desiccant regeneration temperature (NRT) was lowered markedly to merely 36.6 °C in the IH-UARS. Further, better regeneration performance was also presented in the IH-UARS than that of the IH-FPR, with the NRT dropped by 5.9 °C. The results may help improve the performance of the liquid desiccant system and facilitate its fuller utilization of low-grade thermal energy.

Comparison of internally cooled and adiabatic liquid desiccant dehumidification-regeneration system

In the liquid desiccant dehumidification system, there are mainly two types of dehumidifier/regenerator: the adiabatic type, and the internally-cooled/heated type. Thus, it is valuable to compare the performance difference and application scope of these two types. A comparison between the two types should be conducted at a system level, rather than at a component level. Numerical and analytical models were adopted to investigate the performance difference of the two systems. The solution circulating flow rate was the key influencing factor of the performance difference. When ms/ma was approximately 1.1–1.3, the two systems showed similar performance. The internally-cooled system showed better performance under a small flow rate, because of the higher efficiency of the dehumidifier/regenerator and lower heat-cold offset. In contrast, the adiabatic system performed better under a large solution flow rate, owing to a more stable heat and mass transfer driving force. In addition, both systems showed larger efficiencies and higher coefficient of performance (COP) when the outdoor air was relatively dryer. However, the outdoor air status was found to have little influence on the performance difference between the two systems.

Performance enhancement of solar-assisted liquid desiccant dehumidifiers using super-hydrophilic surface

Solar-assisted liquid desiccant dehumidification technology is attracting much attention due to lower energy consumption and less pollution. This study aims at adopting surface-modification technology to improve the dehumidification efficiency and analysing the energy saving of solar-assisted liquid desiccant air-conditioning system. Firstly a novel super-hydrophilic coating was successfully fabricated using nanoscale anatase TiO2 particles with high dispersion. The characterization of the coating was conducted using X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) test. After surface treatment, the surface free energy was significantly increased from 15.71 to 90.15 mJ/m2 and the contact angle was rapidly reduced from 90° to 10° Then, an experimental system with two single-channel plate-type liquid desiccant dehumidifiers was designed. The experimental results indicated that the falling film shrinkage of desiccant solution was effectively prevented and the contact area was rapidly increased by the novel coating. The maximum wetting ratio was increased from 75% to almost 100%. The influence of operating parameters on heat and mass transfer performance was comprehensively analysed. To better evaluate the benefits of the surface-modification technology, a dynamic model of solar-assisted liquid desiccant air-conditioning system was developed. The simulation results indicated that 9% (94MW · h) of electricity could be saved. This finding is very useful in other falling film applications, such as heat exchangers and cooling towers to improve the efficiency and reduce the energy consumption.

Integration of Solid Oxide Fuel Cell with Liquid Desiccant Cooling for Generation of Combined Cooling and Power for a Server

In this study the integration of a solid oxide fuel cell with liquid desiccant system is considered for powering and cooling a data center at the rack level. The novel idea is that each single fuel cell (~12 kW) is used to power one server rack and the waste heat from the fuel cell is used to regenerate a desiccant to provide dehumidification or cooling. The heat concentrates the liquid desiccant solution that is then stored. When moisture must be removed, the high concentration solution is used to dehumidify the outside air. The objective of this study is to theoretically investigate the capability of the integrated system to provide enough cold and dehumidified air to keep the server rack in the safe range of temperature and humidity. Results show that the SOFC waste heat can produce cold and dehumidified air that is in the safe range for server racks. However, with continuous outside air conditions of 25oC and 70% relative humidity the heat from the current SOFC system is enough to provide only 25% of the air flow demand for each server rack.

Performance analysis and mixing ratio selection of mixed liquid desiccant for a dehumidification system

Liquid-desiccant dehumidification system in building air-conditioning has high energy efficiency in comparison to traditional dehumidification technology that needs to cool air below the dew point temperature. Since the dehumidification process dominates the performance of a liquid-desiccant system, this study aims to develop a deeper understanding in the dehumidification performance of some liquid desiccants. Then, an experiment test was conducted to determine the CaCl2-LiCl mixed liquid-desiccant system in a dehumidification system. The effects of the pure and mixture liquid desiccant solution on the moisture removal rate, refrigerating capacity and dehumidification performance were compared and analysed. Results show that with the increase in the LiCl concentration, the dehumidification capacity and dehumidification coefficient first increased rapidly and then increased slowly after 30% LiCl concentration. According to the economic analysis of mixed salt solution, the mixed LiCl and CaCl2 solution with a mixing ratio of 3:1 would achieve the best optimal dehumidification efficiency and would incur a low system cost under the working conditions of a dry-bulb temperature at 26°C and a relative humidity of 60% of fresh and return air.

Exergy analysis of a solar regenerated liquid desiccant assisted air conditioning system for hot and humid climates

In this paper, exergy analysis of a novel solar powered liquid desiccant assisted air conditioning system is presented and simulated. The system aims to provide suitable thermal comfort conditions inside large office buildings with high internal loads situated in the hot and humid tropical/subtropical countries of the world. The system consists of process and regenerating air streams, a liquid desiccant solution loop and a cooling water loop. The primary objective of this study is to present the exergy of cooling capacity along with the overall exergy efficiency of the proposed system. The study helps to quantify the optimum operating and design parameters for system operation based on the second law of thermodynamics. For the base case, which is representative of a hot and humid climatic condition, the proposed system is able to maintain the room air conditions within the moderate thermal acceptability criterion. The exergy of cooling capacity and exergy efficiency for the base case is about 2900 W and 2 % respectively. Parametric analyses show that the system performs the best under conditions of high ambient insolation and temperature, low ambient humidity and a process air to desiccant solution mass flow rate of about 3 in the dehumidifier.

Numerical investigations and performance comparisons of a novel cross-flow hollow fiber integrated liquid desiccant dehumidification system

The heat and mass transfer process of a novel cross-flow hollow fiber integrated liquid desiccant dehumidification system is analyzed numerically. Compared with other porous media or packing towers in dehumidification applications, hollow fibre membranes have significant advantages including low weight, corrosion resistance and no liquid droplet carryover. A novel air-KCOOH cross-flow dehumidification system was designed and manufactured, with 5500 hollow fibres formed into a circular module. The variations of the dehumidification effectiveness and moisture removal rates were studied numerically and validated against experimental results under the incoming air mass flow rates of 0.08–0.26 kg/s and relative humidity from 55% to 75%. The dehumidification performance comparisons for the proposed system using CaCl2, LiCl and KCOOH as the desiccants have been conducted. The results demonstrated that under the same m*(ratio between solution mass flow rate to the air mass flow rate), the proposed system using 62% KCOOH could achieve approximately the same latent effectiveness compared with 40% CaCl2 and 32% LiCl, with at least 3.1% sensible effectiveness improvement. Therefore, it could be concluded that the proposed system using KCOOH as desiccant could be more applicable for dehumidification purpose compared with other systems using conventional liquid desiccants.

Cascaded liquid desiccant system for humidity control in space conditioned by cooled membrane ceiling and displacement ventilation

Displacement ventilation (DV) combined with liquid desiccant membrane cooled ceiling (LDMC-C) is an efficient Heating, ventilation and Air conditioning (HVAC) system. However, the LDMC-C/DV has limited performance in office spaces located in hot and humid climates or characterized by high internal latent loads due to the inability to control the humidity. This study proposes a cascaded liquid desiccant system in which the liquid desiccant flow exiting the LDMC-C is directed into a liquid desiccant heat and mass exchanger in the DV supply duct to control the humidity in the space and conserve energy. To achieve this goal, an integrated model for different components of the cascaded liquid desiccant system is developed to predict the comfort level and the air quality inside the space as well as the energy consumption of the system. The model was validated experimentally in a climatic chamber. The validated integrated system model was applied to a case study to assess the effectiveness of the system in hot and humid climate. Two configurations of the LDMC-C and mass exchanger were simulated and found to provide acceptable humidity for comfort in the occupied zone of the space. Moreover, the cascaded system cooling and heating energy were found lower by 21.25% and 16.23% respectively in August when compared to a LDMC-C/DV, and to DV membrane exchanger system with different configuration. The cooling and heating energies were also lower in the cascaded system compared to a LDMC-C/DV while using conventional supply air dehumidification method.

A proposed hyper-gravity liquid desiccant dehumidification system and experimental verification

Liquid desiccant air conditioning systems have received more attention in recent years for their lower energy consumption. A novel hyper-gravity liquid desiccant dehumidification system was proposed in this paper. Rotating packed bed (RPB) uses a rotating part to create an artificial centrifugal force which is stronger than gravity, thus owning the advantages for gas-liquid heat and mass transfer process intensification. This work considered the feasibility of applying a countercurrent-flow RPB for air dehumidification. An experimental apparatus is established to verify the proposed liquid desiccant dehumidification system. More than 145 reliable experimental data have been collected. The effects of the rotating speed of RPB and inlet parameters of air and desiccant solution are investigated. The results show that both dehumidification effectiveness and moisture removal rate can be improved significantly by increasing the rotating speed of the RPB, solution flow rate and solution inlet concentration. Dehumidification effectiveness can reach 0.621 at the rotating speed of 1500 r/min. Compared with conventional dehumidifiers the desiccant solution consumption of RPB dehumidifier is less under the same dehumidification effectiveness. Besides, the effects of the rotating speed, air flow rate and solution flow rate on the pressure drop of the RPB dehumidifier are presented.

Influence of potential liquid desiccants on solution cooling, heating, and pumping loads of membrane-based liquid desiccant air conditioning system: An analytical study

This paper is intended to study the methodology for choosing the suitable desiccant solution among LiCl, CaCl2, LiBr, and KCOOH solutions (commonly used potential solutions) at optimum heat capacity for an energy-efficient membrane-based liquid desiccant air conditioning system. An analytical model which is validated with existing experimental and numerical results was adopted in this study to find the output parameters. Performance indices considered in this study are solution pumping power, solution heat addition rate (Qadd), and chiller load (Qchiller). Considering the specified solutions at their maximum safe concentrations (0.40, 0.35, 0.55, and 0.75, which are just below their saturation concentrations), performance indices analyzed at fixed ambient condition and dehumidification rate at different heat capacity ratios (Cr* = 4.0, 3.0, 2.5, 2.0, and 1.5). At their corresponding optimum Cr*, Qadd is observed low for CaCl2, whereas it was high for LiBr. Solution pumping power and Qchiller are observed as lesser for the LiBr solution. At their corresponding design parameters, LiBr solution’s investment cost is found to be 30% higher than LiCl solution’s, but its operating cost (pumping power) is nearly 26% to 40% less than the LiCl solution, which suggests that the LiBr solution is the most efficient solution among the four.

Hollow fibre membrane-based liquid desiccant humidity control for controlled environment agriculture

Humidity control is an important factor affecting the overall sustainability, productivity, and energy efficiency of controlled environment agriculture. Liquid desiccants offer the potential for pinpoint control of humidity levels in controlled environments. In the present work, a dehumidification processes utilizing liquid desiccants pumped through the lumens of triple-bore PVDF hollow fibre membranes is implemented in a bench scale controlled environment agriculture system. Hydrophobic hollow fibre membranes were combined into an array and placed near the crops. Concentrated magnesium chloride liquid desiccant solution with a low vapour pressure was pumped through the hollow fibre lumens. The dehumidification permeance rate responded dynamically to the changing transpiration rate of the plants, as influenced by changes in environmental factors such as light, temperature, and vapour pressure. The dehumidification permeance rate increased from an average of 0.26–0.31 g m−2 h−1 Pa−1 as the velocity of the liquid desiccant through the hollow fibres increased from 0.023 to 0.081 m s−1. Humidity levels were targeted to be maintained within a range of 70–90% relative humidity at 23 °C. The membrane-based liquid desiccant system was demonstrated to successfully control humidity within a bench-scale controlled environment agricultural setup.

Experimental and numerical investigation of a novel hybrid deep-dehumidification system using liquid desiccant

Desiccant-based hybrid air-conditioning systems have recently become popular owing to a reduction in their energy consumption. In addition to the dehumidification process, regeneration is a necessary process in a hybrid system, and in all previous studies the regenerator (REG) has been separated from the liquid desiccant dehumidifier (DEH), which implies the need for a complex duct configuration and a large equipment space. In the present study, a novel hybrid liquid desiccant–heat pump air-conditioning system combined with a cooling coil is proposed for deep dehumidification with a supply air humidity ratio of below 5 g/kg, where the REG, condensing DEH (cooling coil), and liquid desiccant DEH are cascaded. This system not only maintains the energy performance advantages of hybrid systems over conventional systems, it also simplifies the layout of the entire system and guarantees a compact system structure, particularly for buildings with an existing central air-conditioning–chilled-water system. Using the proposed system, a coefficient of system performance (COPsys) of 2.4–3.2 has been obtained experimentally with an supply air humidity ratio of 2.1–4.5 g/kg. Despite the compact system structure, when the air is processed from 30 °C and 20 g/kg to 4 °C and 3.6 g/kg, the proposed system achieves a superior COPsys of 3.3 with an improvement of 13.8% when compared with a conventional liquid desiccant dehumidification system.

A Full-Solid-State Humidity Pump for Localized Humidity Control

Summary Humidity management is essential and widely needed in various fields. Traditional moisture removal technologies are mostly based on cooling condensation or desiccant dehumidification. These methods come with inherent defects of energy efficiency and are bulky and complex for small-space humidity-control applications. Here, we demonstrate a full-solid-state humidity pump using the advantages of commercial thermoelectric coolers and silica gel materials. Silica gel is coated on the surface of heat sinks to minimize contact thermal resistance between the material and heat source while maintaining enough heat and mass transfer area between the desiccant and the air. The humidity pump device exhibits 0.61 g W−1 h−1 humidity transfer efficiency, corresponding to a humidity transfer rate of 28.38 g h−1. The proposed device neither uses refrigerant nor introduces liquid water. It opens up the possibility of use in localized humidity control applications with higher efficiency and a wider ambient temperature range than traditional technologies.

Experimental and Numerical Study on Heat and Mass Transfer of Cross-Flow Liquid Desiccant Dehumidifier/Regenerator

A liquid desiccant air dehumidification system driven by heat pump was established. The performance of cross-flow dehumidifier/regenerator was experimentally investigated. The empirical correlations of Sherwood number for dehumidification/regeneration were obtained by fitting the experimental data. On the basis of the empirical correlations of Sherwood number and thermodynamics analysis of heat and mass transfer process for dehumidifier/regenerator, a cross-flow heat and mass transfer model was established. The effects of air and solution parameters on the dehumidification/regeneration performance were analyzed. The number of mass transfer units and the height-to-length ratio of the packing module were also studied. The results show that there exist optimal number of mass transfer units and height-to-length ratio in the dehumidifier/regenerator.

Energy performance comparison of a 3-fluid and 2-fluid liquid desiccant membrane air-conditioning systems in an office building

In this paper, the cooling and dehumidifying capacities of two liquid desiccant membrane air-conditioning (M-LDAC) systems are investigated and compared when installed in an office building located in a hot-humid climate (Miami, Florida). The building HVAC system consists of a radiant cooling system to cover the sensible load and either a 2-fluid or 3-fluid M-LDAC system to meet the latent load. The systems are simulated over the warmest week of the year using TRNSYS simulation software and modelled using a previously developed analytical model for the 2-fluid M-LDAC system and empirical models for the 3-fluid M-LDAC systems. Sensitivity studies are performed where the systems are evaluated under different latent load conditions due to different occupant densities and infiltration rates. The 2-fluid and 3-fluid M-LDAC systems can meet the latent loads at lower occupant densities and infiltration rates but fail to provide thermal comfort conditions as occupancy and infiltration rates increase. It is concluded that the 3-fluid M-LDAC system uses less energy to meet the latent load than the 2-fluid M-LDAC system. The 2-fluid M-LDAC system consumes 50% more energy than the 3-fluid M-LDAC system to provide thermal comfort to the office building during the weekly simulation.

Performance study of the internally-cooled ultrasonic atomization liquid desiccant dehumidification system

Liquid desiccant dehumidification system (LDAC) has drawn much attention in recent years for its great energy-saving benefits in the HVAC industry. However, its performance is severely constrained by the considerable heat released within the dehumidifiers. One solution for this is adding the internal-cooling, but the necessary cooling temperature has remained low due to the dehumidifiers’ limited heat and mass transfer performance. To this end, a novel internally-cooled ultrasonic atomization dehumidification system (IC-UADS) was studied in this work. A model based on the conversion laws of mass and energy, and the sensible heat balance, was developed for performance prediction. Thorough experiments were conducted under various conditions of the airstream, the desiccant solution, and the cooling water to validate the model and good consistency was achieved. The results indicate that dehumidification performance was remarkably improved by 64.9% in the IC-UADS comparing to the adiabatic one. Besides, lower-grade cooling source can be well utilized in the IC-UADS with available cooling-temperature over 25.6 °C, which could even be from the tap water. Moreover, compared to the packed-bed systems, the IC-UADS presented better effectiveness that increased from 47.4% to 80.1%. This study may help enhance the performance and extend the applicability of internally-cooled LDACs.

Performance analysis of vacuum membrane distillation regenerator in liquid desiccant air conditioning system

In order to solve the problem of poor regeneration performance of the solar solution dehumidification air conditioning regeneration system under high temperature and high humidity weather conditions. In this paper, a novel vacuum membrane distillation (VMD) solution regeneration method was proposed for a solution desiccant air conditioning system, and a theoretical study of this novel regenerator is performed. Through the analysis of heat and mass transfer, the mathematical model of VMD regeneration is established, and the mathematical model is verified by experiments. The results indicated that VMD regenerator not only could increase the regenerate rate but also could exhibit higher energy utilization efficiency. At the same concentration, the VMD regenerator has a higher regeneration performance, and the regeneration rate is 1.6–2.4 times that of the internal heat type regenerator, and the 4.5–6 times that of the adiabatic regenerator.

Developing of a novel water-efficient configuration for shower cooling tower integrated with the liquid desiccant cooling system

A novel configuration of shower cooling tower (SCT) with counter-current flow pattern and a liquid desiccant system equipped with the internal cooling system, used lithium chloride as an adsorbent, is developed and modeled separately and mathematically. The governing relations are solved numerically using a finite difference scheme, and the performance of the hybrid system is parametrically studied. The precision of numerical modeling is increased considering the air humidity impact on the air temperature changes during the moisture absorbing and changes in the diameter of the water droplets along the SCT. A novel configuration is developed by the combination of an SCT and a liquid desiccant system as a hybrid system, and the influences of geometric, physical and environmental parameters on the efficiency of the system are scrutinized. The results show that when the initial diameter of water drops is decreased, the temperature of outlet water reduces; whereas, using the hybrid system reduces the process water temperature up to about 32 °C and 37 °C with and without dehumidifier system at 10 m height of shower cooling tower. In hot and humid regions, reducing the process water temperature of shower cooling tower utilized in the hybrid system is greater compared to a single cooling tower.

Modeling and performance analyses of a batch-wise liquid desiccant air conditioning system

Numerous efforts have been made and reported to improve the energy efficiency of liquid desiccant air conditioning (LDAC) in recent studies. This study proposes a novel batch-wise operation strategy for LDAC that desiccant solutions in dehumidifier and regenerator are circulating independently and exchanging intermittently to get a full utilization of solution dehumidification capacity, cooling and heating energy. System models, including heat and mass transfer models of dehumidifier and regenerator, heat exchanger models and solution transfer model, are developed and verified to analyze and estimate the performance of proposed batch-wise strategy. The maximum relative errors between experimental and model predicting results are within 9.69% for outlet air temperature and within 7.97% for outlet air relative humidity. Performance and energy efficiency of proposed strategy are analyzed and compared with normal on-going solution exchange strategy. According to the simulation results, dehumidification rate and regeneration rate can be improved by average of 5.00% and 3.50% due to multiple solution usages in dehumidifier and regenerator. Moreover, the proposed batch-wise strategy demonstrates considerable energy savings with up to 16.81% and 18.60% from dehumidifier and regenerator, respectively.

Thermal energy analysis on liquid desiccant air conditioning system at different desiccant solution parameters

Selection of suitable liquid desiccant operating parameters plays a significant role in the design of energy efficient liquid desiccant air conditioning system. To achieve same dehumidification rate from ambient air, different combinations of solution parameters (heat capacity ratio, concentration, and vapor pressure) could be employed in the system. Considering dehumidifier air inlet condition and dehumidification rates are fixed, an analytical study is carried out on the thermal energy analysis of the system at different solution operating parameters. Operating parameters considered in this study are solution concentrations ( Cs = 0.25, 0.3, 0.35 and 0.40) and heat capacity ratios ([Formula: see text] = 2.5, 3, 4 and 5). Control volume which includes a pair of air and solution channels (half width channels) of full scale liquid-to-air membrane energy exchangers (LAMEE) has been chosen to analyze the energy transfer between air and solution. The results indicate system requires lesser chiller load ( Qchiller) at high concentration and low heat capacity ratio ( Cs = 0.40 and [Formula: see text] = 2.5) which is 0.29 kW to achieve 0.61 kW cooling load. This is 99% lesser than the Qchiller at high concentration and high heat capacity ratio ( Cs = 0.40 and [Formula: see text] = 5) and 30% lesser than the Qchiller at low concentration and low heat capacity ratio ( Cs = 0.25 and [Formula: see text] = 2.5). Solution heat addition rate ( Qadd) per kW cooling capacity ( Qcc) at this solution condition is found as 0.85 kW.