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[email protected] Falling film liquid desiccant air dehumidification Chuanshuai Dong 1 (), Takashi Hibiki 2 , Lizhi Zhang 1 , Lin Lu 3 1. Key Laboratory of Enhanced Heat Transfer and Energy Conservation of Education Ministry, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China 2. School of Nuclear Engineering, Purdue University, 400 Central Drive, West Lafayette, IN 47907-2017, USA 3. Renewable Energy Research Group, Department of Building Services Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China Abstract Falling film liquid desiccant dehumidification technology is attracting more and more attention due to lower energy consumption, less pollution, and more flexible humidity control in recent years. This paper conducts a comprehensive review on falling film liquid desiccant dehumidification systems. Firstly, the working principles and features of the liquid desiccant dehumidification are introduced to describe the dehumidification process. The existing liquid desiccants including organic and inorganic desiccants are reviewed. Then, the structures of falling film dehumidifiers including both adiabatic dehumidifiers and internally-cooled dehumidifiers are described. Besides, the simulation models of falling film dehumidifiers, such as finite difference models, effectiveness NTU (ε–NTU) models and simplified simulation models are summarized. Finally, the exiting performance enhancing methods of falling film dehumidifiers are collected, which provide valuable guidance to researchers and engineers to improve the dehumidification performance. Keywords falling film liquid desiccant dehumidification dehumidification performance energy consumption Article History Received: 30 March 2019 Revised: 4 June 2019 Accepted: 4 June 2019 Review Article © Tsinghua University Press 2019 1 Introduction The building energy consumption has increased rapidly during the past decades (Tuominen et al., 2014). In developed countries, the global contribution from buildings towards energy consumption, both residential and commercial, had steadily increased reaching figures between 20% and 40% and exceeded the major sectors: industry and transportation (Pérez-Lombard et al., 2011). Among the building energy services, the Heating, Ventilation and Air-Conditioning (HVAC) systems are the most energy consuming devices, accounting for more than 50% of the total building energy consumption. It was reported that the energy consumed by air-conditioning systems took up 54% and 23% of the total building energy consumption for the typical official and residential buildings, respectively, in hot and humid regions (EMSD, 2017) The function of air conditioning system is to reduce both sensible and latent heat of air. The latent heat can be handled by the air dehumidification technologies, such as vapour condensation, solid adsorption, liquid absorption, and membrane dehumidification (Brundrett, 1989), etc. Amongst the above dehumidification technologies, liquid desiccant air dehumidification is attracting more and more attention recently due to many advantages, such as lower energy consumption, less pollution, and more flexible humidity control. By handling extra air humidity with desiccant absorption, liquid desiccant dehumidifier is an energy-efficient and eco-friendly alternative to traditional air dehumidification approaches. More than 30% of energy use of traditional air-conditioning system and even up to 50% if solar energy or waste heat is utilized for desiccant regeneration can be saved with liquid desiccant air dehumidification system (LDACS) (Ge et al., 2011). In LDAC systems, the extra moisture is removed by liquid desiccant absorption and the sensible and latent heat can be handled separately. This paper aims at conducting a technical review on falling film liquid desiccant air dehumidification system. Firstly, the working principles and features of falling film liquid desiccant dehumidification technology are introduced. Then, the types of existing liquid desiccant are reviewed. Vol. 2, No. 4, 2020, 187–198 Experimental and Computational Multiphase Flow https://doi.org/10.1007/s42757-019-0036-8
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Page 1: Falling film liquid desiccant air dehumidification · summarized. Finally, the performance enhancing methods for falling film applications are reviewed. This paper is very useful

[email protected]

Falling film liquid desiccant air dehumidification

Chuanshuai Dong1 (), Takashi Hibiki2, Lizhi Zhang1, Lin Lu3

1. Key Laboratory of Enhanced Heat Transfer and Energy Conservation of Education Ministry, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

2. School of Nuclear Engineering, Purdue University, 400 Central Drive, West Lafayette, IN 47907-2017, USA 3. Renewable Energy Research Group, Department of Building Services Engineering, The Hong Kong Polytechnic University, Kowloon, Hong

Kong SAR, China Abstract Falling film liquid desiccant dehumidification technology is attracting more and more attention due to lower energy consumption, less pollution, and more flexible humidity control in recent years.

This paper conducts a comprehensive review on falling film liquid desiccant dehumidification systems. Firstly, the working principles and features of the liquid desiccant dehumidification are introduced to describe the dehumidification process. The existing liquid desiccants including

organic and inorganic desiccants are reviewed. Then, the structures of falling film dehumidifiers including both adiabatic dehumidifiers and internally-cooled dehumidifiers are described. Besides, the simulation models of falling film dehumidifiers, such as finite difference models, effectiveness

NTU (ε–NTU) models and simplified simulation models are summarized. Finally, the exiting performance enhancing methods of falling film dehumidifiers are collected, which provide valuable guidance to researchers and engineers to improve the dehumidification performance.

Keywords falling film

liquid desiccant dehumidification

dehumidification performance

energy consumption

Article History Received: 30 March 2019

Revised: 4 June 2019

Accepted: 4 June 2019

Review Article © Tsinghua University Press 2019

1 Introduction

The building energy consumption has increased rapidly during the past decades (Tuominen et al., 2014). In developed countries, the global contribution from buildings towards energy consumption, both residential and commercial, had steadily increased reaching figures between 20% and 40% and exceeded the major sectors: industry and transportation (Pérez-Lombard et al., 2011). Among the building energy services, the Heating, Ventilation and Air-Conditioning (HVAC) systems are the most energy consuming devices, accounting for more than 50% of the total building energy consumption. It was reported that the energy consumed by air-conditioning systems took up 54% and 23% of the total building energy consumption for the typical official and residential buildings, respectively, in hot and humid regions (EMSD, 2017)

The function of air conditioning system is to reduce both sensible and latent heat of air. The latent heat can be handled by the air dehumidification technologies, such as vapour condensation, solid adsorption, liquid absorption,

and membrane dehumidification (Brundrett, 1989), etc. Amongst the above dehumidification technologies, liquid desiccant air dehumidification is attracting more and more attention recently due to many advantages, such as lower energy consumption, less pollution, and more flexible humidity control. By handling extra air humidity with desiccant absorption, liquid desiccant dehumidifier is an energy-efficient and eco-friendly alternative to traditional air dehumidification approaches. More than 30% of energy use of traditional air-conditioning system and even up to 50% if solar energy or waste heat is utilized for desiccant regeneration can be saved with liquid desiccant air dehumidification system (LDACS) (Ge et al., 2011). In LDAC systems, the extra moisture is removed by liquid desiccant absorption and the sensible and latent heat can be handled separately.

This paper aims at conducting a technical review on falling film liquid desiccant air dehumidification system. Firstly, the working principles and features of falling film liquid desiccant dehumidification technology are introduced. Then, the types of existing liquid desiccant are reviewed.

Vol. 2, No. 4, 2020, 187–198Experimental and Computational Multiphase Flow https://doi.org/10.1007/s42757-019-0036-8

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The structures of falling film liquid desiccant dehumidifiers are comprehensively collected. Afterwards, the existing simulation models of the falling film dehumidifiers are summarized. Finally, the performance enhancing methods for falling film applications are reviewed. This paper is very useful to fully understand the falling film liquid desiccant dehumidification systems and efficiently improve the dehumidification performance.

2 Principles and features of falling film liquid desiccant dehumidification

Figure 1 presents the schematic diagram of a basic liquid-desiccant dehumidification air-conditioning system (Lowenstein, 2008). The whole system is composed of four sub-systems, i.e., a dehumidification system, a regeneration system, a heat exchange system, and an air cooling system.

The task of dehumidification system is to remove the extra moisture of air through absorption process. The moisture exchange from the humid air to liquid desiccant solution is driven by vapour pressure difference between liquid-desiccant solution and humid air. In liquid desiccant dehumidification system, the vapor pressure of air which is in equilibrium with the desiccant solution is lower than that of the humid air. Therefore, the moisture of humid air is absorbed by the liquid desiccant solution and the air is dehumidified. In addition, the latent heat released during the dehumidification process may increase the temperature of liquid desiccant solution and the equilibrium vapor pressure accordingly. To maintain the low surface equilibrium vapor pressure, new dehumidification systems, such as internally-cold liquid desiccant dehumidification system, were developed (Park et al., 1994; Ali et al., 2004). In internally-cold liquid desiccant dehumidification system, the latent heat can be removed

Fig. 1 Schematic diagram of liquid-desiccant dehumidification air-conditioning system (Lowenstein, 2008; reproduced with permission © Taylor & Francis 2008).

by the cooling fluid, which could effectively prevent the temperature increase of desiccant solution.

The liquid desiccant solution is recycled in liquid desiccant dehumidification system. The typical cycle of liquid desiccant solution is present in Fig. 2 (Mazzei et al., 2005). From A to B, the liquid desiccant solution at Stage A is cold and concentrated. As the equilibrium surface vapor pressure is lower than that of humid air, the extra moisture of the air is absorbed by desiccant solution. Therefore, the concentration of desiccant solution decreases and the desiccant moisture content increases. From B to C, the liquid desiccant solution at Stage B is weak and needs to be re-concentrated. The weak desiccant solution is firstly heated to improve the equilibrium surface vapor pressure. Then, the moisture content of the weak desiccant solution is removed by processed air and the desiccant solution is regenerated. From C to A, the liquid desiccant solution at Stage C is concentrated but hot. Thus, the desiccant solution is pre-cooled before being supplied to the dehumidifier. Finally, the desiccant solution returns to Stage A.

The air cooling system is to handle the rest of sensible load and cool the air temperature to the required values. Then, the comfortable (cool and dry) air is supplied to the air-conditioned rooms. Compared with traditional air dehumidification technologies, such as air-condensation dehumidification technology, liquid desiccant air dehumidifi-cation has many advantages: (1) Lower energy consumption. As the latent heat and sensible heat are handled separately, the energy consumption of the air-conditioning system is reduced. Besides, the use of low-grade thermal energy also contributes to the lower energy consumption of air- conditioning system. (2) More environment-friendly. (Chlorofluorocarbons) CFCs and (hydro chlorofluorocarbons)

Fig. 2 Schematic of the typical cycle of liquid desiccant solution (Mazzei et al., 2005; reproduced with permission © Elsevier Ltd. 2004).

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HCFCs, which may have serious impacts on the ozone layer, are no longer required in liquid desiccant air-conditioning system. (3) Easier maintenance. As the whole system is operated under atmospheric pressure or near atmospheric pressure, the maintenance of the system is simplified. Therefore, the liquid desiccant dehumidification system (LDDS) is a promising alternative to conventional air conditioning system (CACS). The comparison between LDDS and CACS is summarized in Table 1 (Li, 2003; Jiang et al., 2004).

3 Existing types of liquid desiccant

The liquid desiccant is one of the key components in liquid desiccant dehumidification system. As the properties of liquid desiccant can affect the dehumidification performance directly, the type selection of liquid desiccant is critical. The liquid desiccant solution absorbs the extra moisture due to the vapour pressure difference. Therefore, an ideal liquid desiccant should possess the following properties: low surface vapour pressure, high solubility, low viscosity, low regeneration temperature, high energy storage density, non-volatile, non-toxic, stable and inexpensive, etc. (Yang et al., 2000).

The types of liquid desiccant have been investigated for several years and several liquid desiccants have been developed. The common liquid desiccants are mainly categorized into two types, i.e., organic desiccant and inorganic desiccant. The inorganic liquid desiccants include triethylene glycol (TEG), diethylene glycol (DEG), tetraethylene glycol (T4EG), dipropylene glycol (DPG), propylene glycol (PG), dipropylene glycol (DPG), etc. (Lof, 1955). The triethylene glycol (TEG) is one of the earliest liquid desiccants, but it has its limitations, such as easy adhesion to equipment or pipes due to high viscosity, which might affect the stability of the dehumidifi-cation system. In addition, the triethylene glycol is volatile because of the low surface vapor pressure and it may evaporate with air and enter the air-conditioned rooms (Elsarrag, 2006). As the operating concentration of triethylene glycol can be as high as 90%–98% by weight, the annular loss of triethylene glycol is very high.

The inorganic liquid desiccants such as lithium chloride (LiCl), lithium bromide (LiBr), and calcium chloride (CaCl2)

Table 1 Comparison between LDDS and CACS

Type CACS LDDS

System initial investment Similar

Operation cost High Save around 40% of cost

Driving energy source Electricity, natural gas Low-grade energy

Control over humidity Average Accurate

Indoor air quality Average Good

System instalment Average Slightly complicate

Energy storage capacity Bad Good

are very popular due to their high absorption capacity, good stability, low viscosity and volatility. The LiCl and LiBr can dry air to 15% and 6% relative humidity, respectively, but these salts are naturally corrosive (Rafique et al., 2016). In addition, the high price of LiCl and LiBr may limit their wide application (Lowenstein et al., 2006). Some researchers selected CaCl2 as liquid desiccant due to the lower price. However, the vapor pressure of CaCl2 is relatively high and CaCl2 solution is unstable, which limits its widespread use.

The properties of liquid desiccants have been investigated for several years (McNeely, 1979; Ertas et al., 1992; Chung and Luo, 1999; Kaita, 2001; Conde, 2004). Conde (2004) developed formulations for the thermal properties, including solubility boundary, vapour pressure, density, surface tension, dynamic viscosity, thermal conductivity, specific thermal capacity, and differential enthalpy of dilution, of lithium and calcium chlorides.

As the liquid desiccant shows direct effect on the heat and mass transfer between the processed air and desiccant solution, it is very important to choose suitable liquid desiccant. Several figures of merit have been proposed by researchers. Table 2 lists the weighting factors and figures of merit for the selection of desiccant solution (Studak and Peterson, 1988).

4 Structures of falling film liquid desiccant dehumidifiers

The liquid desiccant dehumidifier is one of the core com-ponents in liquid desiccant dehumidification system. Various types of dehumidifiers have been developed, which are mainly classified into two types, i.e., adiabatic dehumidifier and internally-cooled dehumidifier, according to whether cooling unit is equipped or not, as shown in Fig. 3 (Luo et al., 2014).

In internally-cold liquid desiccant dehumidifiers, the latent heat released during dehumidification process is removed by cooling fluid, which helps to maintain the temperature of desiccant solution and improve the dehumidification efficiency.

Table 2 Weighing factors and figures of merit for desiccant selection (Studak and Peterson, 1988)

Characteristics Weighting factor Figure of merit

Safety 1.0 Lethal dose (LD50)

Corrosion 0.8 Corrosion rate

Mass transfer potential 0.8 Equilibrium vapor pressure

Heat of mixing 0.6 Energy/kg of absorbed water

Cost of desiccant 0.5 Cost/100 kg of solution

Heat transfer potential 0.5 Thermal conductivity

Parasitic power loss 0.3 Viscosity

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Many researches have investigated the internally-cold liquid desiccant dehumidifiers for decades (Pesaran et al., 1994; Kessling et al., 1998; Chung and Wu, 2000; Jain et al., 2000; Saman and

Alizadeh, 2002; Zhang et al., 2013; Liu et al., 2015). The summary of the internally-cold liquid desiccant dehumidifiers is present in Table 3.

Fig. 3 Structure diagram of two dehumidifiers (Luo et al., 2014; reproduced with permission © Elsevier Ltd. 2013).

Table 3 Summary of internally-cold falling film liquid desiccant dehumidifiers

Source Structure Desiccant type Material Flow pattern Cooling type

Specific surface area (m2/m3) Operating conditions

Pesaran et al. (1994)

Heat pipe LiCl (37 wt%) and TEG (95 wt%)

— Co-current — — a 35 C,T = a 0.283 kg/s,m = s 228 408 L/sm = -

Jain et al. (2000)

Tubular absorber

LiCl — Co-current — — —

Saman and Alizadeh

(2002)

Plate heat exchanger

CaCl2 (40 wt%) Plastic tubes Cross-flow Indirect evaporation

— a 28.5 38 C,T = - a 12 20 g/kg,ω = - a 0.15 0.50 kg/s,m = - s 0.06 kg/sm =

Kessling et al. (1998)

Parallel plate exchanger

LiCl (40.2 wt%) Polypropylene plates

Counter-current Cooling water

121 a 23.9 26.9 C,T = - a 14.5 g/kg,ω = a 6.36 24.83 g/s,m = - s 0.116 1.242 kg/s,m = -

f 24.1 CT =

Liu et al. (2015) Tubes with fins

LiBr (38.8– 42.4 wt%)

Thermally conductive

plastic

Cross-flow Cooling water

342 a 33.5 36.3 C,T = - a 14.2 24.6 g/kg,ω = - a 0.118 0.180 kg/s,m = -

s 0.017 0.059 kg/s,m = -

s 28.6 31.5 C,T = - f 11.1 17.0 C,T = -

f 0.126 0.292 kg/sm = -

Jain and Bansal (2007)

Packed tower CaCl2 (37 wt%) Cellulose paper pads

Cross-flow Cooling water

608 a 20.9 30.3 C,T = - s 30.4 C,T =

s 2 10 L/min,m = - f 4.8 8.7 CT = -

Zhang et al. (2013)

Fin heat exchanger

LiBr (45 wt%) Stainless steel Cross-flow Cooling water

790 a 29.6 33.4 C,T = - a 15.4 17 g/kg,ω = - a 0.358 0.370 kg/s,m = -

s 0.041 0.178 kg/s,m = -

s 24.2 28.5 C,T = - f 16.4 25.5 C,T = -

f 0.101 0.358 kg/sm = -

Chung et al. (2000)

Fin coils TEG (88– 95.2 wt%)

Stainless steel Co-current Cooling refrigerant

160 a 21 26.9 C,T = - a 13.6 17.8 g/kg,ω = - a 1.94 3.77 kg/min,m = -

s 22.3 30.0 C,T = - f 18.4 21.9 CT = -

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5 Simulation models of falling film dehumidifier

Apart from the experimental study of liquid desiccant dehumidifier, several simulation models of liquid desiccant dehumidifier have been developed to investigate the heat and mass transfer mechanism in liquid desiccant dehumidifier. The well validated simulation models could provide researchers and engineers valuable guidance to better understand the heat and mass characteristics. The development of the simulation models for heat and mass transfer in liquid desiccant dehumidifier can be summarized and categorized into four stages, as below:

(1) In the 1980s, the parameters of the liquid desiccant in the thickness direction were considered constant and the thickness of the falling film was ignored (Queiroz et al., 1988).

(2) In Hellmann and Grossman’s model (1995), the thickness of the falling film was considered in the model, which meant that the heat and mass transfer resistance inside the liquid desiccant was taken into consideration. However, the plate or tube surface in the dehumidifier was assumed to be fully wetted by the liquid desiccant, which did not accord with the actual situation.

(3) To improve the prediction accuracy of the simulation models, the incomplete wetting conditions was taken into consideration. Jain et al. (2000) proposed a wetting factor in the simulation model to investigate the effect of incomplete wetting condition on the dehumidification performance. Ren et al. (2007) developed a 2D model of liquid desiccant dehumidifier considering the heat and mass transfer resistance inside the liquid falling film and the incomplete wetting conditions.

(4) Ali et al. (2004) developed a theoretical model to investigate the heat and mass transfer of liquid desiccant dehumidifier concerning the un-uniform distribution of thermal properties in the thickness direction. However, the falling film thickness was assumed constant. Mesquita et al. (2006) developed an improved model based on Ali et al. (2004) model considering the variable falling film thickness along the flow direction caused by moisture exchange.

According to the simulation methods, the simulation models can also be categorized into three types, i.e., the finite difference model, the effectiveness NTU (ε–NTU) model, and simplified simulation models.

5.1 Finite difference model

In finite difference models, the heat and mass transfer is calculated within the control volumes. Many researchers investigated the heat and mass transfer characteristics using finite difference model due to high predictive accuracy. Amongst these existing finite different models, Jain et al.

(2000) model is famous for considering the incomplete wetting conditions, which is present in detail as follows.

Jain et al. (2000) developed a simple steady state model of heat and mass transfer between processed air and liquid desiccant solution for both dehumidifier and regenerator. Two wetness factors, Fw and Fh, were introduced in Jain et al. (2000) model to account for the effect of improper wetting on dehumidification/regeneration performance. On taking suitable values of Fw and Fh, the theoretical model showed good agreement with the experimental results. To simplify the model, several assumptions were made:

(1) The air and desiccant flows were assumed as slug flows.

(2) The contractor was assumed to be well insulated and adiabatic.

(3) The properties of desiccant solution, air, and cooling water were considered constant within the control volume.

(4) The packing was assumed fully wetted and the heat and mass transfer area was assumed equal to the specific value of the packings.

(5) The heat and mass transfer only occurred in thickness direction and no heat and mass transfer occurred in the flow direction.

(6) The heat and mass transfer resistance inside the liquid desiccant was neglected.

For liquid desiccant dehumidifier, the mass and energy conservation equations between processed air, liquid desiccant, and cooling water in the typical control volume, as shown in Fig. 4, are shown as follows.

Mass conservation: For processed air,

aa m w a e

d ( )dWm h P W WZ =- - (1)

Fig. 4 Control volume for liquid desiccant dehumidifier (Jain et al., 2000; reproduced with permission © Elsevier Science Ltd. 1999).

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For desiccant solution,

ss m w a e

d ( )dCm h P W WZ

= - (2)

Energy conservation: For processed air,

aa m a a s a

d ( )dpTm C U P T TZ

= - (3)

For desiccant solution,

s a w as s m w w a fg

d d d dd d d dp pH T T Wm m C m C m HZ Z Z Z

=- - - (4)

For cooling water,

ww w w a s w

d ( )dpTm C U P T TZ

=- - (5)

where eW is the equilibrium humidity of desiccant solution, which is given as follows:

ee

atm e0.62197 PW P P=

- (6)

The mass transfer coefficient mh is calculated as follows:

D a a2/3m ( )

h J ρ VSc

= (7)

where DJ is the dimensionless Colburn factor (Treybal, 1981).

0.17D a0.023J Re-= (8)

Sc is Schmidt number.

a

a AB

μSc ρ D= (9)

Besides, the heat transfer coefficient on air side can be calculated using mass transfer coefficient as follows (Stoecker, 1989):

2/3ca m m ( )ph C h Le= (10)

where Le is Lewis number, a/Le Sc Pr= . The heat transfer coefficient on the desiccant solution

side is given as follows (Ramm, 1968):

0.33

0.11 s rs s

a0.67 , 2300Pr SNu Re ReL

æ ö÷ç= <÷ç ÷çè ø (11)

where sRe is the desiccant solution flow rate and Γ is the linear wetting density.

sa

4ΓReμ

= (12)

s

TπmΓ N D= (13)

rS is the reduced film thickness, which is obtained as follows:

1/3

so s2

s

3 , 16009.8

ΓμS Reρ

æ ö÷ç= <÷ç ÷çè ´ ø (14)

a or 1/3

s

[1 0.022( 4)]0.9085

V SSRe

- -= (15)

Therefore, the heat transfer coefficient on desiccant solution side sh is obtained as follows:

s ss

r

Nu KhS

= (16)

where sK is the thermal conductivity of desiccant solution. The heat transfer coefficient of cooling water wh is

obtained as follows (Kern, 1997):

w ww

eq

Nu KhD

= (17)

0.55 1/3 6w w w w=0.36 , 2000 1 10Nu Re Pr Re< < ´ (18)

where eqD is the equivalent diameter and can be calculated as follows for triangular pitch.

( )2 2

t oeq

o

8 0.423 π / 8=

πP D

DD

´ - (19)

In addition, the flow area is given as follows:

( )s t o ss

t

D P D BaP-

= (20)

Finally, the overall heat transfer coefficient from the cooling water to the desiccant solution is obtained as follows:

o o o

w w i w i s

1 1 1ln 0.00022D D D

U h D K D hæ ö÷ç= + + +÷ç ÷çè ø

(21)

5.2 Effectiveness NTU (number of transfer unit) model

Stevens et al. (1989) developed a heat and mass transfer model for cooling towers based on a simple effectiveness model. Finite difference element was used to solve the mode. In addition, two more assumptions were made including the assumption of the linear relationship of the saturation enthalpy and temperature and the ignorance of moisture removal in the calculation of solution energy conservation. In ε–NTU model, the effective heat and mass transfer process

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was assumed. The equations and calculation process of ε–NTU model were shown as follows:

(1) The calculation of NTU,

= D v T

a

h A VNTUG

(22)

(2) The calculation of effectiveness for the dehumidifier,

(1 )

(1 m )

1 e1 e

NTU m

NTUε

m

*

*

- -

* - -

-=

- (23)

(3) The calculation of outlet air enthalpy,

( )a,o a,i Ts,sat a,ih h ε h h= + - (24)

(4) The calculation of effective saturation enthalpy,

a,o a,iTs,sat,eff a,i 1 e NTU

h hh h -

-+

-= (25)

(5) The calculation of outlet air humidity ratio,

( )a,o Ts,sat,eff a,i Ts,sat,eff e NTUω ω ω ω -= + - (26)

5.3 Simplified simulation models

As both finite difference models and effective NTU models consume lots of numerical power and iterative calculations. It is hard to get the final results directly. Therefore, simplified simulation models are necessary to simplify the calculation process.

Khan and Ball (1992) developed a simplified model to predict the annual energy consumption of packed liquid dehumidifiers and regenerators based on their experimental results. In the simplified model, the outlet air temperature and humidity ratio can be quickly calculated as below:

2o 0 1 i 2 si 3 siW s s W s T s T= + + + (27)

2o 0 1 i 2 ao 3 aoW a a W a T s T= + + + (28)

where siT is the solution spray temperature (°F). oW and iW are the process air humidity ratios of the exit and outlet

air ( 1lb lb⋅ ). aoT is the air exit temperature (°F). 0 3a a and 0 3s s are constant factors determined by a least-squares

analysis. However, as the constant factors depend heavily on the experimental database, it is hard to extend the application of the model to other operating conditions beyond the database.

Qi et al. (2013) developed a semi-empirical model to quickly predict the performance of plate dehumidifiers and regenerators. Three types of effectiveness were introduced to represent the heat and mass transfer during the dehumidification and regeneration process. Enthalpy effec-tiveness, haε , is the ratio of the actual energy change of air

to the maximum possible one. Moisture effectiveness, maε , is the ratio of the actual mass change of air to the maximum possible one. Temperature effectiveness, tfε , is the ratio of the actual heat change of desiccant to the maximum possible one.

a,in a,out a,in a,out f,in f,outha ma tf

a,in I,in a,in I,in f,in s,in; ;h h ω ω t tε ε εh h ω ω t t

- - -= = =

- - - (29)

where h means the enthalpy, ω is the moisture content, and t is the temperature. The subscripts a, s, f, I stand for the air, desiccant solution, cooling/heating fluid, and the interface between air and solution, respectively. The subscripts in and out mean the inlet and outlet characteristics.

Based on the experimental results, the correlations for the three types of effectiveness were developed by linear regressions. For dehumidifiers:

de,ha

0.0486 0.525 0.349 0.0773 0.0764f,in s,in a,in I,in s a f w

0.547 0.378a,in I,in

0.0407d

a

10 ( ) ( ) ( / )( )

( 0.0022 0.0102 )

εt t ω ω m m m a

h δhW H

=

⋅ - --

´ - ⋅ + ⋅

(30) 0.195 0.350 0.0619 0.07

a,in I,in s a f wde,ma 0.009 0.0441 0.375

f,in s,in a,in I,in0.0854

d

a

0.0829 ( ) ( / )( ) ( )

( 0.0032 +0.0248 )

h h m m m aεt t ω ω

W Hδ

⋅ -=

- -´ - ⋅ ⋅

(31) 0.304 0.394 0.757 0.754

a,in I,in a,in I,in w dde,tf 0.857 0.261 0.759 0.602

f s a f,in s,in

f,in s,in

f,in s,i

a

n

0.115 ( ) ( ) ( )( / ) ( )

h h ω ω a W Hεm m m t t

t tδ

t t

⋅ - - ⋅=

--

´-

(32)

For regenerators:

0.987 0.770 0.0838I,in a,in s a w

re,ha 1.532 0.0038 0.012 0.368a,in I,in f,in s,in f

0.1a

25d

27.061 [1000 ( )] ( / )( ) ( )

(0.196 0.287 )

ω ω m m aεh h t t m

W Hδ-

⋅ ⋅ -=

- -´ ⋅ + ⋅

(33)

0.728 0.844I,in a,in s a

re,ma 1.332 0.0168a,in I,in f,in s,in

0.001 0.0691 0.0324 0. 4da

02 2f w

46.219 [1000 ( )] ( / )( ) ( )

( )

ω ω m mεh h t t

m a Wδ H

⋅ ⋅ -=

- -

´ ⋅ (34)

0.794 0.622 0.896a,in I,in w d

re,tf 0.636 0.908 1.07 0.480I,in a,in f f,in s,in

f,in s,in0.0032s a

f,in s,in

a

0.0554 ( ) (1.552 +0.118 )[1000 ( )] ( )

( / )

h h a W Hεω ω m t t δ

t tm mt t

⋅ - ⋅ ⋅=

⋅ - --

´-

(35)

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As discussed above, several models have been developed to estimate the dehumidification performance of falling film liquid desiccant dehumidifiers. The summary of existing models of falling film dehumidifiers are presented in Table 4.

6 Performance enhancing methods for falling film applications

The dehumidification efficiency is the most important index for a dehumidifier. Increasing the dehumidification efficiency can reduce the size of the dehumidifier and therefore reduce the initial cost. Investigators have investigated the performance enhancing methods for several years. The methods can be mainly summarized and categorized into four types, i.e., modification of working surface, addition of surfactant, optimization of the absorber structure, and addition of nanofluid.

6.1 Surface modification

The microstructure of plate surface can significantly affect the flow patterns of liquid desiccant and the dehumidification efficiency accordingly. Many researchers adopted enhanced tubes or plates with complex microstructures instead of the conventional smooth tubes or plates to increase the heat and mass transfer performance between the liquid desiccant and process air. On one hand, the micro-structures of the working surface can increase the mixture of the liquid

desiccant and process air at low flow rates and enlarge the contact area and time. In addition, the capillary effect can accelerate the replenishment of the interfacial liquid desiccant and reduce the heat and mass transfer resistance between the liquid desiccant and the process air. On the other hand, the complex micro-structure of the working surface can reduce the heat transfer boundary layers and enhance the heat transfer between the liquid desiccant and the internally- cooled unit. Thus, the latent heat released during absorption process can be effectively removed by the cooling water flowing at the back of the working surface, which strengthens the driving force of the heat and mass transfer.

Nakao et al. (1986) also investigated the heat and mass transfer process between LiBr aqueous solution and vapour using different types of tubes, including smooth tubes, corrugated tubes, and finned tubes, as shown in Fig. 5. The fin was equipped on the outside surface of the tubes with 45° upward. Through experiment, they found that the best heat and mass transfer performance was achieved with finned tubes. The performance enhancement is attributed to the mixing effect of finned tubes on liquid falling film.

Kang et al. (2002) and Cho et al. (2002) investigated the effect of micro-scale surface treatment on heat and mass transfer performance for a falling film H2O/LiBr absorber. Through experimental analysis, they found that increasing the surface roughness could significantly improve the surface wettability and then increase the absorption rates. The heat and mass transfer performance of tubes with rough surface

Table 4 Summary of simulation models for falling film liquid desiccant dehumidifiers

Classification Film thickness Source Flow pattern Dimensionality Wetting area

Jain et al. (2000) Co-current 1D Constant wetness factors

Liu et al. (2009) Six different configurations 2D Complete wetting

Ren et al. (2007) Four different configurations 2D Complete wetting Not considered

Yin et al. (2009) Co-current 2D Constant wetness factor

Ali et al. (2004) Co-current/ Counter-current/ Cross 2D Complete wetting Constant film thickness Dai and Zhang (2004) Cross 2D Complete wetting

Mesquita et al. (2006) Counter-current 2D Complete wetting

Peng and Pan (2009) Counter-current 2D Complete wetting

Hueffed et al. (2009) Cross 2D Complete wetting

Finite difference model

Variable film thickness

Dong et al. (2017a) Counter-current 2D Incomplete wetting

ε–NTU model — Stevens et al. (1989) Counter-current 1D Complete wetting

— Liu et al. (2008) Counter-current/ Cross — —

— Khan and Ball (1992) Counter-current — —

— Chen et al. (2006) Counter-current/ Co-current — —

— Dong et al. (2018) Counter-current — —

Simplified model

— Qi et al. (2013) Counter-current — —

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could be significantly improved by 100% compared with that of the smooth tubes.

Chen (2002) investigated the heat and mass performance of falling film with four low ribs enhanced tubes. Chen (2002) indicated that the heat transfer coefficient and mass transfer coefficient of the DAC enhanced tubes were improved by 100% and 50%, respectively, compared with those of the smooth tubes.

Dong et al. (2017b) adopted TiO2 super-hydrophilic coating to improve the surface wettability of working plate and therefore enhance the dehumidification performance. The experimental results indicated that the dehumidification efficiency could be increased by 60%. Zhi et al. (2019) and Qi et al. (2019) also used surface-modification technology to improve the dehumidification performance of falling film dehumidifiers.

In summary, the function of surface modification is to improve the surface wettability and therefore increase the contact area between liquid desiccant and processed air. Super-hydrophilic coating is a common surface modification method in falling film dehumidification due to the simplicity. However, the weak stability of the coating might deteriorate the wide application of this technology.

6.2 Addition of surfactant.

Kang and Kashiwagi (2002) investigated the heat transfer enhancement by Marangoni convection process. NH3 aqueous solution was used in the experiment and the images of eddies were captured by holographic interferometer caused by the surfactant. Similar images were also captured by Nishimura et al. (2002) in LiBr aqueous solution.

Hihara and Saito (1993) investigated the effect of surfactant on falling film absorption. Ethyl alcohol and octanol were selected as the surfactant in the experiment. The results indicated that the addition of ethyl alcohol could significantly

increase the absorption rates of the LiBr aqueous solution while the octanol could reduce the absorption rates.

Cheng et al. (2006) experimentally investigated the effect of octyl alcohol and isooctyl alcohol on the absorption capability of NH3 aqueous solution. Cheng et al. (2006) found that for a single absorption cycling, the additives with a suitable concentration range might enhance the heat and mass transfer performance. In addition, Cheng and Chen (2002) also investigated the effect of octyl alcohol and isooctyl alcohol on the absorption capability of the LiBr aqueous solution. The experimental results indicated that the surfactant could significantly improve the heat and mass transfer process.

Wen et al. (2018a, 2018b) adopted surfactant PVP-K30 to improve the dehumidification performance of falling film dehumidifiers. The experimental results indicated that the dehumidification performance could be effectively improved by the addition of surfactant.

In summary, the aim of surfactant is to improve the absorption ability of the liquid desiccant as well as the flow turbulence of falling film. Alcohol surfactants are commonly used in falling film liquid desiccant dehumidification. However, the volatilization of the alcohol surfactants might be a problem.

6.3 Optimization of the absorber structure

In traditional absorbers, heat and mass transfer only occurs in the thin layer between the liquid solution and air. The majority of the liquid solution cannot contact with the air, which reduces the heat and mass transfer rates. To increase the contact area and reduce the heat and mass transfer resistance inside the liquid solution, investigators tried to develop optimized absorber structures.

Islam et al. (2003, 2006) developed two falling film absorbers with film-inverting configuration, as shown in

Fig. 5 Representative image of different types of tubes (Nakao et al., 1986).

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Fig. 6. Baffles were used in the absorber to invert the falling film. The inverting of the falling film successfully achieves the contact between the processed air and the inner liquid desiccant. The simulation and experimental results indicated that the absorption rates were significantly increased by 100% compared with those of the smooth plates.

Another film-inverting absorber was developed by Cui et al. (2009), as shown in Fig. 7. Cui et al. (2009) also investigated the plate falling film absorber with film inverting configuration theoretically and experimentally. Through analysis, the inverting structures could significantly increase the heat and mass transfer coefficients.

In summary, the optimization of the absorber structure could rapidly improve the contact area and enhance the flow turbulence of falling film. But, the cost of the optimized falling film absorbers is higher compared with the common falling film dehumidifiers.

Fig. 6 Schematic diagram of the film-inverting absorbers (Islam et al., 2003; reproduced with permission © Elsevier Ltd and IIR 2003).

Fig. 7 Schematic diagram of plate falling film absorber with the film-inverting configuration (Cui et al., 2009; reproduced with permission © American Society of Mechanical Engineers 2009).

6.4 Addition of nanofluid

Kang et al. (2006) developed binary nanofluid using Cu/CuO nanoparticles and applied it into NH3 aqueous solution

for absorption application. The heat and mass transfer enhancement caused by the binary nanofluid was investigated experimentally and the results indicated that the binary nanofluid could significantly improve the performance. In addition, Lee et al. (2007) developed nanofluid to increase the heat and mass transfer of the bubble absorption using Fe/CNT (carbon nanotube) nanoparticles. Lee et al. (2007) found that the enhancing performance of CNT nanoparticles was better than that of the Fe nanoparticles.

Ma et al. (2008) investigated the effect of CNTs–NH3 binary nanofluid on bubble absorption process with NH3 aqueous solution. Through experimental analysis, an optimized amount of CNTs was proposed, at which they could achieve the best enhancing performance. In addition, the experimental results indicated that the enhancing ratio of the bubble absorption increased as the initial concentration of the NH3 aqueous solution increased.

Pang et al. (2012) adopted mono silver (Ag) nanoparticles to enhance the mass transfer during the ammonia-water bubble absorption. The results indicated that the absorption rate with 0.02wt% Ag nanoparticles was significantly enhanced by 55% compared with that of base fluid. Besides, it was found that the mass transfer performance in the binary nanofluids with the coolant is enhanced more than that without the coolant.

In summary, the main function of the nanofluid is to improve the heat conductivity of the liquid desiccant and enhance the heat and mass transfer performance between liquid desiccant and processed air. However, the sediment of the nanofluid might occur and reduce the performance enhancing effect in practical applications.

7 Conclusions

Falling film liquid desiccant dehumidification system is a promising alternative to traditional air conditioning system due to lower energy consumption, less pollution, and more flexible humidity control. To sufficiently understand the new technology, this paper conducts a comprehensive technical review on the liquid desiccant dehumidification system. The working principles and features of the liquid desiccant dehumidification technology are described and the existing types of liquid desiccant including both organic desiccant and inorganic desiccant are reviewed, which helps the researchers to select the proper liquid desiccant. Afterwards, both adiabatic dehumidifiers and internally-cooled dehumi-difiers are introduced. The internally-cooled dehumidifiers could achieve better dehumidification efficiency due to the removal of latent heat by the internally-cooling water. Besides, the simulation models including the finite difference models, the effectiveness NTU (ε–NTU) models and simplified simulation models are reviewed. Finally, to improve the

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dehumidification efficiency, the existing performance enhancing methods are summarized. This paper could provide valuable guidance to researchers and engineers in liquid desiccant dehumidification systems.

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