Desalination 332 (2014) 1–6

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Desalination

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Experimental investigation of a two-stage solarhumidification–dehumidification desalination process

M. Zamen a,b, S.M. Soufari a, S. Abbasian Vahdat a, M. Amidpour b,⁎, M.A. Zeinali a, H. Izanloo c, H. Aghababaie d

a Iranian Institute of Research and Development in Chemical Industries (ACECR), Karaj, Iranb Department of Mechanical Eng., K.N. Toosi University of Technology, Tehran, Iranc Research Center for Environmental Pollutants and Department of Environmental Health Engineering, Health Faculty, Qom University of Medical Sciences, Qom, Irand Qom Payame Noor University, Qom, Iran

H I G H L I G H T S

• A multistage HD process was investigated to improve efficiency of HD desalinator.• 2-Stage process is the best choice from cost and performance point of view.• A 2-stage solar HD desalination unit constructed and tested in an arid area• Production is presented and compared during hot and cold days.• Improvement in productivity and reduction of solar collector area were obtained.

⁎ Corresponding author at: Energy system EngineeEngineering Faculty, K.N. Toosi University of TechnMollahsadra Ave., Tehran, Iran. Tel.: +98 21 88677272; fa

E-mail address: amidpour@kntu.ac.ir (M. Amidpour).

0011-9164/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.desal.2013.10.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 May 2013Received in revised form 7 October 2013Accepted 18 October 2013Available online 9 November 2013

Keywords:Solar desalinationHumidification–dehumidificationTwo-stage process

This paper experimentally evaluates a two-stage technique to improve the humidification–dehumidificationprocess in fresh water production from brackish water. According to modeling results of multi-stage processand on the basis of construction cost estimation, using a two-stage process is the most suitable choice that canimprove important parameters such as specific energy consumption, productivity and daily production persolar collector area and thus, investment cost. A pilot plant was designed and constructed in an arid area with80m2 solar collector area to evaluate the two-stage process. This unit was tested on cold and hot days. The effectof main parameters on fresh water production of the unit is studied. Experimental results show that two-stageHD desalination unit can increase heat recovery in condensers and hence, reduce thermal energy consumptionand investment cost of the unit. Moreover, productivity can be increased by 20% compared with the single-stage unit.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

With diminishing water resources, the problem of drinking waterseems to be the most important issue in the near future. It is predictedthat more than half of the world's population will suffer from watershortage by 2025 [1]. One of the best solutions to this problem isbrackish/seawater desalination.

Air humidification–dehumidification (HD) desalination is a suitablechoice for producing fresh water when demand is decentralized.Conventional desalination methods such as MSF (Multi-Stage Flash),ME (Multi-Effect), VC (Vapor Compression) and RO (Reverse Osmoses)are suitable for large and medium capacities of fresh water production.But most remote arid areas need low capacity desalination systems.

ring Department, Mechanicalology, No. 15 Pardis Street,x: +98 21 88677273.

ghts reserved.

Simplicity of the process and availability of high levels of solar energyin most of the areas that need fresh water make this system suitablefor these areas.

Most of investigations on HD desalination process are concernedabout productivity and efficiency improvement and have been doneafter 1990 and several units have been built which are similar in basebut different in type of equipment.

Farid and Hamad [2] constructed an HD desalination unit in Basrah,south of Iraq. The unit produced 12 l/day·m2 of solar collector surfacewhich was about three times the production of a single-basin solarstill under similar solar conditions. However, the pressure drop in thecondenser and the humidifier was too high, increasing the electricalpower consumption by the blower to a level that makes such a processuneconomic.

Then, two units of different sizes were constructed and operated inJordan described by Farid et al. [4] and Al-hallaj et al. [3]. They foundthat the effect of water flow rate on heat and mass transfer coefficientis more significant than air flow rate.

SolarCollector

DehumidifierHumidifier

Salt Waterinlet

DistillateWaterDrain

1

2

3

4

5

6

Fig. 1. A schematic diagram of a single HD process.

2 M. Zamen et al. / Desalination 332 (2014) 1–6

Ben Bacha et al. [5] presented a perfect study including modeling,simulation, and experimental validation of a solar HD called SMCEC(Solar Multiple Condensation Evaporation Cycle). They concluded thatperfect insulation of the unit, high water temperature and flow rate atthe entrance of the evaporation tower, low temperature of water atthe entrance of the condenser and hot water recycling by injection atthe top of the evaporation tower can improve operation and productionof the system.

To consider the simultaneous effect of parameters, Hou et al. [6] usedpinch technology to optimize the performance of the HD process. Theymaximized condenser heat recovery through composite curves. Theyfound that there is an optimum value for water to air flow rate ratio(L/G). But the effects of humidifier inlet temperature and solar collectorefficiency were not studied. In another study, Hou [7] evaluates a two-stage solar HD desalination process using pinch technology.

A global optimization of the HD process was developed by Soufariet al. [8]. Effects of different parameters were analyzed and amathematical programming model was presented to optimize theprocess with different objective functions. Then, the model wasdeveloped by adding the solar part and finally a low-cost design forsolar HD desalination was obtained [9]. In the next step, a unit withthe capacity of 10 l/hr, which has been located at the Iranian Researchand Development Center for Chemical Industries (IRDCI), Karaj, Iran,was constructed according to optimization results [10]. In continuationof the previouswork, Zamenet al. [11] developed amathematicalmodelto study the effect of multi-stage technique on process parameters.Their results show that multi-stage technique has good improvementsin comparison with the single-stage technique. They concluded that 2-stage unit will be the best choice for a multi-stage HD desalinationunit. McGovern et al. [12] investigated the effect of air extraction–injection on the performance and energy recovery of an HD systemthrough developing the saturation curve and pinch methodology.They found that a two-stage system would be improved by decreasingthe temperature range of the cycle and pinch point temperature.

In this paper, experimental results of a two-stage desalination unitwith solar collectors designed and constructed on the basis ofmathematical modeling and optimization will be represented.

2. Process description

The single-stage HD unit is distillation under atmospheric conditionby an air loop saturated with water vapor, and has three main sections:Humidifier, dehumidifier and heat source as shown in Fig. 1. First, saltywater heated by an external heat source such as solar collectors (points:2–3 on Fig. 1) enters to the humidifier section. In the humidifier (points:3–4 on Fig. 1) hot water is in contact with air in a packed bed and acertain quantity of vapor is extracted by air (points: 5–6 on Fig. 1).Then, hot humid air leaves the humidifier and enters to thedehumidifier. In this section, water vapor is distilled by bringing thehumid air in contact with a cooled surface which causes condensationof vapor in the air and production of fresh water (points: 6–5 onFig. 1). Generally, the latent heat of condensation is used for preheatingthe salty feed water (points: 1–2 on Fig. 1).

As mentioned in the previous study [11], the process can beimproved by increasing energy recovery between air and hot waterand consequently reducing external heat requirement. This can beachieved by reducing minimum temperature approach throughdividing air loop to some smaller loops where each loop may beconsidered as a single stage. Based on this fact, Fig. 2 shows the newprocess with a number of closed air loops named multi-stage HDdesalination. Feed salt water enters to the first stage (right hand sidein Fig. 2) and after passing through the first condenser, enters to thesecond stage and so on. Salt water temperature increases after eachstage. The outlet of the last stage enters to the solarwater heater systemand then, hot water enters to the last stage and cascades in lower stages

to exit from the first stage. Temperature of the hot water outlet could becontrolled via the design of packed bed and condenser.

The main reason for low thermal energy recovery rate of a single-stage HD unit is the shape of the saturated air curve. If the air enthalpycurve could be flattened, hot and cold water lines could be closer, andthe thermal energy recovery rate could be higher. This issue isextensively discussed in the previous study [11]. The shape of aircurve could be improved if the mass flow rate of air would vary. Themass flow rate of air could be changed for each stage and set to anoptimum value. Hence, the heat recovery section is extended andenergy consumption will be reduced. More details of this issue arepresented in the previous study [11].

In this paper, at first, brief results from modeling the multi-stageprocess described in the previous study [11] are presented. Relatedequations used in previous studies for single-stage HD [8–10] are usedfor multi-stage process modeling. After that, experimental results of a2-stage unit constructed in Qom, Iran will be presented.

3. Modeling results

A multi-stage process with separated closed air loops and solarcollector as heat source was selected as the base case. Main equationsof each stage are based on heat and mass balance in humidifier anddehumidifier towers presented in [8]. Also, additional equations forsolar collector and packing are used according to relations presentedby Zamen et al. [9]. As shown in Fig. 2, the hot water outlet of eachhumidifier is the humidifier inlet of the next (lower) stage and thecondenser outlet of each stage is the inlet of the upper stage. Theseconditions are inserted to the model as new constraints. By addingrelated constraints to the closed air cycle (such as top and bottomtemperature limit and flow rate of air in each loop) and inlet and outletconditions of hot and cold water between stages, the mathematicalmodel of the system would be completed.

The mathematical programming model is solved for one- to four-stage processes with the mentioned constraints to achieve theminimum energy consumption per kg of fresh water production. Thetemperature of the humidifier inlet water and cooling water enteringthe dehumidifier is considered to be 70 (°C) and 20 (°C), respectively.

Collectors

DistillateWater

Third Stage Second Stage First Stage

Salt Water

Drain

Solar

Fig. 2. A simple multi-stage HD desalination unit (with 3stages).

3M. Zamen et al. / Desalination 332 (2014) 1–6

Fig. 3a shows the effect of increasing the number of stages onminimumspecific thermal energy consumption of the process and dailyproduction per solar collector area. It is observed that increasing thenumber of stages from one to two reduces energy consumption bymore than 35%. But, this reduction will be small for systems withmore than two stages. The same effect will be seen in productivity(ratio of fresh water production to salty feed water) and total distillatewater. As shown in Fig. 3b, daily production per unit solar collectorarea increases by more than 40% in the two-stage process, but thisincrement will be 4% and 1% for 3- and 4-stage processes, respectively.The reason why three-stage and higher processes contribute less toproductivity is that the temperature of inlet water of each stage,which is the outlet of the previous stage, gradually decreases. The lessthe temperature of inlet water, the less the productivity of each stage.Generally when the number of stages increases, energy consumptionper kg of fresh water, productivity and daily production per solarcollector area are enhanced, but the main enhancement occurs fromone- to two-stage process.

On the basis of the mentioned modeling, more results are obtainedthat can be summarized as follows:

- Stages with higher temperature have higher productivity.- In themulti-stage process, higher temperature stages need lower airmass flow rates. In other words, liquid to gas flow rate ratio (L/G)reduces with reduction of the humidifier inlet water temperature.In low temperature stages, therefore, a forced draft made by a fanmay be required.

(a)

0

400

800

1200

1600

1 2 3 4

Ene

rgy

Con

sum

ptio

n (k

J/kg

)

Number of stages

Fig. 3. Specific thermal energy consumption: (a) and daily producti

It should be noted thatwhen the number of stages increases, the costof desalination unit increases. According to the above explanations, thetwo-stage process seems to be the most suitable choice. And finally onthe basis of these results, a two-stageHDdesalination unitwas designedand constructed in an arid village near Qom at the center of Iran.

4. Pilot plant description

A two-stage HD desalination unit utilizing solar collectors wasdesigned to produce 500 l/day distillate water required for 100 peopleresiding in an arid village near Qom. A simple process flow diagram ofthe two-stage HD is shown in Fig. 4. Salty water from domesticresources is pumped from feed water tank to the first and secondcondensers successively and heated to the required temperature ofprocess in heat exchange with the hot water of solar collectors. Then,hot salty water is sprayed on the second humidifier packed bed. Theoutlet of this humidifier could be pumped either to the humidifier ofthe first stage or to the hot water storage tank (for daily demand ofhotwater). If the temperature of the outlet streamof thefirst humidifieris low enough, a section of that can be recycled to Tank-1 to reduce theamount of rejected water and also feed the water required. Twoelectrical fans with low and medium speeds facilitate air circulation inthe second and first stages, respectively. The power consumption ofthese fans is 100 and 150W, respectively that can be supplied by solarPV panels. Fig. 5 presents some pictures from the pilot plant constructedin an arid village in Qom, Iran.

(b)

0

2

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10

1 2 3 4

Dai

ly p

rodu

ctio

n pe

r co

llect

or a

rea

(L/d

/m2)

Number of stages

on per collector area, (b) as a function of the number of stages.

Salt Water Make up

Drain

Tank2Distillate water Tank

Recycle20°

40°

55°

70°

55°30°

Hot waterconsumption

PU-101

PU-102

PU-103

Second Stage(Low Temp.)

First Stage(Hight Temp.)

Tank1Salt Feed Water

25°1

2

3

4

5

6

8

7

Solar Collectors

Tank3SolarHeaterTank

Fig. 4. Process flow diagram of the two-stage HD unit.

4 M. Zamen et al. / Desalination 332 (2014) 1–6

According to modeling results for the required amount of freshwater production and solar radiation of the selected site, dimensionand capacity of equipments were obtained. The desalinator wasconstructed as two adjacent towers, each of which count as onestage that comprises of humidifier and dehumidifier sections. Thehumidifier consists of a polypropylene packed bed with a specificsurface area of 240 m2/m3. Cross section area of the humidifier inthe first and second stages is 0.3 and 0.5m2, respectively. The heightof both stages is equal to 2.4 m. The dehumidifier is a finned tubecondenser with copper tube and aluminum fins with a heatexchange surface area of 30 m2 for both of the condensers. A small

Fig. 5. Pilot plant of the two-stage

low speed fan is placed at the top of the condenser of each stage(as showed in Fig. 4). The salty feed water was heated in a tank bysolar heated water circuits. Solar collectors were divided into twosections, some directed to south and others to south–east to gainsolar heat almost uniformly during all day and benefit afternoonsun heat. An automatic control system was used to control theoperation of the unit according to changes in the temperature ofhot water. When temperature reaches 70 °C in the morning, thesystem starts and when temperature drops under 50 °C, the systemstops because of low production. The system is designed to workwith various flow rates and a maximum of 1400 l/h. Digital

HD unit constructed in Qom.

45.0

50.0

55.0

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0Stor

age

tank

Tem

para

ture

(c)

time

Hot days Cold days

Start Stop

Stop

Start

Fig. 6. Storage tank temperature variation during a hot and a cold day.

0.0

10.0

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80.0

Fres

h W

ater

Pro

duct

ion(

Lit/

hr)

Inlet Hot Water Temperature (ºC)

50.0 60.0 70.0 80.0 90.0

Fig. 8. Effect of inlet hot water temperature on production rate (water flow rate: 1350 lit/h).

5M. Zamen et al. / Desalination 332 (2014) 1–6

thermocouples are used in various points (as depicted in Fig. 4) formeasuring temperature and two rotameters are used for feed lineof each stage. Flow rate is regulated manually by means of aregulating valve in the entering line of the second stage.

5. Experimental results

The pilot plant was tested in a certain period of time includingsummer (hot) and winter (cold) days. More than 200 tests wereperformed starting in the morning and continuing to evening.Examining the performance of setup for each test takes one day in realconditions of various days and in each test, about 15 parameters ofwater and air streams were recorded. Therefore, more than 3000 datawas gathered during the test period on hot and cold days. In earlyhours of the day, solar collectors heated the water of storage tank.Desalination unit starts when water in the storage tank reaches anappropriate temperature and fresh water production begins. Insummer, production can start early in the morning before 11 o'clockwhen thehotwater temperature of the tank reaches 70°C. But inwinter,it takes longer till about 12 o'clock or even later. The temperaturevariation of the storage tank for a hot and a cold day are shown in Fig. 6.

The hourly production rate of the unit onwinter and summer days isshown in Fig. 7. On both of these days, the unit starts at 11 o'clock.Because of higher solar intensity and length of hot days, the productionrate on these days is higher than that on cold days. Fresh waterproduction depends on hot water temperature of the storage tank andreduces when it drops. On hot days, the temperature of the storagetank does not change considerably during daylight hours and therefore,

0.010.020.030.040.050.060.070.080.090.0

100.0

11:30

12:00

12:30

13:00

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14 14:30

15 15:30

16 16:30

17 17:30

Prod

uctio

n ra

te (

liter

s pe

r ho

ur)

Time

Cold Days Hot Days

Fig. 7. Hourly production rate on hot and cold days.

production is almost uniform during the operation of the unit, but oncold days, because of lower solar intensity and thus, reduction of thetemperature of the storage tank, production sharply reduces afterabout 2 h from the start time. The total production of the unit duringthese two days was measured. On summer days, the total productionof the unit is more than twice that of winter production and reachedabout 580 l/day.

In this pilot plant, 80 m2 solar collectors were used and therefore,production reached 7.25 l/day·m2 solar collector that is about 40%higher than the previous pilot plant (Soufari et al., 2009 b). Hence, thisresult confirms the theoretical calculation and benefits of the two-stage process in reduction of specific energy consumption and thus, inreduction of the initial investment cost.

Fig. 8 shows the effect of the inlet hot water temperature onproduction rate. It can be seen that this temperature has an importanteffect on production rate. On summer days, the inlet hot watertemperature may exceed 80 °C. This increases fresh water production.Also on winter days, this temperature has the most effect in decreasingfresh water production.

The effect of inlet water flow rate is presented in Figs. 9 and 10 atthe inlet water temperature of 70 °C. As shown in Fig. 9, increase inwater flow rate increases the production rate. But, when waterflow rate is more than 1200 l/h, its increasing rate will decrease.Also, change of productivity (ratio of the fresh water productionrate to the inlet salt water flow rate) has been shown in Fig. 10 as afunction of inlet water flow rate. It can be seen that an increase in

30

32

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40

42

44

46

48

Fres

h w

ater

Pro

duct

ion

(lit/

hr)

Inlet water flow rate (lit/hr)

600 800 1000 1200 1400

Fig. 9. Effect of inlet water flow rate on fresh water production (inlet water temperature:70 °C).

3

3.5

4

4.5

5

600 800 1000 1200 1400

Prod

uctio

n R

ate

(%)

Inlet Water Flow Rate (Lit/hr)

Fig. 10. Effect of inlet water flow rate on productivity (inlet water temperature: 70 °C).

6 M. Zamen et al. / Desalination 332 (2014) 1–6

water flow rate decreases productivity. Productivity changes from3.6 to 4.8 depending on water flow rate. On the basis of the lastexperimental results (Soufari et al. [10]), productivity of the singlestage unit can reach 3.5–4% for inlet hot water temperature of70 °C. Therefore, using two-stage process can enhance productivityby up to 20% depending on water flow rate.

Fig. 11 presents the contributions of the first and second stages tototal production of the unit in various salty feed water flow rates. Asdepicted in this diagram, more than 75% of the total productionoccurs in the stage with higher temperature (the second stage).Mass flow rate of air circulated in this stage is about half of that inthe first stage which has lower temperature. Results show that 50%of the required thermal energy can be supplied by heat recovery incondensers to preheat the inlet feed water: 35% in the second stagecondenser and 15% in the first stage condenser. Because of this,fewer heat sources are needed for the same production and that iswhy multi-stage system has higher productivity in comparison tosingle-stage unit. Since production is more than the desired valueand the temperature of the outlet water of the second humidifier ismore than 50 °C in summer, it may be possible to use this streamas hot water required in the village. Hence, this method can beused as a combined system for simultaneous production of freshwater and hot water.

Total dissolve solids (TDS) of salty feed water in village was morethan 4500ppm and TDS of fresh water during the test period was lessthan 100 ppm. This shows the ability of this process in desalination ofbrackish water to produce potable water in arid area.

Although using multi-stage process can decrease the investmentcost of the system, more complexity of the system may be a problem

0% 20% 40% 60% 80% 100%

720

780

1080

1200

1350

Salt

wat

er f

low

rate

(lit

/hr)

First Stage production Second stage production

Fig. 11. Contribution of each stage to the production of fresh water.

in using even two-stage process. Designing a suitable controller andusing sufficient sensors such as level switches for storage tanks orthermocouples in various points of the system can help to decreaseoperational problems and increase the reliability of the unit. Cost ofthese instruments can be easily compensated with reduction ofinvestment cost of solar collector area.

6. Conclusion

The multi-stage HD process for desalination was introduced.Theoretical results show that important parameters of the processsuch as specific energy consumption, productivity and daily productionper solar collector area improved when multi-stage process was usedinstead of single-stage process. But, this improvement is negligiblewhen the number of stages ismore than two. According to constructioncost, a two-stage unit seems to be the most suitable choice. Hence, atwo-stage pilot plant was designed and constructed in an arid areawith 80m2 solar collector. This unit was tested on cold and hot days.In summer, the total production of the unit is more than twice that ofwinter production and reached about 580 l/day. According to theseresults, fresh water production can reach 7.25 (l/day·m2) that wasabout 40% higher than single-stage unit tested in the previous study[10]. This led to reduction of required solar collectors and thus,reduction in investment cost. Also, productivity can reach 4.8% that isat least 20% higher than the rate obtained in the single-stage unit [10].These results confirm the theoretical calculation and the benefits ofthe two-stage process compared with the single-stage process.Moreover, quality control of inlet and outlet water shows the ability ofthis process in desalination of brackish water in arid area to producepotable water.

Acknowledgment

The authors acknowledge the financial support provided for thiswork by the Rural Water & Wastewater Company of Qom province.

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