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Applied Thermal Engineering xxx (2014) 1e9

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Experimental investigation on integrated liquid desiccant e Indirectevaporative air cooling system utilizing the Maisotesenko e Cycle

W.Z. Gao a, *, Y.P. Cheng b, A.G. Jiang a, T. Liu a, Keith Anderson c

a Merchant Marine College, Shanghai Maritime University, Shanghai, 201306, Chinab School of Energy, Power and Mechanical Engineering, North China Electric Power University, 102206, Chinac Department of Mechanical Engineering e Engineering Mechanics, Michigan Technological University, 49931, USA

a r t i c l e i n f o

Article history:Received 16 June 2014Received in revised form18 August 2014Accepted 28 August 2014Available online xxx

Keywords:M-CycleLiquid desiccantIndirect evaporative coolerHeat and mass transfer

* Corresponding author.E-mail addresses: wzgao@shmtu.edu.cn, gofree@1

http://dx.doi.org/10.1016/j.applthermaleng.2014.08.061359-4311/© 2014 Elsevier Ltd. All rights reserved.

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a b s t r a c t

Liquid desiccant indirect evaporative cooling is an ideal alternative system for conventional vaporcompression systems to meet new economic, environmental, and regulatory challenges. This alternativesystem consists of two air-handling processes: moisture removal in the dehumidifier and sensible heatremoval in the M-cycle indirect evaporative cooler. The performance of the first stage influences thecooling capacity of the second stage. SHR (sensible load divided by total load), dew point effectiveness,moisture reduction, and temperature reduction were adopted as indices to describe the heat and masstransfer performance of the integrated liquid desiccant and the M-cycle indirect evaporative cooler. Theeffects of air and desiccant inlet parameters, as well as the working air ratio, on the performance of thehybrid were experimentally investigated. The results showed that the variation of dehumidificationcapacity in the first stage directly affected the cooling capacity in the second stage when increasing theinlet parameters of the air or desiccant. The energy balance in both the dehumidifier and the M-cycleindirect evaporative cooler were in the range of ±20% for all the experiment runs. To achieve perfor-mance in the second stage, the supplied water flow rate to the wick surface had to be approximately fivetimes that of the evaporative water.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The air handling process in conventional vapor compression air-conditioning systems is usually divided into two stages. In the firststage, the air is cooled below its dew point in order to makemoisture condensed, which leads to a low evaporating temperatureand a poor coefficient of performance. In the second stage, the coldair, dehumidified to the desired moisture, is reheated to acomfortable temperature, resulting in a higher energy consumptionof the whole system.

The integrated liquid desiccant hybrid system is a promisingalternative to avoid the aforementioned problems. This systemcombines liquid desiccant devices with an indirect evaporative aircooling system through utilizing the former to remove the moistureload and the latter to deal with the sensible load. The significantcharacteristic of the system is to control the moisture load and sen-sible load independently, avoiding unnecessary cooling and heating.

63.com (W.Z. Gao).

6

et al., Experimental investigaApplied Thermal Engineering

In fact, liquid desiccant cooling systems is popularly used. Themain advantage of the system is that no CFC (Chlorofluorocarbons)refrigerants and mechanical compressors are involved in thecooling process. Desiccant cooling technology can handle the wetload independently without reducing the air temperature belowdew point. Unlike conventional air conditioning systems, this sys-tem can be driven by low-grade heat sources, such as industrialwaste heat and solar energy with temperatures between 60 �C and100 �C. Many experimental and theoretical investigations [1,2] havebeen carried out on liquid desiccant systems. Grossman and Factor[3] studied a packed bed regenerator/dehumidifier for solar con-ditioning with liquid desiccant, and developed a theoretical modelto predict the performance of the packed bed dehumidifier andregenerator under various operating conditions. Oberg and Gos-wami [4] and Martin and Goswami [5] included the liquid phasemass transfer resistance in their finite differencemodel to study theperformance of adiabatic absorption and regeneration with TEGrespectively. In recent years, some investigators have been focusingon liquid desiccant-based hybrid systems. For example, Ahmedet al. [6] investigated numerically a hybrid liquid desiccant systemand open cycle vapor absorption using lithium bromide solution for

tion on integrated liquid desiccant e Indirect evaporative air cooling(2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.08.066

Nomenclature

C specific heat capacity (kJ/kg �C)h enthalpy (kJ/kg)t temperature (�C)u humidity ratio (kg/kg)ε Schmidt number

Subscriptsa airdp dew pointin inletmid position between the two partsout outlet

W.Z. Gao et al. / Applied Thermal Engineering xxx (2014) 1e92

the process of absorption, dehumidification, a lithium bromidesolution was regenerated in a solar regenerator, and COP (Coeffi-cient of performance) ranged from 0.96 to 1.25 in the experiments.Dai et al. [7] proposed ECOP (Electric coefficient of performance),TCOP (Thermal coefficient of performance), and System COP topredict the performance of the combined hybrid system that con-sisted of the liquid desiccant and vapor compression refrigerator.

The process of indirect evaporative cooling of air is well known,where air or water approaches the wet bulb temperature of theambient air. However, Maisotesenko [8e10] developed a newconfiguration, called M-cycle, shown in Fig. 1. This M-cycle wasclaimed to enable the harnessing of the ambient air to reach adesirable temperature using a dedicated flat plate, cross-flow, andperforated heat exchanger. Compared with the conventional indi-rect evaporative cooler, the M-cycle uses the same dry side and wetside of a plate but with a much different airflow configurationcreating a new thermodynamic cycle, which allows the product air(point 1) to be cooled below the wet-bulb and towards the dew-point temperature (point 1 / 2) of the incoming air. The detailedheat and mass transfer mechanism can be found in Ref. [11].Zhan et al. [12] conducted a comparison between an M-cycle cross-flow heat exchanger and a conventional indirect evaporative cooler.The result indicated that the effectiveness of the former goes up by16.7% when compared to the effectiveness of the latter. A test [9] onan M-cycle based heat exchanger indicated that the device couldreach a wet-bulb effectiveness of 81e91% and dew-point effec-tiveness of 50e60%. To improve the cooling effectiveness of thedew point cooling system, an M-cycle counter-flow heat exchangerwas recently developed and tested numerically and experimentally.The results indicated that the counter-flow M-cycle exchangerincreased the cooling capacity by around 20% and increased web-bulb and dew-point effectiveness by 15e23% when compared tothe cross-flow M-cycle exchanger of the same geometrical size andunder the same operating conditions [13]. Although the M-cycle

Fig. 1. Principle of the heat and mass exchanger based on M-

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heat exchanger has achieved a dramatic increase in the coolingeffectiveness compared with the conventional compressor airconditioning system, it is less effective in humid climates becausethe ambient air already has a high relative humidity, which greatlylimits its energy saving capacity.

To make the M-cycle evaporative heat exchanger more practicalfor humid climates, liquid desiccant integrated evaporative coolingsystems have been investigated. Desiccant solution is designed toabsorb moisture in the passing air, yielding dry hot air. Then the airis cooled in an M-cycle evaporative heat exchanger. The desiccantsolution then needs to be regenerated for reuse by reheating andcounter flowing with ambient air. A prototype of a desiccant-enhanced evaporative air conditioner has been developed [14], inwhich a conventional indirect evaporative cooler was designed tocool the dehumidified air in the second-stage of the whole process.The numerical and experimental results indicate that the DEVAP(Desiccant enhanced evaporative air-conditioning) model used inprevious energy savings estimates [15] is correct within 10%.

In this paper, a prototype was built up and experimentallyinvestigated with integrated liquid desiccant and M-cycle indirectevaporative cooler which is mainly different with DEVAP in itssecond part, M-cycle indirect evaporative cooler. The purposes ofthis study are to observe the temperature and humidity change inboth the air and the desiccant solution, to discuss how the initialparameters of solution and air influence the final temperaturebehavior, and to analyze the ability of handling air and savingenergy.

2. Integrated liquid desiccant and M-cycle indirectevaporative cooler

Schematics of the integrated liquid desiccant and M-cycle in-direct evaporative cooler are shown in Fig. 2. The whole system isdesigned with two parts: the first part is for dehumidification andthe second part is for temperature decreases.

The dehumidifier (the first part) is made up of heat and massexchanger in which target air (① / ②) and desiccant solutioncross flows in one channel, while water and ambient air counterflows in the adjacent channel (⑤/⑥). In order to evaporate morewater for cooling the desiccant solution, nozzles are used to spray ahigh-pressure water flow resulting in a two-phase flow of waterand ambient air in airstream.

The heat and mass transfer process in the whole device can bedescribed as follows (see air state change in Fig. 3). For the purposeof the experiment, it is assumed that the air in state① is pretreatedat an expected humidity and temperature, and then flows throughthe channels in the dehumidifier, where water vapor in the air isabsorbed by the desiccant solution releasing the heat of vapor-ization. Part of the heat is absorbed by the desiccant solution andtransferred to the water fall through the plate, where it is cooled bythe water evaporation. This heat transfer process from desiccant

cycle and its representation on a psychometric chart [8].

tion on integrated liquid desiccant e Indirect evaporative air cooling(2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.08.066

Fig. 2. Schematic of integrated liquid desiccant and M-cycle indirect evaporative cooler.

W.Z. Gao et al. / Applied Thermal Engineering xxx (2014) 1e9 3

solution to evaporating water maintains the dehumidification ca-pacity of solution with a relatively lower vapor pressure.

The dehumidified air exits the dehumidifier at state ②, and isbrought into both the product channel and working channel of theM-cycle indirect evaporative cooler, where it is cooled whenflowing along the flow path, due to the temperature differencebetween the two channels created by the evaporation of water in

Fig. 3. Air state changes on a psychometric chart.

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the wet channel. When flowing through the holes in the plasticplate between theworking channel and thewet channel, part of theair is diverted into the adjacent wet channel, where the air flows inan opposite direction, taking away the sensible heat transferredfrom the adjacent channels of the two sides and the evaporatedwater from the saturated net surface. As a result, the air in the wetchannel gradually approaches saturation when coming across theflow paths, and is finally discharged into the atmosphere at a mixedstate ④. At the same time, the air in the working channel andproduct channel is gradually cooled by the adjacent wet channel toa state below its wet bulb temperature and close to its dew pointtemperature. The fraction of the air that is divided into the workingchannel is dependent on the required sensible cooling capacity inthe dry channel.

3. Experimental setup

3.1. Test chamber

Fig. 4 shows the experimental setup schematic. The test facilityconsists of several parts: a liquid desiccant system, an air handlingsystem, a secondary air system, and a water spraying system. Theliquid desiccant system includes a solution pump, solution tanks Aand B, solution cooler and heater B, and solution valve. The airhandling system consists of the cooling coil A, heater A, humidifier,and fan. The aforementioned devices were used to handle the liquid

tion on integrated liquid desiccant e Indirect evaporative air cooling(2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.08.066

Fig. 4. Schematic of experimental rig.

W.Z. Gao et al. / Applied Thermal Engineering xxx (2014) 1e94

desiccant and target air in order to achieve the expected parame-ters, before being transported into the integrated liquid desiccantand M-cycle indirect evaporative cooler. The secondary air systemincludes a fan and a valve, which is not detailed in Fig. 4. The waterspraying system is made up of a water pump, a water valve andsome sprinklers. Lithium chloride solution with lower vapor pres-sure was chosen as desiccant.

The integrated liquid desiccant andM-cycle indirect evaporativecooler is the core device of the whole system, in which heat andmass exchange occurs between the pretreated air and liquiddesiccant. The detailed parameters of the cooling device are shownin Table 1. Lithium bromide aqueous solution, having lower surfacevapor pressure, was selected as the desiccant in the experiment.

During the experiment, four steps were required to achieve theexpected experimental parameters: (1) The target air was pretreatedto the required temperature and humidity ratio in the air handlingsystem. The ratio between the product air and working air in the M-cycle indirect evaporative cooler was controlled by modulation ofthe exhaust air. (2) The liquid desiccant in tank Awas adjusted to therequired concentration and handled to the set temperature bymodulating the capacity of cooling coil B and heater B, and pumped

Table 1Cooling device parameters.

Parameters Value Units

DehumidifierTotal length 400 mmTotal width 500 mmTotal height 500 mmProduct air channel thickness 3.45 mmWater fall channel thickness 3.45 mmPlastic layer 0.10 mmM-cycle indirect evaporative coolerTotal length 500 mmTotal width 500 mmTotal height 500 mmProduct air channel thickness 2.2 mmWorking dry air channel thickness 2.2 mmWorking wet air channel thickness 2.2 mmPlastic plate thickness 0.3 mmWick thickness 0.3 mm

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to the dehumidifier with an adjustable flow. (3) The secondary airsystem was then used to exchange heat and mass with sprayingwater, resulting in a higher heat and mass transfer capacity betweenthe target air and liquid desiccant. (4) The water systemwas used toadd water to the wicked surface on the auxiliary channel of thedehumidifier and the working channels of the M-cycle indirectevaporative cooler. Once all the parameters reached their set valuesand maintained stability, the tests were carried out.

3.2. Measuring instruments

Desiccant and air parameters before and after each part of theintegrated liquid desiccant and M-cycle indirect evaporative coolerwere measured to investigate its performance. The measured pa-rameters consisted of air flow rate, dry-bulb temperature and wet-bulb temperature. The rate of the inlet air and supply air wereindirectly obtained by using standard nozzles, with which testedpressure drop was captured and converted into flow rate. T-typethermocouples were used to measure the temperature of both theair and the desiccant. The measured desiccant parameters con-sisted of flow rate, densities, and inlet and outlet temperature. Theflow rate was measured by an electromagnetic flowmeter, whichwas specially treated to resist corrosion of the desiccant. The den-sity of the desiccant was measured by a hydrometer, and then theconcentration could be found in the physical property chart oflithium chloride solution according to the measured temperatureand density. Table 2 shows the specifications of the differentmeasuring devices.

Table 2Specifications of the different measuring devices.

Parameters Devices Accuracy Operational range

Air flow rate Standard nozzles 1% 0e1500 m3/hAir temperature Thermocouple ±0.1 �C@40 �C �200e350 �CDesiccant

temperatureThermocouple ±0.1 �C@40 �C �200e350 �C

Desiccantflow rate

Electromagneticflowmeter

±2.5% 100e1000 L/h

Desiccant density Specific gravityhydrometer

1 kg/m3 1100e1900 kg/m3

tion on integrated liquid desiccant e Indirect evaporative air cooling(2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.08.066

Table 3Parameter range of heat and mass condition.

Parameters Unit Initial value Ranges

Air temperature �C 32 24e38Air moisture g/kg 14 9.5e19.5Air flow kg/s 0.2 0.1e0.45Desiccant temperature �C 28 22e35Desiccant flow rate kg/s 0.2 0.12e0.26Desiccant density % 35 28e42Working air ratio % 50 30e70

W.Z. Gao et al. / Applied Thermal Engineering xxx (2014) 1e9 5

3.3. Evaluation on heat and mass transfer performance

The total load of the processed air consists of latent and sensibleloads, which can be respectively handled in the dehumidifier andM-cycle indirect evaporative cooler of the whole device. The sen-sible heat ratio (SHR, the sensible load divided by the total load) ofthe air reflects the required handling capacity of the different partsof the air conditioner. Therefore, the calculated SHR based on theinlet and outlet air parameters could be used to evaluate the per-formance of simultaneous reduction in humidity and temperature.The expression of SHR is shown in Eq. (1).

SHR ¼ Ca�ta;out � ta;in

�

ha;out � ha;in(1)

Typically, moisture reduction is used to determine the amountof vapor removed per kilogram of air in the dehumidifier of thehybrid, which is calculated as follows:

Dua ¼ ua;in � ua;mid (2)

Similarly, the temperature reduction of the indirect evaporativecooler of the hybrid can be described by the following relationship:

Dta ¼ ta;mid � ta;out (3)

It must be pointed out that the temperature difference in Eq. (3)is greater than that of the outlet and inlet air of the hybrid, becausethe air in the dehumidification part is heated by the latent heatreleased by the moisture.

Dew point effectiveness is another parameter used for judgingthe performance of the hybrid describing the extent of theapproach of the outlet air temperature against the dew point of theinlet air, expressed as follows:

Fig. 5. Energy balance in tw

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εdp ¼ ta;in � ta;outta;in � ta;dp

(4)

4. Experimental conditions and experimental methods

The effects of the air and liquid desiccant inlet parameters onthe performance of the hybrid were first tested, and then experi-ments based on the different working air ratio in the M-cycleevaporative cooler were conducted under the initial value condi-tion of the inlet air and liquid desiccant. The initial values and valueranges are shown in Table 3.

The experiment system was run by the initial value in Table 3until it reached stable state, and then the data acquisition systemwas carried out while one parameter was changed within theranges. When finished, the value of the parameter was set back tothe initial value, and then similar operations were made with otherparameters.

5. Experimental results and analysis

5.1. Energy balance in the hybrid

Fig. 5 shows the energy balance in the dehumidification partand sensible cooling part of the hybrid for all the experimentalruns. The figure indicates that the energy differences not only in thedehumidifier but also in the M-cycle indirect evaporative cooler arealmost all within ±20%, with an average difference of 8.6% and 5.3%respectively. One phenomenon to note is that, in the dehumidifier,the energy obtained by desiccant and water is more than thatreleased from the air in most cases. This is because the releasedheat taken away by the evaporative cooling in the adjacent layer isdifficult to be measured, since the water flow was limited andunstable and the air parameters were greatly influenced by theenvironment.

5.2. Performance analysis

5.2.1. Effect of the airflow rateThe effect of the airflow rate on SHR, dew point effectiveness,

moisture reduction and temperature reduction is shown inFig. 6(a). The figure indicates that the dew point effectiveness,moisture reduction, and temperature reduction all increase, whilethe SHR decreases with increasing airflow rate. The decrease of themoisture reduction is due to the decrease of the heat and mass

o parts of the hybrid.

tion on integrated liquid desiccant e Indirect evaporative air cooling(2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.08.066

Fig. 6. Effect of air inlet parameters on the performance of the hybrid.

W.Z. Gao et al. / Applied Thermal Engineering xxx (2014) 1e96

transfer time in the dehumidifier between the air and desiccant.Also, the relatively higher moisture content in the air at state 2inevitably leads to the decrease of the water evaporation intensityin the wet channel of the indirect evaporative cooler, resulting in ahigher outlet supply air temperature. Throughout the process, thechange of the latent heat was significantly more than that of thesensible heat, leading to the increase of the SHR. Note that the dewpoint effectiveness in all airflow rate conditions are greater than 1,even though they are slightly decreasing with the increase of theairflow rate. This phenomenon indicates that the hybrid air con-ditioning system can achieve closer dew point effectiveness withthe M-cycle evaporative cooler directly used in dry area.

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5.2.2. Effect of the air temperatureThe variations of SHR, dew point effectiveness, moisture

reduction, and temperature reduction as a function of the airtemperature are shown in Fig. 6(b). The figure indicates that, withthe increase of the inlet air temperature, the dew point effective-ness and temperature reduction increase slightly while the SHRand moisture reduction show an opposite change. When the inletair was supplied at a higher temperature, the temperature increasein the desiccant accelerated because of the extra heat exchangebetween them, resulting in a decreasing dehumidification ability ofthe desiccant, and then leading to a reduction in the cooling abilityin the indirect evaporative cooler. However, the changes are

tion on integrated liquid desiccant e Indirect evaporative air cooling(2014), http://dx.doi.org/10.1016/j.applthermaleng.2014.08.066

W.Z. Gao et al. / Applied Thermal Engineering xxx (2014) 1e9 7

relatively mild when compared with the influence of the airflowrate. The reason is supported by the fact that, in the present ex-periments, the desiccant flow rate was so large (hence a largethermal capacity), that the influence of the air inlet temperaturewas relatively weak.

5.2.3. Effect of the air humidity ratioFig. 6(c) illustrates the effect of the air humidity ratio on the

performance of the hybrid. With an increase in air humidity ratio,the humidification capacity of the desiccant can be improved forthe increased surface vapor pressure difference between air and

Fig. 7. Effect of air inlet parameters o

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desiccant. However, the air humidity ratio from the dehumidifier isstill higher than that of the low humidity ratio inlet air condition.Therefore, as the inlet air of the indirect evaporative cooler, it had anegative influence on the cooling capacity, causing a higher tem-perature of the supplied air. Thus, the SHR decreases quicklybecause the ratio of the latent heat increases in the whole airhandling process.

5.2.4. Effect of the desiccant inlet temperatureDew point effectiveness, moisture reduction and temperature

reduction decrease while SHR decreases with increasing desiccant

n the performance of the hybrid.

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W.Z. Gao et al. / Applied Thermal Engineering xxx (2014) 1e98

inlet temperature, as shown in Fig. 7(a). An increase in desiccantinlet temperature caused a decrease in desiccant surface vaporpressure and hence a decrease in mass transfer potential within thedehumidifier. Similarly, as the inlet air of the indirect evaporativecooler, an increase in temperature and humidity of the air from thedehumidifier weakened the potential evaporation of water in thewet channel. Although moisture reduction and temperaturereduction decreased at the same time, the decrease of the formeronewasmore obvious than the latter onewhen converted into heatchange, which caused SHR to increase with the desiccant inlettemperature.

5.2.5. Effect of the desiccant flow rateMoisture reduction and temperature reduction both increased

with increasing desiccant flow rate, as shown in Fig. 7(b). Withincreasing desiccant flow rate, the variation of the desiccant tem-perature could be restrained for the increased heat capacity. Lowtemperature of the desiccant within the dehumidifier could ensurea high vapor difference between desiccant and air. Thus, the airhumidity ratio out of the humidifier became lower, which couldobviously increase the cooling ability of the indirect evaporativecooler. Although both moisture reduction and temperature reduc-tion increased significantly, the dew point effectiveness was almostunaffected. The reason for this is that the increase of the outlet airtemperature from the dehumidifier offset the increase of temper-ature reduction in the indirect evaporative cooler. In addition, thedecrease of SHR also indicates the increased capacity ofdehumidification.

5.2.6. Effect of the desiccant inlet concentrationFig. 7(c) shows the effect of desiccant inlet concentration on the

hybrid performance. Compared with the figure in Fig. 7(b), similartendency with different value change of the four performance in-dicators in Fig. 7(c) can be found. High concentration desiccantwould result in a mass transfer potential driven by the moisturedifference between the desiccant surface and air.

5.2.7. Effect of the working air ratioFig. 8 illustrates how the working air ratios within the indirect

evaporative cooler influence the performance of the hybrid. Sincethere were no variations in the working condition of the dehu-midifier, the only changed value of the supply air in the secondstage of the hybrid was temperature, enabling it to meet a rangeof supply air condition. The figure shows that although thevariation of the working air ratio covers a range from 0.3 to 0.7,the temperature reduction only changes approximately from15 �C to 20 �C. This phenomenon can be seen as proof that the

Fig. 8. Effect of working air ratio on

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water evaporation in the wet channel and heat transfer betweenthe dry channel and wet channel are not significantly affected byworking airflow rate. When increased working air flowedthrough the wet channel, more water evaporated into the air;however, this caused the exhaust air to become less and lesssaturated, resulting in very little help to the heat transfer be-tween the two channels.

5.2.8. Effect of the other factorsIn the dehumidifier, the heat released from the mass transfer

was mostly absorbed by the evaporating water on the exhaust side,and carried away by the exhaust air at a state closer to saturation.This could keep the dehumidification ability of the liquid desiccantfor more time, and the air flowing out from the dehumidifier at alower temperature. In the present experiments, the air used toevaporate the water came from the atmosphere, whose tempera-ture and humidity ratio kept changing during the six days of theexperiment. In order to reduce this effect, circulating water wassupplied to the wick surface and a longer time run before otherconditions was ensured. These measures to some extent eliminatedthe adverse effects of the instability. Further testing will be taken toassess their effects in the next step of research.

In the M-cycle indirect evaporative cooler, the water flow ratehas some influence on the water evaporation inwet channel, whichindirectly affects the heat transfer across the channel wall. Theresults of the pre-experiments show that, when the water flow ratewas less than two times the evaporation rate, the wick surface wasnot sufficiently wetted, resulting in a negative effect on the coolingcapacity of thewet channel. Once thewater flow rate was increasedto more than eight times, the wick surface was well wetted;however, the evaporation efficiency was reduced because of thereduction of the heat and mass transfer area. In order to compro-mise the above two factors in the present tests, a water flow ratefive times the evaporation rate was chosen to achieve the opti-mized evaporation.

6. Conclusion

This paper describes advancement in liquid desiccant coolingsystems through the M-cycle. The main advantage that the hybridoffers is the capability of using only one apparatus to removemoisture in the fist stage and remove sensible heat in the secondstage. SHR, dew point effectiveness, moisture reduction and tem-perature reduction were adopted as indices to describe the heatand mass transfer performances of the integrated liquid desiccantand M-cycle indirect evaporative cooler. Obtained results aresummarized as follows:

the performance of the hybrid.

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(1) The energy balance in both the dehumidifier and M-cycleindirect evaporative cooler are within the range of ±20% forall the experiment runs, despite that in the dehumidifier theenergy obtained by desiccant and water was always morethan the energy released from the air.

(2) Increasing the inlet air flow rate and humidity ratio improvesthe dehumidification ability of the dehumidifier, butweakens the cooling ability of the M-cycle indirect evapo-rative cooler because of its high temperature and/or highhumidity ratio out of the dehumidifier. Increasing the inletair temperature has negative effects on the heat and masstransfer both in the dehumidifier and in the M-cycle indirectevaporative cooler.

(3) The removal capacity of both moisture in dehumidifier andsensible heat in M-cycle indirect evaporative cooler can beimproved significantly at the same time while increasing theliquid desiccant flow rate or inlet concentration. However,the influence of the desiccant temperature shows an oppo-site direction.

(4) Long time pre-running of the cycling water in the dehu-midifier is necessary to eliminate the influence of the airuncertainty. The optimized ratio between the supply waterflow and evaporative water flow was proven by experimentto be five, which can achieve high heat and mass transfer inthe M-cycle indirect evaporative cooler.

Acknowledgements

The research described in this manuscript was supported byNational Natural Science Foundation of China (Grant No. 51106094),Innovation Program of Shanghai Municipal Education Commission(Grant No. 13YZ089) and Fundamental Research Funds for theCentral Universities (JB2014239). Their support is gratefullyacknowledged.

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