solar stills made with tubes for sea water desalting

Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society

Mario Realia*, Giovanni Modicab aV.G.B. Angioletti 5, 20151 Milano, Italy

Tel. +39-02-4521488; email: [email protected] bDipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano,

Piazza Leonardo da Vinci 32, 20133 Milano, Italy

Received 17 December 2006; accepted 3 February 2007

Abstract

The report concerns basic technological features of simple solar stills utilizing tubes for sea water desalting.The evaporation section comprises horizontal transparent thin-walled plastic or glass tubes, of ~0.10–0.25 minner diameter, half-filled with sea water which absorbs solar radiation. The condensation section is physicallyseparated from the evaporation section, in a shaded space below it, and comprises horizontal plastic or metal tubesof ~0.01 m inner diameter. The wall thickness of condenser plastic tubes is rather small, ~50 µm.

Water vapour released by solar radiation in evaporator tubes flows into condenser tubes to be condensed intoproduced fresh water by delivering condensation latent heat to atmospheric air. Heat transfer by air convectionmay be helped by surface winds, often available in coastal areas. Enhanced fresh water productivity is expectedwith respect to conventional solar stills in which sea water evaporation and water vapour condensation occur inone confined space. Technological features of the proposed solar stills are analysed in some detail and specificexperimental work is suggested on prototype solar stills in view of clarifying relevant aspects concerningtransparent and opaque construction materials, assembling procedures, and the role of the various operativeparameters vis-à-vis energy efficiency and fresh water productivity.

Keywords: Solar still; Glass/plastic/metal tubes; Plastic welding technology

1. Introduction

Fresh water can be separated from sea waterthrough different technologies which requiresuitable amounts of energy inputs in the formof mechanical, electric, and thermal energy[1,2]. Complex technologies based on advanced

industrial know-hows characterise sea waterdesalting plants of large fresh water productioncapacity (hundreds or thousands of m3/d), butfor very small fresh water productions (tens of l/d),the relatively simple technologies of solar stills(which have been in use since 1872 when a4000 m2 solar still was built in Chile to be oper-ated for many years) are of special interest.

*Corresponding author.

Solar stills made with tubes for sea water desalting

Desalination 220 (2008) 626–632

and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007.

doi:10.1016/j.desal.2007.02.0610011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.

M. Reali, G. Modica / Desalination 220 (2008) 626–632 627

The operation of the solar still is illustratedin Fig. 1. In a blackened tray covered with asloping transparent glass panel and filled withsea water, the absorption of solar radiationreleases water vapour which rises to the glassroof where it condenses and flows down as aliquid film into a collecting trough from where itis recovered for use.

Solar stills have two important features: tech-nological simplicity and exploitation of a freeheat source such as the sun. However, their pro-ductivity (a few l/(m2 d)) is relatively small and,throughout their operational history, variousattempts, even via relatively complex structures,have been made to enhance their performance[1–7].

Simplicity of design and construction is anuseful feature of any contraption which is toimprove living conditions. In the case of solarstills such a feature would be most valuable sincefresh water is essential for sustaining human/animal life.

This report is focused onto solar still technol-ogy development and analyses solar stills whichcan be simply constructed with tubes of suitablematerials and are expected to have enhancedfresh water productivity. The solar still evapora-tion and condensation sections are physicallyseparated and comprise, respectively, horizontaltransparent tubes of ~0.10–0.25 m inner diameterand horizontal tubes of ~0.01 m inner diameter.The evaporator tubes, blackened in their lower

half, are contained in an insulation tray on a table,and the condenser tubes are affixed on a supportnet underneath as schematically illustrated inFig. 2. Water vapour is released in horizontaltransparent tubes half filled with sea water andis condensed in small-diameter horizontal tubesin shaded space underneath. Insulation tray exter-nal walls (parallel to evaporator tubes) havereduced height to avoid shading sea water. A thintransparent plastic foil, placed ~0.01 m abovethe evaporator tubes, hinders air convection andthus heat loss to the atmosphere. Two very simplepressure regulators operating as one-way valves(one for inflow and the other for outflow) keepthe pressure inside the solar still in equilibriumwith external atmospheric pressure.

The table area is ~1.0 m2 (width ~0.5 m;length ~2.0 m), and the legs are ~1.0 m long.

As construction materials for solar stills,plastic pipes appear to have advantageous fea-tures (are relatively cheap, free from corrosion,easily cut and joined, and do not cause galvaniccorrosion when coupled to other materials), butalso two critical drawbacks (relatively smallthermal conductivities and relatively large coef-ficients of thermal expansion). However, forseveral applications in the desalination sector,heat exchangers utilizing specially designedheat transfer surfaces (plates and tubes) madewith high quality polymeric materials are anindustrial reality [8,9]. Desalting applicationsand manufacture of heat-exchanger thin-walled

Fig.1. Schematic representation of a conventional solarstill for the recovery of fresh water from sea water.

Fig. 2. Schematic representation of a solar still madewith tubes for the recovery of fresh water from sea water.

628 M. Reali, G. Modica / Desalination 220 (2008) 626–632

plastic tubes obtained from thin foils via plasticwelding technology are treated in Refs. [10,11].Relevant information on properties of materialsand on production technologies is available inseveral bibliographic references (e.g. [12,13]).

Water vapour is released in horizontal trans-parent tubes half filled with sea water and iscondensed in small-diameter horizontal tubes inshaded space underneath.

2. Transparent tubes for sea water evaporation

Several materials appear to be suitable forthe construction of solar still tubular evaporatorsand are to be chosen by taking into account theirspecific physical characteristics (transparency tosolar radiation, mechanical, and thermal proper-ties), and critical aspects like economy (pur-chase, manufacturing, and transportation costs),and availability (established commercial links)vis-à-vis the local situation. For the clarificationof all relevant aspects, an overall feasibilitystudy would be required.

2.1. Glass

Glass has been the basic material for thetransparent roofs of conventional solar stills andevaporator tubes made with transparent glassappear to offer a practical technical solution forthe present solar stills. Glass is superior to mostplastics in heat resistance but has, generally,greater cost, weight, and fragility.

2.2. Polycarbonate

Polycarbonate is a relatively high tempera-ture plastic: it can be used up to 150°C.

2.3. Polyester

This polymer is produced either as a fibre(Dacron) or as a thin film (Milar). Foils of Milar

show good mechanical strength and great trans-parency to solar radiation.

2.4. Polyethylene (PE)

Pipes (up to ~1 m diameter) are produced byextrusion from resins [13]. Wall thickness andphysical properties depend on the particularresin used (addition of about 3% carbon blackprovides resistance to ultraviolet light).

2.5. Polyvinyl chloride (PVC)

Tubes of suitable dimensions (20–30 cm dia-meter) are commercially available [13]. Tubularstructures could also be made with foils of PVCof 1 m height and 0.25 or 0.80 mm thickness:this material is very transparent, resists UV radi-ation, and can be affixed by nailing or riveting[14]. Price is 3.00 h/m for the smaller thicknessand 8.00 h/m for the larger thickness.

2.6. Polypropylene (PP)

Pipes with diameters in the range 1–15 cmare available [13].

2.7. Reinforced-thermosetting-resin (RTR)

Glass-reinforced epoxy resin is many timesstronger at room temperature than plastics. Glass-reinforced polyester resin is the most widelyused reinforced-resin system with diameter range5–30 cm [13].

For the sake of definiteness, the solar stillevaporation section is assumed to be made with5 cylindrical transparent tubes having 2.0 mlength, 0.10 m inner diameter, and wall thick-ness of 2.5 mm for glass and 1.0 mm for plastic.The tubes are contained in an insulating tray, arehalf filled with sea water and have their lowerhalf blackened.

In transparent plastic tubes, a thin wall wouldfavour the absorption of solar radiation withoutcritical failure risks: the solar still operates at


ambient pressure with internal pressure practicallyequal to external atmospheric pressure so that anytube wall is maintained almost stress-free.

3. Tubes for water vapour condensation

For making water vapour condenser tubes,transparent and opaque plastic materials and glassmay be assumed as well as metals (particularlythose used in sea water desalting; cupro-nickel,titanium, stainless steel, aluminium alloys) andplastic-coated metals.

For the sake of definiteness, plastic con-denser tubes made with PTFE (polytetrafluoro-ethylene) will be assumed. The condenser sectionwill thus comprise a suitable number Nc of thin-walled PTFE plastic tubes in parallel, of length~1 m and of inner diameter ~0.01 m, connectedto the evaporation section and to the fresh waterstorage tank through suitable inlet and outletdistribution manifolds.

Condenser tube number Nc may be determinedpractically, in first approximation, by assumingthat condenser tubes are equivalent, in conductingheat, to a conventional solar still flat glass coverplaced at 45° on the horizontal plane.

The following equality applies:

As × (0.7)−1 × lg × (dg)−1 = Nc × 2prc × lc

× l c × (dc)−1 (1)

where lg, dg, and rc, lc, lc, l c, and dc, represent,respectively, the thermal conductivity and wallthickness of the glass cover, and the innerradius, length, thermal conductivity, and wallthickness of a condenser tube.

With assumed values: As = 1.0 m2; lg =1.0 W m−1 °C−1 [12]; dg = 2.0 × 10−3 m; rc = 5.0 ×10−3 m; lc = 1.0 m; lc = 0.25 W m−1 °C−1 [11], anddc = 5.0 × 10−5 m, one obtains:

Nc = (0.7)−1 × (2.0)−1 × 103 × (2p)−1 × (5.0)−1

× 103 × (0.25)−1 × 5.0 × 10−5 = 4.5

4. Operational features

Solar still operation is introduced via an over-all simplified thermodynamic analysis whichappears adequate for theoretical/experimentalevaluations of basic design parameters. • Solar still operates at essentially constant

pressure, the local atmospheric pressure dueto water vapour and air.

Inside evaporator tubes: pev + peg = pa (2)

• Air is displaced from (or into) evaporatortubes while the evaporation rate of watervapour is increasing (or decreasing).

• Water vapour partial pressure pev may be rep-resented, for a specified 24 h time interval,with a simple curve exhibiting one definitemaximum.

• The solar radiation flux density I, availableon a horizontal surface at the solar still site,is assumed to vary sinusoidally from sunriseto sunset [3,15].

• The variability of the solar radiation fluxdensity induces a related variable evapora-tion rate of sea water:

yev = h × I × As × (Λes)−1 (3)

where h is a time dependent operative param-eter and Λes is the time (via temperature)dependent latent heat of evaporation of seawater (almost identical to that of fresh water).

• The condensation rate of water vapour (i.e.,the fresh water production rate) equals theevaporation rate of sea water:

ycd = yev (4)

• Water vapour is assumed to be saturatedvapour in equilibrium with pure water and tocondense without any external loss in con-denser tubes of length lc. Any temperaturedecrease (due to frictional pressure drop) ofwater vapour flowing in condenser tubes is


neglected, and the following equality isassumed:

Tcv(x) = Tev (5)

where x represents the axial coordinate (0 ≤x ≤ lc).

• In view of (5), latent heat of condensationLcv, viscosity mcv, and density rcv of watervapour inside condenser tubes are assumedto have constant values independent of axialposition at any time instant.

• Water vapour flow is assumed to be laminarwith maximum velocity at inlet and nullvelocity at outlet of any condenser tube.

• For practical evaluations, water vapour inletvelocity ucv is assumed to obey the followingequation:

yev = Nc × rcv × p (rc)2 × ucv (6)

• Condenser tubes are assumed to be equivalentamong themselves as fresh water distillers.

• The condensation of water vapour is drivenby the thermodynamic disequilibrium relatedwith the difference, Tcv – Ta, between thetemperature of water vapour in condensertubes and atmospheric temperature:

ycd × Λcv = ac,0 × Nc × 2prc × lc × (Tcv – Ta) (7)

where ac,0 is a time dependent operativeparameter representing an overall-heat-trans-fer coefficient.

• For a condenser tube having a very smallwall thickness dc (dc/rc << 1), ac,0 may be rep-resented as follows:

(8)

where ac,1, and ac,2 are heat transfer coeffi-cients for, respectively, water vapour con-densation inside condenser tube, and externalair (free) convection.

Suitable average values of ac,0 for anyprototype solar still may be obtained via direct

experimentation. Theoretical analyses of heattransfer processes are relatively complex butuseful information may be obtained via simi-larity theory (see, e.g. [16–18]).

• The mass of daily produced fresh water, Mcd,is proportional to the solar radiationabsorbed:

Mcd = dt = z × Ω × As × (Les* )−1 (9)

Ω = (10)

where the integrations concern a 24 h timeinterval (a = 0 s ≤ t ≤ b = 86,400 s), coeffi-cient z represents the daily energy efficiencyof the solar still, and (Λes

* ) is the latent heatof vaporization of sea water at the highestdaily temperature.

Efficiency z can be easily evaluated viaexperimental field work with solar still pro-totypes equipped with suitable instruments(in particular, thermometers and solar radia-tion meters).

5. Experimental work

For the technological development of theproposed solar stills a comprehensive experi-mental work appears necessary. Researches ofspecific interest concern: • Construction materials and assembling pro-

cedures. • Field tests with prototype solar stills to ana-

lyse daily fresh water productivity vis-à-visrecorded values of operative parameters.

• Experimental tests and theoretical analysesfor achieving suitable design-oriented meanheat transfer coefficients.

The use of plastic materials should helpreduce construction and maintenance costs andthus make the present solar stills attractive devices

1 1 1

0 1 2a adl ac c

c

c c, , ,

= + +

ycd

a

b

∫

I ta

b

∫ d


to be used in coastal areas. Also, evaporator andcondenser tubes made from thin polymeric filmsmay present an economic advantage vis-à-visthe use of ready made plastic tubes and mayoffer a practical choice for people interested inmaking solar stills by themselves. Condensertubes made with metal or plastic-coated metalmay also be considered for solar stills operatingin selected environments.

Field tests appear necessary to check thepractical validity of the theoretical simplifica-tions introduced and to specify the actual rolesof the various operative parameters: evaporationtemperature Te, condensation temperature Tc,solar radiation flux density on horizontal surfaceI, atmospheric temperature Ta, evaporating seawater surface area As, mean surface area of con-denser tubes Ac, atmospheric specific humidityca, wind horizontal velocity magnitude and ori-entation uw and jw, water vapour pressures inevaporation and condensation sections pev andpcv, and daily produced fresh water mass Mcd.

Carefully designed field tests should providesuitable mean values of coefficient ac,0 for over-all heat transfer between condensing watervapour and atmospheric air (in Eq. (7)), and thusallow the design of efficient solar stills in vari-ous environments.

Nomenclature

A area, m2 I solar radiation flux density on horizon-

tal surface, kW m−2 l length, m M mass, kg N number p pressure, kPa T temperature, °C u velocity, m s−1

Greek symbols

a heat transfer coefficient, W m−2 °C−1

d wall thickness, m z solar still energy efficiency h operative parameter l thermal conductivity, W m−1 °C−1 Λ latent heat of evaporation/condensa-

tion, kJ (kg)−1 m dynamic viscosity, kg m−1 s−1 r density, kg m−3 j reference angle in horizontal plane c specific humidity (ratio between water

vapour mass and air mass in specifiedvolume)

y mass flow rate, kg s−1 Ω 24 h integral of solar radiation flux

density on horizontal surface, kJ m−2

Subscripts

a atmospheric c condensation, condenser tube d fresh (distilled) water e evaporation, evaporator tube g air, glass s sea water v water vapour w wind 0 overall 1 water vapour condensation 2 air (free) convection

References

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[12] Handbook of Chemistry and Physics, The Chem-ical Rubber Publishing Co., Cleveland, OH,1961.

[13] R.H. Perry and D.W. Green, Perry’s ChemicalEngineers’ Handbook, 7th edn., McGraw-Hill,New York, 1997.

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solar stills made with tubes for sea water desalting

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