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

Applied Thermal Engineering 163 (2019) 114211

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Research Paper

Experimental study on the performance of the internally-heated ultrasonic atomization liquid desiccant regeneration system Zili Yanga,b, Zhiwei Lianb, Ruiyang Taoa, Hui Nia, Ke Zhonga, a b

T

⁎

Department of Civil Engineering, College of Environmental Science and Engineering, Donghua University, Shanghai 201620, PR China Department of Architecture, Shanghai Jiao Tong University, Shanghai 200240, PR China

H I GH L IG H T S

of IH-UARS was studied via experiments and model predictions. • Performance of the operating parameters on the performance of IH-UARS were clarified. • Effects efficiency was 34.6% higher in IH-UARS than the adiabatic UARS. • Thermal desiccant temperature (36.6 °C) was required for regeneration in IH-UARS. • Lower • Regeneration performance was improved by 48.2% in IH-UARS, compared to IH-FPR.

A R T I C LE I N FO

A B S T R A C T

Keywords: Liquid desiccant Ultrasonic atomization Internal-heating Regeneration performance Thermal efficiency

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

1. Introduction Liquid desiccant dehumidification systems (LDAC) have been attracting increasing research interests [1–3] in the HVAC industry due to its great energy saving potentials [4–6]. Instead of overcooling the humid air below the dew point in the traditional vapor-compression refrigeration dehumidification, the moisture in the airstream is absorbed effectively by the desiccant solution in the LDAC, which can substantially improve the refrigeration system’s coefficient of performance. Furthermore, the diluted desiccant solution can be regenerated in the regenerator and realize the recycling use. This makes the

⁎

regenerator play a dominating role in the overall performance of the LDAC. Given this, a wide variety of regeneration methods have been come up with for better regeneration performance [7–10]. Among the methods, the thermal-driven regeneration is one of the most attractive since it was thought to be able to regenerate the desiccant solution with low-grade thermal energy, such as solar energy or the industrial waste heat. Plenty of studies have also been conducted in various thermal regenerators. For example, the regenerators configured with crossflow/counter-flow packed-beds were widely investigated by Yin et al. [11], Bassuoni et al. [12], Kabeel et al. [13] and Longo et al. [14] while

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

https://doi.org/10.1016/j.applthermaleng.2019.114211 Received 12 June 2019; Received in revised form 28 July 2019; Accepted 3 August 2019 Available online 05 August 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

Subscripts

IH-UARS internally-heated ultrasonic atomization desiccant regeneration system Cp specific thermal capacity, [kJ/(kg·K)] d humidity ratio, [g/kg dry air] D droplets diameter, [m] DMFI desiccant mass fraction increase, [%] G mass flow rate, [kg/h] or [kg/s] h enthalpy, [kJ/kg] H equivalent height, [m] L equivalent length, [m] Mol molar mass, [g/mol] RR regeneration rate, [g/s] n desiccant mass fraction, [%] P pressure, [Pa] t temperature, [oC] TE thermal efficiency, % EE energy efficiency, %

0 a AT d equ i l o w UA

initial status air atmospheric dry air equilibrium inlet liquid desiccant outlet water ultrasonic atomizer

Greek symbol ρ μ

density, [kg/m3] dynamic viscosity, [Pa·s]

1. What are the regeneration performance and thermal efficiency of the internally heated UARS (i.e., IH-UARS)? Is there any performance improvement of the IH-UARS compared to the adiabatic one, and how much? 2. What are the effects of the operating parameters, such as the conditions of the heating water, the airstream, and the desiccant solution, on the regeneration performance of the IH-UARS? Also, how to predict the effects? 3. Compared to the conventional IH-FPR, is it possible to regenerate the desiccant solution at lower desiccant temperature?

Duong et al. [15] looked into the performance of the regenerator equipped with the distillation membrane. Besides, the desiccant regeneration coupled with the heat pump system was studied by Zhang et al. [16] and Yon et al. [9] proposed to improve the performance by operating under vacuum conditions. Furthermore, Wen et al. [17–19] put forward an interesting method to enhancing the performance of the falling-film regenerator by adding surfactant PVP-K30 and carbon-nanotubes into the desiccant solution to improve its mass transfer area with the airstream. Their findings showed that with the contact area increased by 20.1%, the regeneration performance could be effectively improved by 26.3%. More recently, Lian et al. [20] proposed a new liquid desiccant regeneration system integrated with the ultrasonic atomization technology (i.e., UARS) by which the desiccant solution was atomized into numerous droplets, and the heat and mass transfer area was expanded substantially. A series of studies [20–22] were carried out in the UARS, and it has been validated that the regeneration performance was promoted significantly. However, since the heated desiccant solution could be significantly cooled by the scavenging airstream through the sensible heat transfer, fast temperature drop may happen to the warm desiccant solution inside the regenerator. By narrowing the vapor pressure difference between the desiccant solution and airstream, the undesired temperature drop can greatly impede the regeneration process and therefore, severely restrain the system’s performance. One promising approach to address the issue is to introduce the internal-heating [23,24] into the regenerator to prevent or curve the temperature drop. Studies have also been carried out within such internally-heated regenerators to verify the performance improvement. For instance, experiments were performed by Yin et al. [25] and Mun et al. [26] in the regenerators integrated with the plate-fin heat exchanger as the internal heater. Though it was reported that the performance was indeed improved from the adiabatic ones, unfortunately, the necessary regeneration temperature was still too high, which was more than 72 °C in Yin et al.’s system [25] and 70 °C in Mun et al.’s regenerator [26]. This makes it prohibitively challenging for the regenerator (or the LDAC) to be efficiently driven by the low-grade thermal energy, such as solar heating. Also, while the focus was placed on the performance of the internally-heated flat-plate regenerator (IHFPR), little attention has ever been paid to the performance of the internally-heated UARS considering its impressive regeneration performance under the adiabatic condition [20]. Therefore, the following technical issues are to be concerned in this work:

To answer the above questions, a new internally-heated ultrasonic atomization liquid desiccant regeneration system (IH-UARS) was built in this work. A model based on the laws of conservation of mass and energy, and the sensible heat transfer balance, was developed to predict the performance of the IH-UARS. Extensive experimental runs were carried out under various conditions to validate the model and clarify the effects of these conditions on the regeneration performance. At the end, the regeneration performance of the IH-UARS was compared with that of the conventional IH-FPR under the same conditions. The results may help improve the performance of the liquid desiccant system and facilitate its fuller utilization of low-grade thermal energy. 2. Experimental study To clarify the regeneration performance of the IH-UARS, an experimental set-up of the IH-UARS was first constructed (Figs. 1 & 2) where extensive experiments were conducted in comparison with the adiabatic UARS [20]. LiCl aqueous, as same in the adiabatic UARS, was adopted as the desiccant solution. The experimental system and testing methods can be introduced as follows. 2.1. Experimental setup Fig. 1 shows the schematic of the IH-UARS with the experimental components displayed in Fig. 2. As can be seen in the figures, the regeneration system was mainly composed of five parts, namely, the airhandling system, the liquid desiccant system, the ultrasonic atomization system, the internal-heating system, and the regenerator. In the airhandling system, the scavenging airstream was supplied and adjusted with the fan, the cooling coil, heater A, and the humidifier to realize the desired air temperatures and humidity. The liquid desiccant system involved the solution tank A, the pump, the valves, the heat-exchanger and the solution tank B. The diluted 2

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Fig. 1. Schematic of the IH-UARS: (a) system setup; (b) the simplified 3D view.

Fig. 2. Experimental set-up of the IH-UARS: (a) covered with thermal insulation layer, (b) without isolation layer, (c) [20] the ultrasound-atomized droplets. 3

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well cleaned and dried before being sealed and stored. Meanwhile, the regenerator of the IH-UARS was built with 10-mm-thick transparent PMMA (polymethyl methacrylate), which is of excellent corrosion resistance and thermal resistance. The regenerator was further covered with a 10-mm-thick thermal isolation layer on its outside surface, as shown in Fig. 2(a), to minimize the potential heat dissipation from the regenerator to the ambient. By doing so, the heat loss was found smaller than 15%, as validated in the authors’ previous work [20], and satisfactory thermal isolation was realized. In the meantime, to ensure the atomized desiccant droplets being regenerated in the form of droplets and prevent being captured by the regenerator’s walls, the regenerator was carefully designed with the optimization method proposed in the author’s earlier work [30], with the equivalent size (length – L and height – H) determined as Eqs. (1) and (2) show.

desiccant solution was prepared in the solution tank A and can be pumped into the ultrasonic atomization system with the flow rate and temperatures regulated, respectively, by the valves and the heat exchanger. After the heat and mass transfer with the airstream in the regenerator, the desiccant solution was concentrated and regenerated. It was soon collected and sampled in the solution Tank B for further analysis. The ultrasonic atomization system is the core of the IH-UARS. It was composed of the ultrasonic signal generator, the transducer, and the voltage stabilizer. Within the present system, the ultrasonic signals with the frequency of 30 kHz were produced consecutively by the generator and then transmitted to the ultrasonic transducer. As soon as the signals were received by the transducer, the transducer’s conical head, made of TC4 titanium alloy for its good ultrasound transduction ability, started high-frequency vibrating with the amplitudes of only a few microns. When the desiccant solution flowed by the conical head’s vibrating surface, it was atomized immediately into tiny desiccant droplets under the cavitation effect [27]. In the present study, the ultrasonic atomizers, model FYCG YPW-59, identical with that of the adiabatic UARS, were employed to atomize the desiccant solution. The average diameters of the atomized desiccant droplets were around 50 μm as measured by the manufacturer with the method introduced in Ref. [28]. Thus, the atomized liquid desiccant was contacting with the airstream and the internal heaters in the form of tiny droplets, and the heat and mass transfer can be promoted significantly. Though energy was consumed to drive the ultrasonic atomizers, the consumption was quite limited owing to the unique atomization mechanism of the cavitation effect. In the present system, the power consumption was merely 50 W for each transducer with the atomization capacity reached 50 L/h, according to the manufacturer. To validate this, the energy efficiency, concerning the power consumption of the ultrasonic atomizers and their proportion, in the IH-UARS were carefully analyzed in the present work, as shown in Fig. 11(b). Furthermore, the authors have also investigated the operating economics of the UARS when compared with the conventional packed-bed system under the adiabatic conditions [29]. It was found that good operating economics were presented by the UARS, and more detailed information can be found in Ref. [29]. The internal-heating system was made up of the constant temperature hot water tank (Model CH1015, with a 1.5 kW heater and the pump built in), the flowmeters, the valves and the four fin-tube heatexchangers with the size of 0.65 m × 0.5 m × 0.08 m (L × H × W, and fin-height of 10 mm) inside the regenerator. To minimize the pressure drop and droplets capture resulted from the internal heating, the heat exchangers were mounted in parallel with the airflow direction inside the regenerator, as shown in Figs. 1 & 2. In view of the desiccant solution’s possible corrosion effect, protection measures were also taken to the system’s main components. For instance, the upper shell of the ultrasonic transducer, which was made of metal alloy and on the outside top of the chamber, was well covered with the PE layer (i.e., the plastic wrap) to keep it from the direct simultaneous exposure to the air and the desiccant solution (i.e., the water and electrolytes mixture). The transducer’s lower vibrating conical head, which was made of TC4 titanium alloy and covered by the continuously flowing desiccant solution at the inside top of the regenerator, was carefully cleaned after the running of the system to avoid the possible desiccant residue. Then, the whole transducer was

18Ga ·μa Y ·Ga2·ρ l ·D1 ⩽L⩽ 2 6·G l·ρa ·X ·Hmin·W ρa ·D1 ·g ·(ρ l − ρa )·(1 −

H⩾

1 − X )·W

(1)

3 Y ·(1 − 1 − X ) Ga D1 ·ρ l ·(ρ l − ρa )·g · · 108X Gl μa

(2)

In addition, to avoid the possible carryover of the droplets, a meshtype mist eliminator was installed at the regenerator’s outlet. Due to the dense packed-bed or flat-plate, which used to be employed in the conventional regenerators, were completely removed in the present system, its air pressure-drop (with the mist eliminator installed) was measured to be only 56.2 Pa under nominal conditions. Consequently, a small fan, with the power of only 38 W and the pressure head of 180 Pa, was affordable to cover all possible air pressure drops in the present IHUARS. To further verify the possibilities of the potential droplets’ carryover, a field investigation has been conducted in the laboratory where the desiccant concentration in the outflow air was sampled and analyzed with the high-precision Inductively Coupled Plasma Mass Spectrometer (ICP-MS) [31]. It was validated that little impact has been exerted by the outflow air [31] where the desiccant concentration was far below the threshold. More detailed information can be found in the authors’ earlier work [31]. 2.2. Operating conditions and experimental method 2.2.1. Operating conditions To realize the direct comparison, the operating conditions of the IHUARS (Table 1) were consistent with that of the adiabatic UARS [20]. Meanwhile, the slightly low liquid-to-gas ratios, same with the adiabatic UARS, were adopted here to ensure the fair performance comparison and help avoid the possible carryover of the desiccant solution [32]. In addition, the identical ultrasonic atomizers (Model FYCG YPW59) from the conventional UARS regenerator was also adopted in the present system where the desiccant solution was atomized into the droplets with the diameter same with the conventional UARS, which was around 50 μm [20]. The main measured parameters include the temperatures and the flow rates of the desiccant solution, the airstream, the internal-heating water, together with the airflow’s humidity. In addition, the desiccant densities before and after the regeneration were measured through the specific gravity hydrometer, as shown in Table 2, together with its

Table 1 Operating parameters and their ranges for IH-UARS. Parameters

Gl,i

tl,i

ni

Ga,i

ta,i

di

Gw,i

tw,i

Gl,i/Ga,i

Unit Initial value Ranges

kg/h 44 16–65

o C 60 36–65

% 26 24–32

kg/h 92 68–115

o C 32 26–37

g/kg 18 9.5–21.5

kg/h 210 138–276

o

– 0.48 0.14–0.75

4

C 50 40–60

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Table 2 Specification of the measuring devices. Parameters

Devices

Accuracy

Operational range

Desiccant temperature Desiccant flow rate Desiccant density Air flow rate Air temperature Air relative humidity Hot water temperature Hot water flow rate

PT100 RTD Glass rotor flow meter Specific gravity hydrometer Hot-wire anemometer (TESTO 425) PT100 RTD, (ROTRONIC HC2A-S) ROTRONIC HC2A-S PT100 RTD High-precision digital flowmeter

± 0.2 °C ± 2.5% 1 kg/m3 0.03 m/s ± 5% of measured value ± 0.2 °C @10–30 °C ± 0.8% @23 °C, 10–80% RH ± 0.2 °C 0.1 kg/min

0–100 °C 0.1–1 L/min 1100–1500 kg/m3 0–20 m/s −50–100 °C 0–100% RH 0–100 °C 0.5–10 kg/min

temperatures simultaneously tested with the RTD. Then with the measured densities and temperatures, the concentrations of the LiCl solution can be accurately calculated via the well-recognized correlations and method developed in M. Conde’s work [33]. Real-time measurements were realized with the devices specified in Table 2, most of which were connected to a PC-based data acquisition system (DAQ, Agilent 34972A & 34901A). The basic procedure of the experimental process can be introduced as follows.

thermal efficiency (abbr. TE) and the necessary desiccant regeneration temperature to realize the target performance (abbr. NRT), were adopted in the present study.

2.2.2. Experimental method The experimental process comprised three stages, namely the preparation stage, the running stage, the finishing and recovery stage. The operation process of each stage was as follows. During the preparation stage, the laboratory’s background air-conditioning system was turned on to make the air conditions close to the desired status. Then, the fan was switched on and the air, which was adjusted by the air-handling system via the collaboration of the heater, the cooling coil and the humidifier, was supplied to the regenerator. Besides, the dilute desiccant solution with the target mass fraction was prepared and stored in Tank A with the temperature slightly adjusted with the heat-exchanger in the liquid desiccant system. Meanwhile, the internal-heating system was started warming up, with the constant temperature water tank (CTWT) set to the desired temperature. At the moment, the heater built in the CTWT kept heating the water automatically until the water temperature reached and stabilized at the preset value. With the flow rate regulated by the valves, the heating water was pumped to the internal-heaters and circulated within the heating water loop. Furthermore, the measuring devices and the DAQ were switched on, with the scanning interval of 5 s, and ready for the upcoming regeneration stage. As soon as the conditions of the airstream, the desiccant solution, and the hot water loop met the target status and stabilized, the ultrasonic atomization system was switched on. With the well-atomized solution droplets distributed and exchanging heat and mass with the scavenging air inside the regenerator, the desiccant solution started being regenerated while the system’s status was monitored by the DAQ. Once the steady state achieved, the regeneration was considered to reach the balance. Then, the system’s parameters were carefully measured while the desiccant solution at the outlet was multiply sampled and tested for its densities and temperatures, which were later used for assessing the solution’s concentrations. All the monitored data were well logged in the computer as the basis for analyzing the performance of the IH-UARS. After the experimental run, the system was recovered with one operating parameter changed within its range and others reverted to the nominal conditions. The experimental process was repeated until all the running conditions (Table 1) were finished.

RR = Ga,d × (da,o − da,i)

2.3.1. Regeneration rate (RR) The desiccant regeneration rate was utilized to evaluate how much moisture can be removed from the dilute desiccant solution per second. It has been widely employed in evaluating regeneration performance and is defined as Eq. (3) [34] shows. (3)

As can be inferred from Eq. (3), with a higher RR, more moisture will be transferred from the dilute desiccant solution to the airflow. Consequently, the desiccant solution would be regenerated more efficiently. 2.3.2. Desiccant mass fraction increase (DMFI) Since the aim of the regeneration system is to recover the concentration of the diluted desiccant solution for recycling use, the desiccant mass fraction increase (abbr. DMFI) is usually adopted to estimate the regenerators’ performance. It can be evaluated as Eq. (4) shows. Apparently, with a higher DMFI, better regeneration performance is illustrated by the regeneration system.

DMFI = no − n i

(4)

where the no and ni stands for the desiccant mass fraction after and before the regeneration, respectively. 2.3.3. Thermal efficiency (TE) In the IH-UARS, extra power was required for the internal heating, which is absent in the adiabatic system. To fairly appraise the regeneration performance of the IH-UARS and the adiabatic system, the thermal efficiency, defined as Eq. (5) [25] shows, was adopted in this work:

Ga,d·rW ·(da,o − da,i) × 100\% Q w + Qs Ga,d·rW ·(da,o − da,i) × 100\% = Cp,w ·Gw ·(tw,i − tw,o) + Cp,l·G l,i·(tl,i − tl,0 )

TE =

(5)

where rw represents the latent heat of water; Qw and Qs, respectively, represents the heat for the heating water and warming up the desiccant solution from its initial temperature (tl,0). By contrast, there is no internal heating in the adiabatic system and its Qw = 0. Meanwhile, energy was also consumed by the ultrasonic atomizers during the IH-UARS operation. Given this, the energy efficiency (abbr. EE) including the power consumption of the ultrasonic atomizers as well as the thermal energy was employed here, as shown in Eq. (6), to evaluate the energy efficiency in the IH-UARS.

2.3. Performance indices

Ga,d·rW ·(da,o − da,i) × 100\% Q w + Qs + PUA Ga,d·rW ·(da,o − da,i) × 100\% = Cp,w ·Gw ·(tw,i − tw,o ) + Cp,l·G l,i·(tl,i − tl,0 ) + PUA

EEUA = To assess the regeneration performance of the IH-UARS, four indices, namely the regeneration rate of the dilute desiccant solution (abbr. RR), the desiccant mass fraction increase (abbr. DMFI), the 5

(6)

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respectively. Since the present study was focused on clarifying the regeneration performance of the IH-UARS, the inlet and outlet temperatures of the hot water (i.e., tw,i and tw,o) were measured spontaneously with the resistance temperature detector (PT100, Table 2) to obtain the practical heating power into the regenerator. A more sophisticated model, considering the heat transfer process in the fin-tube internal-heaters, should be further developed in the future. Furthermore, with the ultrasonic atomization technology, the dilute desiccant solution was atomized into numerous tiny microns droplets, and consequently, the heat and mass transfer process between the airstream and the desiccant solution can be promoted significantly. Meanwhile, the well-atomized tiny droplets were found moving together with the airflow inside the regenerator. Because of this, the vapor pressure of the desiccant solution could be balanced with that of the airstream at the same position of the system’s outlet, as expressed by Eq. (10).

where the PUA denotes the power for ultrasonic atomizers, which was 50 W per atomizer (with the atomization capability of 50 L/h), in the adiabatic or the internally heated UARS, but equals to zero in the IHFPR. 2.3.4. The necessary desiccant regeneration temperature (NRT) During the regeneration, the desiccant solution needs to be warmed up to increase its vapor pressure. In this condition, with the lower desiccant temperature, less and lower-grade thermal energy would become affordable to drive the regenerator, which indicates a high potential of energy-saving as well as the better regeneration performance. Because of this, to intuitively illustrate the effects from the ultrasonic atomization on the desiccant regeneration, the necessary desiccant regeneration temperatures (abbr. NRT) for realizing the same target regeneration performance under the same operating conditions were compared among the internally heated regenerators and adopted as one of the performance indices.

P l,o (nl,o , tl,o) = Pa,o

3. Prediction model and its validation

where Pl,o can be calculated with the method in Ref. [33] and Pa,o is obtained by Eq. (11).

To predict the regeneration performance of the IH-UARS under various running conditions, a model, based on the conservation laws of energy and mass within the regenerator, was also developed here and validated with the experimental results.

Pa,o =

Mola,d ·da,o·PAT Mola,d ·da,o + Mola,q

(11)

By combining the four established governing equations (Eqs. (7)–(10)), the four outlet parameters of the IH-UARS, namely ta,o, da,o, tl,o, nl,o, can be figured out with the help of the auxiliary equations shown in Eqs. (11)–(15) and the properties of LiCl detailed in Ref. [33]. Finally, the regeneration performance of the IH-UARS can be obtained.

3.1. Prediction model In the present IH-UARS, the moisture was effectively absorbed from the diluted desiccant solution by the airstream within the thermallyinsulated regenerator where little heat loss, which was less than 15% in average and might be neglectable, occurred [20]. Given this, the mass and energy balance equations within the regenerator can be depicted, respectively, as Eqs. (7) and (8) shows.

Ga,d·(da,o − da,i) = G l,i·(1 − nl,i) − [G l,i − Ga,d·(da,o − da,i)]·(1 − nl,o) (7)

ha,i = 1.01ta,i + da,i (2501 + 1.85ta,i)

(12)

ha,o = 1.01ta,o + da,o·(2501 + 1.85ta,o)

(13)

hl,i = f (tl, i, nl, i )

(14)

hl,o = f (tl, o, nl, o)

(15)

Before the model could be applied to predicting the performance, it was verified with the experimental results as introduced in the following section.

Ga,d·ha,i + G l,i·hl,i + Gw ·h w,i = Ga,d·ha,o + [G l,i − Ga,d·(da,o − da,i)]·hl,o + Gw ·h w,o

(10)

(8)

Besides, with the internal-heating, significant sensible heat transfer may take place between the hot water, the desiccant solution, and the airstream inside the regenerator, which can be derived as Eq. (9) shows.

3.2. Model validation Fig. 3 illustrates the comparison of the model predicted performance and the experimental results. As can be seen in the figure, good agreement was achieved between the predicted performance and the experimental data, where the maximum deviation was less than 15% for most conditions. In the meanwhile, the average difference between the predicted and the experimental performance was merely 0.84% for

Cp,w ·Gw ·(tw,i − tw,o ) + Cp,l·(G l,i·tl,i − G l,o·tl,o ) + Cp,a ·Ga,d·(ta,i − ta,o) = 0 (9) where Cp, t, and G, separately, stands for the specific heat capacity, the temperature, and the mass flow rate while the subscript w, l, and a represents the hot water, the liquid desiccant, and the airstream,

Fig. 3. Comparison between the experimental results and the predicted performance from the proposed model. 6

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that was enhanced by 77.8%. Furthermore, with peak DMFI reached 1.08%, the IH-UARS seems to be capable of concentrating desiccant solution more effectively than the adiabatic system whose DMFI was 0.36% under the same condition. Though the RR was rising significantly with the growth of Gl,i in both the IH-UARS and the adiabatic UARS, the increasing trends were slightly different. Within the adiabatic system, the RR seemed to increase linearly, whereas the growth rate was dropping in the IH-UARS. This different trends resulted in a narrowing performance gap between the two systems. The reason may be explained as follows. In the adiabatic UARS, the temperature of the desiccant solution may drop significantly as it can be quickly cooled by the airstream. Owing to this, the adiabatic system’s regeneration performance was relatively limited, compared to that of the internally-heated UARS. Thus, less moisture was removed by the airstream in the adiabatic UARS, as proven by the lower RR in Fig. 5. Consequently, the airstream may still occupy the strong ability to absorb the moisture from the desiccant solution in the adiabatic system. Thus, with the growing Gl,i, more desiccant solution was regenerated by the unit airflow, which will promote the moisture transfer to the airstream. As a result, the RR was increasing with a fast-linear trend in the adiabatic UARS. By contrast, with the internal-heating added in the IH-UARS, the temperature drop of the warm desiccant solution was curved and its vapor pressure difference to the airstream remained broad. This leads the IH-UARS to start with a relatively higher RR. Accordingly, the airstream, which has already carried much moisture, is of little ability to further remove more moisture from the desiccant solution in the fixed contact time. Therefore, the RR of the IH-UARS was increasing, but at a slower rate, with the further rise of the Gl,i. Besides, with the further rise of Gl,i, the DMFI was found descending markedly in the IH-UARS, while it was dropping slightly in the adiabatic system. This may be due to the following reasons. As can be inferred from Eqs. (16) and (17), the DMFI was directly related to the RR/ Gl,i when under the fixed ni. Although more moisture was removed from the desiccant solution with the rise of the desiccant flow rate, the rising rate of RR was dropping significantly in the IH-UARS, as indicated in Fig. 5(a). Given this, the regeneration rate for the unit mass desiccant solution (RR/Gl,i) could decrease markedly with the further growth of the Gl,i, but kept almost constant considering the linear trends of RR shown in the adiabatic system. Therefore, the DMFI drops significantly with rising of Gl,i in the IH-UARS but decreased slowly and even remained almost stable in the adiabatic UARS.

RR and 5.04% for DMFI. This manifests that the model is reliable to predict the regeneration performance of the IH-UARS. 4. Results and discussion 4.1. Regeneration performance of IH-UARS under various operating conditions In total, 63 experimental runs were carried out within the present IH-UARS to clarify its regeneration performance under the effects of various operating conditions, as shown in Figs. 4–10. Performance of the adiabatic UARS [20] under the similar conditions was employed as the baseline to verify the performance improvement of the IH-UARS. The uncertainties of the experimental results were estimated with the method proposed in Kline and McClintock’s work [35]. The results can be introduced as follows. 4.1.1. Effect of heating water temperature To begin with, the effects of the heating water temperature on the regeneration performance were investigated to determine the optimal internal-heating temperature for the IH-UARS, as shown in Fig. 4. It is clear in Fig. 4 that the regeneration performance of the IH-UARS (indicated by RR and DMFI) was growing exponentially with the temperature rise of the heating water (i.e., tw,i). However, the growth rate was dropping significantly, especially in the higher tw,i range. For instance, the RR was enhanced markedly by 0.104 g/s from 0.158 g/s to 0.262 g/s when tw,i increased from 41.6 °C to 50.5 °C (i.e., the lower temperature range). With tw,i further climbed from 50.5 °C to over 61.1 °C (i.e., the higher temperature range), the RR was, by contrast, only enhanced by 0.059 g/s. In view of this weakening improvement effects as well as the lower accessibility and energy efficiency to produce hotter water, there may exist an optimal temperature range of the heating water for balancing the performance improvement and the easiness to generate the hot water with renewable energy. In the present IH-UARS, tw,i = 50 °C, which can be easily generated with the solar water heating system, was adopted as the nominal water temperature. 4.1.2. Desiccant flow rate Fig. 5 illustrates the effects of desiccant flow rate on the regeneration performance of the IH-UARS comparing to that of the adiabatic UARS. As indicated in Fig. 5, the RR, together with the DMFI, was significantly higher than that of the adiabatic system, especially when regenerating lower flow rate of desiccant solution. In the present study, the average RR was enhanced from 0.158 g/s in the adiabatic system to over 0.231 g/s in the IH-UARS, with the improvement amplitude over 46.2%. Similarly, marked improvement can also be found for the DMFI

no =

G l,i·n i G l,i − RR

Fig. 4. Effect of heating water temperature on the regeneration performance: (a) RR (b) DMFI. 7

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Fig. 5. Effects of desiccant flow rate on the regeneration performance: (a) RR (b) DMFI.

Fig. 6. Effects of desiccant temperature on the regeneration performance: (a) RR (b) DMFI.

DMFI =

RR/ G l,i G l,i·n i RR − ni = ni· = ni· 1 − RR/ G l,i G l,i − RR G l,i − RR

Besides, the regeneration performance of the IH-UARS was in line with the trends presented by the adiabatic UARS, where the RR and DMFI were increasing exponentially, as fitted, in both systems. With the further rise of the desiccant temperature, though the regeneration performance could still be boosted, the increasing rate was descending. This suggests that there may exist an optimal range of the desiccant temperature to realize the balance between performance improvement and the energy-efficiency of the system, which would be discussed in the future.

(17)

4.1.3. Desiccant temperature Fig. 6 shows the effects of the desiccant temperature on the regeneration performance of the IH-UARS in comparison with the adiabatic system. Better regeneration performance was demonstrated in the IH-UARS than that of the adiabatic UARS under different desiccant temperatures. In the present study, the average RR was improved from 0.136 g/s in the adiabatic system to more than 0.216 g/s in the IHUARS. Meanwhile, the DMFI was also enhanced in the IH-UARS with the improvement amplitude of over 45.4% compared to the adiabatic system. Apart from that, it was found that the IH-UARS was able to regenerate the desiccant solution with lower temperatures. In this work, the desiccant solution was efficiently regenerated at the tl,i = 36.6 °C with a better regeneration performance realized, compared to the adiabatic UARS. This will help in promoting the fuller utilization of low-grade thermal energy or renewable energy within the LDAC.

4.1.4. Desiccant mass fraction The effects of desiccant mass fraction on the regeneration performance of the UARS with and without internal-heating are plotted in Fig. 7. According to Fig. 7, the IH-UARS demonstrated a much better regeneration performance than that of the adiabatic UARS. In the present study, the RR was improved from 0.141 g/s in the adiabatic system to 0.205 g/s in the IH-UARS, with the improvement amplitude reached 45.4%. Meanwhile, owing to the effectively curved temperature drop of the desiccant solution by the internal-heating, the DMFI was also enhanced dramatically from 0.31% to over 0.46% in the IH8

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Fig. 7. Effects of desiccant mass fraction on the regeneration performance: (a) RR (b) DMFI.

Fig. 8. Effects of air flow rate on the regeneration performance: (a) RR (b) DMFI.

4.1.5. Air flow rate Fig. 8 shows the effects of the air flow rate on the regeneration performance of the IH-UARS in comparison with adiabatic UARS. As evident in the figure, the performance of the IH-UARS was markedly higher than that of the adiabatic system. For instance, when the air flow rate increased from 68 to 115 kg/h, the RR of the IH-UARS was enhanced from 0.231 to 0.259 g/s while it was growing merely from 0.167 to 0.184 g/s in the adiabatic system. The average improvement amplitude of RR reached 42.3%. A similar rising trend can also be observed for the DMFI, as shown in Fig. 8(b). Besides, the performance trends of the IH-UARS were consistent with the adiabatic UARS, where both the RR and DMFI increased significantly with the growth of the air flow rate. This may be due to the vapor pressure of the airstream was kept low and even reduced as more air was adopted to remove the moisture from the dilute desiccant solution. In this condition, the vapor pressure difference between the desiccant solution and airstream can be extended and therefore, the regeneration performance was enhanced with the rising air flow rate in both systems.

UARS. In addition, with the further increase of the initial mass fraction of the desiccant solution, the regeneration performance of the IH-UARS was decreasing significantly, which was consistent with the adiabatic system. The reason for the dropping regeneration performance may be due to the significantly reduced vapor pressure of the desiccant solution, and its pressure difference to the scavenging air, with the rise of the desiccant mass fraction. Thus, the driving force of the moisture transfer process was weakened, and therefore, the performance trends were descending with the desiccant mass fraction. Nevertheless, it was noted that the dropping trend of DMFI was slightly lower in the IHUARS than that of the adiabatic system. This implies that the IH-UARS may be more applicable to regenerate the desiccant solution with a relatively higher initial mass fraction. At the meantime, the regeneration performance predicted by the proposed model seems to be in line with the trends of the experimental data, as shown in Fig. 7. This corroborated that the effects of the desiccant mass fraction can be well predicted with the proposed model.

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Fig. 9. Effects of air temperature on the regeneration performance: (a) RR (b) DMFI.

Fig. 10. Effects of air humidity ratio on the regeneration performance: (a) RR (b) DMFI.

solution may be strongly curved. Thus, the desiccant solution would maintain a high vapor pressure and so is its vapor pressure difference with the air flow. Consequently, the regeneration process may be promoted, and the increasing regeneration performance was observed. This may also explain why the adiabatic UARS seems to be more sensitive to the temperature rise of the airflow, as shown in Fig. 9. In the meantime, the predicted regeneration performance was in good agreement with the experimental results under various air temperatures. This proves the feasibility and reliability of the proposed model to predict the effects of the air temperatures on the regeneration performance of the IH-UARS.

In addition, the predicted regeneration performance by the proposed model compares well with the experimental results, as shown in Fig. 8. This validates that the model is of good reliability and consistency for predicting the effects of the air flow rate on the regeneration performance of the IH-UARS.

4.1.6. Air temperature The effects of air temperature on the regeneration performance of the IH-UARS is given in Fig. 9. It is apparent in Fig. 9 that the regeneration performance, both the RR and DMFI, was enhanced substantially after adding the internal-heating in the UARS. In the present study, the average improvement amplitude of RR in the IH-UARS was over 39.9% compared to that of the adiabatic system. A significant enhancement was also observed to the DMFI. In the meantime, the regeneration performance trend of the IH-UARS was consistent with that of the adiabatic UARS, where both RR and DMFI was increasing continuously with the air temperature rise. This may be due to the sensible heat transfer from the desiccant solution to the airstream was reduced with the rise of ta,i. In this case, the temperature drop of the desiccant

4.1.7. Air humidity ratio Fig. 10 displays the regeneration performance of the IH-UARS in comparison with the adiabatic system under different air humidity. As illustrated in Fig. 10, markedly higher RR and DMFI were presented in the IH-UARS. In the present study, the average RR was enhanced from 0.168 g/s in the adiabatic system to over 0.231 g/s in the IH-UARS in the same air humidity range. Similarly, significant improvement of 10

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Fig. 11. Thermal (energy) efficiency: (a) before/ after adding internal-heating; (b) with/ without considering ultrasonic atomizers’ power consumption.

Fig. 12. Comparison of the necessary desiccant temperature (NRT) under various regeneration loads.

the dropping performance in both the IH-UARS and adiabatic UARS. Besides, it is apparent in Fig. 10 that the predicted trends of the RR and DMFI was in good agreement with the experimental results, with the deviation of less than 4.4%. This validates that the proposed model is of good reliability to predict the effects of the air humidity on the regeneration performance of the IH-UARS.

Table 3 NRT within IH-UARS and IH-FPR for different regeneration loads. Regeneration load (RR, g/s)

0.105 (Low) 0.151 (Medium) 0.192 (High)

NRT (oC)

Temperature lowered (oC)

IH-FPR

IH-UARS

50.0 55.0 60.0

45.3 49.6 54.1

4.7 5.4 5.9

4.2. Thermal efficiency comparison Though better regeneration performance was reported in the IHUARS than the adiabatic system (Figs. 5–10), extra thermal energy was also provided for the internal heaters. To further clarify the system’s cost-efficiency after introducing the internal-heating, the thermal efficiency of the IH-UARS was compared with the adiabatic UARS under various regeneration loads, as shown in Fig. 11(a). As evident in the figure, the thermal efficiency of the IH-UARS was considerably higher than that of the adiabatic system with the average improvement by 34.6%. The reason for this, as can be referred from Eq. (5), may be due

DMFI, by 27.8%, was also observed in the UARS with internal heating. Meanwhile, the varying trends of the RR and DMFI in the IH-UARS, which seem to decrease linearly with the growth of the inlet air humidity, were parallel with that of the adiabatic system. This may be due to the vapor pressure of the airstream was raised considerably with the rise of air humidity, referring to Eq. (11). Consequently, the vapor pressure difference between the air and the dilute desiccant solution was reduced, which hindered the regeneration process and resulted in

11

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Fig. 13. Comparison of the regeneration performance between the IH-UARS and IH-FPR under different desiccant temperatures.

4.3. Comparison with the conventional internally-heated flat-plate regenerator

Table 4 Comparison of DMFI between IH-UARS and IH-FPR. Desiccant regeneration temperature (oC)

50 55 60

DMFI (%)

Performance Improvement (%)

IH-FPR [25]

IH-UARS

0.56 0.82 1.04

0.83 1.08 1.30

Fig. 12 presents the comparison of the necessary desiccant regeneration temperature between the IH-UARS and the conventional IHFPR to finish various regeneration loads under the same initial conditions. As shown in Fig. 12, the necessary desiccant regeneration temperature (NRT) was increasing exponentially, as well fitted by Eqs. (18) and (19), to achieve higher regeneration performance. However, the NRT in the IH-UARS seems to be significantly lower than that of the IHFPR. For instance, when to meet the regeneration load RR = 0.192 g/s under the conditions shown in Fig. 12, the NRT was around 60 °C in the IH-FPR whereas it was merely 54.1 °C in the IH-UARS where the NRT dropped considerably by 5.9 °C. Besides, the dilute desiccant solution could even be effectively regenerated at 45 °C in the present IH-UARS and achieved satisfying performance. These advantages make it possible for the IH-UARS to make use of lower-temperature thermal source for the desiccant regeneration. The necessary desiccant temperature of the IH-UARS and IH-FPR is further compared in Table 3 under three regeneration loads, namely the low, medium, and high load. As listed in Table 3, significant temperature drop, ranging from 4.7 °C to 5.9 °C, was observed in the IH-UARS to realize the same regeneration performance, especially under the higher regeneration load.

48.2 31.7 25.0

to the substantial enhancement of the regeneration performance via employing the internal heating. This higher thermal efficiency indicates that it is cost-effective to enforce internal heating to improve the regeneration performance of the UARS. Meanwhile, the energy efficiency of the IH-UARS with/ without consideration of the ultrasound power consumption was compared in Fig. 11(b). As shown in Fig. 11(b), when taking the ultrasound’s power consumption into account, the energy efficiency of IH-UARS slightly decreased by merely 1.5%, compared to the situation without considering the ultrasound. This very limited decline indicates that the energy cost from the ultrasonic atomizers was playing a minor role in the present IH-UARS.

Table 5 Performance trends of IH-UARS and the conventional regenerators under various operating conditions. Reference

System type

Parameter

tl,i

ni

ta,i

di

tw,i

Gl,i/ Ga,i

Gw/ Gl,i

( C)

(%)

( C)

(g/kg)

( C)

–

–

o

o

o

Yin et al. [25]

IH-FPR (LiCl)

Range RR

50–100 ↑

30–40 ↓

20–40 ↔

– –

– –

1.09–1.64 ↗

0.5–15 ↑

Liu et al. [24]

IH-FTR* (LiBr)

Range RR

20.6–25.1 –

39.1–42.6 –

15.1–19.1 –

6.8–11.3 –

31.1–41.8 ↑

0.2–1.0 ↑

0.8–10.5 ↗

Kim et al. [2]

Adiabatic-Packed-bed (LiCl)

Range RR

36.6–49 ↑

35.5–39.8 ↓

33.2–42.1 ↑

11.7–14.1 ↓

– –

3.7–9.4 ↑

– –

Present study

IH-UARS (LiCl)

Range RR

36–65 ↑

24–32 ↓

26–37 ↑

9.5–21.5 ↓

40–60 ↑

0.14–0.75 ↑

5.5–11 ↑

↑Significant increase (> 15%); ↓Significant decrease (> 15%); ↗Slight increase (5%-15%); ↘ Slight decrease (5%-15%); ↔No significant effect (< 5%); – Not mentioned. * Fin tube made of thermally conductive plastics. 12

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consumption of the fan, the solution pump, and the heating water, was still unrevealed. This would be further explored in future work.

In the meantime, the regeneration performance of the IH-UARS and the IH-FPR was compared under three typical desiccant temperatures, as shown in Fig. 13. It was found in Fig. 13 that the regeneration performance of the IH-UARS was constantly better than that of the IH-FPR, as summarized in Table 4. For instance, when tl,i = 50 °C, the DMFI was about 0.56% in the IH-FPR, while it climbed to over 0.83% in the IHUARS. The improvement amplitude was around 48.2%. However, it was found that though better regeneration performance was presented by the IH-UARS, its performance advantages over the IH-FPR were shrinking with the rise of the desiccant temperature, as shown in Fig. 13 and Table 4. This manifests that the IH-UARS could be of more capability to regenerate desiccant solution at a lower temperature.

Declaration of Competing Interest None. Acknowledgment This work is financially supported by the Fundamental Research Funds for the Central Universities of China (No. 2232018D3-36), the Shanghai Sailing Program (No. 19YF1401800) and the China Postdoctoral Science Foundation (No. 2018 M630385).

4.4. Comparison of performance trends under different operating conditions Appendix A. Supplementary material Table 5 shows the performance trends of the present IH-UARS, in comparison with the conventional regeneration systems in the open literature. As detailed in Table 5, the performance tendency of the IHUARS was generally consistent with that of the existing studies. For instance, the RR was increasing with the tl,i, Gw/Gl,i, and descending with ni and di in all systems. Meanwhile, the performance seems to be more sensitive in the IH-UARS than that of the IH-FPR. For example, ta,i presents a promoting effect on improving the RR in the IH-UARS, while its effect was almost insignificant in Yin et al.’s [25] IH-FPR. This could be ascribed to the dramatically expanded heat and mass transfer area in the IH-UARS where the heat transfer was promoted substantially, and thus more sensitive to the temperature changes of the airstream.

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5. Conclusions In this work, a new internally-heated ultrasonic atomization liquid desiccant regeneration system (IH-UARS) was proposed and thoroughly investigated. A model based on the conservation laws of mass and energy was developed to predict the performance of the IH-UARS. Extensive experiments were carried out under various operating conditions of the airstream, the desiccant solution, and the heating water to validate the model and clarify the performance of the IH-UARS, in comparison with the adiabatic UARS. Furthermore, the regeneration performance and the necessary desiccant temperature were compared between the IH-UARS and the conventional IH-FPR at the end of the paper. The main conclusions can be drawn as follows. (1) The regeneration performance, as well as the thermal efficiency, was improved dramatically in the UARS after introducing the internal heating. In the present work, the average thermal efficiency was enhanced by 34.6% after adopting the internal heating in the UARS. Meanwhile, the average RR was improved with the amplitude of 45.2%. (2) Lower desiccant regeneration temperature was required in the IHUARS. In the present study, the desiccant solution can be regenerated effectively at the temperature as low as 36.6 °C while satisfactory regeneration performance was realized. (3) Good consistency was achieved between the model predicted performance and the experimental results. In the present work, the average deviation between the experimental performance and predicted values was only 0.84% for RR and 5.04% for DMFI. This indicates that the proposed model is of good reliability to predict the performance of the IH-UARS. (4) Compared with the conventional IH-FPR, better regeneration performance and lower desiccant regeneration temperature were achieved in the IH-UARS. In the present work, the necessary desiccant temperature was lowered by over 5.9 °C in the IH-UARS to obtain the same regeneration performance with the IH-FPR. It should, however, be noted that the operating economy of the IHUARS and other internally heated regenerators considering the power 13

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atomization liquid desiccant dehumidification system, Proc. Eng. 205 (2017) 2879–2886. Z. Yang, K. Zhang, M. Yang, Z. Lian, Improvement of the ultrasonic atomization liquid desiccant dehumidification system, Energy Build. 85 (2014) 145–154. X. Li, K. Zhang, Z. Yang, Z. Lian, Indoor air quality affected by ultrasonic atomization liquid desiccant dehumidification system, J. Shanghai Jiaotong Univ. 51 (2017) 257–262. A. Lowenstein, S. Slayzak, E. Kozubal, A zero carryover liquid-desiccant air conditioner for solar applications, Proceedings of the ASME International Solar Energy Conference, 2006, pp. 397–407. M.R. Conde, Properties of aqueous solutions of lithium and calcium chlorides: formulations for use in air conditioning equipment design, Int. J. Therm. Sci. 43 (2004) 367–382. T. Wen, L. Lu, H. Yang, Y. Luo, Investigation on the regeneration and corrosion characteristics of an anodized aluminum plate regenerator, Energies 11 (2018) 1209. S.J. Kline, F. McClintock, Describing uncertainties in single-sample experiments, Mech. Eng. 75 (1953) 3–8.