Performance evaluation of PVA-LiCl coated heat exchangers for next-generation of energy-efficient dehumidification

Applied Energy 237 (2019) 733–750

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Performance evaluation of PVA-LiCl coated heat exchangers for nextgeneration of energy-efficient dehumidification P. Vivekh, D.T. Bui, Y. Wong, M. Kumja, K.J. Chua

T

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Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore

H I GH L IG H T S

composite polymer desiccants for use in coated heat exchangers. • Developed discussion on desiccant’s fundamental characterization studies. • Detailed analysis on the effect of operating parameters on dynamic performance. • Thorough savings in specific power consumed by using composite polymer desiccant. • 54% • 20–60% improvement in dehumidification capacity and process efficacy.

A R T I C LE I N FO

A B S T R A C T

Keywords: Experiments Desiccant coated heat exchangers Composite polymer Dehumidification

The conventional vapor-compression air-conditioner operates with low efficiency because of the intrinsic coupling between sensible and latent cooling. Its efficiency can be improved via employing solid desiccant coated heat exchangers (DCHEs). Dehumidification performance of a DCHE is influenced by the nature of the selected desiccant material. The key attributes of a desiccating material include higher sorption capacity and faster kinetics coupled with its ability to regenerate at a low temperature. In this paper, we developed different concentrations of composite polymer desiccant with polyvinyl alcohol (PVA) and lithium chloride (LiCl). Experiments on isotherms indicated that the composite PVA with a greater concentration of LiCl displayed superior sorption capacity; however, due to the occurrence of deliquescence phenomenon, the most effective concentration of LiCl was observed to be 50w%. The equilibrium sorption capacity of PVA-LiCl (50w%) was 177.2% in contrast to only 28% for silica gel. Further, kinetics revealed that silica gel would take twice the time to adsorb an equivalent amount of water vapor as absorbed by composite polymer desiccant. Our experimental findings on dehumidification performance and process efficacy revealed that the use of composite PVA on DCHEs yielded about 20–60% improvement in moisture removal capacity and thermal coefficient of performance. Lastly, energy analysis indicated that the novel composite polymer DCHE enabled high moisture removal rate even at lower regeneration temperature and recorded a significant saving of 54% in specific power consumption.

1. Introduction In the next three decades, the demand for air-conditioning is estimated to be 2.5 times the values observed in 2010 [1]. Currently, mechanical vapor compression (MVC) technology is the de-facto choice for air-conditioning. Despite its wide success, two significant drawbacks associated with its operations have encouraged the need to seek for alternatives. First, the refrigerant usage in MVC systems is a source of global warming. Second, its process efficiency is low due to the coupling between sensible and latent cooling loads. In tropical climates,

⁎

MVC needs to overcool the air below its dew point temperature to remove moisture and further reheat it to achieve the required supply temperature [2]. As latent load may contribute up to 70% of the total cooling load, the prescription of a lower cooling coil temperature to condense moisture results in reduced energy efficiency [3]. Therefore, de-coupling between sensible and latent load using a desiccant dehumidification system is a necessary step to promote the energy efficiency of the air-conditioning process. Thus far, three types of solid desiccant dehumidifiers have been developed, namely, fixed bed, rotary bed/desiccant wheel, and

Corresponding author. E-mail address: [email protected] (K.J. Chua).

https://doi.org/10.1016/j.apenergy.2019.01.018 Received 22 October 2018; Received in revised form 5 December 2018; Accepted 1 January 2019 Available online 15 January 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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ω

Nomenclature A cp,w D d Ea f h H k L m ṁ n Nt q Qa Qw R R2 t T tcyc W Wd x y

pre-exponential factor in Arrhenius equation, s−1 specific heat capacity of water at constant pressure, kJ/ kg K mass diffusivity, m2/s tube diameter, mm activation energy, kJ/mol parameter used in the diffusivity model/function of independent variables for error analysis specific enthalpy, kJ/kg height, mm kinetic constant, s−1 length, mm mass, kg mass flow rate, kg/s or kg/h number of directly measured parameters for error analysis number of tube pass mass uptake by the desiccant, kg/kg or % average cooling capacity of the air, kW regenerating energy requirement, kW universal gas constant, kJ/mol K coefficient of determination time, s temperature, oC or K cycle time, min width, mm mass of water absorbed by the DCHE, g directly measured parameters for error analysis derived parameters for error analysis

Subscripts 0 a app cw d e f hw i in o out w

initial time air apparent cooling water desiccant/dry desiccant equilibrium fin hot water inner inlet outer outlet wet desiccant

Abbreviations Al COPth Cu DCHE LDF LiCl MRC MRR MVC NRMSE PLCHE PVA SAP SGCHE SPC

Greek symbols δ ΔP

humidity ratio, g/kg

thickness, mm or m pressure drop, Pa

desiccant coated heat exchangers (DCHEs). In the fixed bed systems, the desiccant is tightly packed in a bed. The desiccant adsorbs moisture from the air and provides dry air. Because of poor contact between the desiccant and the surface of the bed, the heat and mass transfer efficiency of the system is compromised [4]. For rotary bed/ desiccant wheel dehumidifiers, the desiccant is impregnated on a wheel. One portion of the wheel is used for dehumidification while the other undergoes regeneration. Lower pressure drop and large surface area for heat and mass transfer make the rotary wheel dehumidifier an improved performer as compared to the fixed bed dehumidifiers. However, in these systems, the outgoing air from the dehumidifier gets heated because of the sorption heat released by the desiccant during dehumidification process [5]. The total cooling load in air remains constant as the dehumidification process is isenthalpic, and the decrease in latent load is converted into an equivalent increase in sensible load. DCHEs were developed to reduce the total cooling load alongside realizing improved dehumidification [6]. A schematic representing the working principle of the DCHEs under dehumidification and regeneration processes is given in Fig. 1. The desiccants are coated on heat exchanger fins, and isothermal dehumidification is achieved by passing cooling water through the tubes to capture the heat of sorption. The regeneration efficiency is also enhanced as the hot water internally heats the heat exchanger. In addition, copper (Cu) tube and aluminum (Al) fins with excellent thermal conductivity promote the heat transfer rate. These inherent advantages over fixed-bed and desiccant wheels establish an extraordinary potential of DCHEs as an energy efficient standalone all-weather air-conditioning technology [7]. In addition, with appropriate selection of desiccants, DCHEs can be specifically

aluminum thermal coefficient of performance copper desiccant coated heat exchanger linear driving force lithium chloride moisture removal capacity, g/kg moisture removal rate, g/h mechanical vapor compression normalized root mean square error PVA-LiCl (50w%) coated heat exchanger polyvinyl alcohol super absorbent polymer silica gel coated heat exchanger specific power consumption, Wh/g

engineered and employed for many real-life applications including heat pumps [8], adsorption chillers [9], and atmospheric water harvesters [10] etc. The performance of DCHEs is highly dependent on the choice of the desiccant material. Important characteristics of a desiccant include high sorption capacity, fast kinetics, high stability, and ability to regenerate at a low temperature [11]. Over the years, several desiccants have been developed to realize improved energy efficiency and dehumidification

Fig. 1. Schematic representation of (a) dehumidification and (b) regeneration processes in DCHEs. 734

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polymers, when combined with an appropriate concentration of hygroscopic salts, possess greater water sorption capacity along with their affinity to retain water up to 2–3 times of its weight. The dehumidification and energy efficacy of pure/composite silica gel DCHEs are limited by the desiccant’s pore characteristics and the requirement of higher regeneration temperature. Since DCHEs’ performance depends on desiccant’s density and coating effectiveness in addition to its water sorption capacity, a comprehensive dynamic performance analysis is essential to determine the viability of this novel composite polymer desiccant. The existing literature on polymer desiccants is only limited to desiccant wheels, thin film and electrospun fiber-based membrane dehumidifiers. No research has been carried out so far to evaluate the key performance parameters of composite polymer based DCHEs. In the present work, we aim to develop composite polymer desiccants using PVA and LiCl and determine the most effective salt concentration suitable for the DCHEs. We then proceed to coat a heat exchanger with the selected composite desiccant and study its dehumidification performance and energy efficiency under different operating conditions. Lastly, we benchmark the performance of the novel composite polymer DCHE with the widely used silica gel coated heat exchangers. The following questions will be addressed in detail to clarify the novelty of this work: (1) What is the most favorable concentration of the hygroscopic salt to be used in the polymer desiccants that is suitable for DCHEs? (2) What is the effect of the salt concentration on polymer desiccant’s kinetics, and how does it differ from the kinetics of silica gel?; and finally, (3) To what extent does the moisture removal capacity, thermal coefficient of performance, and specific power consumption improve by using the novel composite PVA coated heat exchangers?

effectiveness. Silica gel is one of the most popular solid desiccants used in DCHEs due to its low cost, high porosity, and excellent stability [12]. However, its water sorption capacity is limited to 20–30% of its initial weight, and it also has higher requirements of regeneration temperature 60–100 °C [13]. Zhao et al. [5] and Li et al. [14] carried out dehumidification performance analysis of different types of silica gel based DCHEs. SAPO-34 and FAPO-34 were developed to realize superior sorption capacity. Their incorporation in DCHEs improved dehumidification performance by about 20–30% as compared to silica gel DCHEs [15,16]. Hu et al. [17] infused lithium chloride (LiCl) and calcium chloride (CaCl2) salt solution in silica gel and observed 30–45% increase in the sorption capacity. Also, composite silica gel coated heat exchangers infused with LiCl performed better than the corresponding CaCl2 ones. Zheng et al. [18] recorded a marked increase in thermal conductivity of about 90% from 3Wm - 1K - 1 for pure silica gel to 5.8 W m - 1 K - 1 for composite silica gel with 40w% LiCl. A 25–45% increase in dehumidification capacity was also recorded with 40w% LiCl in silica gel. Ge et al. [19] impregnated 75w% potassium formate with silica gel and recorded 2–3 times increase in the adsorption capacity. Zheng et al. [20] combined activated carbon/activated carbon fiber with LiCl and developed composite carbon desiccants, which demonstrated 3–4 times enhancement in the sorption capacity as compared to pure silica gel. Despite this significant increase, due to the low density of activated carbon fiber, the improvement in its dehumidification performance was minimal. This observation highlights that in addition to equilibrium capacity, the performance of a DCHE also depends on its coating effectiveness and desiccant’s density. As a result, a detailed dynamic performance analysis is imperative while studying the feasibility of a desiccant in DCHEs. With continuous advancements made in material science, synthetic polymers were considered as alternatives to conventional desiccants. In the late 1960s, the United States Department of Agriculture developed new cross-linked polymers using acrylates and acrylamides capable of holding water molecules up to 400% of its initial weight. These materials were coined as superabsorbent polymers (SAP) and have been adopted for use in agriculture and diaper industries [21]. In the early 1990s, polymers were studied as possible materials to evolve the nextgeneration of advanced desiccants. As many as 30 different types of amines, acrylates, and cross-linked polymers were synthesized because of their excellent hydrophilicity and high sorption capacity [22,23]. However, these polymers showed degradation in their sorption capacity after a few months of storage. The reason for such a deterioration of the capacity was not studied thereafter [24,25]. More recently, White et al. [26] compared the dehumidification performance of silica gel, zeolite, and SAP desiccant wheel and concluded that the SAP-based desiccant wheel recorded the highest dehumidification performance at low regeneration temperature and high humidity. Higashi et al. [27] coated SAPs on the DCHEs and reported that the SAP coated heat exchangers have better mass diffusivity than silica gel coated heat exchangers by 1–2 orders of magnitudes. Lee et al. [11] extended the idea of using SAP with hygroscopic salts and developed a composite super desiccant polymer for desiccant wheels. Its sorption capacity was 2–3 times better than silica gel, and the performance of the desiccant wheel was found to be sustainable even after 40,000 operating cycles. Yang et al. [28] impregnated SAP in 10.6w% LiCl solution and obtained doubled sorption capacity when compared to pure SAP desiccants. Chen et al. [29] prepared a composite desiccant comprising silica gel and SAP and found that the sorption capacity was enhanced with higher ratios of polymer content in the composite desiccant. Cao et al. [30] observed improved dehumidification performance of thin polymer desiccant wheels at high humidity ratio and low inlet air temperature. Bui et al. [31,32], Yao et al. [33], and Zhang et al. [34] used polyvinyl alcohol (PVA) due to its high water absorption ability and developed composite thin film polyvinyl alcohol with LiCl for membrane dehumidification applications. Results from the above studies indicate that superabsorbent

2. Experimental methods 2.1. Desiccant solution preparation Silica gel (RD 780), hydroxyethyl cellulose (HEC), anhydrous lithium chloride (LiCl), and polyvinyl alcohol (PVA) powders were used to prepare desiccant coating solutions. A 5w% silica gel suspension solution was prepared by adding silica gel to water under continuous stirring at room temperature. 3.3w% HEC powder was added to the silica gel solution to achieve suitable binding of silica gel on the Al fin [14]. PVA and appropriate amounts of LiCl were mixed with water to produce PVA/LiCl solutions with five concentrations (0w%, 16.7w%, 33.3w%, 50w%, and 66.7w%) of LiCl.

2.2. Isotherms and kinetics Water vapor sorption isotherms describe the equilibrium amount of water vapor absorbed/adsorbed by the desiccant as a function of relative humidity (RH). While the isotherms point out the equilibrium amount, kinetics determine the rate at which the desiccant attains its equilibrium capacity. Water sorption isotherms and kinetics were measured using AQUADYNE DVS gravimetric dynamic water sorption analyzer. The accuracies of gravimeter, relative humidity, and temperature are ± 1 μg, < ± 2% and ± 0.2 °C respectively. Before any measurements were conducted, the gravimetric chamber was purged with N2 gas and degassed at 80 °C for about 5 h to ensure that the desiccant is completely dry. To obtain the water sorption isotherms, the gravimetric chamber was kept at a constant temperature, and RH was increased in steps from 0 to 95%. The maximum time for equilibrium was specified as 6 h. The instantaneous water uptake, as defined in Eq. (1), was either recorded once in 20 s or when the mass change was over 0.01%. The gravimetric analyzer was programmed to increase the RH stepwise when the change in the mass recorded by gravimeter was less than 0.001% per minute. 735

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qt =

m w − md md

and convection between air and desiccant. The kinetic constant of the moisture sorption process represents the overall mass transfer coefficient between the air and the desiccant [38]. However, the moisture diffusion inside the desiccant is a dominating factor that limits the reaction rate and quantifying this diffusion resistance is a key issue in developing a consistent heat and mass transfer model. To deduce the diffusivity of the composite PVA desiccant from the kinetics experiments, a modified version of the one-dimensional Fick’s 2nd law of diffusion is employed and represented in Eqs. (6)–(8) [11,39]. The modification is necessary to account for the effect of swelling in the synthetic polymers. Gouanve et al. [39] introduced the divided time region approximation and proposed that the apparent diffusivity ob-

(1)

where qt (kg/kg) is the instantaneous water uptake by the desiccant; mw (kg) is the mass of the wet desiccant during the sorption/desorption process; and md (kg) is the mass of the dry desiccant. Water vapor sorption/desorption kinetics depend on the surface area and the thickness of the desiccant. To accurately compare the kinetics of different desiccants, their geometrical aspects were made to be identical. Al sheets of 10 mm × 10 mm × 0.5 mm were cut and a few drops of the desiccant solution were evenly spread on their surface. The coated sheets were then placed in the oven at 100 °C for 5–6 h to obtain a thin desiccant layer. The amount of the desiccant coated on the sheet was in the range of 15–20 mg and the desiccant thickness was measured to be 0.14–0.2 mm. To study the sorption kinetics, the temperature of the gravimeter was regulated between 20 and 40 °C at a constant RH of 80%. For desorption kinetics, the temperature was controlled from 40 to 80 °C at 0% RH. The sorption and desorption experiments were performed alternatively, and the instantaneous water uptake profile of the desiccant was recorded. In an effort to represent sorption kinetics mathematically, many models such as linear driving force (LDF) approximation, semi-infinite model, Fickian diffusion, and second order approximation were developed [35–37]. LDF approximation is one of the most widely used models to fit the experimental data on the kinetics of a variety of desiccants and offers considerable benefits due to its simplicity and consistency. Therefore, we employed LDF approximation to model the kinetics of the composite PVA desiccant. The LDF approximation describes the rate of change of the instantaneous uptake of the desiccant

(

tained during the second half of the sorption process, i.e.

q ⎞ ⎛ π 2t 8 = ln 2 − Dapp, d 2 at t50 < t < te ln 1 − t ⎟ ⎜ qt f / f π 4δd ⎠ ⎝

(6)

where qt f (kg/kg) is the mass absorbed by the desiccant at time tf, Dapp,d (m2/s) is the apparent diffusivity, and δd (m) is the thickness of the desiccant layer.

f=

qt f qe

(7)

f is valid under the condition that the instantaneous water vapor uptake 3 min before tf is equal to 1% of the uptake from the beginning of the sorption process until tf.

dqt dt

vapor sorption/desorption, which is the difference between the equilibrium water vapor absorbed by the desiccant and the instantaneous mass of the water vapor absorbed by the desiccant (qe − qt ) . The rate of water vapor absorbed/adsorbed by the desiccant at any time, t, can be represented as

qt f − qt f − 180 qt f − q0

= 0.01 (8)

where qt f (kg/kg) is the instantaneous water uptake at time tf, qt f − 180 (kg/ kg) is the instantaneous water uptake 180 s before tf.

(2) 2.4. Desiccant coating on fin-tube heat exchangers

where qt (kg/kg) is the instantaneous mass absorbed/adsorbed by desiccant, qe (kg/kg) is the equilibrium mass absorbed/adsorbed by desiccant, q0 (kg/kg) is the initial water content present in desiccant, and k (1/s) is the kinetic constant. Appropriate limits are applied to the sorption and the desorption processes, and the final form of the equations used for fitting sorption and desorption kinetics are given in Eq. (3) and Eq. (4), respectively.

qt = qe − exp{ln(qe − q0) − kt }

(3)

qt = qe + exp{ln(q0 − qe ) − kt }

(4)

Fin-tube heat exchangers were designed and manufactured based on the geometrical specifications listed in Table 1. Before coating the desiccant, the heat exchangers were cleaned with sodium hydroxide and distilled water. The clean and dry heat exchangers were then dip-coated with the prepared desiccant solutions and dried in the oven at 100 °C for 2–3 h. The heat exchangers were weighed using a weighing balance before and after coating to determine the amount of desiccant coated. The coating process was carried out multiple times until around 50 g of the desiccant was coated on the fins of each heat exchanger. Fig. 2 illustrates the desiccant coating on a typical fin-tube heat exchanger. Further, pure Al fin, silica gel coated Al fin, and composite PVA desiccant with 50w% LiCl concentration were examined under a scanning electron microscope (SEM). Fig. 3 (a) shows smooth nature of the pure Al sheet without any desiccant coating. Fig. 3 (b)-(d) show silica gel

It is apparent that sorption or desorption rate of the desiccant is affected by the variations in kinetic constant and/or driving force for water vapor sorption/desorption. In this study, by varying the desiccant temperature and regulating the concentration of LiCl, the mechanism behind the change in sorption and desorption kinetics rate of the composite desiccant will be investigated. Further, the dependence of the kinetic constant on temperature is studied by Arrhenius equation,

E k = A exp ⎛− a ⎞ ⎝ RT ⎠

)

> 0.5

provides an accurate estimation of the overall diffusivity of moisture within the desiccant. The parameter f is defined as the ratio of instantaneous uptake at the time tf to the equilibrium uptake of the desiccant.

( ) as a function proportional to the driving force for water

dqt = k (qe − qt ) dt

qt qe

Table 1 Geometrical specifications of the heat exchanger. Parameters

Values

where Ea (kJ/mol) is the activation energy, A (1/s) is the pre-exponential factor, R (kJ/mol K) is the universal gas constant, and T (K) is the absolute temperature.

Length (L) Width (W) Height (H) Fin thickness (δf )

200 mm 150 mm 22 mm 0.12 mm

2.3. Moisture diffusion in the desiccant

Fin pitch Inner tube diameter (di ) Outer tube diameter (do) No. of tube pass (Nt )

1.6 mm 9.5 mm 10.5 mm 6

(5)

The moisture transport phenomenon is a combination of diffusion 736

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Fig. 2. (a) Uncoated fin-tube heat exchanger; (b) silica gel coated heat exchanger; (c) PVA-LiCl (50w%) coated heat exchanger; and (d) a schematic showing the desiccant layer on the fin-tube heat exchanger.

DCHE dehumidification system are shown in Fig. 4. It comprises numerous components such as a DCHE chamber, an air heater, an ultrasonic humidifier, two water baths, and a fan. The DCHE chamber consists of two heat exchangers installed in a V-shaped configuration at an angle of 30°. Such an arrangement enhances the frontal interaction between the desiccant and the moist air and develops laminar flow [27]. In the chamber, both heat exchangers undergo either dehumidification or regeneration processes. The interchange between the dehumidification and the regeneration processes is carried out by switching the air and the water valves. During the dehumidification process, the outside air was passed into the air heater and followed by the ultrasonic humidifier to maintain a stable temperature and moisture content. The humid air was then

coating on the Al sheet at different magnification scales. At 200x magnification the silica gel coating appears similar to different granules attached to each other. At 1500x and 10000x magnifications, individual silica gel particles can be observed. The average length of the silica gel particle is about 5 μm. Similarly, SEM images of PVA-LiCl (50w%) coating are illustrated in Fig. 3 (e)-(g). At 200x magnification, the PVALiCl (50w%) coating appears smooth and the particles can be visualized at 3000× and 10,000× magnifications. The PVA-LiCl (50w%) desiccant particles appear cloudlike and their average length is 1 μm. 2.5. Dynamic performance measurement experimental setup The schematic and the photograph of the experimental setup of the

Fig. 3. SEM images of (a) pure Al fin; (b)-(d) silica gel coated Al fin at 200×, 1500×, and 10,000× magnifications, respectively; and (e)-(g) PVA-LiCl (50w%) coated Al fins at 200×, 3000×, and 10,000×, respectively. 737

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Fig. 4. (a) Schematic of the experimental setup for measuring DCHE’s dehumidification performance, and (b) photograph of the experimental setup.

made to exit from the system. During the experiment, dry-bulb and wetbulb temperatures were measured using 10 k thermistors ( ± 0.15 °C) with a 3-s interval time. The temperature sensors were installed at the inlet and the outlet of the DCHE chamber to measure dry and wet bulb temperatures. The other properties of moist air, such as relative humidity, humidity ratio, and enthalpy were derived using traditional psychrometric equations. The airflow rate was measured using a thermal mass flow meter ( ± 0.5%), and the water flow rate was recorded using a vortex flow meter ( ± 2.5%). The pressure drop (ΔP) in the air-flow side of the DCHE was measured using a fluid-filled manometer ( ± 1%). A data acquisition system from Agilent (34970A) and

allowed to interface with the DCHE. The desiccant absorbed the moisture and dehumidified the air. At the same time, the cooling water flowing through the tubes captured the heat of sorption released by the desiccant. The cool and dry air was supplied to the room by the fan. Once the dehumidification process was complete, the outdoor air valve was closed while the indoor air valve was opened. Further, hot water valves were opened, and the cold-water valves were closed. The air was made to flow through the by-pass line until the testing conditions for regeneration were achieved. Subsequently, the by-pass line was closed, and the air was made to pass through the DCHE system to remove the moisture. The hot and humid exhaust air from the DCHE chamber was

Table 2 Specifications of the sensors and measurement instruments used in the DCHE performance testing setup. Parameter

Sensor/Instrument

Range

Accuracy

Temperature Air flow rate Water flow rate Pressure drop

10 k thermistor Thermal mass flow meter Compact vortex flowmeter Fluid filled manometer

−10–100 °C −10–100 °C 3.2–22 L/min 0–50 mm H2O

± 0.15 °C ± 0.5% (Full Scale) ± 2.5% (Full Scale) ± 1.0% (Full Scale)

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specific heat capacity of water at constant pressure, Thw, in (oC) and Thw, out (oC) are the respective inlet and outlet temperatures of the hot water flowing inside the tubes of heat exchanger during the regeneration process. The process efficacy of the DCHE system is expressed in terms of the thermal coefficient of performance (COPth), which is defined as the ratio of the average cooling capacity of air (Qa) to the average heat exchange rate of hot water during regeneration process (Q w ) and is represented as

LabVIEW 2015 were used to record the data on a computer. Table 2 summarizes the details of the sensors and measurement instruments used in the experimental setup. The uncertainties of the directly measured parameters were determined by the accuracy of the sensors while the error propagation method [40] was employed to compute the uncertainties of the derived parameters. n

Δy =

2

⎡ 2⎤ ⎛ ∂f ⎞ ⎢∑ ∂x (Δx i ) ⎥ i ⎠ ⎝ i = 1 ⎦ ⎣ ⎜

⎟

(9)

where Δy is the absolute uncertainty of the derived parameters, Δx is the absolute uncertainty of the directly measured parameters, n is the total number of directly measured parameters, f is the function of the independent variables.

COPth =

Qa Qw

(14)

Ideally, the thermal energy used for regeneration should translate in to an equivalent cooling capacity of the desiccant. However, due to the losses involved in the system and its thermal mass, the cooling capacity obtained by the DCHE system is typically lower than its regeneration energy requirement. For COPth calculation, the electrical power of the fans is considered marginal, and the heat energy is the primary source of energy. Since COPth compares the heating energy requirement with the reduction in the cooling load of the inlet process air, an additional parameter – specific power consumption – is required to calculate the energy efficiency of the dehumidification process. The moisture removal rate (MRR) (g/h) and the specific power consumption (SPC) (Wh/g) of a DCHE based dehumidification system are evaluated using Eq. (15) and Eq. (16), respectively.

2.6. Operating conditions for dynamic performance testing Table 3 specifies the list of different operating parameters controlled in this study. By changing inlet air temperature and humidity ratio, the performance of DCHE under different possible outdoor conditions experienced in tropical climates was studied. Parameters such as air flow rate, cooling water temperature, and hot water temperature were judiciously controlled to identify the most suitable condition of maximum dehumidification capacity, high process efficacy and minimum specific power consumption. Since the cycle time depends on the desiccant’s water sorption characteristics, an appropriate cycle time for silica gel and composite PVA based DCHEs was determined by varying the time between 5 and 30 min. The inlet air temperature was maintained at 30 °C for both dehumidification and regeneration processes. During dehumidification, the inlet air humidity ratio was regulated between 17.5 and 21.5 g/kg as listed in Table 3, whereas it was kept constant at 10 g/kg during regeneration process. The water flow rate was maintained at 4 kg/min throughout. While evaluating the effect of change of a particular condition, the other parameters listed in Table 3 were maintained at their baseline conditions.

MRR = MRC × ṁ a SPC =

(15)

ṁ w cp, w (Thw, in − Thw, out + Tcw, out − Tcw, in ) (16)

MRR

where Tcw, in (oC) and Tcw, out (oC) are the respective inlet and outlet temperatures of the cooling water flowing inside the tubes of heat exchanger during the dehumidification process. 3. Results and discussion

2.7. Performance parameters 3.1. Water sorption isotherms The moisture removal capacity (MRC) (g/kg) of the DCHE system during the cycle time (tcyc) is defined as

MRC =

1 tcyc

∫0

tcyc

(ωa, in − ωa, out ) dt

Fig. 5 (a) shows that silica gel’s isotherm shape is represented by the IUPAC’s Type-I classification [41], and its maximum equilibrium sorption capacity at 30 °C is about 25–30% of its initial weight. Because silica gel is a highly porous desiccant with large BET surface area, the overall moisture adsorption mechanism involves monolayer/multilayer surface adsorption and capillary condensation in the pores [42]. During the onset of its adsorption process, the pores are empty, and thereafter, water vapor molecules condense and occupy the pores. When the pores are completely filled with water molecules, the silica gel can no longer adsorb moisture. It is apparent that the sorption capacity of silica gel is markedly limited by its pore characteristics like porosity, BET surface area, and pore volume. Oh et al. [43] carried out a detailed analysis on the isotherms of silica gel between 25 and 45 °C, and they observed that the equilibrium sorption capacity does not change significantly with the changes in temperature. They further concluded that RH alone facilitates the driving force for moisture sorption, thus, resulting in higher

(10)

where ωa, in (g/kg) and ωa, out (g/kg) are the air humidity ratio values at inlet and outlet of the DCHE chamber, respectively. The total mass of water absorbed is calculated as

Wd = ṁ a (MRC) tcyc

(11)

where Wd (g) is the total mass of water absorbed by the DCHE system and ṁ a (kg/s) is the mass flow rate of air. MRC measures the average amount of moisture removed per kg of the dry air flowing over the DCHE for a given duration. When either tcyc or ṁ a is changed, the amount of moisture absorbed by the DCHE may vary depending on the overall effect caused by MRC. Therefore, Wd is also a key parameter while studying the effect of tcyc or ṁ a . The average cooling capacity of air (Qa) (kW) is calculated using Eq. (12) where ha, in (kJ/kg) and ha, out (kJ/kg) represent the specific enthalpy of process air at inlet and outlet, respectively, during the dehumidification process.

Qa = ṁ a (ha, in − ha, out )

Table 3 Operating conditions for DCHE performance testing.

(12)

Likewise, the heating energy requirement (Q w ) (kW) during regeneration is computed using

Q w = ṁ w cp, w (Thw, in − Thw, out )

(13)

where ṁ w (kg/s) is the mass flow rate of water, cp, w (kJ/kgK) is the 739

Parameters

Units

Baseline conditions

Change range

Cycle time (tcyc) Air flow rate (ṁ a) Inlet air temperature (Ta,in) Inlet air humidity ratio (ωa, in) Cooling water temperature (Tcw,in) Hot water temperature (Thw,in)

min kg/h o C g/kg

– 55 30 21.5

5–30 35–65 30–36 17.5–21.5

o

30 80

25–35 50–80

o

C C

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Fig. 5. Water vapor sorption isotherms at 30 °C for (a) silica gel and (b) PVA with 0w%-66.7w% LiCl.

equilibrium capacities at higher RH. The sorption isotherms of the composite PVA with different concentrations of LiCl at 30 °C are presented in Fig. 5 (b). The water vapor uptake process in the composite polymer desiccant is due to a combination of several mechanisms – monolayer/multilayer adsorption in the pores, disassociation of the polymer ions, swelling of the particles, and absorption of water vapor [37]. When the water vapor occupies the pore of polymeric desiccants, the monomers in the chain dissociate into ions and form a bond with water. At higher RH, as the driving force for sorption becomes more significant, the dissociation of the polymer ions is enhanced. These ions further produce repulsive forces causing the polymer to swell and increase its interstitial volume, which results in attracting more water molecules. In addition, the presence of LiCl in the polymer attracts more amount of water vapor molecules on the surface of the polymer, enhances the dissociation of polymer ions, and consequently, increases the swelling in the polymer molecules. Therefore, the sorption capacity of the composite polymer desiccant is markedly improved at higher RHs. Fig. 6 depicts the state of PVA-LiCl (50w%) and PVA-LiCl (66.7w%) coated Al sheets after moisture sorption. While the PVA-LiCl (50w%) coating remains intact, the PVA-LiCl (66.7w%) liquidifies and is washed off from the Al sheet. Therefore, the most favorable limit is confined to 50w% LiCl in PVA although with respect to the equilibrium mass absorbed, a higher LiCl concentration is deemed to be favorable. At 80% RH, the equilibrium sorption capacity of PVALiCl (50w%) is 177.2% in contrast to only 28% for silica gel. The marked increase in the sorption capacity of the composite polymer

Table 4 Regression results of initial water content, kinetic constant, and equilibrium water sorption capacity obtained by using LDF approximation. Desiccant

Nature

q0 (kg/ kg)

k (1/s)

qe (kg/kg)

R2

NRMSE

Silica gel

Sorption Desorption

0.03 0.27

2.36 × 10−3 3.42 × 10−3

0.26 5.17 × 10−5

0.9942 0.9842

2% 4%

PVA-LiCl (50w %)

Sorption Desorption

0.15 1.78

1.09 × 10−4 5.93 × 10−4

1.78 0.22

0.9866 0.9988

2% 3%

desiccants has paved the way for them to be possible alternatives to silica gel in DCHEs. 3.2. Water sorption-desorption kinetics Table 4 and Fig. 7 portray the experimental and regression results of sorption and desorption kinetics modelled using LDF approximation for silica gel and PVA-LiCl (50w%) coated Al sheets. The coefficient of determination (R2) is close to 1 for both sorption and desorption kinetics of silica gel and PVA-LiCl (50w%), and the normalized root mean square error (NRMSE) varies between 2 and 4%. As a result, the LDF approximation is sufficient to theoretically represent the kinetics of the silica gel and composite polymer desiccant coated Al sheets. Yao et al.

Fig. 6. Photographs of (a) PVA-LiCl (50w%) and (b) PVA-LiCl (66.7w%) coated Al sheet after moisture sorption. 740

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Fig. 7. Experimental and regression results of sorption kinetics of (a) silica gel and (b) PVA-LiCl (50w%) at 30 °C and 80% RH; and experimental and regression results of desorption kinetics of (c) silica gel and (d) PVA-LiCl (50w%) at 60 °C and 0% RH.

3.2.1. Effect of LiCl concentration at constant temperature The effect of LiCl concentration on desiccant kinetics is investigated at 30 °C and 80% RH for sorption process and at 60 °C and 0% RH for desorption process. The experimental results for sorption kinetics are shown in Fig. 8 (a). It is apparent that the sorption rate for the composite desiccants is higher with increasing concentration of LiCl. For

[33] concluded that there is a need for second order kinetics model to accurately represent the sorption and desorption kinetics of composite PVA membranes. However, our inference on the adequacy of the LDF approximation is noteworthy one as it provides a simplistic yet robust approach to capture the kinetics of composite PVA desiccants.

Fig. 8. Experimental result of composite PVA with increasing concentration of LiCl from 0w% to 66.7w% for (a) sorption kinetics at 30 °C and 80% RH and (b) desorption kinetics at 60 °C and 0% RH. 741

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40 °C. Similarly, when the temperature is raised from 40 to 80 °C, the desorption kinetic constants is promoted from 4.2 × 10−4 to 1 8.64 × 10−4 s . The desorption kinetic constant is one-order of magnitude higher than the sorption kinetic constant because of the higher process temperature. Since the equilibrium capacity is kept constant, the enhancement in the sorption and desorption kinetics is attributed to the increase in the kinetic constant. Since the kinetic constant of PVA-LiCl (50w%) depends largely on the process temperature, the Arrhenius equation can be relied on to correlate its dependence and determine the activation energy for water vapor sorption and desorption processes. Fig. 12 shows the Arrhenius plot while Table 5 depicts the activation energy and the pre-exponential factor during sorption and desorption processes. The reason behind increasing kinetic rates during desorption process can be understood by comparing the desorption activation energy with that of the sorption process. As the sorption activation energy is twice as that of the desorption activation energy, the water vapor molecules possess greater inertia for migration during the sorption process resulting in slower kinetics.

instance, in case of the composite PVA-LiCl coated Al sheets, after 5000 s, the amount of mass absorbed is 0.1 kg/kg, 0.4 kg/kg, 0.56 kg/ kg, 0.8 kg/kg, and 0.9 kg/kg for 0w%, 16.7w%, 33.3w%, 50w%, and 66.7w% of LiCl, respectively. The kinetic constant and equilibrium sorption capacity of the desiccants are regressed using Eq. (3) and presented in Fig. 9 (a). The variation in the kinetic constant, due to the increase in LiCl concentration, is observed to be insignificant for PVA based composite desiccants. However, consistent with the results on isotherms, the equilibrium sorption capacity of the composite PVA increases from 0.139 to 2.172 kg/kg, when the LiCl concentration is regulated from 0 to 66.7w%. Improvement in the equilibrium sorption capacity facilitates a greater driving force for water vapor sorption, and accordingly, promotes faster kinetics. In the same vein, for desorption kinetics, the experimental and regression results of composite PVA coated Al sheets with different concentrations of LiCl are shown in Fig. 8 (b) and Fig. 9 (b), respectively. Here, the parameters in the LDF approximation are regressed using Eq. (4). The desorption rate of the composite desiccant is higher with increasing LiCl concentration. This observation is primarily attributed to the higher level of initial water content in the desiccant, which facilitates higher driving force for the moisture desorption. On the contrary, there is no significant trend for desorption kinetic constant with respect to LiCl concentration in PVA. The desorption kinetic constant first in1 creases from 1.0 × 10−3 to 1.31 × 10−3 s when 16.7w% LiCl is added.

3.2.3. Comparison of PVA-LiCl (50w%) with silica gel Fig. 13 depicts the sorption kinetics of PVA-LiCl (50w%) and silica gel coated Al sheets at 30 °C and 80% RH. Silica gel saturates around 2500 s with about 0.27 kg water vapor per kg of dry silica gel, yet PVALiCl (50w%) significantly absorbs water vapor even over 10,000 s. At 2500 s, the amount of water vapor absorbed by PVA-LiCl (50w%) per kg of its dry weight is 0.57 kg, which is double the water vapor absorbed by silica gel. This indicates that silica gel would take twice the time to adsorb an equivalent amount of water vapor as that of composite polymer desiccant. The reason for faster kinetics for composite PVA is attributed to the marked increase in its equilibrium capacity, as observed in Section 3.1. Higher equilibrium capacity creates a stronger driving force for water vapor sorption and thereby, supports faster kinetics.

1

However, it drops to 5.68 × 10−4 s upon further addition of LiCl to 33.3w%. When the LiCl concentration is increased to 50w%, the kinetic 1 constant is marginally improved to 5.93 × 10−4 s and then levels to 1

3.59 × 10−4 s when 66.7w% LiCl is added to PVA. Despite the marginal decrease in kinetic constant, the overall desorption rate is still higher because the improvement in the water vapor desorption driving force outweighs the decrease in the kinetic constant.

3.2.2. Effect of temperature at constant LiCl concentration Experimental results concerned with the sorption/desorption kinetics of PVA-LiCl (50w%) coated Al sheets with respect to increasing temperature are given in Fig. 10. At higher temperatures, both sorption and desorption rates of the composite PVA are enhanced due to better kinetic constants. The equilibrium capacity and the kinetic constant that are regressed from the experimental data are represented in Fig. 11. The equilibrium water sorption capacity of the PVA-LiCl (50w %) remains at 1.78 kg/kg. Apparently, a higher temperature does not affect qe because the equilibrium sorption capacity is primarily a function of the desiccant’s RH. Accordingly, any temperature increase with a constant RH does not affect qe. Nevertheless, the kinetic constant for both sorption and desorption of PVA-LiCl (50w%) appreciates as a function of temperature. The sorption kinetic constant is enhanced from 1 7.5 × 10−5 to 1.8 × 10−4 s when the temperature is regulated from 20 to

3.2.4. Mass diffusivity of PVA-LiCl (50w%) Fig. 14 (a) and Table 6 demonstrate the capability of the modified one-dimensional Fick’s 2nd law to predict the moisture diffusivity in PVA-LiCl (50w%) desiccant with NRMSE of 3–7%. When the RH is maintained at 80%, the driving force for moisture transport is constant, and a temperature change between 20 and 40 °C is able to boost the rate at which water vapor molecules can diffuse in the desiccant medium. Consequently, the apparent diffusivity increases from 8.95 × 10−13 to 1.93 × 10−12 m2s−1 as illustrated in Fig. 14 (b). 3.3. Dehumidification performance evaluation From the results presented in Section 3.1, composite PVA with 50w

Fig. 9. (a) Variation of equilibrium uptake capacity and kinetic constant for sorption process at 30 °C and 80%RH; and (b) variation of initial water content and kinetic constant for desorption process at 60 °C and 0% RH for composite PVA with increasing LiCl concentration from 0w% to 66.7w%. 742

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Fig. 10. Experimental result of PVA-LiCl (50w%) for (a) sorption kinetics with temperature ranging from 20 to 40 °C at 80% RH and (b) desorption kinetics with temperature ranging from 40 to 80 °C at 0% RH.

can absorb moisture for a longer duration. A similar explanation is provided for the regeneration process as ωa, out increases sharply at first, reaches a peak value, and then steadily declines till it levels off with ωa, in . From Fig. 15 (a), two important observations can be made by comparing the dehumidification and the regeneration processes of SGCHE and PLCHE. First, as observed in Sections 3.2.2 and 3.2.4, due to the high temperature, the effective time for regeneration is much shorter than the corresponding dehumidification time. SGCHE needs about 220 s to fully regeneration whereas PLCHE takes over 500 s. Second, the magnitude of the peak of ωa, out is higher for regeneration. The positive influence of moisture diffusivity at higher temperatures is the reason for shorter regeneration time and higher peak of ωa, out .

% LiCl is judiciously selected to be coated on a heat exchanger due to its excellent water vapor sorption characteristics and ability to tackle deliquescence. Also, silica gel was coated on a heat exchanger to benchmark the existing DCHE technology’s performance and compare with the novel polymer based DCHE. For easy referencing, silica gel coated heat exchangers and PVA-LiCl (50w%) coated heat exchangers will be abbreviated as SGCHE and PLCHE, respectively. Fig. 15 (a) displays the experimental results of dynamic dehumidification and regeneration processes of both SGCHE and PLCHE systems. During the onset of the dehumidification process, the outlet air humidity ratio (ωa, out ) drops sharply, attains a minimum value, and gradually increases until it equals the inlet air humidity ratio (ωa, in ) . At this point, no further moisture can be removed from the process air. This sharp decrease followed by the gradual increase in ωa, out can be explained by the following factors. Firstly, the desiccant layer in contact with the air is dry during the start of dehumidification; therefore, the driving force is high and the desiccant layer at the interface quickly absorbs moisture. However, as this layer reaches saturation, moisture needs to diffuse inside the desiccant to allow the sorption of new water molecules. Since, the diffusivity of moisture in the desiccant

(D

3.3.1. Effect of cycle time (tcyc ) Fig. 16 illustrates that when the tcyc is regulated from 5 to 30 min, dehumidification capacity and process efficacy are negatively affected. MRC of SGCHE and PLCHE depreciate by 80% and 59%, respectively. In addition, COPth of SGCHE and PLCHE also decline by 86% and 57%, respectively. On an average, MRC and COPth of PLCHE are about 120% and 250% higher than that of SGCHE. Wd of SGCHE increases marginally from 21.5 to 26.2 g. The increase of a mere 4.7 g for six times rise in tcyc is responsible for the poor process efficacy. As a result, the most appropriate tcyc for SGCHE is around 5 min. In contrast, Wd of PLCHE improves from 32.3 g at 5 min to 68.8 g at 30 min. Although an additional 36.5 g of moisture is absorbed by PLCHE between 5 and 60 min, about 60% of this additional moisture is absorbed between 5 and 10 min. Accordingly, the moisture absorbed is insignificant with respect to the longer tcyc. COPth reduces by less than 10% when tcyc is increased from 5 to 10 min. Therefore, a longer tcyc of 10 min is

m2 s

) is much lower than the diffusivity of moisture in the air (D = 2.58 × 10 ), the moisture transfer is constrained, −13 app, d ~10

− 10−10

a

2 −5 m s

and the mass absorbed by the desiccant is gradual. Fig. 15 (b) shows a schematic of the transport of water molecules from the air to the desiccant. The peak ωa, out in the dehumidification process is similar for both SGCHE and PLCHE. However, ωa, out of SGCHE sharply approaches ωa, in and becomes equal at around 300 s. There is a gradual increase in ωa, out for PLCHE this indicating that the composite polymer desiccant

Fig. 11. (a) Variation of equilibrium uptake capacity and kinetic constant between 20 and 40 °C during sorption process at 80% RH, and (b) variation of initial water content and kinetic constant between 40 and 80 °C and 0% RH during desorption process of PVA-LiCl (50w%). 743

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Fig. 12. Arrhenius plot of the PVA-LiCl (50w%) during (a) sorption process and (b) desorption process.

to 55 kg/h, the magnitude of increase in ṁ a outweighs the fall in MRC, as a result, contributes to an overall higher mass absorbed by SGCHE. When the air flow rate is further increased to 65 kg/h, the air contact time is reduced leading to a sharp decline in the Wd. In contrast, Wd of PLCHE improves by about 14% from 45.44 to 51.45 g. From the process efficacy perspective, Fig. 17 (b) shows that COPth decreases from 0.28 to 0.16 for PLCHE and from 0.21 to 0.11 for SGCHE. Fig. 17 (c) shows an increase in pressure drop of SGCHE, PLCHE, and an uncoated fintube heat exchanger as a function of ṁ a . The variation of ΔP for both the coated heat exchangers is almost similar. However, the DCHEs record 7–25% higher pressure drop as compared to the uncoated fin-tube heat exchanger.

Table 5 Activation energy and pre-exponential factor of the PVA-LiCl (50w%) during sorption and desorption process. 1/T (K−1)

ln(k) (1/s)

Ea (kJ/mol)

A (1/s)

PVA-LiCl (50w%) sorption (RH = 80%) 0.003411 20 7.50 × 10−5 25 8.87 × 10−5 0.003354 30 1.06 × 10−4 0.003299 35 1.43 × 10−4 0.003245 40 1.83 × 10−4 0.003193

−9.49765 −9.33063 −9.15207 −8.85267 −8.60641

34.43

97.39

PVA-LiCl (50w%) desorption 40 4.20 × 10−4 50 4.84 × 10−4 60 5.93 × 10−4 70 7.22 × 10−4 80 8.67 × 10−4

−7.77442 −7.63354 −7.43093 −7.23319 −7.04991

16.97

0.278

T (oC)

k (1/s)

(RH = 0%) 0.003193 0.003095 0.003002 0.002914 0.002832

3.3.3. Effect of inlet air temperature (Ta, in ) According to the data displayed in Fig. 18 (a), when Ta,in is regulated from 30 to 36 °C, the MRC is negatively impacted. Higher temperature of moist air raises the desiccant temperature due to its direct contact with the DCHE and also restricts the relative humidity at the surface. As the sorption capacity of the desiccant is proportional to its RH, mass transfer rate is subsequently reduced and its sorption capacity decreases. MRC is lowered by 14% from 5.2 to 4.5 g/kg for PLCHE, and by 9% from 4.7 to 4.3 g/kg for SGCHE. Fig. 18 (b) indicates that a poorer dehumidification performance does not necessarily translate to lower COPth due to a higher enthalpy of the air at elevated temperature. The cooling capacity from Eq. (12) is evidently higher as the higher temperature gradient promotes heat transfer rates and is also responsible for better COPth. COPth of the PLCHE system is about 15–20% higher than that of SGCHE. 3.3.4. Effect of inlet air humidity ratio (ωa, in ) Fig. 19 shows the positive impact of ωa, in on MRC and COPth. When ωa, in is controlled from 17.5 to 21.5 g/kg, MRC increases from 3.8 to 5.2 g/kg for PLCHE and from 2.19 to 4.4 g/kg for SGCHE. The improvement in MRC is purely due to the enhanced driving force for moisture transfer between the air and the desiccant. It can be observed that the COPth values of SGCHE and PLCHE increase from 0.06 to 0.14 and 0.12 to 0.2, respectively. A juxtapose comparison shows that COPth of PLCHE is 50–90% higher than that of SGCHE. The reason for the improvement in COPth is attributed to the enhancement in cooling capacity. At constant temperature, moist air with higher humidity ratio possesses higher enthalpy; elevating the potential for cooling water to absorb energy from the moist air.

Fig. 13. Sorption kinetics comparison of silica gel and PVA-LiCl (50w%) at 30 °C and 80% RH.

favorable for PLCHE to achieve the improved dehumidification performance with high process efficacy. 3.3.2. Effect of air flow rate (ṁ a) Fig. 17 (a) highlights that MRC for PLCHE drops by 40% from 7.79 to 4.75 g/kg when ṁ a is increased from 35 to 65 kg/h. For SGCHE, it falls by 48% from 5.85 to 3.03 g/kg. Lower MRC is attributed to faster desiccant saturation at higher air flow rates when a higher volume of air comes in contact with the desiccant layer. Wd for SGCHE rises from 34 g at 35 kg/h to 38.1 g at 55 kg/h but drops to 32.9 g at 65 kg/h. From 35

3.3.5. Effect of cooling water temperature (Tcw, in ) From Fig. 20, an almost linear decreasing trend is observed for both MRC and COPth as Tcw,in is increased from 25 to 35 °C. This trend is attributed to the ineffective removal of heat of sorption by the cooling water. At higher water temperatures, a lower temperature gradient 744

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Fig. 14. (a) Experimental result and the modified Fick’s 2nd Law’s prediction of the sorption kinetics experiment of PVA-LiCl (50w%) and (b) apparent diffusivity of water vapor in the PVA-LiCl (50w%) desiccant as a function of temperature increase from 20 to 40 °C.

exists between the desiccant and the cooling water. For PLCHE, MRC drops by 40% from 6.7 to 4 g/kg and for SGCHE, MRC declines by 23% from 4.8 to 3.7 g/kg. Since the dehumidification performance is low, the cooling capacity of the air is also reduced; thus, leading to poor process efficacy. COPth of PLCHE is higher than that of SGCHE by 30–60% and the largest difference is observed in the range spanning 25–27.5 °C.

Table 6 Regression results of the apparent mass diffusivity (Dapp,d) obtained by the modified one-dimensional Fick’s 2nd law of diffusion. T (oC)

Dapp,d (m2/s)

R2

NRMSE

20

8.95 × 10−13

0.9588

6%

25

1.03 × 10−12

0.9879

3%

30

1.30 × 10−12

0.9654

5%

35

1.57 × 10−12

0.9451

5%

40

1.93 × 10−12

0.9840

7%

3.3.6. Effect of hot water temperature (Thw, in ) The positive effect on MRC (as shown in Fig. 21 (a)) at higher temperature facilitates more moisture removal from the desiccant. MRC and COPth of PLCHE record an improvement in the range of 15–40% in contrast to SGCHE. MRCs of SGCHE and PLCHE appreciate from 3 to 4.7 g/kg and 4.2 to 5.2 g/kg, respectively. COPth of PLCHE reduces by almost 3 times from 0.58 at 40 °C to 0.2 at 80 °C. Similarly, for SLCHE, COPth drops from 0.42 to 0.14. Since, the thermal energy requirement and the losses to the environment are low at lower regeneration temperature, the process efficacy is deemed to be higher as observed in Fig. 21 (b). Nevertheless, if the regeneration temperature is too low, then the moisture will not desorb effectively from the DCHEs, and in the subsequent cycles, the trapped moisture would impede its performance. However, for PLCHE with Thw,in at 40 °C, the MRC is marginally reduced by less than 20% as compared to 80 °C whereas the process efficacy is improved by as much as 185%. The improvement in the process efficacy supersedes the reduction in MRC. Lower regeneration temperature is available via the employment of solar heating or by harvesting industrial waste heat. Thus, PLCHE has huge potential to replace the conventional SGCHE in the desiccant dehumidification systems. 3.4. Specific power consumption of DCHE based dehumidification system The dehumidification energy efficiency of DCHEs is computed via Eqs. (15) and (16). Fig. 22 (a) illustrates the trend of MRR when ṁ a is controlled between 35 and 65 kg/h. The variation of MRR with respect to ṁ a is less than 10% for both SGCHE and PLCHE. On the other hand, the SPC of SGCHE and PLCHE show an increasing trend by 59% and 24%, respectively. Fig. 22 (b) conveys that 35 kg/h is the most ideal ṁ a as the least amount of power is consumed to dehumidify the air. Specifically, PLCHE records an improvement of 32% in MRR and saves SPC by 11%. However, when the requirement of ṁ a is higher, further improvement of 55% in MRR with power savings of up to 30% can be achieved with the use of PLCHEs. Fig. 23 (a) and (b) demonstrate the trends in MRR and SPC of PLCHE and SGCHE with respect to different Ta,in. Similar to Fig. 18 (a), both DCHEs record their highest MRR at 30 °C, and PLCHE registers 14% superior MRR. The least power consumption is achieved at 30 °C, and it is apparent that SPC of PLCHE is 16% lower than SGCHE. This

Fig. 15. (a) Dynamic dehumidification and regeneration results under the baseline conditions for SGCHE and PLCHE; and (b) schematic representing the moisture transport from the air to the desiccant.

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Fig. 16. Effect of variation of tcyc between 5 and 30 min on (a) MRC and Wd; and (b) COPth of SGCHE and PLCHE.

DCHEs in systematically reducing both sensible and latent cooling load in air. As a result, when DCHE dehumidification systems are integrated with MVC systems, the cooling load on the evaporator coil can be reduced. Cui et al. [44] have reported that by increasing the evaporator coil temperature from 7 to 12 °C, the coefficient of performance of the conventional air-conditioners can improved by 20%. Fig. 23 (c) and (d) illustrate the variation of MRR and SPC with respect to ωa,in. As the driving force for moisture removal is higher, the best dehumidification

observation highlights the key fact that when the humidity content in the air is not changed, improved energy-efficient dehumidification can be achieved at a relatively lower temperature. Nevertheless, by juxtaposing Fig. 23 (b) and Fig. 18 (b), a diverging trend in SPC and COPth can be observed. While the highest dehumidification efficiency is achieved at 30 °C, COPth is maximum at 36 °C because the DCHE captures higher levels of sensible load from the air at higher Ta,in. The contrasting trend in SPC and COPth highlights the dual function of

Fig. 17. Effect of variation of ṁ a between 35 and 65 kg/h on (a) MRC and Wd; (b) COPth; and (c) ΔP of SGCHE and PLCHE. 746

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Fig. 18. Effect of variation of Ta,in between 30 and 36 °C on (a) MRC and (b) COPth of SGCHE and PLCHE.

Fig. 19. Effect of variation of ωa,in between 17.5 and 21.5 g/kg on (a) MRC and (b) COPth of SGCHE and PLCHE.

Fig. 20. Effect of variation of Tcw,in between 25 and 35 °C on (a) MRC and (b) COPth of SGCHE and PLCHE.

Fig. 24 (c) and (d) show that any value of Thw,in spanning 40–80 °C is suitable for operating PLCHE whereas SGCHE must be closely controlled at 70 °C or 80 °C to achieve the desired MRR. As a result, if a 40 °C regeneration temperature is selected for PLCHE and a temperature of 70 °C for SGCHE, significant specific power savings of up to 54% can be achieved simply by employing PLCHE-based dehumidification systems.

performance by both DCHEs is recorded at 21.5 g/kg. Specifically, PLCHE removes 25% more moisture and records 21% savings in SPC as compared to SGCHE. In order to select the best Tcw,in and Thw,in for SGCHE and PLCHE, a specific dehumidification system is chosen with a requirement of at least 200 g/h moisture removal with the least possible power consumption. Fig. 24 (a) and (b) show that the desired MRR can be obtained with PLCHE with any Tcw,in spanning 25–35 °C, whereas 35 °C is not a suitable option for SGCHE as it is unable to meet the threshold MRR. If Tcw,in at 30 °C is selected for both the DCHEs, 23% superior MRR can be achieved by using PLCHE at 17% lower SPC. Further,

4. Conclusions In this paper, composite PVA coated Al sheets were synthesized, and 747

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Fig. 21. Effect of variation of Thw,in between 40 and 60 °C on (a) MRC and (b) COPth of SGCHE and PLCHE.

Fig. 22. (a) MRR and (b) SPC of SGCHE and PLCHE with ṁ a varying between 35 and 65 kg/h.

Fig. 23. (a) MRR and (b) SPC of SGCHE and PLCHE with Ta,in varying between 30 and 36 °C; (c) MRR and (d) SPC of SGCHE and PLCHE with ωa,in varying between 17.5 and 21.5 g/kg.

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Fig. 24. (a) MRR and (b) SPC of SGCHE and PLCHE with Tcw,in varying between 25 and 35 °C; (c) MRR and (d) SPC of SGCHE and PLCHE with Thw,in varying between 40 and 80 °C.

improvement of up to 55% in MRR. Moreover, temperature as low as 40 °C can be used for regeneration, and savings of 54% savings in SPC can be realized.

their water vapor sorption characterization results were judiciously studied. The composite polymer DCHE’s dynamic dehumidification performance, impact of process efficacy, and specific power consumption during dehumidification were benchmarked with the silica gel coated heat exchangers. The key findings that emerged from this work include:

References [1] Goetzler W, Guernsey M, Young J, Fuhrman J, Abdelaziz O. The Future of Air. Conditioning for Buildings; 2016. [2] Ahmadzadehtalatapeh M. An air-conditioning system performance enhancement by using heat pipe based heat recovery technology. Sci Iran 2013;20:329–36. https:// doi.org/10.1016/j.scient.2013.02.004. [3] Tu YD, Wang RZ, Ge TS, Zheng X. Comfortable , high-efficiency heat pump with desiccant-coated , water-sorbing heat exchangers 2017:1–10. https://doi.or/10. 1038/srep40437. [4] Oh SJ, Ng KC, Chun W, Chua KJE. Evaluation of a dehumidifier with adsorbent coated heat exchangers for tropical climate operations. Energy 2017. https://doi. org/10.1016/j.energy.2017.02.169. [5] Zhao Y, Ge TS, Dai YJ, Wang RZ. Experimental investigation on a desiccant dehumidification unit using fin-tube heat exchanger with silica gel coating. Appl Therm Eng 2014;63:52–8. https://doi.org/10.1016/j.applthermaleng.2013.10.018. [6] Vivekh P, Kumja M, Bui DT, Chua KJ. Recent developments in solid desiccant coated heat exchangers – A review. Appl Energy 2018;229:778–803. https://doi.org/10. 1016/j.apenergy.2018.08.041. [7] Kumar A, Yadav A. Experimental investigation of solar driven desiccant air conditioning system based on silica gel coated heat exchanger. Int J Refrig 2016;69:51–63. https://doi.org/10.1016/j.ijrefrig.2016.05.008. [8] Kummer H, Jeremias F, Warlo A, Füldner G, Fröhlich D, Janiak C, et al. A Functional Full-Scale Heat Exchanger Coated with Aluminum Fumarate Metal-Organic Framework for Adsorption Heat Transformation. Ind Eng Chem Res 2017;56:8393–8. https://doi.org/10.1021/acs.iecr.7b00106. [9] AQSOA honeycomb wheel and AQSOA coated heat exchanger n.d. http://www. aaasaveenergy.com/products/001/pdf/AQSOA_1210E.pdf [accessed April 4, 2018]. [10] Kim H, Yang S, Rao SR, Narayanan S, Kapustin EA, Furukawa H, et al. Water harvesting from air with metal-organic frameworks powered by natural sunlight n.d. [11] Lee J, Lee DY. Sorption characteristics of a novel polymeric desiccant. Int J Refrig 2012;35:1940–9. https://doi.org/10.1016/j.ijrefrig.2012.07.009. [12] Bin Ismail A, Li A, Thu K, Ng KC, Chun W. On the thermodynamics of refrigerant + heterogeneous solid surfaces adsorption. Langmuir 2013;29:14494–502. https:// doi.org/10.1021/la403330t. [13] Li Z, Michiyuki S, Takeshi F. Experimental study on heat and mass transfer characteristics for a desiccant-coated fin-tube heat exchanger. Int J Heat Mass Transf 2015;89:641–51. https://doi.org/10.1016/j.ijheatmasstransfer.2015.05.095.

1. Higher concentration of LiCl in polymer enhances its equilibrium water sorption capacity. However, the maximum feasible concentration of LiCl in the polymer desiccant is limited to 50w% due to the occurrence of deliquescence phenomenon. At 80% RH, the sorption capacity of PVA-LiCl (50w%) is 178% of its initial weight. 2. LDF approximation accurately predicts the sorption and desorption kinetics of the composite desiccants. While the sorption and desorption rates are improved by an increase in both LiCl concentration and temperature, the mechanism behind their increment is different. Higher concentration of LiCl significantly enhances water sorption capacity but yields stable kinetic constant, whereas higher desiccant temperature promotes the kinetic constant. 3. The kinetic constant adheres to the Arrhenius equation, and the activation energy for the sorption process is twice as that of the desorption process. In addition, the mass diffusivity of PVA-LiCl (50w%) increases from 8.9 × 10−13 to 1.9 × 10−12 m2s−1 with a temperature rise between 20 and 40 °C. 4. Improved dehumidification performance is achieved by having higher ṁ a , ωa, in , and Thw, in and lower Ta, in, tcyc , and Tcw, in values. Similarly, enhanced process efficacy of the DCHE setup is obtained by increasing ωa, in and Ta, in and decreasing Thw, in, tcyc , ṁ a , and Tcw, in . 5. MRC and COPth of PLCHE have shown improvements in the range spanning 20–60%. Due to its ability to absorb substantial amount of moisture under a longer cycle time, PLCHE undergoes markedly less cyclic regeneration as compared to SGCHE, which translates to a more sustainable air dehumidification. 6. PLCHE has a huge potential to replace the conventional SGCHE in the desiccant dehumidification systems as it provides substantial 749

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enbuild.2015.05.009. [30] Cao T, Lee H, Hwang Y, Radermacher R, Chun HH. Experimental investigations on thin polymer desiccant wheel performance. Int J Refrig 2014;44:1–11. https://doi. org/10.1016/j.ijrefrig.2014.05.004. [31] Bui DT, Nida A, Ng KC, Chua KJ. Water vapor permeation and dehumidification performance of poly(vinyl alcohol)/lithium chloride composite membranes. J Memb Sci 2016;498:254–62. https://doi.org/10.1016/j.memsci.2015.10.021. [32] Bui TD, Wong Y, Thu K, Oh SJ, Kum Ja M, Ng KC, et al. Effect of hygroscopic materials on water vapor permeation and dehumidification performance of poly (vinyl alcohol) membranes. J Appl Polym Sci 2017;134:1–9. https://doi.org/10. 1002/app.44765. [33] Yao Y, Dai L, Jiang F, Liao W, Dong M, Yang X. Kinetic modeling of novel solid desiccant based on PVA-LiCl electrospun nanofibrous membrane. Polym Test 2017;64:183–93. https://doi.org/10.1016/j.polymertesting.2017.10.008. [34] Zhang LZ, Wang YY, Wang CL, Xiang H. Synthesis and characterization of a PVA/ LiCl blend membrane for air dehumidification. J Memb Sci 2008;308:198–206. https://doi.org/10.1016/j.memsci.2007.09.056. [35] Entezari A, Ge TS, Wang RZ. Water adsorption on the coated aluminum sheets by composite materials (LiCl + LiBr)/silica gel. Energy 2018;160:64–71. https://doi. org/10.1016/j.energy.2018.06.210. [36] Yao Y, Zhang W, Peng Y, Wang L, Liu Y, Chen B. Modeling of Silica Gel Dehydration Assisted by Power Ultrasonics. Refrig Air Cond 2010:1–9. [37] Sultan M, El-Sharkawy II, Miyazaki T, Saha BB, Koyama S, Maruyama T, et al. Water vapor sorption kinetics of polymer based sorbents: theory and experiments. Appl Therm Eng 2016;106:192–202. https://doi.org/10.1016/j.applthermaleng. 2016.05.192. [38] Enteria N, Awbi H, Yoshino H. Desiccant heating, ventilating, and air-conditioning. systems. 2016. https://doi.org/10.1007/978-981-10-3047-5. [39] Gouanvé F, Marais S, Bessadok A, Langevin D, Métayer M. Kinetics of water sorption in flax and PET fibers. Eur Polym J 2007. https://doi.org/10.1016/j.eurpolymj. 2006.10.023. [40] Coleman HW, Steele WG. Uncertainty Analysis for Uncertainty Analysis for Engineers; 2009. [41] Ng KC, Burhan M, Shahzad MW, Bin Ismail A. A universal isotherm model to capture adsorption uptake and energy distribution of porous heterogeneous surface. Sci Rep 2017;7:1–11. https://doi.org/10.1038/s41598-017-11156-6. [42] Chen Y. Packaging selection for solid oral dosage forms. Elsevier Inc.; 2016. [43] Oh SJ, Ng KC, Thu K, Ja MK, Islam MR, Chun W, et al. Studying the performance of a dehumidifier with adsorbent coated heat exchangers for tropical climate operations. Sci Technol Built Environ 2017:1–9. https://doi.org/10.1080/23744731. 2016.1218234. [44] Cui X, Mohan B, Islam MR, Chua KJ. Modelling and performance evaluation of an air handling unit for an air treatment system with regulated outdoor-air fraction. Energy Procedia, vol. 105. Elsevier; 2017. p. 4718–23. https://doi.org/10.1016/J. EGYPRO.2017.03.1024.

[14] Li A, Thu K, Ismail A Bin, Shahzad MW, Ng KC. Performance of adsorbent-embedded heat exchangers using binder-coating method. Int. J Heat Mass Transf 2016;92:149–57. https://doi.org/10.1016/j.ijheatmasstransfer.2015.08.097. [15] Zheng X, Wang RZ, Ge TS, Hu LM. Performance study of SAPO-34 and FAPO-34 desiccants for desiccant coated heat exchanger systems. Energy 2015;93:88–94. https://doi.org/10.1016/j.energy.2015.09.024. [16] Munz GM, Bongs C, Morgenstern A, Lehmann S, Kummer H, Henning HM, et al. First results of a coated heat exchanger for the use in dehumidification and cooling processes. Appl Therm Eng 2013;61:878–83. https://doi.org/10.1016/j. applthermaleng.2013.08.041. [17] Leiming H, Tianshu G, Yu J, Ruzhu W. Hygroscopic property of metal matrix composite desiccant. J Refrig 2014;2:12. [18] Zheng X, Ge TS, Jiang Y, Wang RZ. Experimental study on silica gel-LiCl composite desiccants for desiccant coated heat exchanger. Int J Refrig 2015;51:24–32. https:// doi.org/10.1016/j.ijrefrig.2014.11.015. [19] Ge TS, Zhang JY, Dai YJ, Wang RZ. Experimental study on performance of silica gel and potassium formate composite desiccant coated heat exchanger. Energy 2017;141:149–58. https://doi.org/10.1016/j.energy.2017.09.090. [20] Zheng X, Wang RZ, Ge TS. Experimental study and performance predication of carbon based composite desiccants for desiccant coated heat exchangers. Int J Refrig 2016;72:124–31. https://doi.org/10.1016/j.ijrefrig.2016.03.013. [21] Tadao S, Takashi N. Preparation and application of high-performance superabsorbent polymers. Superabsorbent Polym 1994;573:112–27. https://doi.org/10. 1021/bk-1994-0573. [22] Czanderna AW, Neidlinger HH. Polymers as advanced materials for desiccant applications:1988.1988. [23] Czanderna AW. Polymers as advanced materials for desiccant applications: 1987; 1988. [24] Czanderna AW, Tillman NT, Herdt GC. Polymers as advanced materials for desiccant applications part 3: alkali salts of PSSA and polyAMPSA and copolymers of polyAMPSASS. ASHRAE Trans 1995:697–712. [25] Czanderna AW. Polymers as advanced materials for desiccant applications: progress report for 1989; 1990. [26] White SD, Goldsworthy M, Reece R, Spillmann T, Gorur A, Lee DY. Characterization of desiccant wheels with alternative materials at low regeneration temperatures. Int J Refrig 2011;34:1786–91. https://doi.org/10.1016/j.ijrefrig.2011.06.012. [27] Higashi T, Zhang L, Saikawa M, Yamaguchi M, Dang C, Hihara E. Études Théoriques Et Expérimentales Sur Les Caractéristiques D’Adsorption Et De Désorption Isothermes D’Un Échangeur De Chaleur Enrobé De Déshydratant. Int J Refrig 2017;84:228–37. https://doi.org/10.1016/j.ijrefrig.2017.09.007. [28] Yang Y, Rana D, Lan CQ. Development of solid super desiccants based on a polymeric superabsorbent hydrogel composite. RSC Adv 2015;5:59583–90. https://doi. org/10.1039/C5RA04346H. [29] Chen CH, Hsu CY, Chen CC, Chen SL. Silica gel polymer composite desiccants for air conditioning systems. Energy Build 2015;101:122–32. https://doi.org/10.1016/j.

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