A CRITICAL REVIEW ON ENERGY EFFICIENT LIQUID DESICCANT AIR ...€¦ · The effectiveness of the air...

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A CRITICAL REVIEW ON ENERGY EFFICIENT LIQUID DESICCANT AIR

CONDITIONING SYSTEM (LDAS) INTEGRATED WITH VAPOR COMPRESSION

REFRIGERATION (VCR)

KASHISH KUMAR1*& ALOK SINGH2

1Research Scholar, Mechanical Engineering Department, MANIT, Bhopal, India

2Assistant Professor, Mechanical Engineering Department, MANIT, Bhopal, India

ABSTRACT

Over the past few years, air conditioning (AC) required a significant amount of energy out of the total energy

generated in the world. Therefore, it has emerged as a critical issue to minimize the energy consumption and cost of

cooling in a conventionally used air conditioning (AC) system derived from the vapor compression refrigeration

(VCR) system without decreasing the indoor air quality (IAQ) and console conditions due to the increasing cost of

fossil fuels and other environmental troubles. The VCR system is inefficient and environmental-unfriendly because it

exhausts a large amount of energy, also the technology employed by it to regulate the humidity level in the air, as it

utilizes various refrigerants which cause global warming. Thereby, a liquid desiccant air-conditioning system (LDAS)

can be a favorable substitute for the VCR system. In this article initially, the basic principles of liquid desiccants and

LDAS have been studied. Furthermore, the traditional air-conditioning system integrated with VCR system has been

introduced and Investigation of various configurations of the hybrid LDAS has been performed. Additionally, a

precise overview of performance parameters has been discussed to analyze the effectiveness of the system. The novelty

of this article is that it would be worthwhile to recognize the research studies to survey new trending areas for

upcoming research to assist in the advancement in the LDAS.

KEYWORDS: Desiccant material, dehumidifier, energy saving, VCR system& Hybrid LDAS

Received: Jun 09, 2020; Accepted: Jun 29, 2020; Published: Oct 19, 2020; PaperId.: IJMPERDJUN20201531

1. INTRODUCTION

The energy utilization of the world is projected to increase by up to 50%, in the period from 2010-2040,

demonstrated in Figure 1. The CO2 emission in the air is raised rapidly due to global energy utilization and it has

been anticipated that it will be projected to increase up to 10% in the period from 2010 to 2040 [1]. As a matter of

choice, this frightening prediction, certain measures have been taken to decrease CO2 emission up to 45% in a

view of attaining the objectives of the treaty of Paris climate. The above-mentioned data certified that we are

facing dual problems i.e. we must come up with an idea that will have low carbon emission and that would suffice

our future energy demand [1].

Abbreviations Name

AC Air conditioning

CaBr2 Calcium bromide

COP Coefficient of performance

HCFC Hydrochlorofluorocarbon

KCOOH Potassium formate

HCFC Hydrochlorofluorocarbon

Orig

ina

l Article

International Journal of Mechanical and Production

Engineering Research and Development (IJMPERD)

ISSN(P): 2249-6890; ISSN(E): 2249-8001

Vol. 10, Issue 3, Jun 2020, 16143-16172

© TJPRC Pvt. Ltd.

16144 Kashish Kumar*& Alok Singh

Impact Factor (JCC): 8.8746 SCOPUS Indexed Journal NAAS Rating: 3.11

CFC Chlorofluorocarbon

CaCl2 Calcium chloride

IAQ Indoor air quality

LiBr Lithium bromide

KCOOH Potassium formate

LiCl Lithium chloride

ODS Ozone depleting substance

MgCl2 Magnesium chloride

TEG Triethylene glycol

VCR Vapor compression refrigeration

LDAS Liquid desiccant air conditioning system

CO2 Carbon dioxide

From Figure 1, it can be concluded that the buildings sector consumed about 33% of expected worldwide energy

utilization. Furthermore, the annual consumption of energy was increasing since 2017 by 0.5% in the building sector which

was higher than the others, as shown in Figure 2[1]. The anticipation of the degradation of a large part of electricity which

is done by air-conditioning equipment in the buildings is reported by up to 30% of worldwide electricity consumption by

2050[2]. The reasons would be increase in (i) economic growth, (ii) population, (iii) budget of people, (iv) urbanisation, (v)

ageing, (vi) cooling degree days, and (viii) disease[2][3]. The consequence of climate modification because of global

warming has more impact on India in contrast to various countries. As a result, India's electricity consumption for air-

conditioning is anticipated to rise remarkably[3]. Therefore, it is a kind of challenge for the researchers to come up with an

effective system that should be eco-friendly and economically productive and that can encounter the above-mentioned

challenges. The air conditioning system that meets up to 90% of the ongoing demand of our daily requirement in space

cooling is the vapor compression refrigeration (VCR) system but this system is unable to control moisture in energy

efficient manner [4][5].

Figure 1: World Energy Utilization [1]

In air-conditioning, the most familiar refrigeration system is the vapor compression refrigeration (VCR) system.

The basic principle on which the VCR cycle operates is that the vapor is compressed and condensed to a liquid by lowering

the pressure of vapor. The operations that comprise the cycle of the VCR system are as follows: (i) adiabatic

A Critical Review on Energy Efficient Liquid Desiccant Air Conditioning System 16145

(LDAS) Integrated With Vapor Compression Refrigeration (VCR)


compression,(ii) isothermal heat rejection, (iii) adiabatic expansion, and (iv) isothermal heat addition[6]. In the past few

years, numerous researchers have evolved and instituted the system that has performed well by enlarging the effectiveness

of power distribution and should utilize free energy (i.e. waste energy or renewable energy). Thus, most refrigeration

systems employing a vapor compression refrigeration (VCR) system, as it can perform outstandingly by getting involved

in the above-mentioned features. In a study, Zubair et al.[7] executed an experiment on a technique that was employing hot

flue gas bypass so that it can lower the ability of the air condition system when the system is operating on part-load

conditions. In another investigation, Chen and Jianlin [8] compared the two systems working on a new refrigeration cycle

having working refrigerant as R-22 and azeotrope mixture (R-134a+R22) and concluded that the system working on the

mixture has achieved superior performance than the R-22. The effectiveness of the air conditioning or refrigeration system

can be enhanced by employing additional sub-cooler in the system disclosed by Zubair et al.[9]. Wang et al.[10]

investigated an unconventional refrigeration system consisting of compressed air energy storage which was a combined

form of VCR and gas refrigeration cycle and an economical and thermodynamic analysis was done. Furthermore, Toublanc

and Clausse [11] suggested an unconventional carnot cycle that will attain higher performance for the critical operations

and revealed that (COP)system was 4-70% higher than that of the conventional cycle depending on the application.

Figure 2: Expansion of yearly demand by end-use sectors [1]

There is four vital components of the VCR system: (i) evaporator, (ii) condenser, (iii) compressor, and (iv)

expansion valve. The external power (energy) is applied to the compressor for the compression process, although the heat

addition and heat rejection to the system is done in the evaporator and condenser respectively. The process of heat addition

and heat rejection is performed by different refrigerants. It is expected that in the future around three billion units of

refrigeration system, air-conditioning system, and heat pump systems will become functional and around 17% of the

worldwide electricity utilization would be consumed by this many units [12]. However, most of the electricity demand is

generated from fossil fuels, therefore, this space cooling sector can significantly contribute towards the gross greenhouse

gas emissions in the atmosphere. Apart from energy consumption, air conditioning also contributes to environmental

degradation as the working fluids (refrigerants) got leaked from systems into the atmosphere. In the Montreal protocol,

Breidenich et al.[13]submitted a report stating that the draining out of the ozone layer was caused because of the emissions

of chlorofluorocarbons (CFCs) and hydro chlorofluorocarbons (HCFCs) till the 1980s. As a result, researchers agreed that



these ODS (ozone-depleting substances) must be reduced in future applications of space cooling. Additional harm due to

these refrigerants is the global warming potential (GWP). Therefore, due to the aforementioned factors deteriorating the

environment, natural refrigerants need to be reinstituted properly. In recent times, CO2 has been employed widely among

all other refrigerants such as ammonia, water, and hydrocarbons i.e. butane, propane, etc. owing to its superior

thermophysical properties, non-corrosive nature, non-flammability, and non-toxicity [14][15].

ALTERNATIVE REFRIGERATION SYSTEMS AND DEHUMIDIFICATION PROCESS

One of the major concerns for the environment is the exhaustion of conventional energy resources due to the rapid increase

in the world wide population which is over burden on natural resources. Figure 3,[16]is showing the interconnection

between clean energy, clean environment, and clean technology and the impact of these on humanity and nature.

Figure 3: Interconnection between clean energy, environment, and technology and their sub-connections [16]

To maintain superior indoor air quality (IAQ) and thermal solace conditions in space cooling, it has now become

very critical to come up with a substitute system that can decrease the energy utilization and eliminate the greenhouse gas

emissions [17]. These substitute systems can decrease the consumption of electricity in the building sector to a great extent

by employing some modifications. These modifications in the conventional system can be performed as making use of the

freely available energy (i.e. solar or waste heat) as a substitute to conventional energy by which the air-cooling system can

be energy-efficient [18]. Several thermodynamic modified systems that can be employed in a conventional system to

enhance its productivity and effectiveness are desiccant dehumidification system, absorption system, or intercooler system

[19]. However, the utilization of freely available energy in the thermal system is a tough challenge. Therefore, one of the

significant options would be utilizing solar energy as it is freely available mainly in tropical countries (i.e. India, Africa,

etc.) because of the variation of electricity load during the daytime [20]. A few methods can be employed to utilize the sun

as a source of energy in air-conditioning as demonstrated in Figure 4 [21]. To provide efficient space cooling outcomes,

the thermal refrigeration system consumes thermal power. The most common concept of thermal refrigeration is

demonstrated in Figure 5 [22].




Figure 4: Techniques employed in the transformation of Solar Energy into various Thermal Systems [21]

Figure 5: Concept of Thermally Activation ofthe Desiccant Cooling System [22]



3. WORKING PRINCIPLE OF LIQUID DESICCANTS

In the future, renewable energy would emerge as a better alternative for conventionally used energy. By 2040, out

of the total worldwide demand for energy, around 28-32% would be acknowledged by renewable energy, as demonstrated

in Figure 6[1]. Amongst renewable energy resources, solar power is best as it imparts a substantial part in renewable

energy. The LDAS technology could easily effectively utilize this freely available energy in the regeneration process of

desiccant. Consequently, solar energy is an interesting substitute for the conventional system in moisture control[5].

Figure 6: The Proportion of Solar Energy in the Whole Requirement of energy in Distinct Regions [2]

The principal objective of desiccant is to capture moisture from the air so that burden of the air-conditioning

system can be decreased as this desiccant works on the latent heat load of refined air. Commonly, there exist two kinds of

desiccants(i.e. solid or liquid). Due to certain advantages of liquid desiccant over solid desiccant like higher moisture-

holding ability, low-pressure drop, lower temperature of the regeneration process, applicability in performing both

refrigeration/dehumidification process simultaneously, potential to resist microbial bacteria or viruses in the air, and

operational effectiveness in employing free energy (i.e. solar or waste heat) [23]. Many researchers have investigated

various kinds of single desiccant or mixture of two or more desiccants and their properties. Some of the features of liquid

desiccants are shown in Table 1. A single desiccant has various kinds of properties; however, all the functions or

requirements of the air-conditioning systems cannot be fulfilled by a single desiccant such as it should be inexpensive, low

temperature of regeneration, lower viscosity, lower vapor pressure, and higher density. Therefore, aiming to encounter

these voids, various research studies have been performed to scrutinize the properties of various mixed desiccants that

concluded that liquid desiccants must have a low temperature of regeneration around 323-353 K [24], which can be easily

attained by employing a system that could work with only free energy. In a study, Hassan et al. [25] experimented on a

system using mixed desiccant (50% CaCl2 + 20% Ca(NO3)2) and analyzed the properties of mixed desiccants, such as mass

and heat transfer coefficient, vapor pressure, viscosity, and density and concluded that there wasa rise in vapor pressure of

desiccant as temperature increases. The values were 14.7, 20.5, 34.3, and 47.3 mm Hg adjacent to (30, 40, 50, 60)°C. A

modern technique suggested by Xiu Wei Li et al. [26] is to employ a mixture of two desiccants CaCl2 and LiCltogether.




The outcomes revealed that the efficiency of the dehumidification process while using mixed desiccant was 20% higher

than the LiCl2 solution.

Table 1: The Basic Characteristics of Liquid Desiccant

Characteristics CaCl2 TEG LiCl and LiBr

Toxicity Not-evaporate Non-toxic Not-evaporate

Circulation rate Low High low

Crystallization Yes No Yes

Corrosion hazard Medium Medium High

Loss of desiccant

during

evaporation

No low (dehumidification)

high (regeneration)

No

Suitability of

Dehumidification/

Regeneration

process

Inferior at 60°C Average at 66-81°C Better at exceeding

82°C

Cost Inexpensive Expensive Expensive

3.1. DIFFERENTIATION OF DESICCANT DEHUMIDIFICATION PROCESS TO CONVENTIONAL AIR-

CONDITIONING

The desiccant dehumidification process is commonly integrated with a system that works on the sensible heat load [27].

The merging of such two systems i.e. desiccant system and sensible cooling would represent a hybrid LADS. The

differentiation of vapour compression refrigeration (VCR) system and LADS is given in Table 2[24][28][29].

Table 2. Comparison between LDAS and VCR System

SI No. Parameters VCR LDAS

1. Operational cost High Save around 40% of the cost

2. Source of energy Natural gas or Electricity Low-grade energy (e.g. Solar, waste heat)

3. Indoor air quality Medium Better

4. System installment Medium Slight Complex

5. Storage capacity Medium Better

6. Working fluid HFC, HCFC, CFC LiCl, LiBr, CaCl2, TEG

7. Effect on environment Harmful Comparatively Eco-friendly

8. Humidity control Medium Better

3.2. ADVANTAGES OF THE LDAS

As there exist two kinds of loads in the air conditioning system namely latent heat and sensible heat loads were to control

the latent heat load a desiccant dehumidification system is employed in various layouts. Figure 7.[30]shows all the

configurations that can be employed in a desiccant system. The following are the subsequent advantages of liquid

desiccant:

Lower pressure drop along LDAS makesit right to utilize with a lower temperature of regeneration.

The size of LDAS could be compact or concise as they can pump liquid.

While heat source is unavailable for the regeneration process, the desiccant can be stored so that it can be used

when no heat source available.

While using liquid desiccant, the regeneration temperature should be low around 65-80°C. Therefore, low-grade



energy could be employed to complete the regeneration of weak desiccant.

The most advantageous characteristic is that they have the potential to remove contaminated bacterial infective

particles from the air and provide clean indoor air quality.

Figure 7: Various layouts for Desiccant Dehumidification System [30]

3.3. WORKING PRINCIPLE OF LADS

A fundamental principle of the LADS is to absorb the surplus amount of water vapor and heat from processed air using

liquid desiccants through heating and cooling operations. The principle elements of LDAS are dehumidifier and

regenerator. In an LDAS, the latent heat load in the air i.e. moisture is removed by a strong liquid desiccant solution

followed by the system that will take care of sensible cooling load in conditioned space. For sensible cooling, various

systems are employed (i.e. VCR system, vapor absorption system, direct/indirect evaporative cooling systems) followed by

air sending back to the cooling area. After absorbing moisture from the air, a strong desiccant solution becomes weaker

and sent to the regenerator unit for the reactivation process[31].In Figure 8., the fundamental configuration of LDAS that

consists of a dehumidifier, regenerator, and two heat exchangers is demonstrated. The prior is utilized to increase the

temperature of the weak solution and the latter is to decrease the temperature of the strong solution. The desiccant in the

dehumidifier withdraws the moisture from the air and become diluted. Moreover, this degenerated solution is passed

through a heat exchanger to increase its temperature, and then it is sprinkled into the regenerator. In the regenerator, water

vapors are blown away from the solution, consequently, the solution became regenerated and prepared to reuse. After the

regeneration process, the solution moved towards the dehumidifier over the heat exchanger for cooling purposes [32].




Figure 8:Configuration of LDAS [33]

The most common type of dehumidifier or regenerator that is employed today is based on packed bed

configurations. For better dehumidification process in the case of a packed bed, desiccant should have high flow rates in

the absence of internal cooling [33].Figure 9 shows a schematic diagram of LDAS.

Figure 9: Schematic Illustration of LDAS

4.1. CATEGORIZATION OF HYBRID LDAS

LDAS has capabilities to eliminate both the latent heat load and sensible heat load, which makes it an efficient system to

be used in various building sectors. The LDAS has vital components that make this system effective to nurture indoor air

quality (IAQ). The categorization of LDAS can be done based on various cooling sections that are employed to decrease

the temperature of the dehumidified air, as demonstrated in Figure 10. The dehumidification process in a dehumidifier is

required to withdraw as much adequate amount of moisture to suffice the environment of building requirement by

decreasing the dimensions of the dehumidifier unit and the temperature of regeneration required by the LDAS from around

80°C to 60°C [34].



Figure 10: Categorization of Hybrid LDAS and its Elements

5. LIQUID DESICCANT MATERIALS

The liquid desiccants have hygroscopic properties that absorb moisture from the air towards itself. A desiccant has an

application where a lower dew point temperature of the air is required. The vapor pressure is a vital property on which the

intensity of the effectiveness of liquid desiccant depends. The variation of the vapor pressure of liquid desiccant and the

water due to temperature change can be observed as the temperature increases vapor pressure increases exponentially. The

equilibrium vapor pressure of dilute liquid desiccant is greater than that of a concentrated liquid desiccant[35]. Whenever

this hygroscopic liquid is added to water (solvent), it decreases the vapor pressure of the solution lower than the vapor

pressure of the pure solvent. This ability of liquid desiccant to regulate the humidity level of air can be attributed due to its

lower vapor pressure. The characteristic of liquid desiccants that control the performance, handling, and capital costs of

LDAS is shown in Table 4 [36].

Table 4: Properties of Desiccant Material [36]

Sl. No. Classification Property

1. On basis of transfer

Density

2. Thermal conductivity

3. Specific heat capacity

4. Viscosity

5. Surface tension

6. On basis of absorption Heat of absorption

7. Energy storage capacity

8. Equivalent specific humidity

9. Diffusion coefficient

10. On basis of environment and economy Safety

11. Cost

12. Material compatibility




Table 5: Various hazards of the Desiccants [37][38][39]

Sl. No. Desiccant Health hazard Flammability hazard

1. LiCl Harmful when swallowed No

Give rise to skin rashes

Give rise to serious sour eyes

Give rise to respiratory problems

2. LiBr Harmful if swallowed No

Give rise to skin rashes

Reaction in body

Give rise to serious sour eyes

Give rise to respiratory problems

Can cause cancer

3. CaCl2 Give rise to serious sour eyes No

The various features that a liquid desiccant must pose are as follows:

Greater saturation absorption range

Lower temperature of regeneration

Less viscous

Higher rejection rate of heat

Non-volatile

Fragrance-free

Non-poisonous

Non-inflammable

Steady

Economical

Non-corrosive

Figure 11: Classification of Desiccant Materials



As aprime element, liquid desiccant plays a very vital part in comprehensive performance of desiccant air

conditioning system. Hence, this is essential to inspect features and properties of liquid desiccants for sake of selecting the

finest contender for the system. The two kinds of liquid desiccants are widely used, including an aqueous solution of

organic solvent for example tri-ethylene glycol, di-ethylene glycol, and ethylene glycol, as well as inorganic solutions like

as CaBr2, CaCl2, LiBr, and LiCl[40].In a study, Abdul-Wahab et al.[41]experimented with solar operated liquid desiccant

dehumidification air-conditioning system using TEG as a desiccant solution and reported that TEG is a better desiccant

solution than others because the boiling point temperature of the TEG solution is almost like that of water. However, it can

be simply vaporized within the air and the carry-over of this desiccant is very usual.As a result, TEG can not bean

appropriate desiccant to be employed in LDAS. The different kinds of LDAS employed are inorganic salt solutions like

CaCl2, LiCl, LiBr, KCOOH that are broadly investigated. The capabilities of a desiccant dehumidification system mostly

rely on the vapor pressure. The partial pressure of air, water, and desiccant solution performs a vital function in the

dehumidification and regeneration process because the variation in partial pressure is the main cause forvapors that involve

absorption and desorption processes in dehumidifier and regenerator respectively [42]. As the vapor pressure decreases, the

outlet air will become dry, consequently desiccant executes better. Table 6 shows investigated values of vapor pressure

under different temperatures for CaCl2, LiBr, and LiCl.

Table 6: Investigated values of the Vapor Pressure of Aqueous Salt Solutions [43]

Salt Concentration

(%)

Vapor Pressure (KPa) References

298 K 303 K 308 K 313 K

LiCl

30 2.2 2.79

[41]

40 1.79 2.41

40 1.48 1.74 2.13 2.41

44 1.47 2.1

LiBr

31 3.11 3.67

[41]

38 2.9 3.47

40 2.45 2.82 3.08 3.35

44 2.57 3.14

CaCl2

35 2.78 3.36

[42]

40 2.55 3.13

40 2.1 2.53 2.86 3.14

43 2.2 2.8

5.1. HALIDE SALT DESICCANTS

Initially, TEG was used as a halide salt solution. Though, the implementation of this desiccant is insufficient as its

viscosity is very high due to which there is instability in the operation. The glycol is a volatile substance as it has a lower

vapor pressure that is disclosed, due to this glycol is unsuitable for the air conditioning system [44]. Mostly operated halide

salts which are employed as desiccant solutions are LiCl, CaCl2, LiBr. Various research studies have[7][45][46]

determining thermodynamic properties of foresaid liquid desiccant and concluded that among all halide salts, LiClis the

most stable desiccant which provides low vapor pressure and concentration of dehydration around 30-40%. Although the

price of LiCl is comparably higher than other desiccants. In an investigation, Chen et al. [47]examined a heat pump

integrated with LDAS employing LiCl as the liquid desiccant and found that with the inlet humidity level of 13.9-18.2 g/kg

and inlet temperature of air about 25-27°C, the temperature drop canbe attained in the range of 5-7°C, with (COP)system of




4.Among all liquid desiccants, CaCl2 is a very cheap and most accessible desiccant, however, because of its high instability

among others, it is not recommended to utilize in the LDAS.Dai and Zhang [48] have performeda numerical examination

on mass and heat transfer of the cross flow dehumidifier unit which was having a concentration of 40% CaCl2 solution in

the desiccant system. Namvar et al.[49] has performed an experiment using MgCl2 as liquid desiccant so that the

crystallization process can be eliminated. The results revealed that concentration of MgCl2 was lower as compared to

saturated concentration which eliminates the crystallization process. Liu and Jiang[50] carried out an experiment for mass

flow rate of air of 0.23-0.49 kg/s and temperature of incoming air of 24.3-37.7°C and observed mass transfer, the

effectiveness of LiBrand LiCl, and COP of the system were similar even for the two desiccants that were used. In another

investigation, Kornnaki et al. [51] forecasted the efficiency of dehumidification of a dehumidifier employing an adiabatic

counter flow and elaborated a mathematical model of the system utilizing three different desiccants (i.e. LiBr, CaCl2, and

LiCl). It resulted that the absorption effectiveness of LiCl, LiBr, and CaCl2 were 0.144, 0.136, and 0.124 respectively.

5.2. ORGANIC OR IONIC DESICCANTS

The aforementioned salts have drawbacks as they are corrosive, which causes serious destruction to the material of the air

conditioning system. To eliminate this drawback of desiccant material, the potassium formate (KCOOH) solution is

employed which is eco-friendly and less corrosive.It also has some other advantages like as low toxicity, and less

viscosity[52]. In a study, Elmer et al. [52]used KCOOH solution in an innovative integrated system that includeda

regenerator, dehumidifier, and evaporative cooler and described that by incorporating this, a single heat and mass

exchanger can be developed. Moreover, Atkinson et al. [53]examineda higher concentration of potassium formate

(KCOOH) and resulted that it can surpass the efficiency of the conventional LADS by advancing in highervapor pressure

and keeping crystallization temperature lower than 0°C. The performance of LiCl, LiBr, and KCOOH solution was

analyzed by Longo et al. [54] and concluded that LiBr and LiCl solutions displayed superior dehumidification outcomes

compared to KCOOH solution, however, COOH executed better regeneration. Qiu et al. [55]studied the LDAS operating

on waste flue gases generated by a biomass boiler and observed that the moderate air relative humidity (RH) was

approximately 12.9-13.3% with the concentration ratio of KCOOH of 47% and the airflow rate of 24000 liters/min. In

another study, Longo, and Gasparella [56] inspected a flower greenhouse application for three years using three different

liquid desiccants (LiBr, KCOOH, and LiCl)and observed savings in energy of 9.6%, 15.1%, and 11.7% respectively. The

authors reported that KCOOH is inexpensive and environmental-friendly as it showed higher vapour pressure than other

desiccants.

5.3 COMPOSITE DESICCANT

Several composite desiccant materials seemed to be evolved in the recent few years to enhance their performance. The

significant properties required fora composite desiccant are as follows:

Greater boiling temperature

More latent heat of condensation

Reduced vapor pressure

Lower temperature functionality

Lesser viscosity



Greater density

Lower crystallization temperature

Inexpensive

In a study, Aristov et al. [57] utilized composite material consisting of a mixture of inorganic salts like (LiBr,

NaSO4, SrCl2, and CaCl2) and silica gel and revealed that the desorption temperature of these desiccants is very low. If the

temperature is in the middle of 80 to 90°C, 80% of absorbed water can be desorbed. A few salts including LiCl owned low

vapor pressure because it is highly stable, however, it is comparatively costlier desiccant in contrast to other desiccants.

The dehumidification process by desiccant can be advanced by merging several salts simultaneously and there could be a

reduction in cost and energy utilization [58]. Li et al. [59] compared the experimental result with simulation data and

demonstrated that the moisture transfer rate can attain a value of around 0.8-1.0g/kg if the inlet air ratio and inlet air

temperature are 8.3g/kg and 23-28◦C respectively. Ahmed et al. [46]introduced the regulation of the sequence of blending

desiccant materials (50% CaCl2 and 50% LiCl) to forecast the values of density, viscosity, and vapor pressure of desiccant.

The vapor pressure of mixed desiccants (i.e. CaCl2 and LiCl) at 30% concertation and 60°C temperature was about 100

mm Hg[42]. The outcome of the experiment concluded that by mixing (i.e. LiCl+CaCl2), vapor pressure can be

decreasedin anon-linear way. Chen et al. [60][61]examined the mixed desiccant (glycols+water+salts) for vapor pressure

and densities in the temperature ranging as 30-70◦C and resulted in that the vapor pressure of the selected desiccant was

lower than that of the counterpart desiccant. Table 7 and Table 8 outlined the dehumidification performances of single or

mixed desiccants respectively.

Table 7:Summary of the performances of Various Desiccants

Solution Mass Flow

Rate (kg/s)

Moisture

Removal

Rate (g/s)

Incoming air

temperature

(°C)

Incoming

Air

Humidity

Solution

temperature (COP)system Reference

LiCl 1.74-2.03 25.3-27.9 13.8-18.2

g/kg 15.7-19.2 4.0 [47]

LiBr 0.33-0.47 1.37-2.03 25.3-35.6 9.4-18.4

g/kg 19.6-27.2 0.45 [50]

LiCl 0.29-0.50 1.53-2.46 26.9-35.4 9.8-20.4

g/kg 21.8-29.0 0.47 [50]

KCOOH 0.073 0.16-0.5 30.2-34.8 51.3%-

70.5% 25.2-25.7 0.73 [52]

LiCl 0.01-0.10 - 30-42 12.9-14.9 14-30 0.13-0.20 [51]

CaCl2 0.01-0.10 - 30-42 12.9-14.9 14-30 0.10-0.15 [51]

LiBr 0.01-0.10 - 30-42 12.9-14.9 14-30 0.12-0.18 [51]

Table 8:Summary of Various Properties of Composite Desiccants

Temperature

(°C)

Mixed solvents Concentration

(%) or

(mol/kg)

Vapour

pressure

(Pa)

Density (×10-

3 kg/m3)

Viscosity

(mPa-s)

Reference

60 50% CaCl2 + 50%

LiCl

30% 13330 1.181 1.40 [46]

60 50% CaCl2 + 50%

LiCl

30% 17862 1.160 1.37 [42]

43.3 50% CaCl2 + 50%

LiCl

30% 6265 1.187 1.85 [42]




60 50% CaCl2 +

50%LiCl

40% 11997 1.286 3.494 [42]

60 25% LiCl + 50%

DEG + 25% water

7.865 3440 - - [60]

60 25% LiCl + 50%

T4EG + 25% water

7.867 3386 - - [60]

60 25% LiBr + 50%

DPG + 25% water

3.8386 8959 - - [60]

60 25% LiBr + 50%

DEG + 25% water

3.837 8573 - - [60]

60 25% LiCl + 50%

TEG + 25% water

7.862 2702 1.22 77.97 [61]

60 25% LiCl + 50% PG

+ 25% water

7.862 2956 1.16 35.77 [61]

60 25% LiCl + 50%

TEG + 25% water

3.839 8170 1.29 13.35 [61]

6. RECENT DEVELOPMENT IN THE CONFIGURATIONS OF DESICCANT

DEHUMIDIFIER/REGENERATOR

In the liquid desiccant dehumidification process, the dehumidifier is the most dominant unit whose flow patterns and

configurations are being studied by researchers. The most predominant function of the desiccant dehumidification unit is to

maintain movement of heat and mass transfer among processed air and liquid desiccant. The properties that the liquid

desiccant must posse are as follows [62]:

Higher heat and mass transfer

Resistant tomoisture diffusion

Non-corrosive and inexpensive dehumidifier material

Lowerpressure drop of processed air while passing along with the dehumidifier

Complete prevention of leakage of liquid desiccant with the processed air

Higher surface contact area per unit volume.

Desiccant dehumidifier and regenerator can be classified on various bases such as based on contact, flow pattern,

and cooling technology used in the dehumidifier/regenerator.

6.1. CATEGORIZATION BASED ON THE TYPE OF CONTACT BETWEEN DESICCANT AND AIR

Based on the type of contact of air and liquid desiccant, it can be categorized as direct contact and indirect contact type, as

demonstrated in Figure 12. In case of contact type dehumidifier, mass/heat transfer between desiccant and air occurs

through direct contact. However, in indirect contact type dehumidifier, the transfer between air and desiccant occurs

through a membrane having extremely small pores.



Figure 12: Categorization of dehumidifier/regenerator based on contact

6.1.1. Direct Contact Type

Almost all research studies are being focussed on direct contact type dehumidifiers owing to its various advantageous

features like a higher mass transfer rate and ease in fabrication [63]. In this dehumidifier, air and desiccant come in contact

with each other with or without packing fills. Direct contact type dehumidifiers are reclassified as the packed bed, spray

towers, and falling film, as shown in Figure 12. Amongstall, the packed bed is being mostly investigated because it has a

higher mass transfer rate than that of others [5]. The main advantage of the packing fills system is that it providesa larger

contact area for mass and heat transfer to occur between air and desiccant. Furthermore, the packing fills are recateg

orizednamely structured and random packings, as shown in Figure 13 [64]. Also, the correlation between these structured

and random packing is shown in Table 9 [65].

Figure 13. Visualization of (a) structured packing (b) random packing[65].




Table 9: Contrast between structured and Random Packing [65]

SI No. Random Structured

1. Transport and storage Easy Difficult

2. Surface area requirement Low High

3. Airside pressure drop High Low

4. Cost Low High

5. Flow channel Disturbed Uniform

In a study, Longo and Gasparella [66]experimented to see the mass transfer in the packings and resulted that

pressure dropand moisture removal rate were 60–75% and 20–30% respectively which were greater than that of structured

packings. Moreover, the direct contact type dehumidifier was more energy efficient as compared to the indirect contact

type dehumidifier. However, due to the carryover of the desiccant, it is not widely used as it causes corrosion to

downstream equipment and affects the health of occupants [67]. A specific type of filter is employed to eliminate carryover

of desiccant with processed air but this causes a high-pressure drop and therefore becomes costlier in the operation of the

LDAS [5].

6.1.2. INDIRECT CONTACT TYPE

In recent decades, indirect contact type dehumidifiers are employed in LADS because carryover of desiccant can be

avoided unlike in direct contact type dehumidifiers, as the existence of membrane in the indirect contact type dehumidifier

stops desiccant carryover to the processed air [68].Based on the layout, it has mainly two types i.e. flat plate and hollow

fiber dehumidifier [69]. The advantageous features of the intermediate membrane are as follows [73][70]:

Higher robustness

More dirt resistance

Superior stability

Highly porous

Greater modulus of elasticity

Higher penetration pressure for desiccants

Lower resistance to moisture diffusion

Lower tortuosity factor

Lower pressure drop at the airside

Inexpensive

However, it has a low mass transfer coefficient in contrast to direct contact type dehumidifier that needs to be

addressed properly [27]. To avoid this, superior membrane support such as nanofibrous membrane, micro fins can be

employed [27][71][72]. In a study, Ge et al.[27]performed a comparative study of the packed bed dehumidifier with

membrane-based dehumidifier and resulted that for equal contact area and equal pressure drop, the effectiveness of the

packed bed was increased by 16% and decreased by 13-20% respectively as compared to the membrane-based

dehumidifier.



6.2. Categorization Based on Flow Pattern and Cooling Technology used in Dehumidifier/ Regenerator

In a dehumidifier, the inlet hot and humid air is passed and from which moisture is removed by a strong desiccant solution.

Based on the flow of air in the dehumidifier unit, it has four types (i.e. parallel flow, cross flow, counter flow, and counter

cross flow) and based on cooling and heating technology employed in dehumidifier/regenerator, it is classified into two

types (i.e. internally cooled and adiabatic dehumidifier).

Figure 14: Categorization of dehumidifier/regenerator on the bases of the Flow Pattern and Cooling Technology

6.2.1. PARALLEL-FLOW, CROSS FLOW, COUNTER-FLOW, AND COUNTER-CROSS FLOW

Recently, most of the researchers have been focussing on the flow patterns which were employed in

dehumidifier/regenerator. However, several research studies have been carried on the cross flow pattern because it is the

most prominent among all. The flow designs are demonstrated in Figure 15.

Figure 15: Different flow patterns for dehumidifier/regenerator




In a study, Das et al. and Jain et al. [68][73]examined the cross dehumidifier consisting of a sequence of various

desiccants and air channels in cross-flow patterns and resulted that the efficiency of cross-flow dehumidifier was increased

at the lower air channel gap, however, the system was found to be relatively costlier as the pressure drop is higher. Liu et

al. [74] employed an NTU method on LADS and performed theoretical modelling using a cross-flow pattern. It has been

noted that the moisture removal rate was found to be in the range of 30-60%, which was in consent with simulation data.

Yang et al. [75] developed a hypothesis of dehumidification perfectness (i.e. the ratio of dehumidification effectiveness to

the rate of moisture removal, evaluated by mass and heat transfer) while working on various flow patterns. The

effectiveness of the cross-flow pattern was found to be approximately 10% lower than parallel flow type evaluated by

[76][77][78]. The fabrication of a parallel-flow heat exchanger consisting of a simple header in the LDAS was noted to be

comparatively difficult. Moreover, the drawback of a parallel flow dehumidifier is that there is leakage of desiccant. The

integration of both the parallel and cross-flow in a modern mass and heat exchanger was studied by Vali et al.[78][79].

From the Figure 16 demonstrating the S-shape of the heat exchanger, we can confirm the directions of the flow pattern of

both the air and the liquid desiccant and there is a homogeneous straight path from right to left in a dehumidifier, and,

desiccant flow is from the left bottom header and flow of desiccant is in S-shaped in heat and mass exchanger. The

numerical data disclosed that performance of this type of flow was relatively superior in the comparison of cross flow,

however, it was relatively inferiorin the comparison of parallel flow.

Figure 16: The shape of the counter-cross flow heat exchanger

6.2.2 Internally-Cooled And Adiabatic Dehumidifier/Internally-Heated Regenerator

The availability of inter cooling in the dehumidifier or heating in the regenerator, the LDAS is classified into two types i.e.

adiabatic dehumidifier and inter cooling dehumidifier or internally heating regenerator. In an adiabatic dehumidifier,

during the absorption process temperature of both desiccant and the air increases as heat is evolved in the absorption

process. As a result, there is a decrease in mass and heat transfer sequentially. However, in an internally cooled

dehumidifier, the cooling water eliminates the heat that is evolved in absorption endlessly to enhance the dehumidification

process in the dehumidifier [80]. The performance of the packed bed adiabatic dehumidifier was examined as it had a

higher mass transfer, however, there was difficulty while using them as the introduction of such cooling coils could spoil



the shape of the packing fills. Thereby, they are not being widely used as compared to an internally cooled dehumidifier.

Most of the research studies are being done on dehumidifier working on internally cooled technology which carried falling

filmslike as plate-fin, parallel plate,and tube-fin types [81]. The capabilities of the internally-cooled dehumidifiers mainly

depend on the working variables of the air, desiccant, and the cooling water as these factors haveinfluenced the flow

patterns in the middle of the streams, demonstrated by the Liu et al. [82]. It wasreported that the effectiveness of mass

transfer in the internally-cooled dehumidifier was higher than that of the adiabatic dehumidifier that consisted of a

desiccant cooling heat exchanger. In recent decades, membrane dehumidifiers have been evolved to eliminate desiccant

carryover in the processed air. In addition, more experiments were implemented to observe the patterns of the adiabatic and

membrane based internally cooled dehumidifiers and similar results were revealed [67]. Comparing with dehumidifier,

efficiency of the internally-heated regenerator was noted higher than that of the adiabatic dehumidifier [83]. The foremost

superiorities of LDAS working on internally cooled dehumidifier are as follows: (i) higher rate of dehumidification, (ii)

lower pumping cost of desiccant, (iii) lower flow rate of desiccant (iv) lower storage of desiccant, and (v) smaller size. The

effectiveness of LDAS was examined by Abdel-Salam et al. [84], where internally cooled dehumidifier and heated

regenerator were utilized and it was concluded that at the time of estimation of energy utilization of LDAS, the transient

states could be neglected. The thermal and electrical (COP) systemswere noted in the range of 0.35 to 0.52 and 2.7 to 3.6

respectively. In another study, Gao et al. [80]employed an adiabatic dehumidifier with no heat exchanger available during

dehumidification/regeneration and it was found that the temperature of the solution became very high and which resulted

in decreased efficiency of the dehumidification process. To circumvent this internally cooled dehumidifier, advanced

source of cooling to the desiccant solution and the heat can be removed which get evolved during dehumidification.

Whereas lower vapor pressure of desiccant could be carried out, which is beneficial to the dehumidification process. A

diagrammatic differentiation of heat and mass transfer between internally cooled and adiabatic dehumidifier is shown in

Figure 17. Most research studies have been carried out on an internally cooled dehumidifier [85][80]. Bansal et al.

[85]examined both adiabatic and internallycooled packed bed dehumidifiers and revealed that internallycooled packed

dehumidifiers posed (COP)systemof 0.56-0.72, that was relatively higher than that of diabatic dehumidifier (i.e. 0.37-0.54).

Luo et al. [86] carried out an experiment on a dehumidifier which was internally cooled having one single channel being

utilized for climate conditions in Hong Kong and resulted that dehumidification was limited when concentration was 35%

and the most favorable concentration rate was around 36-39%.

Figure 17: Contrast of adiabatic and Internally Cooled Dehumidifier Depicting Mass and Heat Transfer [87]




7. HYBRID LDAS BASED ON VCR SYSTEM

Among all air conditioning systems, VCR (vapor compression refrigeration) system is the most modern and advanced

system as based on the thermodynamic principles, in this system, sub-cooling of liquid refrigerant is done through which

cooling capability and (COP)system can be improved to a great extent. However, the VCR system has some drawbacks as it

is an energyin efficient process because a huge amount of energy is used in the sub-cooling of refrigerant. The integration

of both system (i.e. VCR and desiccant cooling system), a new system has evolved that is known ashybrid LDAS. The

ability of the dehumidifier and VCR system to work against latent heat load and sensible heat load respectively, both can

make an efficient system i.e.hybrid LDAS. The subsystem of the hybrid LDAS needs to reduce enough moisture to assure

comfort indoor air quality (IAQ) in building area. Most research studies have been executed to decrease the size of the

dehumidifier as it decreases the regeneration temperature in the range of (70-80)°C to (50-60)°C [34]. In another study[88],

a VCR system of 5 ton capacity was integrated with the packed bed desiccant system and the measurements of the gauze

type structure packing towers were (diameter: 50 cm, height: 2.6 m and thickness: 0.5 m), as demonstrated in Figure 18.

Here, CaCl2 was employed as liquid desiccant and it was concluded that the (COP)systemof the hybrid LDAS was higher

than that of the conventional system at various modes of regeneration. The various values of (COP)systemobtained while

heating were; air: 1.61, desiccant: 1.16, both: 1.42 and (COP)system of VCR unit: 0.98.

Figure 18: The Experimental Framework of LADS [88].



Dai et al. [89]studied a hybrid LDAS composed of a VCR unit, desiccant dehumidifier, and evaporative cooling

unit as demonstrated in Figure 19. Here, the dehumidifier is embedded with the packing of honeycomb paper. Based on

this study, it was found that while working with this dehumidifier, the (COP)systemwas enhanced by 23.1% as compared to

the conventional system. However, the (COP)systemwas increased by 15.3% when both the dehumidifier and evaporator

section were operated simultaneously.

Figure 19: The Schematic Diagram of the Hybrid LADS[89].

In a study, Yadav [90]analyzed a hybrid LDAS comprising of a VCR unit, operating LiCl as a liquid desiccant

and revealed that the energy-saving was 80% when the temperature ofthe incoming air was 35°C with RH = 40%, and the

concentration of liquid desiccant was 55-59.5%. Khalil [91]worked on a 6.2kW cooling capacity hybrid liquid desiccant

cooling system where various desiccant flow rates were taken into consideration and at different temperatures of

condenser, evaporator, regenerator, specific moisture recovery, and (COP)systemwere examined. It was concluded that

energy saving was found around 53%, as the (COP)system of suggested system was 68% higher than the conventional

system. In another study, Bassuoni [92] also performed similar experiment considering CaCl2 as a liquid desiccant and

concluded that the advancement of a 54% (COP)system could be attained. Li et al. [93] modified the hybrid desiccant system

by integrating a supplementary regenerator to decrease the cooling capacity of the evaporator. Here, experimental data was

found to be contrast to the simulation results. Moreover, concentration ratio was inspected and it was resulted that there

was a reduction in both colling capacity and concentration ratio by 37% and 0.5% respectively. She et al. [94] Investigated

a hybrid liquid desiccant and performed the thermodynamic analysis for the values obtained from the temperature of

ambient, condenser, relative humidity, and concentration of the desiccant. It was revealed that the (COP)system of the hybrid

LDAS was 18.8% superior to the traditional system. Unlike the abovementioned applications, Mohan et al.

[95]recommended a hybrid desiccant dehumidification system as demonstrated in Fig. 20, where the desiccant was rotated

from the evaporator to the condenser at a lower rate of flow to indulge the lower humidity situation. The parametric

analysis on variation in specific humidity and temperature of absorbent due to change in incoming air temperature,




air/solution air flow rate are investigated. It was concluded that to get better dehumidification, specific humidity should be

high, whereas, inlet air temperature should be low.

Figure 20: TheSchematic Diagram of VCRIntegrated withLDAS[95]

Dai et al. [89]describedthe hybrid desiccant dehumidification system integrated with the evaporative cooler and

VCR unitas the evaporative cooler can absorb of moisture fromair, hence, produce cooling effect. The result of simulation

were foundto be 0.512, 0.725, and 0.801(i.e. COPs of VCR, VCR + desiccant system, and VCR + desiccant system +

evaporative cooling) respectively and recommend that the performance of the hybrid system was 56% higher than that of

the conventional system.

Table 10: Working parameters and performances of different hybrid LDA/VCR system

Ambiance

air

temperature

(0 °C)

Ambiance air

RH

Desiccant

solution

type

Desiccant

concentration

ratio

(in %)

Evaporator

temperature

(0°C)

Condenser

temperature

(0°C)

References

35 40% LiCl 55-59.5 17 - [101]

35 30-60% LiCl 33 5 45-52.5 [105]

20-30 35-45% LiCl - 8-16 36-49 [102]

41-42 46-48% CaCl2 - 8-21 37-52 [103]

40-50 - LiCl 26 - 50.6 [104]

30-50 0.014-0.03

g/kg

- 45 - - [106]

7. CONCLUDING REMARKS AND FUTURE RESEARCH OPPORTUNITIES

Currently, many researchers are focussing on the LDAS and its integration with the other cooling system. This review

paper illustrates that LDAS is an uncomplicated energy efficient applied science system that can be upgraded by

integrating with other cooling systems. Moreover, recent advancement and evolution in LDAS have been discussed. The



advantages and disadvantages of liquid desiccant material are explained comprehensively. The diverse layouts of the

dehumidifier and its performance have been encapsulated. A comprehensive overview of a hybrid LDAS integrated with

VCR is demonstrated. LDAS could aid to resolve crucial matter occurring in the various process like ventilation, air

conditioning industries and to control ultimate electrical energy requirement generated by utilizing a conventional VCR

system. Under this comprehensive review of LDAS following salient conclusions can be presented:

The liquid desiccant specified on salts hasa high absorption ability than the organic one. The most commonliquid

desiccants used in the dehumidification process are LiCl, LiBr, and CaCl2. Almost 80-90% of research studies

have been carried out focusing on LiCl as desiccant because it offers excellent dehumidification as compared to

other salts. However, better substitutes for salt solutions are ionic liquids because of fact that they pose low vapor

pressure, low regeneration temperature, and no corrosion at all.

Desiccant like CaCl2 and LiBrare relatively cheaper, however, they have demerits such as instability and

inefficient dehumidification. To eliminate these disadvantages, KCOOH has come up witha better alternative

where the problem of carryover of desiccant solution can be solved by utilizing KCOOH as a liquid desiccant as it

is an environment friendly and nontoxic liquid desiccant. Moreover, experimental and numerical investigation on

KCOOH have been limited, therefore, the composite desiccant can be employed in a LDAS to enhance the

efficiency of the dehumidification process sinceit improved the absorption capacity and maintained a lower

regeneration temperature.

The most predominant feature of liquid desiccant is that it can give effective control over air humidity ratio and

the regeneration process can be accomplished with free energy i.e. solar or waste energy. The LDAS is an energy

efficient and eco-friendly substitute, the logic lies in the technology that is used by dehumidifiers to eliminate

moisture content.

The vital part of the desiccant dehumidification system is the dehumidifier/regenerator, therefore, various

configurations of dehumidifiers based on type of contact, flow patterns, and the availability of internal cooling

system in dehumidifier are studied. Out of all, internally-cooled dehumidifiers with cross-flow pattern has been

investigated more prominently because it provides better dehumidification results. For upcoming researchers, a

more effective and innovative layout of the dehumidifier (parallel plate, packed tower, and fin coil) for advance

enhancement in mass and heat transfer of processed air and desiccant can be recommended through this review

article.

The most investigated air-conditioning system which has effective control on indoor air quality (IAQ) is LADS

integrated with the VCR. The carryover of liquid desiccant in packed bed type dehumidifier can be eliminated by

using membrane-type dehumidifier because it utilizes a microporous membrane. However, the packed bed type

has a relatively higher mass transfer rate. Similarly, the rate of mass transfer in an internally cooled is greater than

that of the adiabatic dehumidifier. The performance of the LADS integrated with VCR can be enhanced by

employing multiple evaporators and condensers in the LADS.

The additional advantage of utilizingLDAS is that it assures quality of the air by eliminating the volatile organic

compounds, captures particulate matters, and decreases the level of bacteria or viruses present in the processed air.




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