energies

Article A Desiccant Enhanced Two Stage Evaporative Cooling System—Development and Performance Evaluation of a Test Rig

M. Mujahid Rafique 1,2 ID , Shafiqur Rehman 3,* ID , Luai M. Alhems 3 and Muhammad Ali Shakir 4 1 Department of , King Fahd University of Petroleum and , Dhahran 31261, Saudi Arabia; mujahidrafi[email protected] 2 Independent Scholar, Toba Tek Singh 36050, Pakistan 3 Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia; [email protected] 4 Department of Mechanical Engineering, University College of Engineering and Technology, University of Sargodha, Sargodha 40100, Pakistan; [email protected] * Correspondence: [email protected]

Received: 14 October 2017; Accepted: 9 November 2017; Published: 29 December 2017

Abstract: Desiccant technology is found to be a good alternative to conventional cooling systems. It can provide better under hot and humid climatic conditions. The major component of a liquid desiccant cooling system is the desiccant dehumidifier which controls the latent cooling load. In this paper, a newly developed liquid desiccant enhanced evaporative cooling system has been tested experimentally. The effects of ambient conditions and other parameters on the performance of the system are investigated. The system performance curves which help to determine the air outlet conditions and coefficient of performance (COP) of the system are drawn for a wide range of ambient air ratios (0.010–0.026 kg/kg), ambient air temperature (25–40 ◦C), process air flow rate (1.5–8.0 kg/m2·s), regeneration air flow rate (1.5–4.5 kg/m2·s), and regeneration temperature (55–85 ◦C). The results showed that better supply air conditions are achieved for hot and humid climatic conditions with effectiveness of the system largely dependent on process and regeneration air flow rates, regeneration temperature, and humidity ratio of process air. The dehumidification performance is increased by 62% for a change of ambient air humidity ratio from 0.01 to 0.025 kg/kg. The thermal coefficient of performance improved by 50% for the above variation in humidity ratio. This shows that such thermally activated systems are feasible options for hot and humid climatic conditions as indicated by better performance under these conditions.

Keywords: liquid desiccant; ; ; rotary dehumidifier; sustainable development

1. Introduction Statistics prove that the performance of an individual human being is more effective in a conditioned space compared to that in an untreated environment, as a result of which the thermal comfort requirement becomes essential because people spend most of the time in confined environments. Human comfort conditions are achieved when temperature and moisture are held within a certain narrow range. The acceptable ranges of temperature and relative humidity for human comfort as given by the ASHRAE standards 55 are 20–26 ◦C and 30–60%, respectively. This increases the demand of air conditioning, both in the residential and commercial sectors. The use of conventional cooling system consumes large amount of energy to fulfill thermal comfort conditions. Furthermore, this technology is not energy efficient for climatic conditions where latent loads are dominant [1].

Energies 2018, 11, 72; doi:10.3390/en11010072 www.mdpi.com/journal/energies Energies 2018, 11, 72 2 of 17

Energies 2018, 11, 72 2 of 17 Furthermore, this technology is not energy efficient for climatic conditions where latent loads are dominant [1]. SomeSome alternative alternative technology technology is is required required in in order order to to overcome overcome the the drawbacks drawbacks and and rising rising demands demands ofof conventional conventional cooling cooling systems systems fo forr residential residential as as well as commerci commercialal buildings [2,3]. [2,3]. The The regional demanddemand of of heating, heating, ventilation, ventilation, and and air air conditioning conditioning (HVAC) (HVAC) equipment equipment is is pres presentedented in in Figure Figure 1 [[4].4]. ItIt can can be be observed observed that that the the HVAC HVAC equipment equipment deman demandd is is growing growing at at a a rapid rapid rate rate for for all all regions. regions. Secondly,Secondly, the consumption of of primary energy energy resour resourcesces has has increased increased significantly significantly in in recent recent years years becausebecause of of growing demand. An An effective effective alternat alternativeive is is desiccant desiccant cooling cooling technology. technology. Due Due to to the the affinityaffinity to absorb/adsorb absorb/adsorb moisture, moisture, desiccants desiccants like like silica , gel, SiO SiO22, ,and and zeolite zeolite attract attract moisture moisture without without anyany change change in in their their chemical and physical composit composition.ion. The The desiccant desiccant material material is is regenerated regenerated using using hothot air air so so that that the the cycle cycle can can be be repeated. repeated. After After the the dehumidification dehumidification of of the the proc processess air, air, the the evaporative evaporative coolercooler is is used used to lower the temperature according to the desired comfort conditions [[5].5]. Desiccant coolingcooling systemsystem is is an an effective effective and cost-efficientand cost-efficient way to way provide to comfortprovide in comfort hot and humidin hot environments.and humid environments.This technology This has technology many potential has many advantages potential over advantages other cooling over techniques other cooling such techniques as, better supply such as,conditions, better supply energy conditions, efficient, effective energy efficient, utilization effective of low gradeutilization energy of sources,low grade etc. energy [6]. sources, etc. [6].

FigureFigure 1. 1. HVACHVAC equipment equipment demand demand and and annual annual growth growth [4]. [4].

The desiccant cooling technology can be categorized based on the type of desiccant material The desiccant cooling technology can be categorized based on the type of desiccant material used. used. Solid- and liquid-based desiccant cooling are two major types of this thermally driven Solid- and liquid-based desiccant cooling are two major types of this thermally driven technology. technology. The solid desiccant cooling technology is more mature as compared to liquid desiccant. The solid desiccant cooling technology is more mature as compared to liquid desiccant. Liquid Liquid desiccants have advantage over the solid desiccants in that these require lower temperature desiccants have advantage over the solid desiccants in that these require lower temperature heat heat sources (60–85 °C) for regeneration. This makes the usage of renewable energy resources like sources (60–85 ◦C) for regeneration. This makes the usage of renewable energy resources like solar, solar, biomass, etc. more feasible and effective [7]. However, most of the studies carried out on liquid biomass, etc. more feasible and effective [7]. However, most of the studies carried out on liquid desiccant dehumidification systems are direct contact type in which there is a direct contact between desiccant dehumidification systems are direct contact type in which there is a direct contact between liquid desiccant and the air streams. Direct contact type systems have the drawback that the supply liquid desiccant and the air streams. Direct contact type systems have the drawback that the supply air can carry some liquid desiccant droplets with it and can cause corrosion in the ducting and other air can carry some liquid desiccant droplets with it and can cause corrosion in the ducting and equipment. This can increase the maintenance costs and decrease the equipment life. Secondly, other equipment. This can increase the maintenance costs and decrease the equipment life. Secondly, desiccant carryover can affect adversely the quality of air and human health. Direct contact type desiccant carryover can affect adversely the quality of air and human health. Direct contact type systems also require high and pumping power because of the higher pressure drop and systems also require high fan and pumping power because of the higher pressure drop and continuous continuous supply of desiccant solution. supply of desiccant solution. Different configurations are employed to overcome the problem of desiccant carryover and Different configurations are employed to overcome the problem of desiccant carryover and issues issues related to packed bed liquid desiccant [8]. The use of internally cooled/heated related to packed bed liquid desiccant dehumidifiers [8]. The use of internally cooled/heated [9,10] and [9,10] and membrane type [11] dehumidifiers are few of the solutions to the problem of desiccant membrane type [11] dehumidifiers are few of the solutions to the problem of desiccant carryover [9,10]. carryover [9,10]. The rotary type liquid desiccant cooling system can also be designed to overcome The rotary type liquid desiccant cooling system can also be designed to overcome drawbacks of packed drawbacks of packed bed liquid desiccant cooling systems. These wheels have large surface area and bed liquid desiccant cooling systems. These wheels have large surface area and less pressure drops as less pressure drops as compared to packed bed dehumidifiers. Some general advantages of using compared to packed bed dehumidifiers. Some general advantages of using rotary desiccant wheel as a rotary desiccant wheel as a dehumidification unit are: dehumidification unit are:

• These systems are more convenient to install.

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• The process of dehumidification and regeneration are synchronized in these systems and operation is continuous. • For same contact surface area, these systems occupy a smaller space compared to packed bed dehumidifiers. • Dehumidification/regeneration capacity of the dehumidifier can be efficiently controlled by changing the rotational speed of the desiccant wheel.

In the past, most of the investigations were carried out for the solid desiccant rotary dehumidifiers and no in-depth work is available for energy analysis of rotary liquid desiccant dehumidifiers. The purpose of this research is to develop rotary type dehumidifier using a liquid desiccant material instead of solid desiccant to lower the required input heat for the operation of the system. In this paper, an experimental unit is designed and built to study the performance of the rotary liquid desiccant dehumidifier operating in conjunction with two stage evaporative cooler considering different parameters. The combination of direct and indirect evaporative cooler is used as a cooling unit. The experimental results show that better supply air conditions could be obtained to achieve human comfort in the hot and humid with effectiveness of the system largely dependent on air flow rate, regeneration temperature and humidity ratio of the process air. The developed rotary liquid desiccant cooling system is expected to overcome the disadvantages of liquid desiccants.

2. Two Stage Evaporative Cooling In a desiccant-based cooling system, the latent load is controlled by the desiccant dehumidifier whereas for sensible load evaporative coolers can be utilized. Different configurations of evaporative cooler can be used in conjunction with desiccant dehumidifier depending upon the climatic conditions. The two basic evaporative cooler configurations are the direct and indirect type. Two stage evaporative coolers are an advanced technique mostly used for hot and humid climatic conditions. In hot and humid conditions, it is difficult to cool the air using only an indirect evaporative cooler and the use of larger size direct evaporative coolers will not be an economical option. Two stage indirect-direct evaporative coolers can cool the air to lower temperatures as compared to one stage evaporative coolers [12]. This cooling option is much more energy efficient than cooling with . According to ASHRAE, 60–75% savings on electricity can be achieved by replacing conventional compression cooling systems with advanced two stage evaporative coolers. Many researchers have investigated the use of the two stage evaporative cooler configuration as a standalone unit and in conjunction with desiccant dehumidifiers. Farahani et al. [13] investigated a two stage evaporative cooling system for the climatic conditions of Tehran (Iran). The results demonstrated that the use of two stage evaporative cooling can fulfill the human comfort demands efficiently. El-Dessouky [14] developed and tested a modified two stage evaporative cooler as an alternative to single stage evaporative cooling. The results showed that efficiency of two stage evaporative cooling system is better than a single stage system. Al-Sulaiman et al. [15] also utilized two stage evaporative cooling system with liquid desiccant dehumidifier. Tashtoush et al. [16] obtained 20% COP with two stage evaporative cooler than that achieved when employing either direct or indirect evaporative cooler systems alone. Different researchers have utilized multistage evaporative cooling systems to achieve desired supply conditions for thermal comfort [17,18]. Rafique et al. [1] mentioned that the effectiveness of direct, indirect and indirect-direct evaporative coolers is 80–90%, 85%, and 110%, respectively. Furthermore, the energy saving potential of two stage evaporative coolers is better compared to single stage evaporative coolers.

3. Materials and Methods The developed desiccant wheel is a rotor with radius R and width L. A number of narrow slots covered with porous media carrying liquid desiccant solution are distributed uniformly over the cross-section of the rotor as shown in Figure2. The wheel is divided into two sections for process and Energies 2018, 11, 72 4 of 17 cross-sectionEnergies of the2018 rotor, 11, 72 as shown in Figure 2. The wheel is divided into two sections4 for of 17 process and regeneration air with both flowing in a counter-current arrangement. The process air is dehumidified after passingregeneration through air the with desiccant both flowing wheel. in a counter-current On the rege arrangement.neration side, The process air is air heated is dehumidified up to the required after passing through the desiccant wheel. On the regeneration side, air is heated up to the required regenerationregeneration temperature temperature and andthen then is ispassed passed throughthrough the desiccantthe desiccant wheel to wheel desorb moistureto desorb from moisture the from the desiccantdesiccant material. material.

FigureFigure 2. Schematic 2. Schematic of ofrotary rotary liquidliquid desiccant desiccant dehumidifier. .

Figure 3aFigure illustrates3a illustrates the the schematic of theof designed the designed and developed and desiccant developed enhanced evaporativedesiccant enhanced cooling system. The system comprises of two major parts: the rotary liquid desiccant dehumidifier and evaporativethe cooling cooling unit, system. including The direct system and indirect comprises evaporative of coolers.two major The dehumidifier parts: the is arotary heat and liquid mass desiccant dehumidifierexchanger and inthe which cooling moisture isunit, absorbed including from process direct air by aand desiccant indirect material. evaporative The dehumidified coolers. The dehumidifierair isis passed a heat through and themass indirect exchanger evaporative in cooler which and themoisture temperature is absorbed of the air is loweredfrom process using air by a desiccant material.cooling water. The Atdehumidified this stage, there air is nois directpassed contact through between the air indirect and cooling evaporative water. The air cooler is and the passed through the tubes with cooling water sprayed over them. For further cooling, air is passed temperaturethrough of the a air direct is evaporativelowered using cooler cooling in which therewater. is a At direct this contact stage, between there air is andno coolingdirect water.contact between air and coolingThe process water. air The is then air suppliedis passed to through the conditioned the tubes room. with In order cooling to remove water the moisturesprayed from over them. For further cooling,the desiccant air is dehumidifierpassed through for continuous a direct operation evaporative of the cycle, cooler the in ambient which air there is heated is ina andirect contact electrical . This hot air from the heater is then passed through the rotary dehumidifier. between airThe and rotary cooling dehumidifier water. rotates The at process very low speedair is for then proper supplied absorption to and the desorption conditioned of the moisture. room. In order to remove theThe moisture generalized from psychrometric the desiccant representation dehumidifi of completeer cycle for iscontinuous illustrated in Figure operation3b. The process of the cycle, the ambient airair is is heated dehumidified in an and electrical its temperature heating is increased system. from This state 1hot to 2.air From from state the point heater 2 to 3, onlyis then passed temperature decreases whereas from state 3 to 4, the temperature further decreases while the humidity through the rotary dehumidifier. The rotary dehumidifier rotates at very low speed for proper increases. On the regeneration side, air is heated sensibly from state 1 to 5 and removes the absorbed absorption moistureand desorption from the desiccant of the wheel moisture. from state 5 toTh 6.e In generalized this test setup, electricpsychrometric heater is employed representation to of complete cycleobserve is theillustrated performance in of Figure the system 3b. but The for a realprocess system, air a renewable is dehumidified energy such asand solar its can temperature be is ◦ increased fromemployed state to 1 achieve to 2. From required state temperature point of2 to regeneration 3, only temperature (60–85 C). decreases whereas from state 3 to 4, the temperature further decreases while the humidity increases. On the regeneration side, air is heated sensibly from state 1 to 5 and removes the absorbed moisture from the desiccant wheel from state 5 to 6. In this test setup, electric heater is employed to observe the performance of the system but for a real system, a renewable energy such as solar can be employed to achieve required temperature of regeneration (60–85 °C).

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1

(a)

4 5

3

6 2

1

FigureFigure 3. 3.( a()a) Schematic Schematic of of developed developed desiccant desiccant enhanced enhanced cooling cooling cycle; cycle; ( b(b)) Representation Representation of of liquid liquid desiccantdesiccant cooling cooling cycle cycle on on psychrometric psychrometric chart. chart.

4.4. Test Test Chamber Chamber AA detailed detailed schematic schematic of of the the experimental experimental setup setup is is shown shown in in Figure Figure4. 4. The The test test facility facility mainly mainly consistsconsists of of the the following following components: components: •• AA rotary rotary liquid liquid desiccant desiccant dehumidifier dehumidifier (30 (30 cm cm diameter). diameter). •• AA direct direct type type evaporative evaporative cooler cooler (30 (30× ×60 60 cmcm22).). •• AnAn indirect indirect type type evaporative evaporative cooler cooler (30 (30× ×60 60 cmcm22).). •• AA water water spraying spraying system system (0.37 (0.37 kW kW ). pump). •• AA variable variable capacity capacity electric electric heater heater (0–4 (0–4 kW). kW). • • AA variable variable speed speed electric electric motor motor (0–4 (0–4 rpm). rpm). • • TwoTwo air air blowers. blowers. TheThe details details of allof otherall other supporting supporting and operation and operat accessoriesion accessories are shown are in shown Figure5 .in The Figure cylindrical 5. The shapecylindrical rotating shape desiccant rotating wheel desiccant of 30 wheel cm diameter of 30 cm is fabricateddiameter is from fabricated flexi glass. from Flowflexi glass. area is Flow created area usingis created pipes using of chlorinated pipes of chlorinated polyvinyl chloridepolyvinyl of chloride diameter of 21 diameter mm with 21 minimummm with minimum spacing between spacing twobetween pipes. two These pipes. pipes These are uniformlypipes are uniformly distributed distributed over the cross-section over the cross-section of the wheel. of the To wheel. form the To absorbingform the absorbing surface, a wicksurface, cloth a wick layer cloth impregnated layer impregnated with calcium with chloride calcium solution chloride is placedsolution inside is placed the inside the pipes using springs. The variable speed motor is used to vary the rotational speed of the

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Energies 2018, 11, 72 6 of 17 Energies 2018, 11, 72 6 of 17 pipes using springs. The variable speed motor is used to vary the rotational speed of the desiccant wheel.desiccant The wheel. regeneration The regeneration heat is provided heat is provided using a variable using a capacityvariable electriccapacity heater. electric In heater. the process In the airprocess stream, air afterstream, the after desiccant the desiccant dehumidifier dehumidifier direct and direct indirect and type indirect evaporative type evaporative coolers are coolers used forare coolingused for purposes. cooling purposes. The packing The material packing for material the direct fo evaporativer the direct coolerevaporative is composed cooler ofis cellulosecomposed pad. of Thecellulose size of pad. the The pad size is 15 of cm the deep pad 30is 15 cm cm wide deep and 30 60cm cm wide high. and The 60 cm water high. is distributedThe water is using distributed spray nozzlesusing spray at the nozzles top of at the the pad. top In of indirectthe pad. evaporative In indirect evaporative cooler, the coolingcooler, the is circulated water is insidecirculated the coilsinside and the the coils air and cools the as air it passes cools as across it passes the tubes. across The the indirect tubes. The evaporative indirect evaporative cooler coils are cooler constructed coils are fromconstructed from tubes copper with bonded tubes with aluminum bonded fins. aluminum Fins are fins. attached Fins are to theattached outside to ofthe tubes outside to enhance of tubes theto enhance surface areathe surface for heat area transfer. for .

FigureFigure 4.4. SchematicSchematic ofof thethe experimentalexperimental setup.setup.

Figure 5. Photographic view of the experimental setup. Figure 5. Photographic view of the experimental setup. A one-fourth section of the wheel is dedicated for regeneration air stream and 3/4 for the process air stream.A one-fourth The process section air of is the dehumidified wheel is dedicated after passing for regeneration through the air dehumidification stream and 3/4 for section the process of the airwheel stream. where The the process heated air air is dehumidified(55 to 85 °C) removes after passing the moisture through theabsorbed dehumidification by the wheel. section For ofbetter the performance of the desiccant cooling system and reliable results, efforts are made to minimize any leakage. The desiccant dehumidifier is located in the middle of two aluminum frames with gaskets on both sides of the desiccant wheel. The dehumidifier rotates with minimum friction because of the

Energies 2018, 11, 72 7 of 17 wheel where the heated air (55 to 85 ◦C) removes the moisture absorbed by the wheel. For better performance of the desiccant cooling system and reliable results, efforts are made to minimize any leakage. The desiccant dehumidifier is located in the middle of two aluminum frames with gaskets on both sides of the desiccant wheel. The dehumidifier rotates with minimum friction because of the smooth surface provided by the glass fiber gaskets. High temperature insulation is provided between the hot and cold side of air to minimize the transfer of heat through the walls.

5. Measuring Instruments Temperatures of the process and regeneration air streams at various state points are measured using k-type thermocouples, as shown in Figures4 and5. Digital hygrometer and anemometer are used to measure the humidity and the velocity of the air, respectively at different points. The details of all the sensors are summarized in Table1. All the sensors are calibrated under testing conditions.

Table 1. Specifications of sensors.

Parameters Devices Model No. Accuracy Temperature K-type thermocouple KK-K-30 ±0.2 ◦C @23 ± 5 ◦C For ≥ 70% RH ±(3% + 1% RH) For <70% Relative humidity Hygrometer RHXL3SD RH ± 3% RH Flow rate Anemometer HHF1001A 1.5% Full scale accuracy Heater capacity Wattmeter GH-019D ±(0.2% reading +0.05% FS) Density Hydrometer - ±1 kg/m3

6. Test Procedure and Conditions Table2 summarizes the ranges of the different operating parameters. The initial temperature and humidity values were set by turning on the test chamber. After that, both process and regeneration air fans and variable speed motor for desiccant dehumidifier were turned on. After certain time, the regeneration temperature was achieved using electric heater. Once the steady state conditions were achieved, multiple measurements were taken with the instruments listed in Table1. The data acquisition (DAQ) was established with LabVIEW 2013, which has the ability to simultaneously control and record the temperature, humidity, and mass flow rates at each state point.

Table 2. Operating parameters.

Parameter Value Range Base Value Ambient air temperature (◦C) 25–40 35 Ambient air humidity ratio (kgv/kga) 0.010–0.025 0.020 Process air mass flux (kg/s·m2) 1.5–8.0 3.0 Regeneration air flow rate (kg/s·m2) 1.5–4.5 1.0 Regeneration temperature (◦C) 55–85 70

7. Uncertainty Analysis No physical quantity can be measured with perfect certainty. There are always errors in any measurement and inaccuracies can and do happen. Measurements should be made with great care to reduce the possibility of error as much as possible. The general error propagation (u) is given by Equation (1). Here u is a function of x, y, and z. ∆u is the uncertainty of u, similar to ∆x, ∆y, and ∆z: s  ∂u 2  ∂u 2  ∂u 2 ∆u = × (∆x)2 + × (∆y)2 + × (∆z)2 (1) ∂x ∂y ∂z

The combined uncertainty (U) is determined by the calibration of all sensors and is calculated using Equation (2): Energies 2018, 11, 72 8 of 17

q 2 2 2 U = (Uinstrument) + (Ucaliberation) + (Urandom) (2) where, Urandom, is calculate with Equation (3):

Sµ,95%ϑ Urandom = √ (3) ND

µ = N − 1 (4)

For all the thermocouples used in this experimental setup, an uncertainty of 0.10–0.25 K was observed. The calculated uncertainties for different instrumentations are listed in Table3.

Table 3. Uncertainties of measuring instruments used in experiment.

Instrument Parameter Uncertainty K-type thermocouple Temperature ±0.2 ◦C Anemometer Velocity of air ±0.1 m/s Desiccant Density meter (hydrometer) ±1 kg/m3 density Electronic weighing machine Desiccant mass ±0.001 kg Rotameter Water flow rate ±0.5 L/min

8. Performance Indicators A number of parameters are used in order to evaluate the performance of desiccant based cooling system. This section describes the performance indices used in the present system. These parameters are used for performance of a liquid desiccant enhanced cooling system to have lower regeneration temperature, no carryover of desiccant solution, and better supply air conditions. The relationships for dehumidification coefficient of performance (DCOP), cooling capacity, coefficient of performance (COP), electric coefficient of performance (ECOP), thermal coefficient of performance (TCOP) and sensible energy ratio (SER) are presented in Equations (5)–(10), respectively [19–21]:

. mp × h f g × (ω1 − ω2) DCOP = . (5) mr × (h7 − h6) . CC = mp(h1 − h4) (6) CC COP = (7) (Ecool+Epumping) Ethermal + β where, the value of equivalent conversion coefficient (β) is taken as 0.3 for the present study [12]:

CC ECOP = (8) Ecool + Epumping

CC TCOP = (9) Ethermal The additional cooling required along with the dehumidification system in order to achieve comfort conditions in a conditioned room is defined by sensible energy ratio (SER) [21]:

. mp × (h2 − h1) SER = . (10) mr × (h7 − h6) Energies 2018, 11, 72 9 of 17

For better performance of the system, the achieved values of DCOP, CC, COP, ECOP, and TCOP should be higher whereas value of SER should be lower. The lower value of SER means less cooling is required after the desiccant dehumidifier.

9. Results and Discussion The performance of the desiccant cooling system is tested to have lower regeneration temperature and better supply air conditions. Experiments were carried out using the ranges of operating parameters listed in Table3. Generally, high values of the determination coefficient or R-squared (R2) were obtained in all the cases with a linear fitted regression line. The high value of R2 indicates that data is uniformly distributed along the regssion line. The effect of the process air inlet humidity on system’s performance was investigated and the results are shown in Figure6. The electrical, thermal, and overall performance increased with the increase in inlet air humidity ratio. The mass transfer potential enhanced with increased humidity ratio of ambient air which in turn improved the capacity and performance of the system. In fact, the driving force for mass transfer is increased with the rise of humidity ratio of inlet air. Although, with the increase in ambient air humidity the required input heat for desorption of moisture also increased. However, this increase is not as high as capacity of the system to remove latent load. Thus, the higher the ambient air humidity, the better the performance (COP, TCOP, and ECOP, DCOP) of the system will be. The COP, TCOP, ECOP, and DCOP represent the overall, thermal, electrical and dehumidification coefficient of performance, respectively. Similar to the moisture removal rate, a higher DCOP was achieved at higher inlet humidity ratio due to an increase in the moisture absorption capacity of desiccant dehumidifier as shown in Figure6. For instance, the DCOP increased by 62% when the humidity ratio is changed from 0.01 to 0.025 kg/kg. With regard to sensible energy ratio, the temperature at the exit of dehumidifier strongly increased with humidity ratio of inlet air because of larger quantity of absorbed by the desiccant. Therefore, temperature at state 2 increased due to the increase in the released heat of absorption. Thus, increase in humidity ratio of the inlet air increases the sensible energy ratio of the system at fixed regeneration value and supply air temperature. Figure7 illustrates the dependence of different performance parameters on air temperature. As regards to DCOP, an increase in the process air inlet temperature causes a slight reduction of moisture removal capacity but an increase in of inlet air increases DCOP. In case of SER, the effect of inlet air temperature is insignificant due to change in temperature of inlet air because of small variations in absorption heat and air temperature at the exit of the dehumidifier. An increase in the mass flux of the air to an improvement in the system performance coefficients COP, ECOP, and TCOP, (Figure8). This is due to the fact that the cooling capacity of the system increases while the input energy is kept constant. It is to be noted that the mass flux of regeneration air is kept constant and so the required thermal load remains the same. Due to respective high and low mass flow rates, the absorbed moisture may not be efficiently removed from the dehumidifier. Similarly, at low flow rate of process air and high flow rate of regeneration air, the required heat input will increase and may cause a decrease in system performance. The moisture absorbed by the desiccant dehumidifier may increase with the process air mass flux due to enhanced driving force for moisture transfer. However, the total moisture removal capacity decreases due to less residence time at higher flow rates. Thus, DCOP decreases with mass flux of process air. Furthermore, increasing the inlet flow rate of process air leads to an increase in both the supply air humidity ratio and temperature inside the dehumidifier. Thus, keeping the regeneration temperature constant, sensible energy ratio increases with inlet process air mass flux as shown in Figure8. Energies 2018, 11, 72 9 of 17 driving force for mass transfer is increased with the rise of humidity ratio of inlet air. Although, with the increase in ambient air humidity the required input heat for desorption of moisture also increased. However, this increase is not as high as capacity of the system to remove latent load. Thus, the higher the ambient air humidity, the better the performance (COP, TCOP, and ECOP, DCOP) of the system will be. Energies 2018, 11, 72 10 of 17

(a) COP [-] Linear (COP [-])

0.8 0.6 0.4

COP [-] 0.2 0 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028 Ambient humidity ratio [kg/kg]

ECOP [-] Linear (ECOP [-]) (b) 6 4 2 ECOP [-] 0 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028 Ambient air humidity ratio [kg/kg]

(c) TCOP [-] Linear (TCOP [-]) 2 1.5 1 0.5 TCOP [-] 0 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028 Ambient air humidity ratio [kg/kg]

DCOP [-] Linear (DCOP [-]) (d) 1

0.5 DCOP [-] 0 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028 Ambient air humidity ratio [kg/kg]

Sensible energy ratio [-] Linear (Sensible energy ratio [-])

0.6 (e) 0.4 0.2 0 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028

Sensible energy ratio [-] Sensible Ambient air humidity ratio [kg/kg]

Figure 6. Variations of (a) COP; (b) ECOP; (c) TCOP; (d) DCOP; (e) SER with ambient air humidity ratio. Figure 6. Variations of (a) COP; (b) ECOP; (c) TCOP; (d) DCOP; (e) SER with ambient air humidity ratio.

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COP [-] Linear (COP [-]) (a) 0.8 0.6 0.4

COP [-] 0.2 0 20 25 30 35 40 45 Ambient air temperature [oC]

ECOP [-] Linear (ECOP [-]) (b) 6 4 2 ECOP [-] 0 20 25 30 35 40 45 Ambient air temperature [oC]

(c) TCOP [-] Linear (TCOP [-]) 2 1.5 1 0.5 TCOP [-] 0 20 25 30 35 40 45 Ambient air temperature [oC]

DCOP [-] Linear (DCOP [-]) (d) 0.6 0.4 0.2 DCOP [-] 0 20 25 30 35 40 45 Ambient air temperature [oC]

Sensible energy ratio [-] Linear (Sensible energy ratio [-])

0.8 (e) 0.6 0.4 0.2 0 20 25 30 35 40 45

Sensible energy ratio [-] Sensible Ambient air temperature [oC]

Figure 7. Variations of (a) COP; (b) ECOP; (c) TCOP; (d) DCOP; (e) SER with ambient temperature. Figure 7. Variations of (a) COP; (b) ECOP; (c) TCOP; (d) DCOP; (e) SER with ambient temperature.

EnergiesEnergies2018 2018, 11, ,11 72, 72 12 of12 17of 17

COP [-] Linear (COP [-]) (a) 0.6 0.4 0.2 COP [-] 0 0.5 1.5 2.5 3.5 4.5 5.5 6.5 Process air mass flux [kg/s.m2]

ECOP [-] Linear (ECOP [-])

6 (b) 4 2 ECOP [-] 0 0.5 1.5 2.5 3.5 4.5 5.5 6.5 Process air mass flux [kg/s.m2]

(c) TCOP [-] Linear (TCOP [-]) 1.5 1 0.5 TCOP [-] 0 0.5 1.5 2.5 3.5 4.5 5.5 6.5 Process air mass flux [kg/s.m2]

DCOP [-] Linear (DCOP [-]) (d) 1.5 1 0.5 DCOP [-] 0 0123456789 Process air mass flux [kg/s.m2]

Sensible energy ratio [-] Linear (Sensible energy ratio [-])

0.6 (e) 0.4 0.2 0 0.5 1.5 2.5 3.5 4.5 5.5 6.5 Sensible energy ratio [-] Sensible Process air mass flux [kg/s.m2]

Figure 8. Variations of (a) COP; (b) ECOP; (c) TCOP; (d) DCOP; (e) SER with process air mass flux. Figure 8. Variations of (a) COP; (b) ECOP; (c) TCOP; (d) DCOP; (e) SER with process air mass flux.

The effect of regeneration air flow rate on performance of the system is presented in Figure9. The COP and TCOP decreased by 25 and 60%, respectively with the regeneration air flow rate changed from 1.5 to 4.5 kg/m2·s. An increase in regeneration flow rate decreases DCOP, as shown in Figure9.

Energies 2018, 11, 72 13 of 17

This is due to a proportional rise in the energy requirement for regeneration. With regards to SER, an increase in regeneration air flow rate resulted in the rise of absorbed moisture and heat of absorption. The regeneration temperature inversely affects the overall COP, TCOP, and DCOP of the system as it can be observed from Figure 10. The variation of regeneration temperature from 55 to 85 ◦C, the values of COP, TCOP, and DCOP of the system decreased from 0.91 to 0.58, 3.4 to 1, and 0.6 to 0.4, respectively. The sensible energy ratio is decreased with the increase in regeneration temperature (Figure 10). This decrease of SER is due to the increase in the difference between the regeneration and the process air inlet temperatures. As described above, the increase in ratio of flow rates causes a decrease in DCOP for desiccant dehumidifier due to an increase in regeneration heat. Also, the increase in this ratio will cause an increase in temperature at the exit of the dehumidifier due to increase in released Energiesheat of 2018 absorption., 11, 72 This increase in exit temperature in turn decreases the SER. Thus, with the increase13 of 17 in ratio of air flow rates and regeneration temperature, a reduction in SER occurs.

COP [-] Linear (COP [-]) (a) 1

0.5 COP [-] 0 11.522.533.544.5 Regeneration air mass flux [kg/s.m2]

(b) TCOP [-] Linear (TCOP [-]) 4

2

TCOP [-] 0 11.522.533.544.5 Regeneration air mass flux [kg/s.m2]

(c) DCOP [-] Linear (DCOP [-]) 1

0.5

DCOP [-] 0 11.522.533.544.5 Regeneration air mass flux [kg/s.m2]

Sensible energy ratio [-] Linear (Sensible energy ratio [-]) (d) 0.3 0.2 0.1 0 11.522.533.544.5 2

Sensible energy ratio [-] Sensible Regeneration air mass flux [kg/s.m ]

Figure 9. Variations of (a) COP; (b) TCOP; (c) DCOP; (d) SER with regeneration air mass flux. Figure 9. Variations of (a) COP; (b) TCOP; (c) DCOP; (d) SER with regeneration air mass flux.

COP [-] Linear (COP [-]) (a) 1 0.8 0.6 0.4 COP [-] 0.2 0 45 50 55 60 65 70 75 80 85 90 Regeneration temperature [oC]

Energies 2018, 11, 72 13 of 17

COP [-] Linear (COP [-]) (a) 1

0.5 COP [-] 0 11.522.533.544.5 Regeneration air mass flux [kg/s.m2]

(b) TCOP [-] Linear (TCOP [-]) 4

2

TCOP [-] 0 11.522.533.544.5 Regeneration air mass flux [kg/s.m2]

(c) DCOP [-] Linear (DCOP [-]) 1

0.5

DCOP [-] 0 11.522.533.544.5 Regeneration air mass flux [kg/s.m2]

Sensible energy ratio [-] Linear (Sensible energy ratio [-]) (d) 0.3 0.2 0.1 0 11.522.533.544.5 2

Sensible energy ratio [-] Sensible Regeneration air mass flux [kg/s.m ]

Energies 2018Figure, 11 ,9. 72 Variations of (a) COP; (b) TCOP; (c) DCOP; (d) SER with regeneration air mass flux. 14 of 17

COP [-] Linear (COP [-]) (a) 1 0.8 0.6 0.4 COP [-] 0.2 Energies 2018, 11, 72 0 14 of 17 45 50 55 60 65 70 75 80 85 90 Regeneration temperature [oC]

TCOP [-] Linear (TCOP [-]) (b) 3

2

1 TCOP [-]

0 45 50 55 60 65 70 75 80 85 90 Regeneration temperature [oC]

DCOP [-] Linear (DCOP [-]) (c) 0.8 0.6 0.4

DCOP [-] 0.2 0 45 50 55 60 65 70 75 80 85 90 Regeneration temperature [oC]

Sensible energy ratio [-] Linear (Sensible energy ratio [-])

1 (d) 0.8 0.6 0.4 0.2

Sensible energy ratio [-] Sensible 0 45 50 55 60 65 70 75 80 85 90 Regeneration temperature [oC]

Figure 10. Variations of (a) COP; (b) TCOP; (c) DCOP; (d) SER with regeneration temperature. Figure 10. Variations of (a) COP; (b) TCOP; (c) DCOP; (d) SER with regeneration temperature.

TheThe developed developed system system showed showed better better performance performance compared compared to to the the results results reported reported in in the the literatureliterature using using other other configurations configurations of of this this technology. technology. The The results results obtained obtained at at a a regeneration ◦ temperaturetemperature of of 55 55 °CC for for the the present present system are; COP = 0.8, 0.8, TCOP TCOP = = 2.4 2.4 whereas whereas the the results results obtained obtained byby Abdel-Salam Abdel-Salam et et al. al. [20] [20] for the membrane liquid desiccant dehumidifier dehumidifier under similar operating conditionsconditions were:were: COP COP = 0.63= 0.63 and and TCOP TCOP = 1.48. = Furthermore,1.48. Furthermore, the value the of COPvalue obtained of COP by obtained Bourdoukan by ◦ ◦ Bourdoukanet al. [22] for et the al. effect [22] offor regeneration the effect of temperature regeneration (55 temperatureC), ambient (55 temperature °C), ambient (35 temperatureC), and inlet (35humidity °C), and ratio inlet (0.014 humidity kg/kg) ratio were (0.014 reported kg/kg) as were 0.42, reported 0.29, and as 0.35, 0.42, respectively, 0.29, and 0.35, whereas, respectively, under whereas,similar operating under similar conditions, operating the presentconditions, system the givepresent COP system values give of 0.8,COP 0.5, values and 0.44of 0.8, for 0.5, the and effect 0.44 of forregeneration the effect temperature, of regeneration ambient temperature, temperature andambient inlet humiditytemperature ratio, and respectively. inlet humidity In comparison ratio, respectively. In comparison to a solid desiccant rotary cooling system, the achieved performance of the proposed system is much better. The dehumidification coefficient of performance achieved by Ge et al. [19] for a solid desiccant cooling system at a regeneration temperature of 60 °C was 0.38 whereas in the present case it was 0.59. The performance of liquid desiccant assisted cooling system is expected to be higher compared to solid desiccant cooling system due to lower regeneration heat requirement. The required regeneration temperature in case of liquid desiccant (calcium chloride) is 60–85 °C [7] whereas for solid desiccant (silica gel) it is 60–120 °C [23]. More fieldwork is required in order to compete with other technologies available in the market. A summary of potential advantages which can be achieved by the implementation of proposed system is provided in Figure 11.

Energies 2018, 11, 72 15 of 17

to a solid desiccant rotary cooling system, the achieved performance of the proposed system is much better. The dehumidification coefficient of performance achieved by Ge et al. [19] for a solid desiccant cooling system at a regeneration temperature of 60 ◦C was 0.38 whereas in the present case it was 0.59. The performance of liquid desiccant assisted cooling system is expected to be higher compared to solid desiccant cooling system due to lower regeneration heat requirement. The required regeneration temperature in case of liquid desiccant (calcium chloride) is 60–85 ◦C[7] whereas for solid desiccant (silica gel) it is 60–120 ◦C[23]. More fieldwork is required in order to compete with other technologies available in the market. A summary of potential advantages which can be achieved Energiesby 2018 the, implementation11, 72 of proposed system is provided in Figure 11. 15 of 17

FigureFigure 11.11. PotentialPotential advantages advantages of of liquid desiccantdesiccant based based cooling. cooling.

10. Conclusions10. Conclusions SolarSolar thermal thermal cooling cooling is not is not a new a new concept, concept, nevertheless, nevertheless, it it has has been been gaining gaining relevance relevance to provide to provide efficientefficient cooling cooling at low at low costs costs without without generating generating CO CO2 emissions.emissions. ThroughoutThroughout history, history, energy energy has has been been used usedin different in different forms forms such such asas mechanical, mechanical, electrical, electrical, chemical, chemical, heat, etc. heat, The etc. use ofThe alternative use of sourcesalternative sourcesof energy of energy can result can result in a more in a energy more efficientenergy andefficie costnt effective and cost systems. effective Many systems. efforts haveMany been efforts made have been tomade make to the make effective the effective use of renewable use of renewable energy sources energy due tosources escalating due oil to prices escalating and the oil cost prices of other and the cost ofprimary other energyprimary resources energy inresources recent years. in recent years. In this paper, the performance of a desiccant enhanced evaporative cooling system is investigated In this paper, the performance of a desiccant enhanced evaporative cooling system is experimentally. The effects of different performance parameters on the system performance have been investigated experimentally. The effects of different performance parameters on the system studied. With the increase in regeneration temperature, DCOP decreased due to increase in input performanceenergy at have higher been regeneration studied. With temperatures. the increase It was in concludedregeneration that temperature, the efficient control DCOP of decreased input and due to increaseoutput in parameters input energy can provide at higher effective regenerati dehumidificationon temperatures. capacity It with was the concluded tested system. that Thethe totalefficient controlmoisture of input removal and output capacity parameters decreases at can high provid processe effective air flow ratesdehumidificati due to the reducedon capacity residence with the testedtime. system. The electrical,The total thermal, moisture and removal overall performancecapacity dec increasereases at with high the process increase air in flow inlet airrates humidity due to the reducedratio. residence The values time. of COP, The ECOP,electrical, and TCOPthermal, increased and overall from 0.41 performance to 0.59, 2.30 increase to 3.6, and with 0.74 the to 1.48,increase in inletrespectively, air humidity when ratio. the ambient The values air humidity of COP, ratioECOP, was and changed TCOP from increased 0.01 to 0.025from kg/kg.0.41 to A0.59, higher 2.30 to 3.6, andDCOP 0.74 was to 1.48, achieved respectively, at higher when inlet humiditythe ambien ratiot air due humidity to an increase ratio was in thechanged moisture from absorption 0.01 to 0.025 kg/kg.capacity A higher of the DCOP desiccant was dehumidifier.achieved at higher For an increaseinlet humidity in humidity ratio ratio due fromto an 0.01 increase to 0.025 in kg/kg the moisture the DCOP increased by 62%. The variations of regeneration temperature greatly affect the values of COP absorption capacity of the desiccant dehumidifier. For an increase in humidity ratio from 0.01 to 0.025 and TCOP. The increase in regeneration temperature from 55 to 85 ◦C lowered the COP and TCOP of kg/kg the DCOP increased by 62%. The variations of regeneration temperature greatly affect the the system by 32% and 53%, respectively. The desiccant-based technology is beneficial from both an valueseconomic of COP as and well TCOP. as an environmental The increase pointin regeneration of view. The temperature system could befrom improved 55 to 85 by °C developing lowered the COP compositeand TCOP desiccant of the materialssystem by and 32% further and lowering 53%, re thespectively. required The regeneration desiccant-based temperature. technology In this is beneficialway better from performance both an economic of the system as well can as be an achieved. environmental point of view. The system could be improved by developing composite desiccant materials and further lowering the required regeneration temperature. In this way better performance of the system can be achieved.

Acknowledgments: The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 10-ENE1372-04 as part of the National Science, Technology and Innovation Plan.

Author Contributions: All authors contributed significantly to this work. Especially, M. Mujahid Rafique (Muhammad Mujahid Rafique) and Shafiqur Rehman developed the idea and carried out the experimentation. M. Mujahid Rafique and Shafiqur Rehman proposed the original idea, developed the experimental setup, conducted calculation and wrote the paper. Luai M. Alhems checked and revised the paper and provided technical and financial support. Muhammad Ali Shakir helped in the evaluation of results and managed the data information. The author M. Mujahid Rafique carried out the experimental work while affiliated with King Fahd University of Petroleum & Minerals and analysis/writing as independent scholar.

Conflicts of Interest: The authors declare no conflict of interest.

Energies 2018, 11, 72 16 of 17

Acknowledgments: The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 10-ENE1372-04 as part of the National Science, Technology and Innovation Plan. Author Contributions: All authors contributed significantly to this work. Especially, M. Mujahid Rafique (Muhammad Mujahid Rafique) and Shafiqur Rehman developed the idea and carried out the experimentation. M. Mujahid Rafique and Shafiqur Rehman proposed the original idea, developed the experimental setup, conducted calculation and wrote the paper. Luai M. Alhems checked and revised the paper and provided technical and financial support. Muhammad Ali Shakir helped in the evaluation of results and managed the data information. The lead author M. Mujahid Rafique carried out the experimental work while affiliated with King Fahd University of Petroleum & Minerals and analysis/writing as independent scholar. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

COP coefficient of performance Cp specific heat (kJ/kg·K) CC cooling capacity (kW) E input energy (kW) h specific enthalpy (kJ/kg) hfg of vaporization (kJ/kg) . m air mass flow rate (kg/s) ND number of data points S student test at a 95% confident interval T temperature (K) U uncertainty error Greek letters ω solute humidity (kg/kg) ϑ standard deviation Subscripts 1, 2, 3 . . . state points a air p process r regeneration µ degree of freedom

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