MATERIAL REQUIREMENTS OF BASIN STILLS[edit | edit source]
The materials used for this type of still should have the following characteristics:
- Materials should have a long life under exposed conditions or be inexpensive enough to be replaced upon degradation.
- They should be sturdy enough to resist wind damage and slight earth movements.
- They should be nontoxic and not emit vapors or instill an unpleasant taste to the water under elevated temperatures.
- They should be able to resist corrosion from saline water and distilled water.
- They should be of a size and weight that can be conveniently packaged, and carried by local transportation.
- They should be easy to handle in the field.
Although local materials should be used whenever possible to lower initial costs and to facilitate any necessary repairs, keep in mind that solar stills made with cheap, unsturdy materials will not last as long as those built with more costly, high-quality material. With this in mind, you must decide whether you want to build an inexpensive and thus short-lived still that needs to be replaced or repaired every few years, or build something more durable and lasting in the hope that the distilled water it produces will be cheaper in the long run. Of the low-cost stills that have been built around the world, many have been abandoned. Building a more durable still that will last 20 years or more seems to be worth the additional investment.
Choosing materials for the components in contact with the water represents a serious problem. Many plastics will give off a substance which can be tasted or smelled in the product water, for periods of anywhere from hours to years. As a general guide, if you are contemplating using any material other than glass or metal in contact with water, you may perform a useful screening test by boiling a sample of the material in a cup of good water for half an hour, then let the water cool, and smell and taste it. This is a considerably accelerated test of what happens in the still. If you can tell any difference between the test water and that you started with, the material is probably safe to use. To get some experience, try this on polyethylene tubing, PVC pipe and fiberglass resin panel.
Basic Components[edit | edit source]
A basin still consists of the following basic components: (1) a basin, (2) support structures, (3) glazing, (4) a distillate trough, and (5) insulation. The section that follows describes these components, the range of materials available for their construction, and the advantages and disadvantages of some of those materials.
The Basin. The basin contains the saline (or brackish) water that will undergo distillation. As such, it must be waterproof and dark (preferably black) so that it will better absorb the sunlight and convert it to heat. It should also have a relatively smooth surface to make it easier to clean any sediment from it.
There are two general types of basins. The first is made of a material that maintains its own shape and provides the waterproof containment by itself or with the aid of a surface material applied directly to it. The second type uses one set of materials (such as wood or brick) to define the basin's shape. Into this is placed a second material that easily conforms to the shape of the structural materials and serves as a waterproof liner. No one construction material is appropriate for all circumstances or locations. Table 1 lists the various materials and rates them according to properties desirable for this application.
Table 1: A Comparison of Various Materials Used in Solar Basin Construction
| Type of Material | Durability | Cost | Local Availability | Skill Needed | Cleaning | Portability | Toxicity |
|---|---|---|---|---|---|---|---|
| Enameled Steel | High | High | Low | Low | High | Medium | Low |
| EPDM Rubber | High | High | Low | Low | High | High | Low |
| Butyl Rubber | High | High | Low | Low | High | High | Low |
| Asphalt Mat | High | Medium | Medium | Medium | Medium | Medium | [a] |
| Asbestos Cement | High | Medium | Low | Medium | Medium | Medium | High |
| Black Polyethylene | Medium | Low | Low | Low | Medium | High | Low |
| Roofing Asphalt on Concrete | Medium | Medium | High | Medium | Medium | Low | [a] |
| Wood | Low | [a] | [a] | Medium | Medium | Medium | Low |
| Formed Fiberglass | Medium | Medium | Low | Low | High | Medium | Low |
[a] = Unknown or depends upon local conditions.
Selecting a suitable material for basin construction is the biggest problem in the solar still industry. The corrosion conditions at the water line can be so severe that basins made of metal--even those coated with anti-corrosive materials--tend to corrode. Basins made of copper, for example, are likely to be eaten out in a few years. Galvanized steel and anodized uncoated aluminum are likely to corrode in a few months. This is also true of aluminum alloys used to make boats. There are many chemical reactions that double in rate with each 10 [degrees] centigrade increase in temperature. Whereas an aluminum boat might last 20 years in sea water at 25 [degrees] C if you increase that temperature by 50 [degrees], the durability of that aluminum may well be only one or two years.
Porcelain-coated steel lasts only a few years before it is eaten out by corrosion. The special glass used for porcelain is slightly soluble in water, and inside a still it will dissolve away. The typical life of stills equipped with porcelain basins is about five years, although several have been kept operating much longer than that by repairing leaks with silicone rubber.
People have also tried to use concrete because it's inexpensive and simple to work with, but the failure rate has been high because it often develops cracks if not during the first year, then later on. Concrete and abestoscement also absorb water. The water may not run right on through, but it does soak it up. Everybody knows that satisfactory cisterns and reservoirs are built of concrete, but in a solar still the rules change. Any part of it that is exposed to outside air will permit evaporation. Since it is salt water that is being evaporated, salt crystals will form in the concrete near the surface and break it up, turning it to powder.
What about plastic? Every few years, someone decides that if we could just mold the whole still--except for the glass and glass seal--out of some plastic such as styrofoam, it would be so easy and inexpensive. But styrene foam melts at about 70 [degrees] Centigrade. Urethane foam is a little more promising, but it tends to be dimensionally unstable, and, if a still is constructed in the inclined-tray configuration, the efficiency suffers, because the non-wetted portions do not conduct heat to the wetted portions very well.
What about fiberglass? People have spent a lot of time trying to build stills from fibreglass resin formulations. Thus far, they have found the material to be unusable for any part of the still (e.g., the basin or distillate trough) that comes in contact with water, either in liquid or vapor form. Epoxy and polyester resins can impart taste and odor to the distilled water, not just for weeks, but for years. Researchers have found that this problem cannot be eliminated by covering these materials with a coat of acrylic br anything else. The odors migrate right through the coating and make the distilled water unsalable, if not undrinkable. Moreover, using fiberglass resin is not a particularly low-cost approach. Finally, a fiberglass basin or trough that is subjected to hot water for many years develops cracks. Unless researchers find a way to solve these problems, fiberglass remains an unsuitable material.
One alternative is ordinary aluminum coated with silicone rubber. The durability of basins made with this material increased into the 10- to 15-year range. For the hundreds of stills one company sold using this material, the coating was all done by hand. With production roll coating equipment, the basin's durability could probably be increased even more.
Although stainless steel has been used, success has been poor.
Support Structures. Support structures form the sides of the still as well as the basin, and support the glazing cover. As noted earlier, some materials used in forming the basin also form the still support structure while other still configurations demand separate structures, especially to hold the glazing cover.
The primary choices for support structures are wood, metal, concrete, or plastics. In most cases the choice of material is based upon local availability. Ideally, the frame for the glazing cover should be built of small-sized members so they do not shade the basin excessively.
Wooden support structures are subject to warping, cracking, rot, and termite attack. Choosing a high-quality wood, such as Cypress, and letting it age may help to alleviate these problems, but, if high heat and high humidity prevail inside and outside the still, the still will require frequent repair or replacement. The main advantage of wood is that it can be easily worked with basic hand tools.
Metal may be used for the supports but is subject to corrosion. Since metals are not subject to warping, they can aid in maintaining the integrity of the seals, although the expansion rate of a metal must be taken into account to ensure its compatibility with the glazing material and any sealants used. Use of metal for frame members is practically limited to aluminum and galvanized steel. Both will last almost indefinitely, if protected from exposure.
Silicone rubber will not adhere well to galvanized steel, but does so very well to aluminum.
Concrete and other masonry materials may form the sides and glazing support of a still as well as the membrane. This is more readily possible in a single-slope still (Figure 6) rather than
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in a double-slope still (Figure 7).
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Glazing Cover. After the pan, the glazing cover is the most critical component of any solar still. It is mounted above the basin and must be able to transmit a lot of light in the visible spectrum yet keep the heat generated by that light from escaping the basin. Exposure to ultraviolet radiation requires a material that can withstand the degradation effects or that is inexpensive enough to be replaced periodically. Since it may encounter temperatures approaching 95 [degrees] celsius (200 [degrees] F), it must also be able to support its weight at those temperatures and not undergo excessive expansion, which could destroy the airtight seals. A film type cover, which must be supported by tension or air pressure, seems like a very poor choice.
Ideally, the glazing material should also be strong enough to resist high winds, rain, hail, and small earth movements, and prevent the intrusion of insects and animals. Moreover, it must be "wettable." Wettability allows the condensing vapor to form as sheets of water on the underside of a glazing cover rather than as water droplets. If the water does form as droplets, it will reduce the performance of the still for the following reasons:
- Water droplets restrict the amount of light entering the still because they act as small mirrors and reflect it back out.
- A percentage of the distilled water that forms as droplets on the underside will fall back into the basin rather than flow down the glazing cover into the collection trough. Except for temporary conditions at startup, such a loss of water should not be tolerated.
Other factors determining the suitability of glazing material include the cost of the material, its weight, life expectancy, local availability, maximum temperature tolerance, and impact resistance, as well as its ability to transmit solar energy and infrared light. Table 2 compares various glazing materials based on these factors.
Of the glazing materials listed in Table 2, tempered glass is the best choice in terms of wettablity and its capability to withstand high temperatures. It is also three to five times stronger than ordinary window glass and much safer to work with. One disadvantage of tempered glass is its high cost. While tempered low-iron glass, in one series of tests, gave 6 percent additional production, it also added about 15 percent to the cost of the still. Moreover, glass cannot be cut after it has been tempered. Nevertheless, it is a valid choice, certainly for a top-quality, appliance type product.
Table 2. A Comparison of Various Glazing Materials Used in Building Solar Stills
| Type of Glazing Material | Estimated Cost ($/Ft^2)(a) | Weight (Lb/Ft^2) | Life Expectancy | Maximum Temperature | Solar Transmittance (%) | Infrared Light Transmittance (%) | Impact Resistance | Wettability | Locally Available |
|---|---|---|---|---|---|---|---|---|---|
| Tempered Low-Iron Glass | 3.60 | 1.6 to 2.5 | 50+ years | 400-600°F, 204-316°C | 91 | < 2 | Low | Excellent | No |
| Ordinary Window Glass | 0.95 | 1.23 | 50 years | 400°F, 204°C | 86 | 2 | Low | Excellent | Yes |
| Tedlar | 0.60 | 0.029 | 5-10 years | 225°F, 107°C | 90 | 58 | Low | Treatable | No |
| Mylar | ? | ? | ? | ? | ? | ? | Low | Treatable | No |
| Acrylic | 1.50 | 0.78 | 25+ years | 200°F, 93°C | 89 | 6 | Medium | Treatable | No |
| Polycarbonate | 2.00 | 0.78 | 10-15 years | 260°F, 127°C | 86 | 6 | High | Treatable | No |
| Cellulose Acetate Butyrate | 0.68 | 0.37 | 10 years | 180°F, 82°C | 90 | ? | Medium | ? | No |
| Fiberglass | 0.78 | 0.25 | 8-12 years | 200°F, 93°C | 72-87 | 2-12 | Medium | Treatable | No |
| Polyethylene | 0.03 | 0.023 | 8 months | 160°F, 71°C | 90 | 80 | Low | Possibly Treatable | ? |
(a) Costs are in U.S. dollars, and were developed based on data published between 1981 and 1983.
Ordinary window glass is the next best choice, except that it has an oily film when it comes from the factory, and must be cleaned carefully with detergent and/or ammonia. If you choose glass as a glazing material, double-strength thickness (i.e., one-eighth of an inch, or 32 millimeters) is satisfactory.
While some plastics are cheaper than either window glass or tempered glass, they deteriorate under high temperatures and have poor wettability. Moreover, under temperature conditions typical of solar stills, the chemicals in plastics are likely to interact with the distilled water, possibly posing a health hazard.
What about the size of the glass? Using a low slope of glass, the goal is to make it as wide from north to south as possible. It doesn't take any more labor to install a 90 centimeter piece of glass than it does to install one of 60 centimeters and you get more absorber area. Also, loss of heat through the walls will be the same whether the still is large or small. Using pieces of glass wider than 90 centimeters (3 ft.) introduces two problems: (1) the price per unit area of the glass goes up; and (2) the labor costs and the danger of handling it increase. On the basis of experience, one optimal size is about 86 centimeters (34"), a size that is commonly stocked and widely available, especially in the solar collector industry.
Distillate Trough. The distillate trough is located at the base of the tilted glazing. It serves to collect the condensed water and carry it to storage. It should be as small as possible to avoid shading the basin.
The materials used for the trough must satisfy the general material requirements outlined previously. Those most commonly used include metal, formed materials used in basin construction (with or without plastic liners), or treated materials.
Stainless steel is the material of choice, although it is expensive. Common varieties, such as 316, are acceptable. Other metals require protective coatings to prevent corrosion. Aluminum is not supposed to corrode in distilled water, but it seems preferable to rub a coating of silicone rubber over it anyway. Galvanized iron probably will not last more than a few years at most, and copper and brass should not be used because they would create a health hazard. Also, steel coated with porcelain is a poor choice because the glass will dissolve slowly and allow the steel to rust.
Basins lined with butyl rubber or EPDM can have their liners extend beyond the basin to form the trough. This method is inexpensive to implement and provides a corrosion-free channel.
No version of polyethylene is acceptable because it breaks up and emits an unpleasant odor and taste. Some people have used polyvinyl chloride (PVC) pipe, slit lengthwise. However, it is subject to significant distortion inside the still, can give off an undesirable gas, and is subject to becoming brittle when exposed to sunlight and heat. Butyl rubber should be okay, but because it is black, the distillate trough becomes an absorber and re-evaporates some of the distilled water (a minor problem).
Ancillary Components
Ancillary components include insulation, sealants, piping, valves, fixtures, pumps, and water storage facilities. In general, it is best to use locally available materials, which are easily replaceable.
Insulation. Insulation, used to retard the flow of heat from a solar still, increases the still's performance. In most cases, insulation is placed under the still basin since this is a large area susceptible to heat loss.
In stills where the depth of water in the basin is two inches or less, performance has been increased by as much as 14 percent, but this gain decreases as the depth of the water in the basin increases. Increases in performance resulting from the installation of insulation materials are also less in those locations where greater amounts of solar energy are available.
The least expensive insulation option is to build a solar still on land that has dry soil and good drainage. The use of sand helps to minimize solar heat losses, and may also serve as a heat sink, which will return heat to the basin after the sun sets and prolong distillation process.
Insulation, which adds approximately 16 percent to construction costs, may be extruded styrofoam or polyurethane (Note: polyurethane in contact with soil will absorb moisture and lose much of its insulation value.)
Sealants. Although the sealant is not a major component of a solar still, it is important for efficient operation. It is used to secure the cover to the frame (support structure), take up any difference in expansion and contraction between dissimilar materials, and keep the whole structure airtight. Ideally, a good sealant will meet all of the general material requirements cited earlier in this paper. Realistically, however, it might be necessary to use a sealant that is of lesser quality and has a shorter lifespan but that may be locally available at prices more affordable to people in developing countries. One major drawback of applying low-cost sealants to stills is the frequent labor input the stills require to keep them in serviceable condition.
Sealing a solar still is more difficult than sealing a solar water-heating panel on two counts: (1) an imperfect seal could cause a drop of rain water carrying micro-organisms to enter the still, which would contaminate the water; and (2) applying a sealant that imparts a bad taste or odor to the distilled water will make it unpalatable.
Traditional sealants that are locally available include:
- window putty (caulk and linseed oil);
- asphalt caulking compound;
- tar plastic;
- black putty.
A wide variety of other caulks sealants is also available. These include latex, acrylic latex, butyl rubber and synthetic rubbers, polyethylene, polyurethane, silicone, and urethane foam. Most of these will be more costly than traditional varieties, but they may wear longer.
Of this group of sealants, molded silicone or EPDM, clamped in place, seems to be the most promising. Silicone rubber sealant, applied from a tube, is certainly a superior choice, although people have reported a few instances of degradation and seal failure after 5 to 15 years when the seal was exposed to sunlight. Covering the sealant with a metal strip should extend its life greatly. Researchers are experimenting with an extruded silicone seal, secured by compression.
One final note: Remember a sealant that works well for windows in a building does not assure that it will work in a solar still, due to higher temperatures, presence of moisture, and the fact that the water must be palatable and unpolluted.
Piping. Piping is required to feed water into the still from the supply source and from the still to the storage reservoir. The general material requirements cited earlier hold true for this component.
While stainless steel is preferred, polybutylene is a satisfactory pipe material. Black polyethylene has held up well for at least 15 years as drain tubing. Nylon tubing breaks up if exposed to sunlight for 5 to 10 years. PVC (polyvinyl chloride) pipe is tolerable, although during the first few weeks of still operation it usually emits a gas, making the distilled water taste bad. Ordinary clear vinyl tubing is unacceptable. There is a "food grade" clear vinyl tubing that is supposed to be satisfactory for drinking water, but the sun's rays are likely to degrade it if it's used in a solar still. Companies sell drinking water and milk in high-density polyethylene bottles, and have had satisfactory results. But put the same plastic bottle filled with water in the sun, and the plastic will degrade, imparting a bad taste to the water. Few plastics can withstand heat and sunlight. Brass, galvanized steel, or copper may be used in the feed system, but not in the product system.
One final note: Although a solar still repeatedly subjected to freezing will remain unharmed, drain tubes so exposed may freeze shut unless you make them extra large. Feed tubes can easily be arranged with drain-back provision to prevent bursting.
Fittings. Fittings are connection devices that hold pipe segments together. If you put a solar still on the market with instructions to consumers that connections be made "finger tight only", people could put a wrench on a connection, loosen it, and be faced with an expensive repair problem. So, the options include having tight control of installation personnel, or doing a thorough training job, or making the equipment rugged enough to withstand ordinary plumbing practice.
A solar still is fed on a batch basis for an hour or two every day. It is necessary to admit some extra water each day, to flush out the brine. There is very little pressure available to get the water to drain, so drainage cannot proceed rapidly. To prevent flooding, it's good practice to insure that the feed rate does not exceed this maximum drainage rate. If one uses needle valves thus to restrict the flow, such valves have been found to be unstable over the years, generally tending to plug up and stop the flow. It has proven to be a satisfactory solution to this problem--when feeding from city water pressure of typically 50 p.s.i.--to use a length of small diameter copper tubing, such as 25 feet or more of 1/8 inch outside diameter, or 50 feet of 3/16 inch outside diameter tubing, to serve as a flow restrictor. It needs to have a screen ahead of it, such as an ordinary hose filter washer, with 50 mesh or finer stainless steel screen, to prevent the inlet end from plugging.
Storage Reservoir. In selecting materials for the storage reservoir, two precautions should be noted.
- Distilled water is chemically aggressive, wanting to dissolve a 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 of different substances, is not well documented, but the practical consequences are that some things, such as steel, galvanized steel, copper, brass, solder, and mortar, which distilled water, resulting in damage or destruction of the tank component, and quite possibly in contamination of the water. Stainless steel type 316) is a good choice. Polypropylene laboratory tanks are okay but must not be exposed to sunlight. Butyl rubber lining of some structural framework should be okay. Galvanized steel would last for only a few years, adding some zinc and iron to the water. Concrete should serve, again with the expectation that the concrete will slowly crumble over many years' time. The tiny amount of calcium carbonate that is leached out can be used by the body in the diet. In fact, one way to prevent such chemical attack is to introduce some limestone or marble chips into the distilled water stream, or in the reservoir itself, to pick up some calcium carbonate on purpose, thus greatly slowing the attack on the tank itself.
- Extreme precautions need to be taken to prevent entry of insects and airborne bacteria. Air must leave the reservoir every time water enters it, and must re-enter every time water is drawn off. Use a fine mesh--50 x 50 wires to the inch--or finer screen covering the vent, and turn the opening of the vent/screen assembly downward, to prevent entry of rain water. If this is ignored for even one hour, an insect can get in, and you have germ soup from then on.
Storage capacity should be adequate to contain four to five times the average daily output of the still.
Factors to Consider in Selecting Materials for Basin Still
Let us review the functions of the basin:
- It must contain water without leaking.
- It must absorb solar energy.
- It must be structurally supported to hold the water.
- It must be insulated against heat loss from the bottom and edges.
An infinite number of combinations of materials will serve those functions. The membrane that holds water, for example, may be stiff enough to support the water, but it doesn't have to be. The basin may be rigid enough to support the glass, but it doesn't have to be. In short, a component need not satisfy two functions at the same time. Indeed, it is usually better to select local material that will best do each job separately, and then put them together. But if you can find a material that does a couple of jobs well, so much the better.
In selecting materials for a solar still, there are almost always tradeoffs. You can save money on materials, but you may lose so much in productivity or durability that the "saving" is poor economy.
Summary of Materials Recommended for Basin Still Construction
Where the objective is the lowest cost of water on a 20-year life cycle cost basis, the best materials for building a basin still are:
- silicone compound coating to blacken the bottom of the basin;
- metal ribs spaced 40 centimeters (16 inches) apart to support the underside of the basin;
- about 25 to 38 millimeters of insulation between the ribs (this may be high-temperature urethane foam, or fiberglass);
- a bottom covering of lightweight galvanized steel, or aluminum sheet (note: if you plan to put the still on the ground and use an insulation that is impervious to water, no bottom sheet is needed);
- metal siding, such as extruded aluminum, to support the still (note: extruded aluminum can be assembled quickly, but it is expensive; thus, you may prefer a lower cost material such as painted steel or aluminum;
- a stainless steel trough;
- tempered low-iron glass, or regular double-strength window glass. (If using patterned glass, put the pattern side down);
- extruded gaskets, compressed into final position;
- type 316 stainless steel fittings (note: brass is not acceptable; PVC is acceptable, but poor in very hot climates);
- a mirror behind the still for higher latitudes.
Although these materials are representative of a high-cost still design, they are probably a good investment since none of the inexpensive designs has stayed on the market. However, we must also ask the question, "Expensive compared to what?" Compared to hauling purified water in bottles or tanks, solar distilled water would almost always be much less expensive. Compared to hauling vegetables by airplane to hot desert places, using a solar still to raise vegetables in a greenhouse should be less expensive.
Compared to the cost of boiling water to sterilize it, the solar still should be competitive in many situations. And although the materials used in building a high-cost still will probably always be expensive, mass production could ultimately drive down the unit cost per still.
