Air Dehumidification Research Articles

Multi-Objective Design Optimization of Hollow Fiber Membrane-Based Liquid Desiccant Module Using Particle Swarm Optimization Algorithm

ABSTRACTHollow fiber membrane-based liquid desiccant air dehumidification is an effective method for air dehumidification. The benefit of this technique is that the carryover problem of air mixing with desiccant solution can be avoided. Optimal design of the hollow fiber membrane-based liquid desiccant module is presented in this paper. Two objective functions are total entropy generation and total annual cost. However, a lower entropy generation often corresponds to a higher cost of the designed hollow fiber membrane module. To balance the total entropy generation and the total annual cost of hollow fiber membrane-based liquid desiccant modules, the multi-objective particle swarm algorithm is developed for the optimal hollow fiber membrane module design problem. The geometric variables like length of fiber, inner diameter of fiber, number of fiber and packing fraction effect on the total entropy generation and total annual cost are studied. The optimal structural parameters of a hollow fiber membrane-based desiccant module are obtained using multi-objective particle swarm algorithm.

International Façades - CROFT

International Façades - CROFT

Fluid flow and heat transfer across a curved hollow fiber membrane tube bank (CHFMTB): Effects of the tube deformations

A cross-flow hollow fiber membrane tube bank (HFMTB) is commonly employed for air humidification or dehumidification. The tubes in the HFMTB are easily deformed due to the gravity of the water/liquid desiccant stream inside the tubes and the scour of the air stream flowing across the tube bank. Fluid flow and heat transfer across a curved hollow fiber membrane tube bank (CHFMTB) with a regularly populated arrangement are investigated. Two unit cells containing two columns of the curved tubes are selected as the computational domains, which include 12 tubes both for the in-line and staggered arrangements. The equations governing the fluid flow and heat transfer are established via a renormalization group k-ε (RNG KE) turbulent model. The mean friction factors (fm) and Nusselt numbers (Num) for the air flowing across the CHFMTB under various tube arrangements (in-line, staggered), Reynolds numbers (Re), tube deformed heights (Δh) and angles (θ) are calculated and analyzed. It can be found that the mean friction factors and Nusselt numbers for the staggered arrangement are about 2.1–2.7 and 1.4–2.2 times of those for the in-line arrangement, respectively. For most of the Reynolds numbers ranging from 68.42 to 342.30, both the mean friction factors and Nusselt numbers for the HFMTB are almost larger than those for the CHFMTB. Further, the larger the Reynolds numbers are, the larger the differences are.

Effects of the Random Distributions on the Laminar Flow and Heat Transfer in a Quasi-Counter Flow Parallel-Plate Membrane Channel

ABSTRACTAn internally cooled parallel-plate membrane contactor has been employed for liquid desiccant air dehumidification. The contactor is comprised of a series of quasi-counter flow parallel-plate membrane channels (QCPMC). The processing air and the liquid desiccant (solution) streams are separated by the membranes. Cooling tubes are installed in the solution channel to take away the absorption heat. The laminar flow and heat transfer in the QCPMC with the cooling tubes in the solution side (QCPMCC) are studied based on a unit cell containing the sandwiched domain outside the cooling tubes between two neighboring membranes. This paper is focused on the effects of the random distributions in the height direction on the transport phenomena in the QCPMCC. The governing equations of the momentum and thermal transports are built up together with a uniform wall temperature boundary condition. The product of the mean friction factor and Reynolds number and the mean Nusselt number are then calculated. Effects of the various random distributions in the height direction on the product of the mean friction factor and Reynolds number and the mean Nusselt number are analyzed. Compared to the regular arrangement, the mean Nusselt number for the random distribution is larger than that for the regular one when the Reynolds number is less than 68.5. However when the Reynolds number is larger than 68.5, the mean Nusselt number for the random distribution is smaller than that for the regular one.

Performance investigation on polymeric electrolyte membrane-based electrochemical air dehumidification system

An electrochemical air dehumidification system with polymer electrolyte membrane (PEM) was developed. The system performance under various operating conditions was investigated experimentally. A semi-empirical prediction model was also developed with multi-parameter linear regressions. Results showed that the novel system could dehumidify the air flow, with a humidity decrease from 90% (inlet) to less than 30% RH (outlet) under a 3V electric field, which is promising as it can achieve an independent, portable and energy-efficient moisture removal. The steady-state dehumidification performance derived was 80kg/(kWh·m2), or 54kg/(h·V·m2), which were advantageous to current electrochemical dehumidifiers. Specially, this simple element has a volume of only 0.001–0.01‰ of traditional ones, which is also suitable for cascading or multilevel assembly to satisfy various requirements for commercial and industrial applications. The moisture transfer, mainly caused by the electrolysis (+), electro-osmosis (+) and back diffusion (−), increased significantly with the increases in anode-side air humidity and flow rates. When the inlet air humidity increased from 70 to 90%, the dehumidification rate increased about 1.5–2 times. However, only 30% of total power input was effectively used in current element, and the rests were lost, leading to a relatively low system COP (≈0.33). The main reason was that the back-diffusion mass transfer was found to be up to 2.3 times larger than expected, causing by the high moisture content in cathode-side layers (mainly the diffusion layer), which seriously deteriorated the system performance. Therefore, enhancing the diffusion layer performance may help to optimize the dehumidification efficiency effectively. The suggested measures include improving the structure, changing the internal surface parameters or adding microporous layers, etc.

Investigation on capacity matching in a heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system

The heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system is promising recently because solution droplets can be prevented from crossing over into the process air. The quasi-isothermal processes are realized by two-stage dehumidification processes and the system performance is improved. In this study, a novel capacity matching index (CMI) is introduced to evaluate the energy capacity matching of the system through modeling study. It is found that CMI is usually lower than 1 under the typical hot and humid weather condition like South China and the demand and supply of energy in the system is mismatching. As inlet air temperature rises, the dehumidification rates, CMI, EER and COP all decrease. But CMI is almost constant with different inlet air humidity. The influence of air inlet temperature to dehumidifiers and regenerators on the system performance is also investigated. The higher the inlet temperature of dehumidifiers is, the larger the CMI, EER and COP are. The dehumidification rates and CMI both grow with an increase in the inlet temperature of regenerators. It is beneficial for energy balance of the system and high moisture loads, but the side effect is that the EER and COP both decrease.

Graphene oxide papers with high water adsorption capacity for air dehumidification

Graphene oxide (GO) has shown a high potential to adsorb and store water molecules due to the oxygen-containing functional groups on its hydrophilic surface. In this study, we characterized the water absorbing properties of graphene oxide in the form of papers. We fabricated three kinds of graphene oxide papers, two with rich oxygen functional groups and one with partial chemical reduction, to vary the oxygen/carbon ratio and found that the paper with high oxygen content has higher moisture adsorption capability. For the GO paper with reduction, the overall moisture absorbance was reduced. However, the absorbance at high humidity was significantly improved due to direct formation of multilayer water vapor in the system, which derived from the weak interaction between the adsorbent and the adsorbate. To demonstrate one application of GO papers as a desiccant, we tested grape fruits with and without GO paper. The fruits with a GO paper exhibited longer-term preservation with delayed mold gathering because of desiccation effect from the paper. Our results suggest that GO will find numerous practical applications as a desiccant and is a promising material for moisture desiccation and food preservation.

Design of Air Ventilation System for Cargo Hold Vessels Using Solar Desiccant

One of the facilities and infrastructure of the vessel is the ventilation system in the cargo hold to maintain the quality. One attempt to avoid high moisture ratios is to provide a dry air supply by using desiccants. The purpose of this thesis is to design the system of air ventilation with solar desiccant by analysis the calculation with decrease air humidity ratio after passing desiccant rotor as well as fulfillment needs of heater and cooling system using heat of exhaust gas and seawater as well as fulfillment of electricity need using solar energy. From the result of analysis obtain to provide air supply in the cargo hold of 437.5 m3 / hour, the specification of rotor desiccant has a diameter of 550 mm with thickness 200 mm to decrease ratio of outside air humidity equal to 83.1% become 46.5%. Dehumidification air temperature of 47.7oC will be lowered to 35oC by using the sea water cooling media. As for the reactivation air heater requirement of 24.292 kW would be to fulfilled by utilizing the exhaust power of 498.12 kW. And for the electric power needs of the syetm is 34,488 wp will be supplied from the total solar module is 33 units with 345 wp per-capacity.

Formaldehyde removal performance analysis of a liquid desiccant dehumidification system

Liquid desiccant (LD) systems are an efficient method for humidity control. Moreover, LD systems also function to purify the air and remove volatile organic compounds such as formaldehyde. This study aims to analyze the performance of a LD system for formaldehyde removal from indoor air while simultaneously considering air dehumidification requirements. A theoretical model is proposed to predict the formaldehyde removal efficiency (εformaldehyde) of the LD system with various flow mediums, such as an air-LiBr solution, air-LiCl solution, and air-CaCl2 solution. Prior to assessing the LD system performance with the model, Henry's law constants (HLCs) of formaldehyde in the three solutions are tested at the salt concentration range of 30–50 wt% and at the temperature range of 10–50 °C. Based on the results, the correlation equation of the HLC's temperature dependence are given, and then adopted for the simulation model. The effects of the number of mass transfer units of formaldehyde (NTUmf), formaldehyde concentration in the return air, solution flow rate, and airflow rate on system performance are investigated and compared with various flow mediums. The results indicate that NTUmf is a key factor influencing the εformaldehyde. However, the NTUmf do not influence the dehumidification performance of the LD system. Both the εformaldehyde and dehumidification performance remain unchanged with the increase of formaldehyde concentration in the return air. Both the εformaldehyde and dehumidification performances decreased when the solution flow rate increased, and they likewise decreased with increased airflow rates.

Experimental study on dehumidification performance enhancement by TiO2 superhydrophilic coating for liquid desiccant plate dehumidifiers

Liquid desiccant air dehumidification is a promising dehumidification technology with lower energy consumption, less pollution and more flexible humidity control. However, the incomplete wetting conditions of liquid desiccant deteriorates the dehumidification performance and limits the development. Therefore, how to enhance the surface wettability is critical to improving the dehumidification performance as well as reducing the building energy consumption. In this paper, a novel TiO2 superhydrophilic coating was applied in a plate dehumidifier to enhance the surface wettability and improve the dehumidification performance. Firstly, the characterization of the coating was conducted. The test results showed the surface wettability was effectively improved with the contact angle dramatically decreasing from 84.6° to 8.8°. Then two single-channel internally-cooled liquid desiccant dehumidifiers were fabricated to investigate the effect of TiO2 coating on dehumidification performance. The experimental results indicated that the dehumidification performance was significantly enhanced with the mean enhancing ratios of 1.60 and 1.63 for moisture removal rate and dehumidification efficiency, respectively. Besides, the influencing factors on dehumidification performance were analysed comprehensively. Based on the analysis, the new empirical correlations of mass transfer coefficients were developed with different contact angles. Lastly, the energy consumption of liquid desiccant air-conditioning system with coated dehumidifiers for a commercial building in Hong Kong was simulated and around 80MW⋅h of the electricity consumption could be saved. This study is helpful to researchers and engineers who are interested in enhancing the dehumidification performance and reducing the building energy consumption.

Laminar flow and heat transfer in an internally-cooled hexagonal parallel-plate membrane channel (IHPMC)

A internally-cooled hexagonal parallel-plate membrane contactor has been proposed and employed for liquid desiccant air dehumidification, which contains a series of internally-cooled hexagonal parallel-plate membrane channels (IHPMC) for the liquid desiccant stream. Several cooling tubes are installed in the solution channel to take away the absorption heat swiftly. The friction factors and heat transfer coefficients in the complex IHPMC are necessary for the structural design and energy analysis. This study will be focused on the influences of the channel structural parameters on the product of mean friction factors and Reynolds number (fRe)m and Nusselt numbers (Num) in the IHPMC, instead of on the coupled heat and mass transports. The laminar flow and heat transfer in the solution channel are studied based on a unit cell containing the sandwiched domain outside the cooling tubes between two neighboring membranes. The momentum and thermal transport governing equations are built up together with a uniform wall temperature boundary condition and solved by a finite volume approach. The mean (fRe)m and Nusselt numbers (Num) are calculated and analyzed. Influences of the tube numbers (N), tube outer diameters (do), tube distributions, Reynolds number (Re), and tube shapes on the (fRe)m and Num are studied. It can be found that when the do is equal to 0.002m, the (fRe)m rises with an increase in the N, while the Num decrease with the N increasing. When the N is fixed as 3, the (fRe)m increase with the do increasing, and the Num decrease with the do increasing. The results are useful for the performance evaluation, structural design of the membrane contactors formed by the IHPMC used for liquid desiccant air dehumidification.

Demonstration of Liquid Desiccant Based Air Conditioning System using Solar Energy and Waste Heat

Objectives: The objective of the current work is to demonstrate Liquid desiccant based air conditioning (LDAC) systems to reduce energy consumption and cater to latent cooling load more effectively. Methods/Statistical Analysis: This work describes three different projects undertaken to demonstrate efficacy of LDAC systems. One of the project demonstrated regeneration of liquid desiccant in a simple but economical solar thermal collector along with integrated liquid desiccant (LD) heat exchanger. Two other projects demonstrated regeneration using waste heat source and dehumidification of air in an internally cooled LD-air contacting device. Experiments were carried out in city of Vadodara in summer months of year2017 and encouraging results are found. Findings: The passive solar regeneration project showed that a latent cooling rate of 96 W can be provided using an LDAC system per m2 area of solar thermal collector. Another useful utility in form of distilled water could be produced at a rate of 1.7 liters in a day per m2 area in this collector. The internally heated device used for demonstrating use of waste heat for LD regeneration could provide moisture removal rate of around 3.0 kg/h from air using aqueous solution of calcium chloride as LD with an average concentration of 37.7%.In the internally cooled air-LD contacting device, 2.0 kW of latent cooling along with 0.3 kW sensible cooling could be provided for air entering at around 34.7o C temperature and 47.5% relative humidity with an average chilled water temperature of 9.5o C and LD concentration of 40.0%. These experiments have thus demonstrated and established efficacy of LDAC system to provide effective dehumidification with use of energy sources with nominal recurring costs and reduced pollutant emission. Application/ Improvements: Reducing heat losses in solar collector, air bypasses in the contacting device and higher residence time of LD in contacting device can help improve these results further. Keywords: Air Conditioning, Dehumidification, Energy Conservation, Liquid Desiccant, Solar Energy, Waste Heat

The development and experimental performance evaluation on a novel household variable refrigerant flow based temperature humidity independently controlled radiant air conditioning system

The present temperature humidity independent control (THIC) air conditioning (A/C) systems are not that applicable in small-to-medium scaled residential and office buildings for their complex configuration and additional requirement of regenerating heat. In this paper, a novel house-hold VRF based radiant A/C system adopting outdoor air dehumidifier (OAD) was developed and experimentally tested. This novel system employed the VRF system, applying one refrigerant cycle to the OAD to cooling and dehumidifying the outdoor air and another to the Refrigerant-Water plate heat exchanger (RWPHE) to providing chilled water for the adopted radiant terminals. Therefore, the supplying outdoor air from the OAD handled the indoor latent cooling load and the radiant terminals the indoor sensible load, so as to realize the independent control of indoor air temperature and humidity. Experiments were carried out at different outdoor air states for examining the performance of this novel THIC system. The experimental results showed that the supply air temperatures were controlled at proper levels with a moderate fluctuation below 2°C and the supply air humidity ratios were within 8–10g/kg, meeting the requirement of ASHRAE’s recommendation. Therefore, the results suggested promising performance of the developed system in temperature and humidity control.

Modelling and experimental investigation of the cross-flow dew point evaporative cooler with and without dehumidification

This paper investigates the cooling potential of a cross-flow dew point evaporative cooler under various conditions. Preliminary study reveals that the dew point evaporative cooler’s performance does not approach its designed capacity when the ambient humidity goes beyond the thermal comfort zone. To address this problem, an air dehumidification process is proposed before the evaporative cooling takes place. It is observed that the cross-flow cooler is able to achieve improved cooling effectiveness when an appropriate inlet air humidity condition is realized. The influence of different dehumidification levels on its cooling capacity and efficiency is subsequently analyzed. Additionally, a mathematical model is developed to predict the cooler performance. Key findings from this study are: (1) The maximum discrepancy between the simulation results and experimental data is within ±3.0%; (2) the overall wet bulb and dew point effectiveness of the cross-flow cooler can reach 1.25 and 0.85 for cooling of the supply air with moderate humidity; (3) the respective wet bulb effectiveness, cooling capacity and COP of the cross-flow system are 0.86, 2.2kW and 4.6 under humid ambient air condition; and (4) the dehumidification of the supply air enables the cooling capacity and energy efficiency to be improved by 70–135%.

Diffusion in Randomly Overlapping Parallel Pore and Fiber Networks: How Pore Geometry and Surface Mobility Impact Membrane Selectivity

Pore-resolved computations are undertaken, within a continuum model framework, to explore surface diffusion as a selective mechanism for gas separations using membranes of randomly overlapping parallel cylindrical pores or fibers. Orders of magnitude of the model parameters are established using an intrinsic gas diffusivity that is self-consistent with the Knudsen diffusivity obtained from Monte Carlo simulations reported in the literature. The relative contributions of surface and gas diffusion to the overall permeation show that the surface-diffusion flux increases with the specific surface area, whereas the gas flux increases with porosity. Thus, gas diffusion that is perpendicular to pores and fibers can be hindered by the increasing tortuosity while simultaneously promoting permeation via surface diffusion. The selectivity of pore structures with fibrous networks is examined for the dehumidification of air, natural gas, and carbon dioxide. Selectivities, defined as the ratio of the effective diffusio...

Effects of material properties on heat and mass transfer in honeycomb-type adsorbent wheels for total heat recovery

Desiccant wheels have been widely used for humidity treatment: air dehumidification and total heat recovery. Since the operating conditions are different, heat and mass transfer behaviors in the wheels are quite different. This study aims at investigating the performance of honeycomb type desiccant wheels for total heat recovery with various solid desiccant wall materials. A one-dimensional, transient heat and mass transfer model is used to predict the cyclic behaviors of the desiccant wheels. Effects of the working conditions, the rotary speed, the inlet velocities of process air (fresh air) and exhaust air on the performance of the wheel are investigated and compared with various desiccant wall materials. Totally ten most commonly used desiccant materials are considered, with different adsorption and thermophysical properties. It is found that of the 10 materials, the zeolites 5A and 13X perform better than other desiccants for total heat recovery under the high rotary speed (higher than 15rpm) under typical working conditions. The results are different from desiccant wheels for air dehumidification.

Investigation of Geometry Effects of Channels of a Silica-gel Desiccant Wheel

Desiccant evaporative cooling systems are potential environment-friendly alternative to energy-intensive vapour compression chillers. They operate on an open heat-driven cycle consisting of a combination of a dehumidifier, a sensible heat exchanger and evaporative coolers. A desiccant wheel is the heart of the heat-driven cooling system and it uses a solid desiccant for dehumidification. The desiccant material is coated onto the supporting rotor structure. The matrix consists of multiple channels in the direction of the axis of the wheel rotation. The flow passage of the desiccant wheel is usually of sinusoidal shape. The performance of the whole cooling system largely depends on the dehumidification ability of the desiccant wheel and mathematical models are an effective tool to predict the heat and mass transfer behaviour of moisture transport in air dehumidification applications with rotary desiccant wheels. In this paper, a heat and mass transfer model incorporating both solid-side and gas-side resistances is presented and compared with experimental data. This model is used to investigate the effect of the geometry and shape of element channels on the transport process and the performance of the wheel.

Transient performance and temperature field of a natural convection air dehumidifier loop

In this paper, transient performance of the previously introduced natural convection heat and mass transfer loop is investigated for an air dehumidifier system. The performance of the loop is studied in different conditions of heat source/heat sink temperature and different startup desiccant concentrations. Unlike conventional loops, it is observed that natural convection of the fluid originates from the heat sink towards the heat source. The proper operation of the cycle is highly dependent on the heat sink/heat source temperatures. To reduce the time constant of the system, a proper desiccant concentration should be adopted for charge of the loop.

Effect of hygroscopic materials on water vapor permeation and dehumidification performance of poly(vinyl alcohol) membranes

ABSTRACTIn this study, two hygroscopic materials, inorganic lithium chloride (LiCl) and organic triethylene glycol (TEG) were separately added to poly(vinyl alcohol) (PVA) to form blend membranes for air dehumidification. Water vapor permeation, dehumidification performance and long‐term durability of the membranes were studied systematically. Membrane hydrophilicity and water vapor sorbability increased significantly with higher the hygroscopic material contents. Water vapor permeance of the membranes increased with both added hygroscopic material and absorbed water. Water permeation energy varied from positive to negative with higher hygroscopic content. This observation is attributed to a lower diffusion energy and a relatively constant sorption energy when hygroscopic content increases. Comparatively, PVA/TEG has less corrosive problems and is more environmentally friendly than PVA/LiCl. A membrane with PVA/TEG is observed to be highly durable and is suitable for dehumidification applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44765.

Molecular dynamics simulation in the application of direct air dehumidification by electrodialysis method

The independent air dehumidification has attracted more and more attention in recent years. In this study, using the method of molecular dynamics simulation, water vapor molecules diffusion in the graphene oxide membrane by electrodialysis method is studied. We hypothesis that graphene oxide membrane can adsorb the water vapor molecules more easily under different electric fields. Meanwhile, the interactions between water molecules and the oxygen-containing functional groups of graphene oxide membrane play an important part in diffusion, including Van der Waals force, coulombic force and hydrogen bonding. However, the result reveals that with the increase of the magnitude of electric field strength, the diffusion coefficients of water vapor molecules change due to the strong hydrogen bonding and the polar orientation of water molecules. Therefore, external electric fields are conductive to the diffusion of water vapor molecules under the specific range of electric intensity value.