Desalination 230 (2008) 281–287

Performance study on an acrylic mirror boosted solar distillationunit utilizing seawater

S. Shanmugan*, P. Rajamohan, D. MutharasuDepartment of Physics, Thiagarajar College of Engineering, Madurai, 625 015 Tamil Nadu, India

Tel. +91 (452) 248-2240; Fax: +91 (452) 248-3427; email: shanmugan@tce.edu

Received 30 March 2006; Accepted 5 December 2007

Abstract

A solar still is a simple desalination unit, but typical configuration is a sealed box with an angled glass top.Sunlight shining into the box heats water or liquid, causing it to evaporate. The moisture condenses on the relativelycool glass cover and runs down the sloped surface for collection. In this paper, the performance of solar still in termsof collection of distilled water have been analyzed and a booster mirror (acrylic) is attached with just above the glasscover of solar still, which will reflect solar radiation in excess to water and it is possible to adjust the booster mirrorfor perfect reflection depending upon the sun moving angle. Low rates of distillation have been observed with theexisting unit. A notable result has been observed with a boosted distillation unit (4.2 L/m2/d at 890 W/m2 max.). Thearrangements have been made by commercial Al sheet material and insulated with a thermocol sheet.

Keywords: Solar still; Acrylic mirror booster; Distilled water

1. Introduction

Water is critical for life and for livelihoods.Water is also a problem for women and childrenbecause they bear the burden of collecting water.In some places, women have to walk nearly10 km to reach a water source. According to theUnited Nations, every day 4,400 children underthe age of 5 die around the world, having fallensick because of poor-quality water. A third of the

*Corresponding author.

world’s population is enduring some form ofwater scarcity. Water scarcity affects some partsof the world more than others. Today, 800 millionpeople live under a threshold of “water stress.”As rivers dry up, lakes shrink and groundwaterreserves are depleted; that figure will rise to3 billion in 2025, especially in parts of Asia andAfrica. There is an urgent need to reduce wasteand invest in infrastructure to “harvest” rainwateror increase storage [1]. Seawater desalination isone of the possible solutions to the severe water

0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.doi:10.1016/j.desal.2007.12.004

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shortage problem our planet is experiencingduring the first half of this century [2], a problemthat is not exclusive to developing countries, asthe appearance of seasonal episodes of persistentdrought in certain regions of the so-calleddeveloped countries is becoming more and morefrequent.

Lack of clean drinking water kills more chil-dren than anything else. Recently inexpensivesolar powered distillation units or stills andpasteurization ponds have been developed toprovide people with fresh water [3]. The simplesolar still of the basin type is the oldest methodand improvements in its design have been madeto increase its efficiency [3,4]. Conventionaldesalination is a phase separation method where-by saline water is heated to produce water vapor,which is then condensed to produce freshwater.The heat source may be like LPG, diesel, wood,and other conventional fuels, etc.

The energy required to evaporate water is thelatent heat of vaporization of water. This has avalue of 2260 kJ/kg. This means that to produce1 L (i.e. 1 kg since the density of water is 1 kg/L)of pure water by distilling brackish water requiresa heat input of 2260 kJ. This does not allow forthe efficiency of the heating method, which willbe less than 100%, or for any recovery of latentheat that is rejected when the water vapor iscondensed [5].

A solar still is a device to desalinate impurewater like brackish or saline water. It is a simpledevice to get potable/fresh distilled water fromimpure water, using solar energy as fuel, for itsvarious applications in domestic, industrial andacademic sectors. It is easy to operate and alsomeans that as there is no messy cleaning or filtersreplacement procedures. It has less maintenance.

Rahim highlights some drawbacks that exist inthe evaporation and condensing zones of thehorizontal solar desalination stills, and introducesnew techniques developed at the University ofBahrain to improve the efficiency of both eva-porating and condensing zones and suggesting a

new cheap method of storing excess solar energyduring the day, for the continuation of the processat night [6].

This paper describes how to build an acrylicmirror boosted solar still, and its performance inseawater desalination is reported here.

2. Materials and method

2.1. Instruments and apparatus

Distillation units routinely use designs thatconserve as much thermal energy as possible byinterchanging the heat of condensation and heatof vaporization within the units. Generally, thesolar still unit is fabricated by using fiber ormetals like iron or aluminium. We have built thesolar still with an Al sheet and a thermocol sheet(insulation material).

The still consists of several parts, which arefitted on top of each other and finally coveredwith a glass cover. The first part is an externalcontainer made from galvanized iron painted bygray color, then packed with a layer of thermocol,which functions as insulation for the containerbottom. A shallow aluminum tray is fitted insidethe container. A hollow space of 2.5 cm betweenthe aluminium and galvanized iron container wasmaintained in all directions. This is tightly packedby thermocol to avoid the heat loss by four sides.

The basic materials needed for the manufac-ture of an acrylic boosted mirror solar distillationis a 16-gauge aluminium sheet for the inner andouter box, 20-gauge G.I. sheet to support thepolyacrylic sheet, 5-mm-thick transparent glass asthe top cover, 3-mm-thick polyacrylic sheet as thereflecting surface (for mirror boosting), siliconesealant to provide the leak proof, PVC pipe forboth collection of distilled water and to make afeed to the solar still, hardware and miscellaneousmaterials to manufacture to the exact dimensions.The inner bottom surface of the solar still wascoated with commercially available black boardpaint. The still we built had a 1 m2 collection area

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Fig. 1. View of solar distillation unit with mirror booster.

Table 1Technical specifications of the solar still

Sl.no.

Specification Dimensions

1. Basin area (m2) 1.00×1.002. Glass area (considering all four

sides insulation thickness (m2)1.12×1.08

3. Transparent glass thickness (mm) 54. Insulation in all five sides (mm) 405. Boosted mirror, length × breath

(mm)1000×1000

6. Thickness of the boosted mirror(mm)

3

7. Depth at north side (mm) 3008. Depth in south side (i.e.,

condensate collection area) (mm)100

9. Inclination angle of the glasscover (E)

11.30

(reference area is the black surface of the still)which is 1 m wide, 1 m long and 30 cm deep atthe north side and 10 cm deep at the condensatecollection side. The transparent glass cover wasfixed by 11.30E as the inclination angle. Speci-fications are given in Table 1. The solar distilla-

tion unit with an acrylic mirror booster is shownin Fig. 1.

The polyacrylic sheet (mirror polished) isfixed in a metal frame by bolts and nuts. Metalscrew arrangement help to move the metal framein an angle (180º). It focuses solar radiation to theclean glass cover area. Any leakage around thepipe, water condensation trough and underneaththe black coated inner aluminium surface couldbe arrested by using a non-toxic silicone sealant.The appropriate fitting is glued to the protrudingend of the inflow tube to attach the garden hose,or whatever tubing used to deliver the untreatedwater or saline water.

2.2. Daily still performanceThe clean glass cover is placed on top of the

container and sealed to the external container bymeans of a rubber gasket followed by an non-toxic silicone sealant all around its edges. Itensure to arrest the air leakage (and vapor withthe air as well) by air gaps. In this way the still isassembled and ready for operation. Feed has been

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in a continuous process by attaching the hosefrom the seawater container to the inflow tube.Attach the tube from the collecting trough to thepure water container used to collect the distilledwater. The saline water flow rate is adjusted tothe desired value by means of a control valve.

Adjust the inflow water volume so that thewater dribbles down the black poly lining. Oncethe still reaches stagnation temperature, adjust thevolume so that the water never quite reaches thelower end of the still. As the water evaporatesaway from the hot aluminium sheet, you will seedroplets form on the underside of the glasssurface. As gravity pulls these drops down to thetrough, check to make sure the distilled water canflow unimpeded out of the trough, through thedischarge tube, and into the collecting and storagecontainer. The still may have to be tilted about 1Eto the discharge side so that the trough does notoverflow.

Experiments were carried out on clear days ata shadow free location covering the month ofApril 2002. The temperatures of the saline waterin the basin, vapor space and ambient air aremeasured and recorded at 15-min intervals.Thermocouples (K type) are used to measuretemperatures and data logger (Dynalab Model:WDL 1002) record temperatures for 24 h atintervals of 1 min. The data are then handled ona PC. The distilled water is collected by a gradu-ated test tube each hour. Seven hours of operationper day were accountable in the performancestudy. The incident global radiation (I0) on theglass cover area is recorded by the same datalogger.

The overall efficiency [7,8] of the solardesalination unit is determined by the ratio ofproduct condensate to the theoretical amount ofsalt water vaporized. In terms of daily perfor-mance, this efficiency can be expressed as:

Table 2Performance of solar still without mirror boosting

Sl. no.

Date Solar radiation I0 (W/m2)

Vapor spacetemp. (ºC)

Saline watertemp. (ºC)

Atm. temp.(ºC)

Distilled watercollection Fp (ml)

Efficiency(%)

1 01.04.02 840 56 52 30 3050 35.072 02.04.02 845 58 51 31 3050 34.863 04.04.02 810 54 50 29 2800 33.384 05.04.02 830 55 50 31 2900 33.755 06.04.02 860 60 53 30 3150 35.386 07.04.02 860 61 52 30 3200 35.947 12.04.02 840 57 51 29 3075 35.368 13.04.02 850 58 52 30 3100 35.239 16.04.02 885 62 52 31 3300 36.0210 18.04.02 865 59 53 30 3150 35.1711 19.04.02 840 56 50 30 3100 35.6512 20.04.02 820 54 49 29 2900 34.1613 24.04.02 890 63 56 32 3400 36.914 28.04.02 825 54 51 31 2950 34.5415 29.04.02 838 55 52 30 3000 34.58

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Table 3Performance of solar still with mirror boosting

Sl.no.

Date Solar radiation I0 (W/m2)

Vapor spacetemp. (ºC)

Saline watertemp. (ºC)

Atm. temp.(ºC)

Distilled watercollection Fp (ml)

Efficiency(%)

1 01.04.02 840 58 54 30 3800 43.692 02.04.02 845 61 53 31 3850 44.003 04.04.02 810 56 52 29 3600 42.934 05.04.02 830 58 53 31 3750 43.645 06.04.02 860 63 55 30 3950 44.366 07.04.02 860 63 55 30 3900 43.807 12.04.02 840 61 53 29 3800 43.698 13.04.02 850 62 55 30 3850 43.759 16.04.02 885 68 54 31 4100 44.7510 18.04.02 865 63 56 30 4000 44.6611 19.04.02 840 59 53 30 3900 44.8412 20.04.02 820 56 52 29 3800 44.7613 24.04.02 890 69 60 32 4200 45.5814 28.04.02 825 56 53 31 3800 44.4915 29.04.02 838 58 55 30 3850 44.37

0Efficiency % = 100vQ p

IΦ

×

where Qv = 2.434×103 kJ/kg, the energy requiredto evaporate 1 kg of brackish water about 40°C;Фp is the still productivity in L/m2/d; I0 the totalradiation in kJ/m2/d which was taken from thedata logger and found that the average efficiencyfor the free condensing without using the boostedmirror was 35.06%. By introducing the acrylicmirror boosted facility, the average efficiencyincreased to 44.22%. The efficiency of the solarstill as reported in literature [9] was 30.56%.

3. Results and discussion

The performance study of the designed solarstill without and with a booster mirror is reportedin Tables 2 and 3. The readings have been tabu-lated and comparative values for understanding

the efficiency of the system have been presentedhere. It is found that the acrylic mirror boostedsolar distillation unit produces good results interms of the yield (distilled water collection),efficiency etc. The results shows good perfor-mance in seawater conversion and quite higheroutput than reported value in the literature [10].

The system shows the efficiencies between35% and 45%. The solar stills installed in thedifferent parts of the world have efficiencies ofthe order of 30–40% [11]. But there are someproblems with insulation, leakage, water collec-tion channels, etc. Further we have to try toimprove reducing losses and related problems.The performance study is being continued toidentify and rectify these types of losses. Figs. 2and 3 show the temperature change within thesolar distillation unit for both without and withthe boosted mirror system. The comparative per-formances of both systems are distinguished in

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Fig. 2. Temperature analysis inside the still withoutmirror booster.

Fig. 4. Comparative efficiency of the solar still with andwithout mirror booster.

Fig. 4. A comparison of outlet from both with andwithout mirror boosted is explained in Fig. 5

4. Conclusions

The performance of an acrylic mirror boostedsolar distillation unit has been studied andreported. The amount of water collected per dayis a maximum of about 4.2 L (maximumachieved). When comparing the solar distillationunit without mirror boosted, it has showed goodperformance and 20 to 26% increased efficiency.This unit may be used in making jaggary

Fig. 3. Temperature analysis inside the still with mirrorbooster.

Fig. 5. Comparison of outlet from both with and withoutmirror booster solar distillation unit.

(unrefined brown sugar made from palm sap),and other purposes which involve a distillationpro-cess. In the future it will become a greatappli-cation in all desert areas. It could be usedfor converting saline water to pure drinking waterin rural areas of India.

5. Symbols

I0 — Total radiation, kJ/m2/dQv — Enthalpy of evaporation, kJ/kgФp — Still productivity, L/m2/d

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