Adaption of an Evaporative Desert Cooler Into a Liquid Desiccant Air Conditioner: Experimental and Numerical Analysis

atmosphere

Article Adaption of an Evaporative Desert Cooler into a Liquid Desiccant Air Conditioner: Experimental and Numerical Analysis

Mustafa Jaradat *, Mohammad Al-Addous and Aiman Albatayneh

Department of Energy Engineering; German Jordanian University, Amman Madaba Street, P.O. Box 35247, Amman 11180, Jordan; [email protected] (M.A.-A.); [email protected] (A.A.) * Correspondence: [email protected]

Received: 25 November 2019; Accepted: 25 December 2019; Published: 28 December 2019

Abstract: Desert coolers have attracted much attention as an alternative to mechanical air conditioning systems, as they are proving to be of lower initial cost and significantly lower operating cost. However, the uncontrolled increase in the moisture content of the supply air is still a great issue for indoor air quality and human thermal comfort concerns. This paper represents an experimental and numerical investigation of a modified desert air cooler into a liquid desiccant air conditioner (LDAC). An experimental setup was established to explore the supply air properties for an adapted commercial desert cooler. Several experiments were performed for air–water and air–desiccant as flow media, at several solutions to air mass ratios. Furthermore, the experimental results were compared with the result of a numerical simplified effectiveness model. The outcomes indicate a sharp reduction in the air humidity ratio by applying the desiccant solutions up to 5.57 g/kg and up to 4.15 g/kg, corresponding to dew point temperatures of 9.5 ◦C and 12.4 ◦C for LiCl and CaCl2, respectively. Additionally, the experimental and the numerical results concurred having shown the same pattern, with a maximal deviation of about 18% within the experimental uncertainties.

Keywords: desert cooler; evaporative cooling; indoor air quality; liquid desiccant; eﬀectiveness model; moisture removal

1. Introduction Air conditioning represents a considerable part of current electrical power consumption due to the signiﬁcant growth in populations, in addition to rapid developments in lifestyle and comfort standards [1,2]. In certain countries of the Middle East and North Africa (MENA), for example, Jordan, Lebanon, Syria, and Egypt, having air conditioners was considered as a luxury in the last few decades. However, the climate changes and hot summers in the last years have made it a must. The recent heatwaves have sharply increased the demand for air conditioners as simple ventilators were not enough to moderate the heat. The total global electrical energy consumption for cooling in buildings using air conditioners accounts for about 20% nowadays [3]. Additionally, the share of energy consumption for space cooling is higher for regions with hot climates. In some of Middle East countries, such as Arab Gulf countries, the domestic ownership of air conditioning units is close to 100%. There are various techniques for space cooling. The most utilized is the conventional vapor compression air conditioners. Although conventional air conditioners have many advantages, such as their reliability, stability, eﬀective heat transfer rates, and compact sizes, they still have the problem of high overall electricity demand and peak electricity loads [4]. Besides, handling the latent load in vapor compression systems requires cooling the supply air beneath its dew point temperature.

Atmosphere 2020, 11, 40; doi:10.3390/atmos11010040 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, x FOR PEER REVIEW 2 of 15 vaporAtmosphere compression2020, 11, 40 systems requires cooling the supply air beneath its dew point temperature.2 of 15 Significant energy is required to reach the apparatus dew point (ADP) and further cooling below the ADP is required to compensate the bypass effect of the evaporator coils [5]. The dehumidified air Significant energy is required to reach the apparatus dew point (ADP) and further cooling below must then be reheated to the required comfort level and this leads to extra energy waste [6]. the ADP is required to compensate the bypass effect of the evaporator coils [5]. The dehumidified Numerous issues have been associated with conventional coolers, starting with their relatively high air must then be reheated to the required comfort level and this leads to extra energy waste [6]. costs, their operational costs, and maintenance costs; it has likewise been understood that recently Numerous issues have been associated with conventional coolers, starting with their relatively high the demand on using air conditioning for cooling has been increasing due to climate change and due costs, their operational costs, and maintenance costs; it has likewise been understood that recently the to increase in the urban heat island impact, particularly in populated territories. demand on using air conditioning for cooling has been increasing due to climate change and due to In some countries, because of the unaffordable conventional air conditioners (ACs) being increase in the urban heat island impact, particularly in populated territories. accompanied by high operating expenses, people are now looking for alternatives of conventional In some countries, because of the unaffordable conventional air conditioners (ACs) being power supply systems. Evaporative coolers have attracted much attention as an alternative to accompanied by high operating expenses, people are now looking for alternatives of conventional power conventional air conditioning systems, as they are considered an economic solution to cool the supply supply systems. Evaporative coolers have attracted much attention as an alternative to conventional air air temperature [7]. Direct evaporative coolers use the latent heat of water to absorb heat from the conditioning systems, as they are considered an economic solution to cool the supply air temperature [7]. supply air as it directly contacts the water and evaporates it. In this way, the system loses sensible Direct evaporative coolers use the latent heat of water to absorb heat from the supply air as it directly heat with a lower dry bulb temperature while maintaining a constant wet bulb temperature. contacts the water and evaporates it. In this way, the system loses sensible heat with a lower dry bulb Simultaneously, the water vapor that evaporated is also added to the air, which increases the water contenttemperature in the supply while maintaining air leading to a constantextreme indoor wet bulb air temperature.quality (IAQ) Simultaneously,issues and can increase the water thevapor risk ofthat respiratory evaporated diseases is also [8,9]. added to the air, which increases the water content in the supply air leading to extremeRegarding indoor the air most quality suitable (IAQ) issuesclimates and where can increase evaporative the risk coolers of respiratory are widely diseases used, [it8 ,9has]. been noticedRegarding that they the operate most suitablemore efficiently climates in where hot, dryevaporative regions; coolers as the temperature are widely used, of ambient it has been air increasesnoticedthat and they its humidity operate more decreas efficientlyes, the inbetter hot, drywill regions;be the performance as the temperature of the ofevaporative ambient air cooler increases at providingand its humidity indoor thermal decreases, comfort. the better Those will regions be the are performance the Middle ofEast, the Far evaporative East, some cooler regions at providingin North andindoor West thermal America, comfort. most Thoseregions regions in Australia, are the in Middle addition East, to Far a number East, some of European regions in Northcountr andies [10]. West FigureAmerica, 1 shows most the regions regions in Australia, that have in hot addition and dry to climates a number (red of European colored) that countries are preferable [10]. Figure for1 desert shows coolers.the regions that have hot and dry climates (red colored) that are preferable for desert coolers.

FigureFigure 1. 1. KöppenKöppen–Geiger–Geiger climate climate classification classiﬁcation map map (1980 (1980–2016)–2016) [10]. [10].

AlthoughAlthough evaporative evaporative cooling cooling systems systems have havebeen used been as aused more as progressive a more and progressive environmentally and environmentallyfriendly replacement friendly for conventionalreplacement for ACs, conventional as it only requires ACs, as electricity it only requires to drive electricity a pump unlike to drive vapor a pumpcompression unlike vapor air conditioning compression systems air conditioning which draw systems more electric which energy draw [more11–13 electric], the issue energy remains [11–13], with thethe issue fact that remains these systemswith the rely fact on that the these ambient systems conditions. rely on The the cooling ambient effectiveness conditions. of desert The cooling coolers effectivenessis low comparing of desert it to coolers conventional is low comparing vapor compression it to conventional systems; theyvapor can compression also have relatively systems; largerthey cansizes also and have most relatively importantly larger the sizes potential and most for importantly affecting human the potential comfort isfor higher affecting as it human can increase comfort the ishumidity higher as levelit can or increase even temperature the humidity may level not or be even reduced temperature enough. may The incrementnot be reduced in the enough. water vapor The incrementlevel in the in supplied the water air vapor can lead level to ain fertile the supplied environment air can to sweatlead to in, a andfertile for environment mold and bacterial to sweat growth. in, andFurthermore, for mold and increased bacterial moisture growth. levelsFurthermore, can affect increased the stability moisture of household levels can hygroscopicaffect the stability material, of householdsuch as woody hygroscopic furniture. material, Figure such2 shows as woody the humidity furniture. levels Figure been 2 shows linked the to comfort, humidity mold levels growth, been linkedand the to incidencecomfort, mold of respiratory growth, and illness the [incidence14,15]. of respiratory illness [14,15].

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Figure 2. Recommended space humidity levels linked to its impact on human health [15]. Figure 2. Recommended space humidity levels linked to its impact on human health [15]. Despite that evaporative cooling is known as an old system, there are few modifications in these systems,Despite and very that littleevaporative regarding cooling the humidityis known as of an the old supply system, air there leaving are thefew cooler.modifications Most of in thethese modificationssystems, and in the very literature little regarding were coordinated the humidity toward of the the determination supply air leaving of air–water the cooler. contact Most surfaces. of the Diffmodiferentications cooling pads in the were literature evaluated were regarding coordinated cooling toward efficiency, the suchdetermination as jute, lu ff ofa fibers, air–water and palm contact ® fiberssurfaces. [16]; coarse Different and cooling fine polyvinyl pads were carbonate evaluated [17]; regarding rice straw cooling and palm efficiency, leaf fibers such [18 as]; andjute, CELdek luffa fibers,, straw,and andpalm slice fibers wood [16]; [19 coarse]. Additionally, and fine polyvinyl several researchers carbonate [1 have7]; rice discussed straw and the mainpalm factorsleaf fibers when [18]; testingand CELdek a cooling® , straw, pad, such and asslice surface wood area, [19]. thickness,Additionally, and several type and researchers size of its have perforations discussed [ 20the–22 main]. Somefactors researchers when testing also further a cooling discussed pad, the such decrease as surface in electricity area, thickness, consumption and by type natural and draughtsize of its withoutperforations the use [20 of– a22]. blower Some [23 researchers]. also further discussed the decrease in electricity consumption byOne natural of the draught promising without and energythe use e offfi cienta blower strategies [23]. for handling the latent load of air conditioning systemsOne is to of apply the desiccant promising systems. and energy By applying efficient desiccants, strategies air for can handling be dehumidified the latent without load of the air needconditioning to cool the systems air below is itsto dewapply point desiccant temperature systems. [24 By]. Theapplying most desiccants, used type of air desiccant can be dehumidified systems is solidwithout desiccants the need and to namely cool the the air desiccant below its wheels dew point [25]. temperature However, liquid [24]. desiccantThe most systemsused type possess of desiccant the advantagessystems is of solid a separation desiccants between and namely the absorption the desiccant and the wheels regeneration [25]. However, processes liquid [26], desi regenerationccant systems at lowerpossess temperatures the advantages [27], and of thea separation possibility between for energy the storage absorption in a thermochemical and the regeneration energy formprocesses [28–30 [26],]. Liquidregeneration desiccants at lower are sorbents temperatures that have [27], a and high the affi possibilitynity to sorbates. for energy In open storage cycle in systemsa thermochemical (under atmosphericenergy form pressure), [28–30]. theLiquid sorbate desiccants is the water are sorbents vapor inthat the have air. a high affinity to sorbates. In open cycle systemsThe desiccant (under atmospheric solution’s eff ectivenesspressure), normallythe sorbate depends is the water on the vapor temperature in the air. and the concentration giving itThe a better desiccant performance solution’s at high effectiveness concentrations normally and lower depends temperatures, on theso the temperature controlling method and the forconcentration a solution can giving be attained it a bett byer varying performance its temperature, at high concentrations concentration, and or bothlower according temperatures, to the so air the specificationscontrolling requiredmethod for for a the solution output. can be attained by varying its temperature, concentration, or both accordingDue the to appealing the air specifications properties of required desiccants for with the theiroutput. affi nity for more than water vapor, such as microorganisms,Due the appealing they are widelypropert usedies of in desiccants the applications with their inwhich affinity air for borne more microorganisms than water vapor, could such causeas microorganisms, major issues, for they example are widely hospitals used and in the medical applications facilities in [ 31which–33]. air borne microorganisms could causeThere major are issues, several for common example hospitals liquid desiccants and medical that facilities were investigated[31–33]. in many researches. Some examplesThere areof se theveral investigated common liquid solutions desiccants are hygroscopic that were salt investigated solutions, suchin many as lithium researches. bromide Some (LiBr),examples CaCl2 of, and the LiCl,investigated or organic solutions compounds are hygroscopic like mono /salttriethylene solutions, glycol such [ 34as– lithium36]. In thebromide literature, (LiBr), LiClCaCl and2, CaCland LiCl,2 solutions or organic are the compounds most investigated. like mono/triethylene glycol [34–36]. In the literature, LiCl andThis CaCl paper2 solutions represents are the an adaptionmost investigated. of liquid desiccant solutions into a commercial desert cooler. LiCl-H2ThisO with paper a massrepresents fraction an ofadaption 0.43 and of liquid CaCl2 -Hdesiccant2O with solutions a mass fractioninto a commercial of 0.45 were desert used cooler. as desiccants.LiCl-H2O The with desert a mass cooler fraction was firstly of 0.43 tested and as CaCl a normal2-H2O evaporativewith a mass cooler fraction and of then 0.45 it were was testedused as withdesiccants. externally The cooled desert desiccants cooler was with firstly temperatures tested as a normal of 20 ◦C evaporative and 24 ◦C. cooler The experiments and then it forwas both tested desiccantswith externally were also cooled performed desiccants by varying with temperatures the solution massof 20 flow°C and rates 24 at °C. eight The values, experiments starting for with both desiccants were also performed by varying the solution mass flow rates at eight values, starting with 50 kg/h with an increment of 50 kg/h. Furthermore, the experimental results were compared with the numerical results of a basic effectiveness model.

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50 kg/h with an increment of 50 kg/h. Furthermore, the experimental results were compared with the

Atmospherenumerical 20 results20, 11, x ofFOR a PEER basic REVIEW eﬀectiveness model. 4 of 15

2. Description of the Modified Desert Cooler 2. Description of the Modified Desert Cooler An experimentalexperimental teat rig was constructed to experimentally study the the air outlet characteristics. The analyses were performed on a typical commercial desertdesert coolercooler thatthat costcost aboutabout US $80. The desert cooler represented a drip drip-type-type direct evaporative cooler with with corr corrugatedugated packing packing at at the the air air outlet. outlet. Water streamsstreams flowflow downdown byby gravitygravity over the structured packing material and are recycled from the basin driven by a pump. The water sprinkled onto the top edges of the pad is distributed further by gravity and capillarity. Figure 33 showsshows thethe innerinner structurestructure ofof thethe desertdesert evaporativeevaporative coolercooler andand thethe setup as modifiedmodified with desiccant solutions.

Figure 3. 3. InnerInner configuration conﬁguration of ofthe the evaporative evaporative cooler cooler and andthe setup the setup with witha desiccant a desiccant solution solution (left), (andleft ), the and modified the modiﬁed desert desert cooler cooler (right (right). Numerical). Numerical labels labels are (1) are concentrated (1) concentrated desiccant desiccant tank, tank, (2) (2)desiccant desiccant solution solution pump, pump, (3) (3) water/desiccant water/desiccant distributor, distributor, (4) (4) packing packing material, material, (5) (5) fan, fan, (6) (6) water circulation/desiccantcirculation/desiccant-outlet-outlet pump, and (7)(7) diluteddiluted desiccantdesiccant tank.tank.

The commercial desert cooler consisted of a corrugated packing material (4) made of cellulose with aa volumetricvolumetric surface surface area area of of 77 77 m2 /mm23/m. The3. The cooler cooler had had a built-in a built pump-in pump (6) and (6) a and fan (5).a fanThe (5). pump The waspump originally was originally used to circulate used to the circulate water in the the water conventional in the evaporative conventional cooler evaporative measurements. cooler Themeasurements. built-in pump The was built used-in pump to extract wasthe used collected to extract diluted the collected solution diluted to its tank solution (7). to its tank (7). The desert cooler cooler inlet inlet and and outlet outlet were were modified modiﬁed by by applying applying two two rectangular rectangular ducts ducts in inorder order to connectto connect it to it to the the laboratory laboratory air air ductwork, ductwork, as as shown shown in in Figu Figurere 3 3 (right). (right). The The desiccant desiccant solution solution is introducedis introduced from from above above and and it flows it ﬂows through through a distributor a distributor above above the cellulose the cellulose packing packing material. material. The Thedistributor distributor is used is used originally originally to distribute to distribute the water the water in the in desert the desert evaporative evaporative cooler. cooler.

3. Instrumentation and Exper Experimentalimental Setup Ambient air was supplied to an air handling unit accompanied by an air heaterheater/cooler/cooler and humidifier/dehumidifier to create the target conditions required for each experiment. Air volume flow humidifier/dehumidifier. to create the target conditions required for each experiment. Air volume flowrate (rateVa) was(푉푎̇ ) was measured measured using using a volume a volume flow flow meter. meter. Air Air temperature temperature (ϑa) ( and휗푎) and relative relative humidity humidity (ϕ) were(휑) were measured measured using using temperature temperature and and humidity humidity sensors. sensors. Two Two temperature temperature andand relativerelative humidity sensors were embedded at the inlet and outlet of the absorber.absorber. The three autonomous properties, namely temperatur temperature,e, relative humidity, and pressure, were used to derive all other air properties, such asas the the air air humidity humidity ratio ratio and and enthalpy, enthalpy, using using humid humid air state air equationsstate equations from thefrom American the American Society Societyof Heating, of Heating, Refrigerating Refrigerating and Air-Conditioning and Air-Conditioning Engineers Engineers (ASHRAE) (ASHRAE) [37]. [37]. The desiccant solution circuit consisted of plastic tanks for the concentrated and diluted desiccant solutions. A pump was used to circulate the liquid desiccant. The volume flow rate of the desiccant solution was measured by a magnetic inductive flowmeter. The temperature of the desiccant solution was measured with Pt100 resistance thermometer sensors connected to the absorber inlet and outlet. The density of the solution (휌) was measured by taking samples of the

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The desiccant solution circuit consisted of plastic tanks for the concentrated and diluted desiccant solutions. A pump was used to circulate the liquid desiccant. The volume ﬂow rate of the desiccant Atmosphere 2020, 11, x FOR PEER REVIEW 5 of 15 solution was measured by a magnetic inductive ﬂowmeter. The temperature of the desiccant solution was measuredsolution withat the Pt100absorber resistance inlet and thermometeroutlet. The desiccant sensors concentration connected was to derived the absorber by the measured inlet and outlet. The densitydesiccant of the density solution and (temperatureρ) was measured by applying by taking the correlations samples given of the by solution Conde [38]. at the Figure absorber 4 shows inlet and outlet. Thethe instrumentation desiccant concentration setup of the wasabsorber derived air circuit. by the measured desiccant density and temperature by applying the correlations given by Conde [38]. Figure4 shows the instrumentation setup of the absorber air circuit.

Figure 4. The experimental setup of the device tested with desiccant solutions. For the air circuit: . volume flow rate (V), relative humidity (ϕ), and temperature (ϑ) were measured. For the desiccant Figure 4. The experimental. setup of the device tested with desiccant solutions. For the air circuit: solutions:volume mass flow flow rate rate (푉̇ ), (m relative), density humidity (ρ), and (휑), temperatureand temperature (ϑ )(휗 were) were measured. measured. For the desiccant solutions: mass flow rate (푚̇ ), density (휌), and temperature (휗) were measured. The relevant parameters of the desiccant solution and air were measured at the inlet and outlet of the modifiedThe desert relevant cooler. parameters Table1 of shows the desiccant the uncertainties solution and air of thewere connected measured at instruments. the inlet and outlet of the modified desert cooler. Table 1 shows the uncertainties of the connected instruments. Table 1. Applied instrumentation and uncertainties as given by the manufacturer. Table 1. Applied instrumentation and uncertainties as given by the manufacturer. Instrument Model Uncertainty Range of Operation Instrument Model Uncertainty Range of Operation Air temperature Hygroflex 420 0.5 K 50–100 C Air temperature Hygroflex 420 0.5 K −50–100 °C− ◦ Relative humidityRelative humidity HygroflexHygroflex 420 420 2% 2% 0%–100% 0%–100% 3 Air volumeAir flow volume rate flow rate Optiswirl Optiswirl 4070 4070 DN 150DN 150 3% 3% 370–4800 m370–48003/h m /h Solution temperatureSolution temperature Pt100Pt100 0.5 0.5 K K Solution volume flow rate Optiflux 1050 1% 20–3900 L/h Solution volume flow rate Optiflux 1050 1% 20–3900 L/h Solution density L-Dens 323 1 g/cm3 0.5–2 g/cm3 Solution density L-Dens 323 1 g/cm3 0.5–2 g/cm3

The modifiedThe modified desert desert cooler cooler was testedwas tested by varyingby varying the the desiccant desiccant solution solution massmass flowflow rate rate in in the the range of 50–400range kg/ hof in 50 increments–400 kg/h in ofincrements 50. The of air 50. mass The flowair mass rate, flow temperature, rate, temperature, and humidity and humidity ratio ratio were kept were kept steady as 400 kg/h, 32 °C, and 13.12 g/kg for the entire experiment. The mass fractions of steady as 400 kg/h, 32 C, and 13.12 g/kg for the entire experiment. The mass fractions of the desiccant the desiccant solutions◦ were 0.43 for the LiCl-H2O solution and 0.45 kg/kg for the CaCl2-H2O solution. solutionsThe were solution 0.43 fortempera the LiCl-Hture varied2O solutionat values and20 °C 0.45 and kg 24/kg °C. for Table the 2 CaCl shows2-H the2O experimental solution. The inlet solution temperatureparameters. varied at values 20 ◦C and 24 ◦C. Table2 shows the experimental inlet parameters.

TableTable 2. Experimental 2. Experimental setup setup of of the the modiﬁed modified desert desert cooler: cooler: inlet inlet conditions conditions..

. . 퐦̇ 퐬퐨퐥 퐤퐠/퐡 퐦̇ 퐚 퐤퐠/퐡 훝퐚 °퐂 훚퐚 퐠/퐤퐠 훝퐬퐨퐥 °퐂 훏퐋퐢퐂퐥 퐤퐠/퐤퐠 훏퐂퐚퐂퐥ퟐ 퐤퐠/퐤퐠 msol kg/h ma kg/h ϑa ◦C ωa g/kg ϑsol◦C ξLiCl kg/kg ξCaCl2 kg/kg 50–400 400 32 13.12 20–24 0.43 0.45 50–400 400 32 13.12 20–24 0.43 0.45 A total of 32 experiments were carried out by varying one of the inlet parameters, denoted by A totalthe blue of 32 shaded experiments cells in Table were 2.carried The change out in by air varying temperature one of Δ휗 the푎, air inlet humidity parameters, ratio ∆휔 denoted, and the by the rate of moisture removal 푚̇ 푣 were assessed. The rate of moisture removal is given as shown in blue shaded cells in Table2. The change in air temperature ∆ϑa, air humidity ratio ∆ω, and the rate of Equations (1). and (2) [31]: moisture removal mv were assessed. The rate of moisture removal is given as shown in Equations (1) and (2) [31]: . . m = m (ω ωo), (1) v,AS da i − Atmosphere 2020, 11, 40 6 of 15

. . m = m (Xo X ), (2) v,SS salt − i . where mv,AS represents the moisture removal rate calculated from the air side, which represents the . . change in the air humidity ratio (∆ω) multiplied by the dry air mass ﬂow rate mda ; and mv,SS . represents the change in water content in the salt (∆X) multiplied by the salt mass ﬂow rate msalt . The water content per mass of salt (X) is given in Equation (3):

1 ξ X = − (3) ξ where ξ is the mass fraction of the salt solution, which represents the mass of the salt divided by the mass of the solution.

4. Numerical Model A basic effectiveness model was modified for a quick estimation of the outlet conditions for the heat and mass exchanger from the entry conditions and the exchange surface. In addition, the model helps to determine the deviations between the measured data and simulation. This paper demonstrates a mathematical model for a cross-flow heat and mass exchanger for determining ε-NTU relations. The present model is based on the work of Stevens [39]. Effectiveness, ε, is defined as the ratio of the actual heat and mass transfer rate to the maximum possible rate, namely,

ωa,i ωa,o ε = − , (4) ω ωeq a,i − where ωeq is the humidity ratio of air at equilibrium with the desiccant solution and is given as ps ϑsol,i, ξsol,i ωe = 0.62198 . (5) p p ϑ , ξ − s sol,i sol,i

The saturated pressure ps is realized as a nonlinear correlation of temperature and mass fraction according to [38] for aqueous solutions of LiCl and CaCl2. By applying the mass transfer coefficient (β) and the area of exchanger surface (A), the number of transfer units is derived: βA NTU = . . (6) ma Applying the Lewis relationship, the rate of mass transfer coefficient to heat transfer coefficient is given in Equation (7) [40]: β DLe1/3 = . (7) α λ The ε-NTU relation for cross-flow configuration depends on the number of rows. For the corrugated packing material, with an infinite number of rows (air passages), the use of the approximate relation is described by Equation (8) and represents the infinite series solution taken from [39].    X Xn m Xn m  1 ∞  NTU (NTU)  C NTU (C∗NTU)  = 1 e− 1 e− ∗  . (8) C NTU  − m!  − m!  ∗ n=0 m=0 m=0 

5. Results and Discussion

5.1. Conventional Direct Evaporative Cooling Prior to the desiccant investigations, the desert cooler was tested as a conventional evaporative cooler using water for the same air properties. Tap water was used with a temperature of 26 ◦C to ﬁll Atmosphere 2020, 11, 40 7 of 15 the desert cooler tank and the water was circulated internally. Table3 shows the inlet parameters and outlet results of the desert evaporative cooler.

Table 3. Inlet and outlet conditions of the desert cooler tested in evaporative cooling mode.

. ma kg/h ϑ C ω g/kg ϑa,o C ωa,o g/kg ε a,i◦ a,i ◦ − 400 32.0 13.12 24.7 16.07 0.68

As shown in Table3, a reduction in the air dry bulb (∆ϑa = 7.3 K) and an increment in the air humidity ratio (∆ω = 2.95 g/kg) were observed as a result of the heat and mass transfer between the air and water. The performance of the desert cooler was assessed by determining the saturation effectiveness. The effectiveness of the desert cooler was ε = 0.68. The saturation effectiveness represents the extent to which the air exiting the desert cooler approaches the wet bulb temperature, as shown in Equation (9):

ϑdb,o ϑdb,i ε = − . (9) ϑ ϑ db,o − wb 5.2. Modified Desert Cooler: Adiabatic Liquid Desiccant Dehumidification Two test sequences were performed to study the effect of solution flow rate and temperature on the absorption process. The test sequences were performed for the desiccant solutions of LiCl and CaCl2. Even though that LiCl is more effective as a desiccant [32], CaCl2 was also applied as it is more accessible and has a low cost compared to LiCl salt. The solution temperatures were set to 20 ◦C and 24 ◦C. The duration of each experiment was set to about 75 min. The shown experiments represent the averaged last 30 min of each experiment with a time-step of 10 s. This time represent a quasi steady-state density of the desiccant solutions at the absorber outlet. The aim of the experiments was to study the supply air conditions represented by the rate of moisture removal and outlet temperature as shown in Figure5, Figure6, Figure7, Figure8, Figure9, and Figure 10. Figures5 and6 show the rate of moisture removal rate from the airstream as a function of the solution mass flow rate using externally cooled desiccant solutions at 20 ◦C (Figure5) and 24 ◦C (Figure6). The experimental results of each of the solutions were compared; LiCl-H 2O (solid red line) and CaCl2-H2O (solid blue line). Moreover, the experimental results were compared with the results from the simplified effectiveness model (the dashed red and blue lines for LiCl and CaCl2 solutions, respectively). Atmosphere 2020, 11, x FOR PEER REVIEW 8 of 15

Figure 5. 5.Absorbed Absorbed water water vapor vapor versus versus the solution the solution ﬂow rate flow for ratethe desiccants for the desiccants with an inlet with temperature an inlet temperature of 20 °C. of 20 ◦C.

Figure 6. Absorbed water vapor versus the solution flow rate for the desiccants with an inlet temperature of 24 °C.

The deviation between the experimental and numerical results was in the range of 3%–18% and decreased by increasing the solution flow rate. An explanation could be that increasing the solution flow rate will increase the surface wetting of the corrugated material and thus coming closer to the assumption of complete surface wetting assumed in the model. The deviation was within the experimental uncertainties of 20%. The rate of moisture removal using the LiCl solution is higher than for the CaCl2 solution due to its higher absorption capacity. The rate has also been noted to be higher for solutions with lower temperatures, hence at 20 °C. For the LiCl solution with an inlet temperature of 20 °C, the moisture removal rate was in the range of 1.05–2.21 kg/h compared to 0.52–1.56 kg/h for CaCl2 for the same inlet temperature. An increment in the solutions outlet temperature was observed as a result of the enthalpy of condensation and enthalpy of dilution. This increment is inversely proportional to the solution flow rate and is lower for the CaCl2 solution compared to LiCl solution, as shown in Figures 7 and 8. An increment in the solution temperature of ∆휗푠표푙 = 20.6 퐾 was observed for the LiCl solution with an inlet temperature 20 °C and a mass flow rate of 50 kg/h compared to ∆휗푠표푙 = 14.8 퐾 for the CaCl2 solution at the same inlet conditions. The increase in the solution temperature decreases with increasing the solution flow rate as a result of the low heat capacity of the desiccant solutions and the reduced exposure time. An increment of the solution temperature of about 7 K was observed for a solution flow rate of 400 kg/h for both desiccants.

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AtmosphereFigure2020 5., 11Absorbed, 40 water vapor versus the solution flow rate for the desiccants with an inlet8 of 15 temperature of 20 °C.

Figure 6. 6.Absorbed Absorbed water water vapor vapor versus versus the solution the solution ﬂow rate flow for therate desiccants for the desiccant with an inlets with temperature an inlet temperature of 24 °C. Atmosphereof 24 20◦C.20, 11, x FOR PEER REVIEW 9 of 15 The deviation between the experimental and numerical results was in the range of 3%–18% and decreased by increasing the solution flow rate. An explanation could be that increasing the solution flow rate will increase the surface wetting of the corrugated material and thus coming closer to the assumption of complete surface wetting assumed in the model. The deviation was within the experimental uncertainties of 20%. The rate of moisture removal using the LiCl solution is higher than for the CaCl2 solution due to its higher absorption capacity. The rate has also been noted to be higher for solutions with lower temperatures, hence at 20 °C. For the LiCl solution with an inlet temperature of 20 °C, the moisture removal rate was in the range of 1.05–2.21 kg/h compared to 0.52–1.56 kg/h for CaCl2 for the same inlet temperature. An increment in the solutions outlet temperature was observed as a result of the enthalpy of condensation and enthalpy of dilution. This increment is inversely proportional to the solution flow rate and is lower for the CaCl2 solution compared to LiCl solution, as shown in Figures 7 and 8. An increment in the solution temperature of ∆휗푠표푙 = 20.6 퐾 was observed for the LiCl solution with an Figure 7. Solutions outlet temperature as a function of the solution flow ﬂow rate for the desiccants with inlet antemper inlet temperatureature 20 °C of and 20 °C.a mass flow rate of 50 kg/h compared to ∆휗푠표푙 = 14.8 퐾 for the CaCl2 an inlet temperature of 20 ◦°C.C. solution at the same inlet conditions. The increase in the solution temperature decreases with increasing the solution flow rate as a result of the low heat capacity of the desiccant solutions and the reduced exposure time. An increment of the solution temperature of about 7 K was observed for a solution flow rate of 400 kg/h for both desiccants.

Figure 8. Solutions outlet temperature as a function of the solution flow rate for the desiccants with Figure 8. Solutions outlet temperature as a function of the solution flow ﬂow rate for the desiccants with an inlet temperature of 24 °C. an inlet temperature of 24 ◦°C.C.

Figure 9 shows the outlet mass fraction of both desiccants as a function of solution flow rate. The spread of desiccant solutions is inversely proportional to the solution mass flow rate, as shown in Figure 9. The spread of the mass fraction was higher for the LiCl solution with a maximal spread of 2.5% compared to the CaCl2 solution with a maximal spread of 1.2%; both for a solution mass flow 2.5% compared to the CaCl2 solution with a maximal spread of 1.2%; both for a solution mass flow rate of 50 kg/h. While, the mass fraction spread was in the range of 0.6% and 0.4% for a solutions mass flow of 400 kg/h for LiCl and CaCl2, respectively. Furthermore, the mass fraction spread was mass flow of 400 kg/h for LiCl and CaCl2, respectively. Furthermore, the mass fraction spread was slightly affected by the solution inlet temperature for the given temperature range.

Atmosphere 2020, 11, x FOR PEER REVIEW 9 of 15

Figure 7. Solutions outlet temperature as a function of the solution flow rate for the desiccants with an inlet temperature of 20 °C.

Figure 8. Solutions outlet temperature as a function of the solution flow rate for the desiccants with an inlet temperature of 24 °C.

Figure 9 shows the outlet mass fraction of both desiccants as a function of solution flow rate. The spread of desiccant solutions is inversely proportional to the solution mass flow rate, as shown in Figure 9. The spread of the mass fraction was higher for the LiCl solution with a maximal spread of 2.5% compared to the CaCl2 solution with a maximal spread of 1.2%; both for a solution mass flow rate of 50 kg/h. While, the mass fraction spread was in the range of 0.6% and 0.4% for a solutions Atmospheremass flow2020 of, 11400, 40 kg/h for LiCl and CaCl2, respectively. Furthermore, the mass fraction spread9 was of 15 slightly affected by the solution inlet temperature for the given temperature range.

Atmosphere 2020, 11, x FOR PEER REVIEW 10 of 15

Figure 9. Mass fraction spread for the LiCl and CaCl2 desiccant solutions at 20 °C and 24 °C as a function of solution flow rate.

Figures 10 and 11 show the air outlet temperature as a function of solution flow rate for a solution inlet temperature of 20 °C and 24 °C. As an adiabatic process, the air outlet temperature decreased Figureby increasing 9. Mass the fraction solution spread flow rate. for theAn LiClincrement and CaCl in the2 desiccant air outlet solutionstemperature at 20was◦C observed and 24 ◦ withC as aLiCl function as the of desiccant solution for ﬂow a solution rate. flow rate of 50 kg/h.

FigureFigure 10. Air 10. Air outlet outlet temperature temperature as as a a function function of the solution solution flow ﬂow rate rate for for the the desiccants desiccants with with an inlet an inlet temperature of 20 °C. temperature of 20 ◦C.

As shown in Figures5 and6, the moisture removal rate increases by increasing the solution mass flow rate. For the LiCl solution there was an increase in the moisture removal rate of 1.16 kg/h (110% increase) for the given range of solution flow rate. This trend is expected as increasing the solution flow rate will lead to lower increments in the solution temperature, as shown in Figures7 and8. Increasing the solution flow rate also leads to lower decrement in the solution concentration, as shown in Figure9. As a result, this will keep the solution at a low vapor pressure and thus improve the mass transfer of water vapor from air to the concentrated solution. Furthermore, an increased solution flow rate will enhance the wetted transfer area of the corrugated packing material. The deviation between the experimental and numerical results was in the range of 3%–18% and decreased by increasing the solution flow rate. An explanation could be that increasing the solution flow rate will increase the surface wetting of the corrugated material and thus coming closer to the assumptionFigure 11. Air of outlet complete temperature surface as a wetting function of assumed the solution in flow the rate model. for the The desiccants deviation with an was inlet within the experimentaltemperature uncertainties of 24 °C. of 20%. The rate of moisture removal using the LiCl solution is higher than for the CaCl2 solution due to its higherA summary absorption of the capacity. performed The experiments rate has also is presented been noted in Figure to be higher12. The forprototype solutions was withtested lower in three modes: in the conventional direct evaporative cooling mode (the red line), and adiabatic temperatures, hence at 20 ◦C. For the LiCl solution with an inlet temperature of 20 ◦C, the moisture dehumidification using LiCl (blue points) and CaCl2 (green points) solutions. removal rate was in the range of 1.05–2.21 kg/h compared to 0.52–1.56 kg/h for CaCl2 for the same inlet temperature.

Atmosphere 2020, 11, x FOR PEER REVIEW 10 of 15

Atmosphere 2020, 11, 40 10 of 15 Figure 9. Mass fraction spread for the LiCl and CaCl2 desiccant solutions at 20 °C and 24 °C as a function of solution flow rate. An increment in the solutions outlet temperature was observed as a result of the enthalpy of Figures 10 and 11 show the air outlet temperature as a function of solution flow rate for a solution condensation and enthalpy of dilution. This increment is inversely proportional to the solution flow inlet temperature of 20 °C and 24 °C. As an adiabatic process, the air outlet temperature decreased rate and is lower for the CaCl solution compared to LiCl solution, as shown in Figures7 and8. by increasing the solution flow2 rate. An increment in the air outlet temperature was observed with An increment in the solution temperature of ∆ϑ = 20.6 K was observed for the LiCl solution with LiCl as the desiccant for a solution flow rate of 50sol kg/h. an inlet temperature 20 ◦C and a mass flow rate of 50 kg/h compared to ∆ϑsol = 14.8 K for the CaCl2 solution at the same inlet conditions. The increase in the solution temperature decreases with increasing the solution flow rate as a result of the low heat capacity of the desiccant solutions and the reduced exposure time. An increment of the solution temperature of about 7 K was observed for a solution flow rate of 400 kg/h for both desiccants. Figure9 shows the outlet mass fraction of both desiccants as a function of solution flow rate. The spread of desiccant solutions is inversely proportional to the solution mass flow rate, as shown in Figure9. The spread of the mass fraction was higher for the LiCl solution with a maximal spread of 2.5% compared to the CaCl2 solution with a maximal spread of 1.2%; both for a solution mass flow rate of 50 kg/h. While, the mass fraction spread was in the range of 0.6% and 0.4% for a solutions mass flow of 400 kg/h for LiCl and CaCl2, respectively. Furthermore, the mass fraction spread was slightly affected by the solution inlet temperature for the given temperature range. Figures 10 and 11 show the air outlet temperature as a function of solution flow rate for a solution inlet temperature of 20 ◦C and 24 ◦C. As an adiabatic process, the air outlet temperature decreased by increasing the solution flow rate. An increment in the air outlet temperature was observed with LiCl Figure 10. Air outlet temperature as a function of the solution flow rate for the desiccants with an inlet as thetemperature desiccant forof 20 a solution°C. flow rate of 50 kg/h.

Figure 11. AirAir outlet outlet temperature temperature as a function of the solution flow ﬂow rate for the desiccants with an inlet

temperature ofof 2424 ◦°C.C.

A summary of the performedperformed experiments is presented in Figure 1212.. The prototype was tested in threethree modes:modes: in the conventional direct evaporativeevaporative cooling mode (the red line), and adiabatic dehumidiﬁcationdehumidification using LiCl (blue(blue points)points) andand CaClCaCl22 (green points) solutions.

Atmosphere 2020, 11, 40 11 of 15 Atmosphere 2020, 11, x FOR PEER REVIEW 11 of 15

Figure 12.12. Illustration of the performed experiments on the psychrometric chart (Mollier diagram): The red lineline representsrepresents thethe conventionalconventional direct evaporative desert cooler, the green dots represents

the performedperformed experimentsexperiments with CaCl2 as a desiccant, and the blue dots represents the performedperformed experiments with LiClLiCl asas aa desiccant.desiccant.

Figure 12 represents a graphical user interface for thermal comfort prediction based on ASHRAE Standard 55 [[41].41]. Summer comfort zone specifies specifies boundaries of air temperature and humidity for people inin typicaltypical summersummer clothingclothing duringduring primarilyprimarily sedentarysedentary activity.activity. Since people feel comfort subjectively and each person has their own preferences for room climate, there is a comfort range of room air air temperature temperature and and relative relative humidity humidity in which in which the vast the majority vast ofmajority room users of room feel comfortable. users feel Thecomfortable. limits of summer The limits comfort of summer zone, of ASHRAE comfort Standard zone, of 55, ASHRAE were developed Standard theoretically. 55, were develoAs shownped intheoretically. Figure 12, As this shown zone in extends Figure from 12, this around zone 23extends◦C to from 27 ◦ Caround and 35%23 °C to to 75% 27 °C relative and 35% humidity; to 75% i.e.,relative the absolutehumidity; humidity i.e., the absolute should be humidity between should 5 and 12 be g between/kg air for 5 comfort.and 12 g/kg air for comfort. The air that left the conventional desert cooler had a sig significantnificant drop in the dry bulb temperature of 7.3 K. However, thethe airair humidityhumidity ratio is significantlysignificantly increased to 16.1 gg/kg;/kg; this can be noticed while observingobserving thethe redred lineline pathpath onon thethe psychrometricpsychrometric chartchart (Mollier(Mollier diagram)diagram) above.above. The air outlet conditions for the liquid desiccant system are shown in Figure 1212,, andand thethe pointspoints werewere determineddetermined based on the experimental evaluation carried out in the paper. The results of absorption using LiCl and CaCl 2 solutionssolutions at at 20 20 °C◦C were were chosen chosen to tobe be represented represented in inthe the psychrometric psychrometric chart. chart. The Theblue blue line lineshows shows the path the path of the of experimental the experimental results results using using the LiCl the LiClsolution solution for the for experiments the experiments 1 to 18 to(by 8 (byincreasing increasing the solution the solution flow flowrate) rate)while while the green the green dots represent dots represent the experimental the experimental results results using CaCl using2. CaClAs shown,2. As shown, the humiditthe humidityy ratio decreasesratio decreases significantly significantly by applying by applying the thedesiccant desiccant solutions solutions and and it is it ishigher higher for for the the LiCl LiCl solution solution compared compared to the to the CaCl CaCl2 solution.2 solution. Additionally, Additionally, the thetemperature temperature does does not notdecrease decrease like like it does it does in a in conventional a conventional desert desert cooler, cooler, the the air air outlet outlet conditions conditions hit hit the the comfort zone margins for a solution toto airair massmass ratioratio ofof r푟m푚==1.1.

5.3. Dehumidification Dehumidification Load For the the dehumidification dehumidification and and cooling cooling by byapplying applying liquid liquid desiccants desiccants in the in modified the modified desert cooler,desert thecooler, cooling the cooling load is calculatedload is calculated by multiplying by multiplying the air mass the air flow mass rate flow by the rate change by the in change specific in enthalpy specific as given in Equation (10) [5]: enthalpy as given in Equation (10) [5]: . . Q = m (h ho). (10) deḣ da i − 푄푑푒ℎ = 푚̇ 푑푎(ℎ𝑖 − ℎ표). (10) kJ The specific enthalpy (h) of the air entering the modified evaporative cooler is hi = 65.7푘퐽kg . The specific enthalpy (ℎ) of the air entering the modified evaporative cooler is (ℎ = 65.7 ). 𝑖 푘푔 For the performed experiments, presented in Figure 12, the dehumidification/cooling load (Qdeh) and theFor airthe outlet performed conditions experiments, are given presented in Table4. in Figure 12, the dehumidification/cooling load (푄푑푒ℎ) and the air outlet conditions are given in Table 4.

Atmosphere 2020, 11, 40 12 of 15

Table 4. Air outlet conditions and the dehumidiﬁcation load for both desiccants at a temperature of

20 ◦C.

Air Outlet Conditions Using LiCl-H2O as Air Outlet Conditions Using CaCl2-H2O as Desiccant Desiccant . . ϑ ω g kJ ϑ ω g kJ Exp. o, °C o, kg ho, kg Qdeh, kW o, °C o, kg ho, kg Qdeh, kW 1 33.8 9.87 59.4 0.71 31.9 11.53 61.6 0.46 2 31.5 8.81 54.3 1.27 30.5 10.67 58.0 0.86 3 30.1 8.22 51.3 1.60 29.5 10.07 55.4 1.15 4 29.2 7.89 49.5 1.80 28.8 9.67 53.7 1.34 5 28.6 7.68 48.4 1.93 28.3 9.41 52.5 1.47 6 28.2 7.54 47.6 2.02 27.9 9.22 51.6 1.57 7 27.9 7.43 47.0 2.08 27.6 9.07 51.0 1.64 8 27.6 7.36 46.6 2.13 27.4 8.97 50.5 1.70

As shown in Table4, the dehumidification load increases by increasing the solution flow rate up to 2.13 kW and 1.7 kW for solution mass flow rates of 400 kg/h of LiCl-H2O and CaCl2-H2O, respectively. The table also shows that the LiCl-H2O is more efficient in dehumidification compared to CaCl2-H2O, as expected. The percentage change in the dehumidification load was up to 25.3% for an air to solution mass ratio of rm = 1. However, an additional load is required for the regeneration of the diluted liquid desiccant solution that can be regenerated by solar energy.

6. Conclusions In this paper, a modification of a direct evaporative (desert) cooler was performed by applying LiCl-H2O and CaCl2-H2O as desiccants. The prototype was tested by running it in a direct evaporative cooler and in an externally cooled (adiabatic) desiccant system. Several experiments were performed by varying the solution flow rate at the two inlet temperatures. The experimental results were also compared with a single-node effectiveness model. The results show more beneficial air outlet conditions by using liquid desiccants regarding the air humidity ratio: A reduction in the air humidity ratio of up to 5.76 g/kg for the LiCl solution and up to 4.15 g/kg for the CaCl2 solution compared to an increase of 2.95 g/kg in the humidity ratio for the conventional desert cooler. Thus, the indoor air quality is significantly improved in the modified desert cooler. The moisture removal rate calculated from the measured data was compared with the numerical results from the NTU effectiveness model. The results show a systematic pattern and the results were − in good agreement. The maximal deviation in the rate of moisture removal between the experimental and the modeled results was in the range of 18%. The results show that the LiCl solution is more effective in moisture removal than the CaCl2 solution. However, the difference in moisture removal between LiCl and CaCl2 solutions decreased by increasing the solution flow rate. The percentage difference was 51% for rm = 0.125 and it was reduced to 28% for rm = 1. Considering the huge price difference between LiCl and CaCl2, it is highly recommended to apply CaCl2 to the conventional desert coolers, which will make the price competitive for the target group of users.

Author Contributions: Conceptualization, M.J.; methodology, M.J. and A.A.; software, M.J.; formal analysis, M.A.-A. and A.A.; investigation, M.A.-A.; resources, A.A.; data curation, M.A.-A.; writing—original draft, M.J.; writing—review and editing, A.A.; supervision, A.A. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Atmosphere 2020, 11, 40 13 of 15

Acknowledgments: The authors are grateful for the support of the Deanship of Graduate Studies and Research at the German Jordanian University. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

Symbol Meaning Unit A Area m2 α heat transfer coefficient W/m2.K β mass transfer coefficient m/s ε effectiveness h f g enthalpy of vaporization kJ/kg . m mass flow rate kg/h p pressure Pa rm air to solution mass flow ratio

X mass of water per mass of salt kgH2O/kgsalt V volume m3 ∆ diﬀerence λ thermal conductivity W/m.K

ω humidity ratio gw/kgda ϕ relative humidity ρ density kg/m3 ϑ temperature ◦C ξ mass fraction of the desiccant kgsalt/kgsol Subscripts a air CaCl2 calcium chloride i inlet conditions da dry air deh dehumidiﬁcation LiCl lithium chloride as salt NTU number of transfer units o outlet conditions s saturated sol solution v water vapor w water

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