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- ........TI OLOGIEs FOR DEVELOPMENT

UNDERSTANDINGSOLAR STILLS

byJoel GordesHorace McCracken

Technical Reviewers:Daniel DunhamJacques Le NormandDarrell G. Phippen

TP# 37:9/85 8 VA technicbl paper

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UNDERSTANDINGSOLAR STILLS

byJoel GordesHorace McCracken

Technical RevieweLs:Daniel DunhamJacques Le NormandDarrell G. Phippen

Published by:Volunteers in Technical Assistance (VITA)1815 North Lynn Street, Suite 200Arlington, Virginia 22209 USATelephone 703/276-1800Cable VITAINCTelex 440192 VITAUI

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PREFACE

This paper is one of a series published by Volunteers in Technical Assistance to provide an introduction to specific state-ofthe-art technologies of interest to people in developing countries. The papers are intended to be used as guidelines to helppeople choose technologies that are suitable to their situations.They are not intended to provide construction or implementationdetails. People are urged to contact VITA or a similar organization for further information and technical assistance if theyfind that a particular technology seems to meet their needs.The papers in the series were written, reviewed, and illustratedalmost entirely by VITA Volunteer technical experts on a purelyvoluntary basis. Some 500 volunteers were involved in the production of the first 100 titles issued, contributing approximately5,000 hours of their time. VITA staff included Maria Giannuzzias editor, Suzanne Brooks handling typesetting and layout, andMargaret Crouch as project manager.The author of this paper, VITA Volunteer Horace McCracken, is thepresident of the McCracken Solar Company in Alturas, California.The co-authcr, VITA Volunteer Joel Gordes, is currently the solardesign analyst for the State of Connecticut's Solar MortgageSubsidy Program. The reviewers are also VITA volunteers. DanielDunham has done consulting in solar and alternative sources ofenergy for VITA and AID. He has lived and worked in India, Pakistan, an6 Morocco. Mr. Dunham has also prepared a state-of-theart survey on solar stills for AID. Jacques Le Normand is Assistant Director at the Brace Research Institute, Quebec, Canada,which does research in renewable energy. He has supervised workwith solar collectors and has written several publiations onsolar and wind energy, and conservation. Darrell G. Phippen is amechanical engineer and development specialist who works withFood for the Hungry in Scottsdale, Arizona.VITA is a private, nonprofit organization that supports peopleworking on technical problems in developing countries. VITA offers information and assistance aimed at helping individuals andgroups to select and implement technologies appropriate to theirsituations. VITA maintains an international Inquiry Service, aspecialized documentation center, and a computerized roster ofvolunteer technical consultants; manages long-term field projects; and publishes a variety of technical manuals and papers.For more information about VITA services in general, or Thetechnology presented in this paper, contact VITA at 1815 NorthLynn Street, Suite 200, Arlington, Virginia 22209 USA.

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UNDERSTANDING SOLAR STILLSby VITA Volunteers Horace McCracken and Joel Gordes

I. INTRODUCTIONNinety-seven percent of the earth's water mass lies in itsceans. Of the remaining 3 percent, 5/6 is brackish, leaving aere .5 percent as fresh water. As a result, many people do notave access to adequate and inexpensive supplies of potablewater. This leads to population concentration around existingater supplies, marginal health conditions, and a generally lowstandard of living.Solar distillation uses the heat of the sun directly in a simplepiece of equipment to purify water. The equipment, commonlyalled a solar still, consists primarily of a shallow basin withLransparent glass cover. The sun heats the water in the basin,ausing evaporation. Moisture rises, condenses on the cover anduns down into a collection trough, leaving behind the salts,minerals, and most other impurities, including germs.Although it can be rather expensive to build a solar stillis both effective and long-lasting, it can thatproduce purified watert a reasonable cost if it is built, operated, and maintainedproperly.This paper focuses mainly on small-scale basin-type solar stillss suppliers of potable water for families and other small users.Of all the solar still designs developed thus far, the basin-typecontinues to be the most economical.HISTORY OF SOLAR DISTILLATIONDistillation has long been considered a way of making salt waterrinkable and purifying water in remote locations. As early ashe fourth century B.C., Aristotle described a method tovaporate impure water and then condense it for potable use.P.I. Cooper, in his efforts to document the development and usef solar stills, reports that Arabian alchemists were thearliest known people to use solar distillation to produceotable water in the sixteenth century. But the first documentedreference for device was made in 1742 by Nicolo Ghezzi oftaly, although it is not known whether he went beyond theconceptual stage and actually built it.

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The first modern solar still was built in Las Salinas, Chile, in1872, by Charles Wilson. It consisted of 64 water basins (aotal of 4,459 square meters) made of blackened wood with slopingglass covers. This installation was used to supply water (20,000iters per day) to animals working mining operations. After thisarea was opened to the outside by railroad, the installation wasallowed to deteriorate but was still in operation as late as912--40 years after its initial construction. This design hasformed the basis for the majority of stills built since thattime.During the 1950s, interest in solar distillation was revived, andin virtually all cases, the objective was to develop large centralized distillation plants. In California, the goal was toevelop plants capable of producing 1 million gallons, or 3,775cubic meters of water per day. However,researchers after about 10 years,around the world concluded that large solar distillation plants were much too expensive to compete with fuel-firedones. So research shifted to smaller solar distillation plants.In the 1960s and 1970s, 38 plants were built in 14 countries,with capacities ranging from a few hundred to around 30,000liters of water per day. Of these, about one third have sincebeen dismantled or abandoned due to materials failures. None inhis size range are reported to have been built in the last 7years.Despite the growing discouragement over community-size plants,McCracken Solar Company in California continued its effortsmarket solar stills for toresidential use. Worldwide interestsmall residential units is growing, and now inthat the price of oils ten times what it was in the 1960s, interest in the largerunits may be revived.Although solar distillation at present cannot compete with oilfired desalination in large central plants, it will surely becomea viable technology within the next 100 years, when oil supplieswill have approached exhaustion. When that day arrives, theprimary question will be, "Which method of solar distillation isest?" Meanwhile, almost anyone hauling drinking water anydistance would be economically better off using a solar still.

NEEDS SERVED BY SOLAR DISTILLATIONSolar distillation could benefit developing countries in severalways:

o Solar distillation can be a cost-effective means ofproviding clean water for drinking, cooking, washing,and bathing--four basic human needs.

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o It can improve health standards by removing impuritiesfrom questionable water supplies.o It can help extend the usage of existing fresh water inocations where the quality or guantity of supplyeteriorating. isWhere sea water is available, it caneduce a developing country's dependence on rainfall.o Solar stills, operating on sea or brackish water, canensure supplies of water during a time of drought.o Solar distillation generally uses less energy to purifywater than other methods.o It can foster cottage industries, animal husbandry, orydroponics for food production in areas where suchctivities are now limited by inadequate supplies ofpure water. Fishing could become important on desertseacoasts where no drinking water is available forfishermen.o Solar distillation will permit settlement in sparselypopulated locations, thus relieving populationpressures in urban areas.

APPLICATIONSThe energy from the sun used to distill water is free.cost of But thebuilding a still makes the cost of the distilledather high, waterat least for large-scale usesand such as agricultureflushing away wastes in industry and homes. Consequently,he solar still is used principally to purify water fornd drinkingor some business, industry, laboratory, andapplications. green-houseIt also appears able to purify polluted water.Solar Distilled Water for IrrigationFor field agriculture, the solar still is not very promising.takes about one Itmeter depth of irrigation water per year toroduce crops in dry climates, whereas the solar still can evapoate about two meters' depth. Thus, one square meter of solartill would irrigate two square meters of land. Unquestionably,the cost of building the still would make water more valuablehan the crops being produced. This may not be true, howevtor,for agriculture in controlled environments, i.e., greenhouses.ell-designed hydroponically-operated greenhouse should be Aableo produce 8 to 10 times as much food, per unit volume of waterconsumed, as field crops.

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Recovery of Salt from a Solar StillSince salt is a very cheap industrial material, and a solar stillannot produce anymore than an open pond, combining the recoveryf salt With the distilling of water iseconomically. not attractiveWhere a family is using a solar still toater valued at provide$1 per day, the amount of salt they need mightcost them half a cent.Recovery of Potable Water from SewageAlthough it seems possible that potable water canrom sewage, be recoveredf contaminants such as odorous gases are present inewage water fed to the still, some portion of those gases willvaporate and condense with the distilled water.robability they could be filtered out with ;:ctivated carbon, buto date, however, no one

In allhas had any experience with this.

Alcohol ProductionIf the "contaminant" is alcohol,water. it can be separated from theut it would take two orto three passes through the stillttain a high enough concentration of alcohol to be used as auel. Considering the current availability ofroducing alcohol in this way is not yet fossil fuels,economical. However,hen fossil fuel supplies run low and thedistillation could play a significant role. price rises, solar

Recovery of Distilled Water From Polluted Water BodiesWhether or not solar distillation can actually purify pollutedater is not yet known. Laboratory tests have shown, however,hat a solar still can eliminate bacteria.research, If after additionala quantity of clean water canpolluted water, this be recovered fromcapability may become economicallyimportant morehan the purification of sea water. It may also besed to reamove toxic substances such as pesticides.Preliminary laboratory tests show that a modified version of thetill--now commercially available--can doremoving a very good job ofuch substances from feed water. Trichloroethylene(TCE), for example, has been removed by a factor of 5,000 to 1;thylene dibromide (EDB) by 100 to 1; nitrates by 50 to 1; andthers within those ranges. Of course, more work must be done touantify these numbers, not to mention the unending listchemicals that need to be tested. of

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Elimination of Algae. While algae will grow in some deep basintills where the water temperature seldom gets very high, in thehallow basin still it is usually killed by the high temperature.II. OPERATING PRINCIPLESGENERAL THEORY OF SOLAR DISTILLATIONDistillation operates by the escape of moving molecules fromater surface into the gases above it. theSensible heat--the kindou can measure with a thermometer--is caused by the movement ofolecules, zig-zagging about constantly, except that they arell moving at the same speed. Add energy and move notand the fastest-moving they faster,ones may escape the surface to becomevapor.It takes a lot of energy for water to vaporize. While a certainmount of energy is needed to raise the temperature of a kilogramf water from 00 to 1000 Celcius (C),times it takes five and one-halfthat much to change it from water at 100'C to water vaport 100 0 C. Practically all this energy, however, is given backwhen the water vapor condenses.The salts and minerals do not evaporate along withOrdinary table salt, the water.for example, does not turn into vapor untilt gets over 1400'C, so it remains in the brine when thevaporates. waterhis is the way we get fresh water in therom the oceans, cloudsby solar distillation. All the fresh water onearth has been solar distilled.It is not necessary for the water to actually boil to bring aboutistillation. Steaming it away gently does theoiling, except that in the solar still, same job asit will usually turn outven more pure, because during boiling the breaking bubblesontaminate maythe product water with tiny droplets of liquid waterswept along with the vapor.THE SOLAR DISTILLATION PROCESSThe solar distillation process is shown in Figureenergy passing through a glass 1. Solarcoverwater in a pan; heats up the brine or seathen this causes the water to vaporize. The vaporises and condenses on the underside of the cover and runsdown into distillate troughs.

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Figure 1. Solar Distillation Process

\.AIATO 3CONIDEN5 ING WATER1RAIDIATION\ \\

fresh Water from the Sun, by Daniel C.Dunham, (Washington, D.C., August 1978),p. 16.

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A more technical description follows.:1. The sun's energy in the form of short electromagneticwaves passes through a clear glazing surface such asgl'ass. Upon striking a darkened surface, this lightchanges wavelength, becoming long waves of heat whichis added to the water in a shallow basin below theglazing. As the water heats up, it begins to evaporate.2. The warmed vapor rises to a cooler area. Almost allimpurities are left behind in the basin.3. The vapor condenses onto the underside of the coolerglazing and accumulates into water droplets or sheetsof water.4. The combination of gravity and the tilted glazingsurface allows the water to run down the covera collection trough, and intowhere it is channeled intostorage.

In most units, less than half the calories of radiantfalling on the still energyare used for the heat of vaporization necessary to produce the distilled water. A commercial stills aresold to date have had an efficiency range of 30 to(The 45 percent.maximum efficiency is just over 60 percent.) Efficiency iscalculated in the following manner:

Energy required for the vaporizationof the distillate that is recoveredEfficiency = Energy in the sun's radiationthat falls on the still.

Providing the costs don't rise significantly, an efficiencyincrease of a few percent is worth working for. Improvements arerincipally to be sought in materials and methods of construction.

III. SOLAR STILL DESIGN VARIATIONSAlthough there are many designs for solar stills, and fourgeneral categories, (concentrating collector stills; multipleray tilted stills; tilted wick solar stills; and basin stills)percent of all functioning stills are of the basin type.

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CONCENTRATING COLLECTOR STILLA concentrating collector still, as shown in Figure 2, usesparabolic mirrors to focus sunlight onto an enclosed evaporationessel. This concentrated sunlight provides extremely highemperatures which are used to evaporate the contaminated water.he vapor is transported to a separate chambercondenses, and is transported to storage. where itThis type of still isapable of producing from .5 to .6 gallons per day per squarefoot of reflector area. This type of output far surpasses otherypes of stills on a per square foot basis. Despite this still'sutstanding performance, it has many drawbacks; includinghigh thecost of building and maintaining it, the need for strong,direct sunlight, and its fragile nature.

/

Figure 2. A Concentrating Collector StillSource: Jim Leckie, Gil Masters, Harry Whitehouse,and Lily Young, More Other Homes andr&rha-, (San Francisco, California:Sierra Club Books, 1981), p. 305.

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MULTIPLE TRAY TILTED STILLA multiple tray tilted still (Figure 3), consists of a series ofshallow horizontal black trays enclosed in an insulated containerwith a transparent top glazing cover. The vapor condenses ontothe cover and flows down to the collection channel for eventualstorage.This sEtill can be used in higher latitudes becabse the whole unitcan be tilted to allow the sun's rays to strike perpendicular tothe glazing surface. The tilt feature, however, is less importantat and near the equator where there is less change in the sun'sposition over the still. Even though efficiencies of up to 50percent have been measured, the practicality of this designremains doubtful due to:

o the complicau.J nature of construction involving manycomponents;o increased cost for multiple trays and mounting requi rements. ,A.a

'-A :oig0tuwc&amei

*aie

Figure 3. A Multiple Tray 'ilted StillSource: Jim Leckie, Gil Masters, Harry Whitehouse,

and Lily Young, More Other Homes andr (San Francisco, California:Sierra Club Books, 1981), p. 304.

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TILTED WICK SOLAR STILLA tilted wick solar still draws upon the capillary actionfibers to distribute feed water over the entire surface ofof thewick in a thin layer. The water is then exposed to sunlight.(See Figure 4.)

Pv* saMaM

Figure 4. A Tilted Wick Solar Still

Source: Jim Leckie, Gil Masters, Harry Whitehouse,and Lily Young, More Other Homes andi(San Francisco, California:Sierra Club Books, 1981), p. 304.

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A tilted wick solar still allows a higher temperature to form onhis thin layer than can be expected fro, a larger body of water.system is as efficienthis as the tilted tray design, but itsuse in th e field remains questionable because of:o in'creased costs due to mounting requirements andessential insulation;o the need to frequently clean the cloth wick of built-upsediments, highlighting the need 'for an operableglazing cover;o the need to replace the black wick material on aregular basis due to sun bleaching and physicaldeterioration by ultra-violet radiation;o uneven wetting of the wick which will result in dryspots, leading to reduced efficiency; ando the unnecessary aspect of the tilt feature except whereit is required higher latitudes.

BASIN STILLA basin still (See Figure 5), is the most common type in use,although not in current production.While the basic design can take on many variations, the actualshape and concept have not changed substantially from the days ofhe Las Salinas, Chile stills built in 1872. The greatestchanges have involved the use of new building materials, whichay have the potential to lower overall costs while providing ancceptably long useful life and better performance.

Figure 5. A Basin StillSource: Jim Leckie, Gil Masters, Harry Whitehouse,and Lily Young, MQre Other Homes andGQxbu (San Francisco, California: SierraClub Books, 1981), p. 304.

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All basin stills have four major components:1. a basin;2. a support structure;3. a transparent glazing cover; and4. a distillate trough (water channel).

In addition to these, ancillary components may include:I. insulation (usually under the basin);2. sealants;3. piping and valves;4. facilities for storage;5. an external cover to protect the other components fromhe weather and to make the still estheticallypleasing; and6. a reflector to concentrate sunlight.

Physical Dimensions of the Basin StillThe actual dimensions of basin stills vary greatly, dependingthe availability of materials, onwater requirements, ownershippatterns, and land location and availability.If the only glazing available is one meter at its greatestimension, the still's maximum inner width will be just under oneeter. And the length of the still will be set according to whats needed to provide the amount of square meters to produceequired amount of water. theLikewise, if an entire village werewn and touse the still, the total installation would have to bequite large.It is generally best to design an installation with many smallmodular units to supply the water. This allows:

o units to be added;0 manageable components to be handled by unskilledpersons without expensive mechanical equipment;o maintenance can be carried out on some units whileothers continue to operate.

Most community size stills 1/2 to 21/2 meters wide, with lengthsanging up to around 100 meters. Their lengths usually run alongn eastwest axis to maxiinizce the transmission of sunlight throughhe equatorialfacing sloped glass. Residential, appliancetypeunits generally use glass about 0.65 to 0.9 meter widelengths withranging from two to three meters. A water depth of 1.5to 2.5 cm is most common.

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The usual argument for greater depths is that thecan stored heatbe used at night to enhance production when the air temperaures are lower. Unfortunately, no deep basin has ever attainedhe 43 percent efficiency typical of a still of minimum waterepth. Th6 results to date are clear: the shallower the depthhe better. Of course, if the basin is too shallow, it will dryut and salts will be deposited, which is not good. Noteolar heat can evaporate about 0.5 cm thatsummer. of water on a clear day inBy setting the initial charge at about 1.5 cmirtually depth,ll of the salts remain in the solution, and can belushed out by the refilling operation.MATERIAL REQUIREMENTS OF BASIN STILLSThe materials used for this type of still should have the following characteristics:

o Materials should have long life under expo.edconditions or be inexpensive enough to be replaced upondegradation.O They should be sturdy enough to resist wind damage andslight earth movements.o They should be nontoxic and not emit vapors or instillan unpleasant taste to the water under elevated temperatures.o They should be able to resist corrosion from salinewater and distilled water.o They should be of a size and weight that can bec'aveniently packaged, and carried by localtransportation.o They should be easy to handle in the field.

Although local materials should be used whenever possible toower initial costs and to facilitate any necessary repairs, keepn mind that solar stills made with cheap, unsturdy materialsill not last as long as those built with more costly, highquality material. want With this in mind, you must decide whether youo build an inexpensive and thus short-lived still thateeds to be replaced or repaired every few years, or buildomething more durable and lasting in the hope that the distilledater it produces will be cheaper in the long run. Of the lowost stills that have been built around the world, many have teenbandoned. Building a more durable still that will last 20 yearsor more seems to be worth the additional investment.

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Choosing materials for the components in contact with theepresents a serious problem. Many plastics will give wateroff aubstance which can be tasted or smelled in the product water,or periods of anywhere from hours to years. As a general guide,f you are contemplating using any material other than glassetal in contact with water, oryou may perform a usefultest screeningy boiling a sample of the material in cup of good wateror half an hour,it.

then let the water cool, and smell and tastehis is a considerably accelerated test of what happenshe still. inf you can tell any difference between the test waternd that you started with, the material is probably safeo get to use.ome experience, try this on polyethelene tubing, PVC pipeand fiberglass resin panel.Basic ComponentsA basin still consists of the following basic components: (1)asin, a2) support structures,trough, and (3) glazing, (4) a distillate(5) insulation. The section that follows describeshese components, the range of materials availableconstruction, for theirand the advantages and disadvantages of ofthose materials. some The Basin. The basin contains the saline (or brackish) water thatill undergo distillation. As such, it must be waterproof andark (preferably black) so that it will bettersunlight and convert it absorb theto heat. It should also haveelatively smooth surface to make it easier to clean any sedimentafrom it.There are two general types of basins. The first is made of aaterial that maintains its owncontainment shape and provides the waterproofby itself or with the aid of aapplied surface materialdirectly to it. The second typematerials (such as wood or uses one set ofbrick) to define the basin's shape.nto this is placed a second material that easily conforms to thehape of the structural materials and serves asliner. a waterproofNo one construction material is appropriate for ,11ircumstances or locations. Table 1 lists the various materialsnd rates them according to properties desirable for thisappl ication.

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Table 1.A Comparison of Various Materials Usedin Solar Basin ConstructionType of Dura- Local Avail- SkillMaterial bility Cost ability Needed

Port-Cleaning ability Enameled High High Low Low High Mediu Steel

EPDM High High Low Low High High RubberButyl High High Low Low High HighRubberAsphalt High Medium Medium Medium Medium Medium MatAsbestos High Medium Low Medium Medium MediumCementBlack Medium Low Low Low Medium High LPolyethyleneRoofing Medium Medium High Medium Medium Low [aAsphaltonConcreteWood Low [a] [a] Medium Medium Medium LFormed Medium Medium Low Low High Medium LFiberglass[a] = Unknown or depends upon local conditions.

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Selecting a suitable material for basin constructionbiggest problem in is thethe solar still industry.conditions at the water line can be The corrosionso severe that basins made ofetal--even corrode. those coated with anti-corrosive materials--tend toeaten out Basins made of copper, for example, are likely to ben'a few years. Galvanized steel and anodized uncoatedluminum are likely to corrode in a few months.true of aluminum This is alsoalloys used to make boats.chemical reactions that double in There are manyrate with each 100 centigradencrease in temperature. Whereas an aluminum boat might last 20ears in sea water at 250C if you increase that temperature00, the durability of that aluminum may well be only one byor twoyears.Porcelain-coated out by steel lasts only a few years before it is eatenorrosion. The special glassslightly soluble in water, used for porcelain isand inside a still it willway. The dissolveypical life of stills equipped with porcelain basinss about five years, although several have been kept operatinguch longer than that by repairing leaks with silicone rubber.People have also tried to use concrete because it'sand inexpensivesimple to work with, but the failure rate has beenecause it often develops cracks if not during the first highthen later on. year,Concrete and abestoscement also absorb water.ater may not run Theright on through,Everybody knows but it does soak it up.that satisfactory cisterns and reservoirsuilt of concrete, arepart it but in a solar still the rules change. Anyf that is exposed to outsideevaporation. Since air will permitsalt crystals it is salt water that is being evaporated,will form in the concrete near the surface andbreak it up, turning it to powder.What about plastic? Every few years, someone decides that if weould just mold the whole still--except for the glass andea!--out of some plastic such as glassstyrofoam,and inexpensive. But it would be so easyUrethane foam styrene foam melts at about 700 Centigrade.is a little more promising,dimensionally unstable, and, but it tends to beif a still is constructed in thenclined-tray configuration, the efficiency suffers, because theon-wetted portions do not conduct heat to the wetted portionsvery well.What about fiberglass? People have spent a lot of time trying touild stills from fibreglass resin formulations. Thus far, theyave found the material to be unusable for any part of the stille.g., the basin or distillate trough) that comes in contact withater, either in liquid or vapor form. Epoxy and polyesteresins can impart taste and odor to the distilled water, notor weeks, but 3ustfor years. Researchers have foundroblem that thisannot be eliminated by covering these materials withoat of acrylic or anything else. aThe odors migrate r:qht

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through the coating and make the distilled water unsaleable, ifnot undrinkable. Moreover, using fiberglass resin aarticularly low-cost approach. is not Finally, a fiberglass basin orrough that is subjected to hot water for many years developscracks. Unless researchers find a way to solve these problems,fiberglass remains an unsuitable material.One alternative is ordinary aluminum coated with silicone rubber.he durability of basins made with this material increasedthe 10- to 15-year range. intoFor the hundreds of stills one companyold using this material, the coating was all done by hand.production roll coating equipment, Withthe basin's durability couldprobably be increased even more.Although stainless steel has been used, success has been poor.Support Structures. Support structures form the sides of thetill as well as the basin, and support the glazing cover. Asoted earlier, some materials used in forming the basin also formhe still support structure while other still configurationsdemand separate structures, especially to hold the glazing cover.The primary choices for support structures are wood, metal,concrete, or plastics.based In most cases the choice of material ispon local availability. Ideally, the frame for theglazing cover should be built of small-sized members so they donot shade the basin excessively.Wooden support structures are subject to warping, cracking, rot,nd termite atack. Choosing a high-quality wood, such asypress, and letting it age may help to alleviate these problems,but, if high heat and high humidity prevail inside and outsidethe still, the still will require frequent repair or replacement.The main advantage of wood is that it can be easily worked withbasic hand tools.Metal may be used for the supports butSince is subject to corrosion.metals are not subject to warping, they can aid inaining the integrity of the seals, mainalthough the expansion ratef a metal must be taken into account to ensure its compatibilitywith the glazing material and any sealants used. Use of metalor frame members is practically limited to aluminumvanized steel. and gal-Both will last almost indefinitely, if protectedfrom exposure.Silicone rubber will not adhere well to galvanized steel, butdoes so very well to aluminum.Concrete and other masonry materials may form the sides andlazing support of a still as well as the membrane. This iseadily possible in a single-slope still (Figure 6) rather morethanin a double-slope still (Figure 7).

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Figure 6. Single-Slope StillSource: U.S. Agency for International Development,Fresh Water from the Sun, by Deniel C.Dunham (Washington, D.C., August 1978),p. 90.

Figure 7. Double-Slope StillSource: U.S. Agency for International Development,Fresh Water from the Sun, by Daniel C.Dunham (Washington, D.C., August 1978),p. 89.

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Glazing Cover. After the pan, the glazing cover isritical component of any solar still. the mostIt is mounted above theasin and must be able to transmit a lot of light in thepectrum yet keep the visibleheat generated by that light from escapinghe basin. Exposure to ultraviolet radiation requires a materialhat can withstand the degradation effects or that is inexpensivenough to be replaced periodically. Since ittemperatures approaching may encounter950 celcius (2000 F), it must also beble to support its weight at those temperatures and not undergoxcessive expansion, which could destroy the airtight seals.ilm type cover, Awhich must be supported by tension or airpressure, seems like a very poor choice.Ideally, the glazing material should also be strong enough toesist high winds, rain, hail, and small earth movements,prevent the intrusion of insects and animals. andbe Moreover, it mustwettable." Wettability allows the condensing vapor tos sheets formof water on the underside of a glazing cover ratherhan as water droplets. If the water does form as droplets,will reduce the performance of the still itfor the followingreasons:

o Water droplets restrict the amount of light enteringthe still because they act as small mirrors and reflectit back out.o A percentage of the distilled water that forms asroplets on the underside will fall back into the basinrather than flow down the glazing cover into thecollection trough. Except for temporary conditions atstartup, such a loss of water should not be to].!rated.

Other factors determining the suitability of glazing materialnclude the cost of the material, its weight, life expectancy,ocal availability, maximum temperature tolerance,resistance, as and impactwell as its ability to transmit solar energy andnfrared light. Table 2 compares various glazing materials basedon these factors.Of the glazing materials listed in Table 2, tempered glass is theest choice in terms of wettablitywithstand and its capability toigh temperatures. It is also three to five timestronger than ordinary window glass and much safer to workOne with.disadvantage of tempered glass istempered low-iron glass, its high cost. Whilein one series of tests, gave 6 percentdditional production, it also added about 15 percent to the costf the still. Moreover, glass cannot be cut after it hasempered. Nevertheless, it is beena valid choice, certainly for atop-quality, appliance-type product.

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Table 2. A Comparison of Various Glazing MaterialsUsed in Building Solar Stills

rwpe of Est mat~;rpIng Solar Infrared LightCst- Weight Life mauimn Transmittance Traneslttance Impact2 2'teriaI (Dollar@/Ft) (I/Ft ) Expectancy Temperature (Percent) (Percent) Resistance Wettabilty

Tempered Low-Iron 01.6 to 40O -600OFGlass 3.60 2.5 50+ years 2040-316oC 91 ess than 2 Low Excellent Ordinary Window 400OFGlass .95 1.23 50 years 204 0C 86 2 Low ExcellentTedlar .60 .029 5-10 years 225 0F

107 0C 90 58 Low TreatableMylar ? ? ? ? ? ? Low TreatableAcrylic 1.50 .78 25+ years 200OF930C 89 6 Medium TreatablePolycarbonate 2.00 .78 10-15 years 260F

127 C 86 6 High TreatableCellulose AcetateButyrate .68 .37 10 years 180OF82'C 90 ? MediumFiberglass .78 .25 8-12 years 200F

930C 72-87 2-12 Medium TreatablePolyethylene .03 .023 8 months 160F

710C 90 80 LowPossiblytreatable

aCosts are in U.S. dollars, and were developed based on data published between 1981 and 1983.

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Ordinary window glass is the next best choice, except that it hasn oily film when it comes from the factory,carefully with detergent and/or ammonia. and must be cleanedIf you choose glass asglazing material, double--strength thickness (i.e., one-eighthof an inch, or 32 millimeters) is satisfactory.While some plastics are cheaper than either window glass orempered glass, they deteriorate under high temperatures and haveoor wettability. Moreover, under temperature tonditions typicalf solar stills, the chemicals in plastics are likely to interct with the distilled water, possibly posing a health hazard.What about the size of the glass? Using a low slope ofhe goal glass,is to make it as wide from northIt doesn't take any to south as possible.more labor to installof glass than it does to a 90 centimeter pieceinstall one of 60 centimeters andet youore absorber area. Also,will be the loss of heat through the wallsame whether the still is large or small. Usingieces of glass wider than 90 centimeters (3 ft.)problems: introduces two1) the price per unit area of the glass goes up; and2) the labor costs and the danger of handling it increase.he basis of experience, On(34"), a size one optimal size ir about 86 centimetersthat is commonly stocked andespecially in the solar collector industry. widely available,Distillate Trough. The distillate trough is located atf the tilted glazing. the baseIt serves to collect the condensed waternd carry it to storage. it should be as small as possible tovoid shading the basin.The materials used for the trough mustmaterial requirements outlined previously. satisfy the generalused include metal, Those most commonlyformed materials used in basin constructicn(with or without plastic liners), or treated materials.Stainless steel is the material of choice, although it is expenive. Common variet4es, such as 316, are acceptable.metals require protective Othercoatings to prevent corrosion. Alumium is not supposed to corrode in distilled water,preferable but it seemso rub a coating of silicone rubberGalvanized over it anyway.iron probably will not lastmost, more than a few years atnd copper and brass should not be used because they wouldreate a health hazard. Also,poor steel coated with porcelain is ahoice because the glass will dissolve slowly and allow thesteel to rust.Basins lined with butyl rubber or EPDM can havextend beyond the their linersbasin to form the trough. This method isnexpensive to implement and provides a corrosion-free channel.

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No version of polyethylene is acceptable because it breaks up andmits an unpleasant odor and taste. Some peoplepolyvinyl chloride have used(PVC) pipe, slit lengthwise. However, it isubject to significant distortion inside the still,an can give offndesirable gas, and is subject to becoming brittleexposed to sunlight and heat. whenButyl rubber should be okay, butecause it is blackthe distillate trough becomes an absorber ande-evaporates some of the distilled water (a minor problem).Ancillary ComponentsAncillary components include insulation,valves, sealants, piping,fixtures, pumps, and water storage facilities.general, it is best to use Inlocally available materials, which areeasily replaceable.Insulation. Insulation, used to retard the flow of heat from aolar still, increases the still's performance.insulation In most cases,is placed under the still basin since this is a largearea susceptible to heat loss.In stills where the depth of water in the basin is two inchesless, performance orhas been increased by as much asbut this 14 percent,gain decreases as increases. the depth of the water in the basinIncreases in performance resulting from thenstallation of insulation materials are also less in thoseocations where greater amounts of solar energy are available.The least expensive insulation option is to build a solar stillon land that has dry soil and good drainage. The use sandfelps to minimize solar heat losses, and may also serve as a heatink, which will return heat to the basin after the sun sets andprolong distillation process.Insulation, which adds approximately 16 percent to constructioncosts, may be extruded styrofoam or polyurethanethane in (Note: polyurecontact with soil will absorb moisture and lose much ofits insulation value.)Sealants. Although the sealant is not a major component of aolarto securestill,the itcoveris importantto the framefor efficient operation. It is used(support structure), take up anyifference in expansion and contraction between dissimilar mateials, and keep the whole structure airtight. Ideally, a goodealant will meet all of the general material requirementsearlier citedin this paper. Realistically, however, it might beecessary to use a sealant that is of lesser quality and has ahorter lifespan but that may be locally available at prices moreffordable to people in developing countries.of applying One major drawbacklow-cost sealants to stills is the frequent labornput the stills require to keep them in serviceable condition.

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Sealing a solar still is more difficult than sealingwater-heating panel a solaron two counts: (1) an imperfect seal couldause a drop of rain water carrying micro-organisms to enterstill, thewhich would contaminate the water;sealant that imparts a bad taste or and (2) applying aodor to the distilled waterwill make it unpalatable.Traditional sealants that are locally available include:

o window putty (caulk and linseed oil);0 asphalt caulking compound;o tar plastic;o black putty.

A wide variety of other caulks sealants is also available. Thesenclude latex, acrylic latex, butyl rubber and synthetic rubbers,olyethylene, polyurethane, silicom, and urethane foam.hese will be Most ofmore costly than traditional varieties, but theymay wear longer.Of this group of sealants, molded silicone or EPDM, clamped inlace, seems to be the most promising. Silicone rubber sealant,pplied from a tube, is certainly a superiorpeople choice, althoughhave reported a few instances of degradation andailure after 5 to 15 years when the seal sealwas exposed toight. Covering the sealant with suna metal strip should extend itsife greatly. Researchers are experimenting with an extrudedsilicone seal, secured by compression.One final note: Remember a sealant that works well for windowsn a building does not assure that it will work in a solar still,ue to higher termperatures, presence of moisture, and the facthat the water must be palatable and unpolluted.Piping. Piping is required to feed water into the still fromsupply source theand from the still to the storage reservoir.general material requirements cited earlier hold true Thefor thiscomponent.While stainless steel is preferred, polybutylenesatisfactory pipe material. is aBlack polyethylene has held up wellor at least 15 years as drain tubing.exposed to sunlight for 5 to 10 years. Nylon tubing breaks up ifPVC (polyvinyl chloride)ipe is tolerable, although during the first few weeks ofoperation it usually emits a gas, stilltaste bad. making the distilled waterOrdinary clear vinyl tubing is unacceptable. Theres a "food grade" clear vinyl tubing that is supposedsatisfactory for drinking water, but the sun's rays to bedegrade it are likely toif it's used in a solar still. Companies sell

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drinking water and milk in high-density polyethylene bottles, andave had satisfactory results. But put the same plastic bottlefilled with water in the sun, and the plasticimparting a bad taste to the water. will degrade,Few plastics can withstandeat and sunlight. Brass, galvanized steel, or copper may besed in the feed system, but not in the product system.One final note: Although a solar still repeatedly subjected Loreezing will remain unharmed, drain tubes so exposed may freezehut unless you make them extra large. Feed tubes can easily berranged with drain-back provision to prevent bursting.Fittings. Fittings are connection devices that hold pipeegments together. If you put a solar still on the marketinstructions to consumers that connections be made withonly", "finger tightpeople could put a wrench on a connection, loosen it, ande faced with an expensive repair problem. So, the optionsinclude having tight control of installation personnel,a thorough training job, or doingor making the equipment rugged enough towithstand ordinary plumbing practice.A solar still is fed on a batch basis for an hour or two everyay. It is necessary to admit some extra water each day, tolush out the brine. There is very little pressure available toet the water to drain, so drainage cannot proceed rapidly. Torevent flooding, it's good practice to insure that the feed rateoes not exceed this maximum drainage rate. If one uses needlealves thus to restrict the flow, such valves have been found toe unstable over the years, generally tending to plug up and stophe flow. It has proven to be a satisfactory solution to thisroblem--when feeding from city water pressure of typically 50.s.i.--to use a length of small diameter copper tubing,25 feet or such asore of 1/8 inch outside diameter, or 50 feet of 3/16nch outside diameter tubing, to serve as a flow restrictor.needs to Ithave a screen ahead of it, such as an ordinary hoseilter washer, with 50 mesh or finer stainless steel screen, toprevent the inlet end from plugging.Storage Reservoir. In selecting materials for the storage reservoir, two precautions should be noted.1) Distilled water is chemically agressive, wanting to dissolvea little of practically anything, until it gets "satisfied,"and then the rate of chemical attack is greatly slowed.What this number is, in terms of parts per million ofdifferent substances, is wellot documented, but thepractical consequences are that some things, such as steel,galvanized steel, copper, brass, solder, and mortar, which

distilled water, resulting in damage or destruction of theank component, and quite possibly in contamination of the24

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water. Stainless steel (type 316) is a good choice. Polypropylene laboratory tanks to sunlight. Butyl are okay but must not be exposedrubber lining of some structuralframework should be okay. Galvanized steel would lastonly a few years, adding some zinc and iron to the forwater.Concrete should serve, again with the expectation that theoncrete tiny will slowly crumble over many years' time. Themount of calcium carbonate that is leached out can besed by the body in the diet. In fact, one way to preventsuch chemical attack is to introduce some limestonemarble orchips into the distilled water stream,reservoir itself, or in theto pick up some calcium carbonate onurpose, thus greatly slowing the attack on the tank itself.2) Extreme precautions need to be taken to prevent entry ofinsects and airborne bacteria. Air must leave the reservoirevery time water enters it, and must re-enter every timeater is drawn off. Use a fine mesh--50 x 50 wires to theinch--or finer screen covering the vent, and turn the opening of the vent/screen assembly downward, to prevent entryf rain water. If this is ignored for even one hour, annsect can get in, and you have germ soup from then on.Storage capacity should be adequate to contain four to five timesthe avcerage daily output of the still.Factors to Consider in Selecting Materials for Basin StillLet us review the functions of the basin:

o It must contain water without leaking.o It must absorb solar energy.o It must be structurally supported to hold the water.o It must be insulated against heat loss from the bottomand edges.

An infinite number of combinations of materials will serve thoseunctions. The membrane that holds water, for example, may betiff enough to support the water, but it doesn't havehe basin may to be.be rigid enough to support the glass, butdoesn't have to be. itIn short, a component need not satisfy twounctions at the same time. Indeed, it is usually better toelect local material that will best do each job separately,then put them together. andBut if you can find a material that doesa couple of jobs well, so much the better.In selecting materials for a solar still, there are almost alwaysradeoffs. You can save money on materials, but you may lose so

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much in productivity or durability that the "saving" is pooreconomy.Summary of Materials Recommended for Basin Still ConstructionWhere the objective is the lowest cost of water on a 2 0-year lifeycle cost basis, the best materials for building a basin stillare:

0 silicone compound coating to blacken the bottom of thebasin;o metal ribs spaced 40 centimeters (16 inches) apart tosupport the underside of the basin;o about 25 to 38 milimeters of insulation between theribs (this may be high-temperature urethane foam, orfiberglass);o a bottom covering of lightweight galvanized steel,aluminum sheet (note: orif you plan to put the still onthe ground and use an insulation that is impervious towater, no bottom sheet is needed);o metal siding, such as extruded aluminum, to support thestill (note: extruded aluminum can be assembledquickly, but it is expensive; thus, you may prefer alower cost material such as painted steel or aluminum;o a stainless steel trough;o tempered low-iron glass, or regular double-strengthwindow glass. (If using patterned glass, put thepattern side down);o extruded gaskets, compressed into final position;o type 316 stainless steel fittings (note: brass is notacceptable; PVC is acceptable, but poor in very hotclimates);o a mirror behind the still for higher latitudes.

Although these materials are representative of a high-cost stilldesign, they are probably a good investment since none olinexpensive designs has stayed on the market. theHowever, we mustalso ask the question, "Expensive compared to what?" Compared toauling purified water in bottles or tanks, solar distilled waterwould almost always be much less expensive. Compared to haulingvegetables by airplane to hot desert places, using a solar stillto raise vegetables in a greenhouse should be less expensive.26

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Compared to the cost of boiling water to sterilize it, the solartill should be competitive in many situations. And although theaterials used in building a high-cost still will probably alwayse expensive, mass production could ultimately drive down theunit cost per still.IV. OPERATION AND MAINTENANC, rF SOLAR STILLSOPERATING REQUIREMENTS OF BASIC STILLSProtecting Distilled Water from ContaminationProtecting a solar still against the entry of insects and poluted rainwater is important. After your still is installed, youmust:

o disinfect the interior of the still and tubing withhlorine compounds (adding a few spoonfuls of laundrybleach to a few liters of water does the job nicely);and0 provide a vent* in the feed tube at the still, screenedwith fine stainless steel screen filter washer in aipe fitting, turned downward to prevent entrycontaminated ofrainwater. If these precautions are notaken, flying insects, attracted by the moisture, mightfind their way in and die in the distillate trough.

Preventing contamination in a storage reservoir is a little moreifficult, as the daily high temperature are not availablepasteurize the water. toNevertheless, with diligent attention toetail, the system can be used for decades without contamination.Filling and Cleaning a Basin StillFilling a basin still is a batch process*,night or in the morning. done once a day, at7 With a still of this design, about 5 toercent of the day's total distilled water is producedsundown, afterso it is important to wait until the stillRefilling it between three hours or is cold.more after sundown and up tone or two hours after sunrise will cause little, if any, loss ofproduction.

* A vent allows air to enter and exit the still daily during theoperation and refilling.

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It is not necessary to drain the still completely. Refilling itith at least twice as much as it produces will normally dilutend flush it adequately. Three times as much would keepittle cleaner, it aand could be worth doing, provided the cost ofeed water- is nominal. A rapid mechanical flushing is not.!quired; a gentle trickle does the job.Feeding Hot Water to a Basin StillIf a basin still is fed water that is hotter thanair, the ambienthe unit befcmes a conventional distiller,uses glass instead of copper except that itas the condenser. If the hot waters virtually cost-free, as is geothermal or waste water,e well worth doing. it canIf the feed water is heated by fossil fuelsr by separate solar panels, the economics look doubtful, and thefeed line tends to plug up with scale.FACTORS INFLUENCING SOLAR STILL OPERATING PERFORMANCEIn this section, we discuss some important factors that influencehe rate of production of distilled water.climatic factors, These includethermal loss factors, and solar still designfactors.Climate FactorsRadiation: Its Effect on Efficiency. The amount of solar radiation a solar still receives isaffecting the single most important factorts performance. The greater the amount ofreceived, energythe greater will be the quantity of waterFigure 8 shows the rate distilled.of production of a basin still on thebasis of specific solar inputs.Solar stills produce less distilled water in winter thansummer, inwhich is a problem. To some extent, the demand forrinking water also varies with the seasons,haps 2 to 1, by as much as persummer over winter. But thevariation annual sunlightaffecting a still's solar distillation ratethan that, at least is greaterin tropics, regions well outside the tropics. thenat latitudes of less than 200,variation the annual sunlightis probably well under 2 to 1, so it mayproblem there. not be aThe farther away from the equator, the greaterhe annual sunligfht variation, to perhaps 7latitudes. to 1 at 40'This is unacceptable, making use of a solar stilldifficult in winter at high latitudes.

* Note that there are other methods available for largedistillation plants. However, because they fall outsidescope of this paper, they theare not discussed here.

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Many approaches have been tried to solve this problem.the whole still up to more or Tiltingless an equatorial mourt brings theatio down very nicely. This is called the "inclined-tray"still, and is accomplished by using many small pans in a stairtep arrangement. With this arrangement, total sunlighttriking the aperture of the glass remains more constant, and theight which glances off the water ofbottom of the one one small tray warms theabove it, improving performance. While this issubstantial advantage, it is the only advantage of this design,nd it must be weighed against the disadvantages of higherssociated costswith putting many small pans vs. only onenclosure, and, in themost probably, higher installation costs due toolding the end of the pan higher off the supporting surface, androtecting it against wind loads. In latitudes perhaps 200p, it oneems possible that the inclined-tray will find a place inthe market.Using an inclined-tray still is only one solution to theof problemannual variation in higher latitudes. Some other steps thatcan be taken 'nclude:

o buying an extra large still that produces enoughdistilled water in winter, resulting in a likelihoodthat you will have more water than you need in summer;o using less water in winter and/or using some tap water;o buying supplemental water in winter; oro saving some of the excess distilled water made insummer or fall for use in winter;o installing a mirror behind the basin to reflectadditional sunlight back into the still in winter.reflect back much Toas light as possible, useeflective asurface of about one-third to one-half ofthe aperture of the glass cover, tilted forward 100from the vertical, mounted at the rear edge of thestill. In latitudes between 300 and 400, this givesfrom 75 to 100 percent more yield in mid-winter.

Condensing-Surface Temperature. Much work has been done too trybtain lower condensing temperatures,temperature thereby increasing theifference between the heated feed water andondensing surface. theThis approach undoubtedly derives from 100ears of steam power engineering, in which it is mostto importantet the steam temperature high and the condensing temperatureow to gain efficiency. But this principle does not holdor a solar still. truecontents of Steam for power is pure steam, whereas thea solar still are both air and water vapor.been demonstrated repeatedly that the higher It hasthe operating29

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temperature of the still--insolation being equal--the higherefficiency. For theeach 6' celsius (100 F) increasetemperature, in ambientthe production of a still increases by 7 to 8 perent. The practical effect of this is that a still operating inhot desert climate will produce typically as much as one-thirdmore water than the same unit in a cooler climate.(By the same token, cooling the glazing cover of a solar still bypraying water on it or blowing air overstill produce more it does not help thedistillate. In an experimentUniversity of at theCalifornia in the United States, two identicalstills were built. The glazing cover of the first still wasooled; the cover of fanthe second still was not.stills, Of the twohe cooled unit produced significantly lessConsequently, distillate.it's better to put the still in protected arearather than a windy area.)Thermal Loss FactorsProduction is also associated with the thermal efficiency of thetill itself. This efficiency may range from 30 to 60depending on percent,still construction, ambient temperatures,velocity, and windsolar energy availability. Thermal losses for aypical still vary by season, as shown in Table 5.

Table 5. Distribution of Incoming Solar Radiationin the Distillation ProcessDecember May{Percent) (Percent)Reflection by Glass 11.8 11.8Absorption by Glass 4.1 4.4

Radiative Loss from Water 36.0 16.9Internal Air Circulation 13.6 8.4Ground and Edge Loss 2.1 3.5Re-Evaporation and Shading 7.9 14.5[Remainder of Energy Used to Distill Water] 24.5 40.5Direct Useof the Sun's Energy, Daniels, Farrington, 1964,Ballantine Books, page 124.

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Solar Still Design FactorsSlope of the Transparent Cover. The angle at which thearent cover transis set influences the amountentering a solar still. of solar radiationWhen sunlight strikes glass straight on,t 900 to- the surface, about 90 percent of thehrough. light passesip the glass a little, so that it strikes at a "graing angle" of 80', and only a few percent is lost.few more But tilt itens of0, and the curve goes overoff to practically zero at 200 grazing angle, the hill, droppingdirect where virtually noight gets through. In a greenhouse-type still, for aarge part of the year the half of the glass that is facing awayrom the equator is receiving sunlight at very low grazinggles. an-t is actually shadowing the back one-third of the still.t is more efficient to make that half of the glassquator facings long as possible, theback wall and put a more or less reflectiveto the rear. This was one of the significant stepshat has increased the efficiency of basin stillsabout 43 percent, from 31 tousing a single slope of glass. And it costsless to build.The slope of the glass cover does not affect the ratehe distillate at whichruns down its inner surface totrough. the collectionA common misconception was that the glass cover must beilted to get the water to run off.the fact This may have arisen fromhat ordinary window glass, as it comes fromactory, has thea minute oily film on it.clean, the But if the glass isater itself will form filmwise condensation onnd will be able to run off at it,a slr-e as little as 10.There are three reasons why it is best topossible: use as low a slope as1) the higher the slope,ing materials are needed to cover the more glass and supporta given area of the basin;he higher slope increases the volume and weight [of tihe (2)and therefore shipping costs; and still](3) setting the glass at a highlope increases thje volume of air inside the still,the effficiency which lowersof the system.than A glass cover that is no to 7 centimeters from the water surface will morestill to operate efficiently. allow theConversely, as glass-to-waterdistance increases, heat loss due to convection becomes greater,causing the still's efficiency to drop.Some important design stills have been built following the low-slopeconcept for the glass cover,sloping piece of glass at the yet using a short, steeplyrear. This requires eitheriding an extra collection trough at prosuccessive troughs touching the rear, or else making theheel and toe,exceedingly difficult to get out in the middle ofsothethat it isservice anything. array toIt also increases the condensing surface relaive to the absorber, which reduces operating temperatures in thetill, and is clearly disadvantageous. A reflective andnsulated back may be preferable to glass.

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Some years ago at the University of California, researchers builtn experimental multiple tray tilted still with an average glasso-water distance of about 30 millimeters,of 62 percent, one showing an efficiencyof the highest ever recorded.efficiency The loss ofs greatest the first centimeter, rather less theecond cm, and so on, tailing off to smaller rates of loss per cmistance as far as the test was carried. This is onerinciple reasons a high slope of glass is to be avoided. of theIn sum, it is clear that a solar still should be built in a wayhat will get the water as hot as possible,to the glass as and keep it as closepossible. This is achieved by keeping the glassover at a minimum distance from the waterpractical terms, falls between 5 and 7 cm., surface, which inand by minimizing theepth of water in the pan, to about 1.5 cm.Wicks and Related TechniquesResearchers have tried to improve the efficiency ofy enhancing a solar stillts surface evaporation areaside-by-side using wicks. In aest of two identical stills at theCalifornia, University ofsing a floating black synthetic fabric in one stillnd nothing in the other, the difference in productionthe stills betweenwas indistinguishable,reported some improvement. though similar tests haveIt seems exceedingly difficultind a toick material that will last for 20 years in hotater, salinend that will not get crusted up with salts over a periodf time. As for putting dye in the water,the slight improvement in performance studies suggest thatincreased does not justify theost and maintenance and operating problems associatedwith this technique.Putting dark-colored rocks in the feedwater to store heat forfter nightfall has been reported by useimprove performance by 40 percent, Zaki and his associates tobut he does not give theeference point from which this is measured. Ifone still containing 4 cm. of he was comparingwater with anotherbut containing same water depthblack stones, the productivitysomewhat would increasedue to the decrease in thermal mass andrease in resulting inperating temperature. Reducing the initial waterepth might have accomplished the same result. For this reason,lacing dark-colored rocks in the feedwater does not appear to be promising technique for improvements in solar still performance.

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MAINTENANCE REQUIREMENTS OF BASIN STILLSWays of Handling the Buildup of Mineral DepositsIt is inevitable that some minerals are deposited on thef the basin. In bottommost situations, including sea water and cityap water, the amount deposited is so small that it createsroblem for decades. noOne still in particular has been operatedor 20 years without ever having been opened or'cleaned.as As longhere is not an excessive buildup of deposits,formation of indicated bydried-out island in the afternoon,problem. they create noaccumulationSuch mineral deposits become the normal absorber. Anf these deposits changes the interior surface ofasin from its original black color to aa darkreflecting earth brown,some sunlight, causing a 10 percent droproduction. in stillTo offset this reduction, simply make the still 10ercent larger than it would need to be if it were cleaned outperiodically.Some desert waters high in alkalis will depositscale a whitishon the bottom and sides of graya basin.feed water will do so, In fact, almost anyespecially if the basin is allowed to dryut. In some cases, the alkaline water may form a crust of scalehich is held on the water's surface by airdischarged when the feed water bubbles that aresuch as is heated. Light-colored depositshese may reduce production ofmore. the still by 50 percent orhose that settle to the bottom of the basincoated black can be easilyby mixing one tablespoon of blackconcrete coloring powder with about 10 iron oxideor 15 liters of water anddding the solution to the still by means ofto the feed water pipe. a funnel connectedThis blackening agent ismparts no bad taste or odor inert, andto the distilled water.solution After theeaches the basin through the feed water pipe, itettles on the bottom of the basin and restores it to its orginallack color. Some owners do this each fall,begins to drop. when productionCost is only pennies per application.Deposits that float on the surface of the water in a basin areougher problem aand one that requiresAustralian more research. Anolar still expert suggests agitating the contents ofhe still by recirculating, or stirring, the water in the pan forne hour each night, to minimizedeposits. Adding a pint the buildup of floatingor two of hydrochloricacid to (swimming pool)he still whenever the bottom becomes grayish-white-every year or two, maybe oftener in some cases--is a satisfactoryway of removing practically all of the scale.

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Accumulation of Dust on the Glazing Cover: What to DoIn the vast majority of stills, dust accumulates on theover. glassut-it does not keep building up; it's held more oronstant by the action lessof rain and dew. This "normal"ccumulation causes production to drop from 5 to 15 percent.ffset this, simply Tomake your still 10 percent larger thanould need to be itif kept clean. However,unusually dusty area, if the still is in anor if itis available is large enough that a caretakerat modest cost, cleaning the glazingjustified. percent of 10,000 liters per day may becoverenoughisen to justify cleaning the cover once a month in the dry season.Repair and Replacement of Basin Still ComponentsAs with all devices, the components of a basin still may need toe repaired or replaced from time to time. The frequency dependsn the type of material used to construct the still.with premium materials will require almost One builtno maintenance,will butntail a higher capital cost because many of the materialsust be imported materials. Use of cheaper materials subject toegradation will almost certainly lower the initial cost, butill increase the amount of maintenance. Even so, if the longerm cost of maintenance and the lower initial cost are less thanhe higher initial cost for premium materials, this may present aetter option, especially if cost of capital is high.called "life cycle This iscost analysis," and it is strongly recommended.SKILLS REQUIRED TO BUILD, OPERATE, AND MAINTAIN A BASIC STILLCraftmanship and attention to detail in construction areimportant for an efficient, cost-effective still.In addition, supervisory personnel must be on hand who know howo size stills to meet a community's water supply needs; who knowow to orient stills; who are techniques; and familiar with required constructionwho have the ability to train others in theonstruction, operation and maintenance of stills.Finally, it is important to ask local workers to participatethe planning and construction phases of ina solar still project toet the indigenous population to accept the technology. A sensef pride in the building of the project may well mean the differnce between long-term success or failure of the project.COST/ECONOMICSThe cost and economics of solar stills depend on many variables,including:

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it paid for itself in one year; if five times that much, thenive years, etc.--not counting interest.into Cost of feeding watert is pretty small, but will increase the payout periodittle also.' aIn the United States,run the payout period tends toetween two and five years, depending on the still's size andfeatures.SPECIAL DESIGN VARIATIONSThe majority of information presented thus far has centeredhe basin-type solar still because it is the easiest to constructnd may use a wide range of materials,

ondifferent locales., making it adaptable toBut variations of thepossible, such basin still areas the double-slopedepicted earlier in this paper. and single-slope stillsIn addition to thesehere options,re other ways to design the stillefficiency or potential to increase itsto produce potable water. Some of theseare discussed below.Basin Stills Equipped with ReflectorsSome stills have been eq'uipped with reflective materialsave the potential whichto increase the amount of sunlight fallinghe ontill without having to increase the area of the still.atitudes Atn the thirties, performance increases in00% winter ofave been achieved with a mirror of less than 1/2 thef the glass. areaIn the tropics, of course,required. this function is not second question arises about usingenhance production year round. mirrors tolector, which This becomes a focusing colintroduces substantial additionalproblems. costs andIf the mirror assembly is cheaperassembly, then it deserves to be looked at than the panfurther, but it is notttractive at this time. Tentatively, reflective aluminum sheethas the most advantages.Basin Stills Equipped with Insulated Glazing CoversAnother innovation is the use of an insulated glazing covere put over the glazing at night toweather. This or during extremely coldcuts continue longer, heat losses, allowing distillation toand retains heat overnight,to start earlier the next day. causing productionCost-benefit analysis ofapproach has not been made. this

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V. COMPARING THE ALTERNATIVESFor a couple of gallons of purified water a day, there is noethod that can compete with solar distillation.million gallons a day--AS LCIG For a couple ofAS WE ARE WILLING TO BURN UPNHERITANCE OF FOSSIL CHEMICAL BUILDING BLOCKS JUST TO OURWATER--boiling EVAPORATEdistillation is the cheapest way to purify seawater.In sum, solar stills have:

o high initial costs;o the potential to use local materials;o the potential to use local labor for construction andmaintenance;o low maintenance costs (ideally);o no energy costs (not subject to fuel supplyinterruptions);o few environmental penalties; ando in residential sizes, no subsequent costs fordelivering water to the end user.

Most competing technologies are:o low in initial costs;o dependent on economy of scale;o high in operating and maintenance costs;o high in energy input costs;o low in local job creation potential;o vulnerable to changes in energy supply and costs; and

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-Groove for Swa r Isoft woer 8-aa5iIXSocnsur.. ' \\,, r 8r~k~s -- ,ti:hen.,sigtns

Grc eH-r'dror rrj;n\ . "," -Wtlekish,~edl . -_.=water~

Manua/on Sola Dit"lt') i I tpr Conc roteFigure 8. A Basin Still at Chakmou- Tunisia

Source: S.G. Talbert, J.A. Eibling, and G L6f,eorge (Springfield, Virginia, Virginia: National TechnicalInformation Service, April 1970), p. 46.

To-'/n C t

-- ,'!

Figure 9. Roofwasher(Midwest Plan Service, 1968)Source: Murray, Milne, Residenotial WaterConserva-ig1nReport No. 35 (Davis, California: University ofCalifornia, 1976), p. 108.

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'--- 10"- I",

L 9-9

Atchd

Pto. plywood

]/4" x4latlattrtcIa

wide-- ,"t

P l Iorr)Ps/'l %&b ?.v ,,hfiv tap*

c

IPi lirin boll

Figure 10. A Basin Still on Petit St. Vincent IslandSource: S.G. Talbert, J.A. Eibling, and George L~f,Manual on SolarDistillationofSaline Water,(Springfield, Virginia: National Technical InformationService, April 1970), p. 60.

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VI. CHOOSING THE TECHNOLOGY RIGHT FOR YOUFACTORS TO CONSIDERSolar energy is an excellent choice for waterhose distillationreas inf the Third World that meet the followingconditions:

0 expensive fresh water source (US) $1 or more per 1,000gallons);

o adequate solar energy; ando available low-quality water for distillation.

Other conditions suitability for solar stills are:o competing technologies that require expensive

conventional wood, or petroleum fuels;o isolated communities that may not have access to cleanwater supplies;o limited technical manpower for operation and maintenance of equipment;o areas lacking a water distribution system; ando the availability of low-cost construction workers.

The greater the number of these conditions present,solar stills are likely to be the morea viable alternative.of the water produced by a still over If the costits useful lifethan by alternate methods, it is lessis economical to pursue.Other factors to consider are the availabilitycapital, as and cost ofell as the local tax structure, which may allow taxredits and depreciation allowances as a meansportion of the cost. to recover aThis has proved to be a major incentive inthe United States.Finally, the acceptance of solar distillation will depend greatlyn how well one understands and handles the many social issuesnd cultural constraints that technologies. Some of the can hamper the introduction of newmore important issues that may affecthe acceptance of solar distillation are outlined below.

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o Stills built for village use require communitycooperation that may be foreigh to some culturalgroups. If the distilled water is incorrectlydistributed, causing a family unit not to receivefair share of water, itsthis could become a source ofconflict. For this reason, a family-sized solar stillunit, which a household has complete control over, maybe more practical than a unit that serves an entirevillage.0 Potential users who think they will find distilledwater tasteless or not in keeping with what they areaccustomed to may become disappointed and possiblyabandon altogether the thought of drinking the water.The problem of taste must be dealt with early on so asnot to give people a reason to respond negatively tothe technology as a whole.o In some societies, conflicts may arise over whether itis the responsibility of the man or the womanhousehold to operate the solar still. of theNot dealing withthis issue early on could result in the household'stotal rejection of the technology.o If solar distillation is perceived to be a threat to acommunity's traditional lifestyle, the community mayreject the technology. Such concerns can be headed offif the technology is designed appropriately from thestart and introduced at the proper time. Moreover, acommunity is more likely to accept the technology if itrecognizes the importance of clean water and considersit a priority to the degree that it is willing tochange certain aspects of its lifestyle.

MARKET POTENTIALThree potential markets exist for solar stills. First, a solarstill can be economically attractive almost any place in theorle where water is haled and where a source of water isavailable to feed the still.Second, many people who boil their water to kill germs could solar still for the same usepurpose. It will take more work toemonstrate this function adequately, but early tests have madeit seem highly promising.A third market is in arid regions, whose untapped water resourcessufficientay be to economically provide a population withpotable water.

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CONCLUSIONWorldwide experience in researching and marketingover solar stillshree decades has provided an ample foundation for atill industry. solarNo inherent techaical oreen identified. economi2 barriers have solar still is suited(manufacturing] to villagetechniques and to mass production. Around theorld, concerns over water quality are increasing, and in specialituations a solar still can provide aeconomically water supply morehan any other method. Commercial activitiespicking areup after a lull during thepossible late 1970s. It is nowo predict a rapid increase in the manufacturemarketing of solar stills. and

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SUPPLIERS AND MANUFACTURERS OF SOLAR STILLSLodestone EngineeringP.O. Box 981Laguna Beach, California 92652-0981USA

SOLEFILTour Roussel-NobelCEDEX No. 3F. 92080 Paris La DefenseFRANCECornell Energy, Inc.4175 South FremontTucson, Arizona 85714USA

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BIBLIOGRAPHYCooper, P."., Solar Distillation--State of the Art and Futurerospects." Solar Energy and Arabhe WoTId (1983): 311-30.Daniels, Farrington. Direct Use of the Sun's Energ-y. New York,New York: Ballantine Books, 1975.El-Rafaie, M.E.; El-Riedy, M.K.; and El-Wady, M.A. "Incorporationof Fin Effect in Predicting the Performance of CascadedSolar Stills." SolarErgy and the Arab World (1983): 336-Goetchew, Martin. "Shedding Light on Solar Collector Glazing."Materials R n* 90 (September 1979): 55-58.Langa, Fred; Flower, Bob; and Sellers, Dave. "Solar Glazzings:roduct Review." New Shelter (January 1982): 58-69Leckie, Jim; Master, Gil; Whitehouse, Harry; and Young, Lily.More Other Hoes and arbage. San Francisco, California:Sierra Club Books, 1981.Mohamed, M.A. "Solar Distillation Using Appropriate Technology."SolarE gy and the Arb Worl (1983): 341-45.Talbert, S.G.; Eibling, J.A.; and L6f, George. Manual on SolarDistillation of Saline Water. Springfield, Virginia:National Technical Information Service, ?pril 1970.Dunham, Daniel C. PR From the Sun. Washington, D.C.:U.S. Agency for Internation Development, August 1978.Zaki, G.M.; El-Dali, T.; and El-Shafiey, M. "Improved Performanceolar Stills." qnjar EnerLv and the Arab World (1983):331-35.

McCracken, Horace: Only a small amount of McCracken's work has beepublished, but the data are available. Inquiriwill be welcomed:McCracken Solar Co.P.O. Box 1008Alturas, California 96101USA

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VOLUNTEERSINTECHNICALASSISTANCE1815 N. Lynn St., Suite 200Arlington, Virginia 22209 USA