IV. OPERATION AND MAINTENANCE OF SOLAR STILLS[edit | edit source]
OPERATING REQUIREMENTS OF BASIC STILLS[edit | edit source]
Protecting Distilled Water from Contamination
Protecting a solar still against the entry of insects and polluted rainwater is important. After your still is installed, you must:
- disinfect the interior of the still and tubing with chlorine compounds (adding a few spoonfuls of laundry bleach to a few liters of water does the job nicely); and
- provide a vent(*) in the feed tube at the still, screened with fine stainless steel screen filter washer in a pipe fitting, turned downward to prevent entry of contaminated rainwater. If these precautions are not taken, flying insects, attracted by the moisture, might find their way in and die in the distillate trough.
Preventing contamination in a storage reservoir is a little more difficult, as the daily high temperature are not available to pasteurize the water. Nevertheless, with diligent attention to detail, the system can be used for decades without contamination.
Filling and Cleaning a Basin Still
Filling a basin still is a batch process (*), done once a day, at night or in the morning. With a still of this design, about 5 to 7 percent of the day's total distilled water is produced after sundown, so it is important to wait until the still is cold. Refilling it between three hours or more after sundown and up to one or two hours after sunrise will cause little, if any, loss of production.
(*) A vent allows air to enter and exit the still daily during the operation and refilling.
It is not necessary to drain the still completely. Refilling it with at least twice as much as it produces will normally dilute and flush it adequately. Three times as much would keep it a little cleaner, and could be worth doing, provided the cost of feed water is nominal. A rapid mechanical flushing is not required; a gentle trickle does the job.
Feeding Hot Water to a Basin Still
If a basin still is fed water that is hotter than the ambient air, the unit becomes a conventional distiller, except that it uses glass instead of copper as the condenser. If the hot water is virtually cost-free, as is geothermal or waste water, it can be well worth doing. If the feed water is heated by fossil fuels or by separate solar panels, the economics look doubtful, and the feed line tends to plug up with scale.
FACTORS INFLUENCING SOLAR STILL OPERATING PERFORMANCE[edit | edit source]
In this section, we discuss some important factors that influence the rate of production of distilled water. These include climatic factors, thermal loss factors, and solar still design factors.
Radiation: Its Effect on Efficiency. The amount of solar radiation a solar still receives is the single most important factor affecting its performance. The greater the amount of energy received, the greater will be the quantity of water distilled. Figure 8 shows the rate of production of a basin still on the basis of specific solar inputs.
Solar stills produce less distilled water in winter than in summer, which is a problem. To some extent, the demand for drinking water also varies with the seasons, by as much as perhaps 2 to 1, summer over winter. But the annual sunlight variation affecting a still's solar distillation rate is greater than that, at least in regions well outside the tropics. In the tropics, at latitudes of less than 20 [degrees], the annual sunlight variation is probably well under 2 to 1, so it may not be a problem there. The farther away from the equator, the greater the annual sunlight variation, to perhaps 7 to 1 at 40 [degrees] latitudes. This is unacceptable, making use of a solar still difficult in winter at high latitudes.
(*) Note that there are other methods available for large distillation plants. However, because they fall outside the scope of this paper, they are not discussed here. Many approaches have been tried to solve this problem. Tilting the whole still up to more or less an equatorial mount brings the ratio down very nicely. This is called the "inclined-tray" still, and is accomplished by using many small pans in a stair-step arrangement. With this arrangement, total sunlight striking the aperture of the glass remains more constant, and the light which glances off the water of one small tray warms the bottom of the one above it, improving performance. While this is a substantial advantage, it is the only advantage of this design, and it must be weighed against the disadvantages of higher costs associated with putting many small pans vs. only one in the enclosure, and, most probably, higher installation costs due to holding the end of the pan higher off the supporting surface, and protecting it against wind loads. In latitudes perhaps 20 [degrees] on up, it seems possible that the inclined-tray will find a place in the market.
Using an inclined-tray still is only one solution to the problem of annual variation in higher latitudes. Some other steps that can be taken include:
- buying an extra large still that produces enough distilled water in winter, resulting in a likelihood that you will have more water than you need in summer;
- using less water in winter and/or using some tap water;
- buying supplemental water in winter; or
- saving some of the excess distilled water made in summer or fall for use in winter;
- installing a mirror behind the basin to reflect additional sunlight back into the still in winter. To reflect back as much light as possible, use a reflective surface of about one-third to one-half of the aperture of the glass cover, tilted forward 10 [degrees] from the vertical, mounted at the rear edge of the still. In latitudes between 30 [degrees] and 40 [degrees], this gives from 75 to 100 percent more yield in mid-winter.
Condensing-Surface Temperature. Much work has been done to try to obtain lower condensing temperatures, thereby increasing the temperature difference between the heated feed water and the condensing surface. This approach undoubtedly derives from 100 years of steam power engineering, in which it is most important to get the steam temperature high and the condensing temperature low to gain efficiency. But this principle does not hold true for a solar still. Steam for power is pure steam, whereas the contents of a solar still are both air and water vapor. It has been demonstrated repeatedly that the higher the operating temperature of the still--insolation being equal--the higher the efficiency. For each 6 [degrees] celsius (10 [degrees] F) increase in ambient temperature, the production of a still increases by 7 to 8 percent. The practical effect of this is that a still operating in a hot desert climate will produce typically as much as one-third more water than the same unit in a cooler climate.
(By the same token, cooling the glazing cover of a solar still by spraying water on it or blowing air over it does not help the still produce more distillate. In an experiment at the University of California in the United States, two identical stills were built. The glazing cover of the first still was fan-cooled; the cover of the second still was not. Of the two stills, the cooled unit produced significantly less distillate. Consequently, it's better to put the still in a protected area rather than a windy area.)
Thermal Loss Factors[edit | edit source]
Production is also associated with the thermal efficiency of the still itself. This efficiency may range from 30 to 60 percent, depending on still construction, ambient temperatures, wind velocity, and solar energy availability. Thermal losses for a typical still vary by season, as shown in Table 5.
Table 5. Distribution of Incoming Solar Radiation in the Distillation Process
|Thermal Loss Factors||December (%)||May (%)|
|Reflection by Glass||11.8||11.8|
|Absorption by Glass||4.1||4.4|
|Radiative Loss from Water||36.0||16.9|
|Internal Air Circulation||13.6||8.4|
|Ground and Edge Loss||2.1||3.5|
|Re-Evaporation and Shading||7.9||14.5|
|[Remainder of Energy Used to Distill Water]||24.5||40.5|
Direct Use of the Sun's Energy, Daniels, Farrington, 1964, Ballantine Books, page 124.
Solar Still Design Factors
Slope of the Transparent Cover. The angle at which the transparent cover is set influences the amount of solar radiation entering a solar still. When sunlight strikes glass straight on, at 90 [degrees] to the surface, about 90 percent of the light passes through. Tip the glass a little, so that it strikes at a "grazing angle" of 80 [degrees], and only a few percent is lost. But tilt it a few more tens of [degrees], and the curve goes over the hill, dropping off to practically zero at 20 [degrees] grazing angle, where virtually no direct light gets through. In a greenhouse-type still, for a large part of the year the half of the glass that is facing away from the equator is receiving sunlight at very low grazing angles. It is actually shadowing the back one-third of the still. It is more efficient to make that half of the glass facing the equator as long as possible, and put a more or less reflective back wall to the rear. This was one of the significant steps that has increased the efficiency of basin stills from 31 to about 43 percent, using a single slope of glass. And it costs less to build.
The slope of the glass cover does not affect the rate at which the distillate runs down its inner surface to the collection trough. A common misconception was that the glass cover must be tilted to get the water to run off. This may have arisen from the fact that ordinary window glass, as it comes from the factory, has a minute oily film on it. But if the glass is clean, the water itself will form filmwise condensation on it, and will be able to run off at a slope as little as 1 [degrees].
There are three reasons why it is best to use as low a slope as possible: (1) the higher the slope, the more glass and supporting materials are needed to cover a given area of the basin; (2) the higher slope increases the volume and weight [of the still] and therefore shipping costs; and (3) setting the glass at a high slope increases the volume of air inside the still, which lowers the efficiency of the system. A glass cover that is no more than 5 to 7 centimeters from the water surface will allow the still to operate efficiently. Conversely, as glass-to-water distance increases, heat loss due to convection becomes greater, causing the still's efficiency to drop.
Some important stills have been built following the low-slope design concept for the glass cover, yet using a short, steeply sloping piece of glass at the rear. This requires either providing an extra collection trough at the rear, or else making the successive troughs touching heel and toe, so that it is exceedingly difficult to get out in the middle of the array to service anything. It also increases the condensing surface relative to the absorber, which reduces operating temperatures in the still, and is clearly disadvantageous. A reflective and insulated back may be preferable to glass.
Some years ago at the University of California, researchers built an experimental multiple tray tilted still with an average glass-to-water distance of about 30 millimeters, showing an efficiency of 62 percent, one of the highest ever recorded. The loss of efficiency is greatest the first centimeter, rather less the second cm, and so on, tailing off to smaller rates of loss per cm distance as far as the test was carried. This is one of the principle reasons a high slope of glass is to be avoided.
In sum, it is clear that a solar still should be built in a way that will get the water as hot as possible, and keep it as close to the glass as possible. This is achieved by keeping the glass cover at a minimum distance from the water surface, which in practical terms, falls between 5 and 7 cm., and by minimizing the depth of water in the pan, to about 1.5 cm.
Wicks and Related Techniques
Researchers have tried to improve the efficiency of a solar still by enhancing its surface evaporation area using wicks. In a side-by-side test of two identical stills at the University of California, using a floating black synthetic fabric in one still and nothing in the other, the difference in production between the stills was indistinguishable, though similar tests have reported some improvement. It seems exceedingly difficult to find a wick material that will last for 20 years in hot saline water, and that will not get crusted up with salts over a period of time. As for putting dye in the water, studies suggest that the slight improvement in performance does not justify the increased cost and maintenance and operating problems associated with this technique.
Putting dark-colored rocks in the feedwater to store heat for use after nightfall has been reported by Zaki and his associates to improve performance by 40 percent, but he does not give the reference point from which this is measured. If he was comparing one still containing 4 cm. of water with another same water depth but containing black stones, the productivity would increase somewhat due to the decrease in thermal mass and resulting increase in operating temperature. Reducing the initial water depth might have accomplished the same result. For this reason, placing dark-colored rocks in the feedwater does not appear to be a promising technique for improvements in solar still performance.
MAINTENANCE REQUIREMENTS OF BASIN STILLS[edit | edit source]
Ways of Handling the Buildup of Mineral Deposits
It is inevitable that some minerals are deposited on the bottom of the basin. In most situations, including sea water and city tap water, the amount deposited is so small that it creates no problem for decades. One still in particular has been operated for 20 years without ever having been opened or cleaned. As long as there is not an excessive buildup of deposits, indicated by formation of a dried-out island in the afternoon, they create no problem. Such mineral deposits become the normal absorber. An accumulation of these deposits changes the interior surface of a basin from its original black color to a dark earth brown, reflecting some sunlight, causing a 10 percent drop in still production. To offset this reduction, simply make the still 10 percent larger than it would need to be if it were cleaned out periodically.
Some desert waters high in alkalis will deposit a whitish gray scale on the bottom and sides of a basin. In fact, almost any feed water will do so, especially if the basin is allowed to dry out. In some cases, the alkaline water may form a crust of scale which is held on the water's surface by air bubbles that are discharged when the feed water is heated. Light-colored deposits such as these may reduce production of the still by 50 percent or more. Those that settle to the bottom of the basin can be easily coated black by mixing one tablespoon of black iron oxide concrete coloring powder with about 10 or 15 liters of water and adding the solution to the still by means of a funnel connected to the feed water pipe. This blackening agent is inert, and imparts no bad taste or odor to the distilled water. After the solution reaches the basin through the feed water pipe, it settles on the bottom of the basin and restores it to its original black color. Some owners do this each fall, when production begins to drop. Cost is only pennies per application.
Deposits that float on the surface of the water in a basin are a tougher problem and one that requires more research. An Australian solar still expert suggests agitating the contents of the still by recirculating, or stirring, the water in the pan for one hour each night, to minimize the buildup of floating deposits. Adding a pint or two of hydrochloric (swimming pool) acid to the still whenever the bottom becomes grayish-white--every year or two, maybe oftener in some cases--is a satisfactory way of removing practically all of the scale.
Accumulation of Dust on the Glazing Cover: What to Do
In the vast majority of stills, dust accumulates on the glass cover. But it does not keep building up; it's held more or less constant by the action of rain and dew. This "normal" accumulation causes production to drop from 5 to 15 percent. To offset this, simply make your still 10 percent larger than it would need to be if kept clean. However, if the still is in an unusually dusty area, or if it is large enough that a caretaker is available at modest cost, cleaning the glazing cover is justified. Ten percent of 10,000 liters per day may be enough to justify cleaning the cover once a month in the dry season.
Repair and Replacement of Basin Still Components
As with all devices, the components of a basin still may need to be repaired or replaced from time to time. The frequency depends on the type of material used to construct the still. One built with premium materials will require almost no maintenance, but will entail a higher capital cost because many of the materials must be imported materials. Use of cheaper materials subject to degradation will almost certainly lower the initial cost, but will increase the amount of maintenance. Even so, if the long-term cost of maintenance and the lower initial cost are less than the higher initial cost for premium materials, this may present a better option, especially if cost of capital is high. This is called "life cycle cost analysis," and it is strongly recommended.
SKILLS REQUIRED TO BUILD, OPERATE, AND MAINTAIN A BASIC STILL[edit | edit source]
Craftmanship and attention to detail in construction are important for an efficient, cost-effective still.
In addition, supervisory personnel must be on hand who know how to size stills to meet a community's water supply needs; who know how to orient stills; who are familiar with required construction techniques; and who have the ability to train others in the construction, operation and maintenance of stills.
Finally, it is important to ask local workers to participate in the planning and construction phases of a solar still project to get the indigenous population to accept the technology. A sense of pride in the building of the project may well mean the difference between long-term success or failure of the project.
COST/ECONOMICS[edit | edit source]
The cost and economics of solar stills depend on many variables, including:
- cost of water produced or obtained by competing technologies;
- water requirements;
- availability of sunlight;
- cost of locally-available materials;
- cost of local labor;
- cost of imported materials; and
- loan availability and interest rates.
Table 6 shows the variation in costs for stills built in the 1970s in the Philippines, India, Pakistan, and Niger. Note that stills built in Niger in 1977 cost twice as much as those built in the Philippines in the same year, reflecting the wide variation in local cost.
Table 6: Variation in Costs for Stills Built in the 1970s
|Location||Year Built||(Dollars/Square Foot)|
(Costs today are undoubtedly higher.)
WHY BUY A STILL?--It saves money.[edit | edit source]
A solar still must operate with extremely low costs for maintenance arid operation. Over a long period according to a study by George Lof, it is valid to assume that 85 percent of the cost of water from the still will be chargeable to the costs of buying it; the remainder to operation and maintenance.
It is easy to calculate the return on investment in a solar still. Say you have one that produces a daily amount of water that would cost you $1 to buy in bottles: then that still returns you $365 per year. If the still had cost you $365, then it paid for itself in one year; if five times that much, then five years, etc.--not counting interest. Cost of feeding water into it is pretty small, but will increase the payout period a little also. In the United States, the payout period tends to run between two and five years, depending on the still's size and features.
SPECIAL DESIGN VARIATIONS[edit | edit source]
The majority of information presented thus far has centered on the basin-type solar still because it is the easiest to construct and may use a wide range of materials, making it adaptable to different locales. But variations of the basin still are possible, such as the double-slope and single-slope stills depicted earlier in this paper. In addition to these options, there are other ways to design the still to increase its efficiency or potential to produce potable water. Some of these are discussed below.
Basin Stills Equipped with Reflectors
Some stills have been equipped with reflective materials which have the potential to increase the amount of sunlight falling on the still without having to increase the area of the still. At latitudes in the thirties, performance increases in winter of 100% have been achieved with a mirror of less than 1/2 the area of the glass. In the tropics, of course, this function is not required. A second question arises about using mirrors to enhance production year round. This becomes a focusing collector, which introduces substantial additional costs and problems. If the mirror assembly is cheaper than the pan assembly, then it deserves to be looked at further, but it is not attractive at this time. Tentatively, reflective aluminum sheet has the most advantages.
Basin Stills Equipped with Insulated Glazing Covers
Another innovation is the use of an insulated glazing cover to be put over the glazing at night or during extremely cold weather. This cuts heat losses, allowing distillation to continue longer, and retains heat overnight, causing production to start earlier the next day. Cost-benefit analysis of this approach has not been made.
V. COMPARING THE ALTERNATIVES[edit | edit source]
For a couple of gallons of purified water a day, there is no method that can compete with solar distillation. For a couple of million gallons a day--AS LONG AS WE ARE WILLING TO BURN UP OUR INHERITANCE OF FOSSIL CHEMICAL BUILDING BLOCKS JUST TO EVAPORATE WATER--boiling distillation is the cheapest way to purify sea water.
In sum, solar stills have:
- high initial costs;
- the potential to use local materials;
- the potential to use local labor for construction and maintenance;
- low maintenance costs (ideally);
- no energy costs (not subject to fuel supply interruptions);
- few environmental penalties; and
- in residential sizes, no subsequent costs for delivering water to the end user.
Most competing technologies are:
- low in initial costs;
- dependent on economy of scale;
- high in operating and maintenance-costs;
- high in energy input costs;
- low in local job creation potential;
- vulnerable to changes in energy supply and costs; and
VI. CHOOSING THE TECHNOLOGY RIGHT FOR YOU[edit | edit source]
FACTORS TO CONSIDER[edit | edit source]
Solar energy is an excellent choice for water distillation in those areas of the Third World that meet the following conditions:
- expensive fresh water source (US) $1 or more per 1,000 gallons);
- adequate solar energy; and
- available low-quality water for distillation.
Other conditions suitability for solar stills are:
- competing technologies that require expensive conventional wood, or petroleum fuels;
- isolated communities that may not have access to clean water supplies;
- limited technical manpower for operation and maintenance of equipment;
- areas lacking a water distribution system; and
- the availability of low-cost construction workers.
The greater the number of these conditions present, the more solar stills are likely to be a viable alternative. If the cost of the water produced by a still over its useful life is less than by alternate methods, it is economical to pursue.
Other factors to consider are the availability and cost of capital, as well as the local tax structure, which may allow tax credits and depreciation allowances as a means to recover a portion of the cost. This has proved to be a major incentive in the United States.
Finally, the acceptance of solar distillation will depend greatly on how well one understands and handles the many social issues and cultural constraints that can hamper the introduction of new technologies. Some of the more important issues that may affect the acceptance of solar distillation are outlined below.
- Stills built for village use require community cooperation that may be foreign to some cultural groups. If the distilled water is incorrectly distributed, causing a family unit not to receive its fair share of water, this could become a source of conflict. For this reason, a family-sized solar still unit, which a household has complete control over, may be more practical than a unit that serves an entire village.
- Potential users who think they will find distilled water tasteless or not in keeping with what they are accustomed to may become disappointed and possibly abandon altogether the thought of drinking the water. The problem of taste must be dealt with early on so as not to give people a reason to respond negatively to the technology as a whole.
- In some societies, conflicts may arise over whether it is the responsibility of the man or the woman of the household to operate the solar still. Not dealing with this issue early on could result in the household's total rejection of the technology.
- If solar distillation is perceived to be a threat to a community's traditional lifestyle, the community may reject the technology. Such concerns can be headed off if the technology is designed appropriately from the start and introduced at the proper time. Moreover, a community is more likely to accept the technology if it recognizes the importance of clean water and considers it a priority to the degree that it is willing to change certain aspects of its lifestyle.
MARKET POTENTIAL[edit | edit source]
Three potential markets exist for solar stills. First, a solar still can be economically attractive almost any place in the world where water is hauled and where a source of water is available to feed the still.
Second, many people who boil their water to kill germs could use a solar still for the same purpose. It will take more work to demonstrate this function adequately, but early tests have made it seem highly promising.
A third market is in arid regions, whose untapped water resources may be sufficient to economically provide a population with potable water.
CONCLUSION[edit | edit source]
Worldwide experience in researching and marketing solar stills over three decades has provided an ample foundation for a solar still industry. No inherent technical or economic barriers have been identified. A solar still is suited to village [manufacturing] techniques and to mass production. Around the world, concerns over water quality are increasing, and in special situations a solar still can provide a water supply more economically than any other method. Commercial activities are picking up after a lull during the late 1970s. It is now possible to predict a rapid increase in the manufacture and marketing of solar stills.
SUPPLIERS AND MANUFACTURERS OF SOLAR STILLS[edit | edit source]
P.O. Box 981
Laguna Beach, California 92652-0981
CEDEX No. 3
F. 92080 Paris La Defense
Cornell Energy, Inc.
4175 South Fremont
Tucson, Arizona 85714
BIBLIOGRAPHY[edit | edit source]
Cooper, P.I., "Solar Distillation--State of the Art and Future Prospects." Solar Energy and the Arab World (1983): 311-30.
Daniels, Farrington. Direct Use of the Sun's Energy. New York, New York: Ballantine Books, 1975.
El-Rafaie, M.E.; El-Riedy, M.K.; and El-Wady, M.A. "Incorporation of Fin Effect in Predicting the Performance of Cascaded Solar Stills." Solar Energy and the Arab World (1983): 336-40.
Goetchew, Martin. "Shedding Light on Solar Collector Glazing." Materials Engineering 90 (September 1979): 55-58.
Langa, Fred; Flower, Bob; and Sellers, Dave. "Solar Glazzings: A Product Review." New Shelter (January 1982): 58-69.
Leckie, Jim; Master, Gil; Whitehouse, Harry; and Young, Lily. More Other Homes and Garbage. San Francisco, California: Sierra Club Books, 1981.
Mohamed, M.A. "Solar Distillation Using Appropriate Technology." Solar Energy and the Arab World (1983): 341-45.
Talbert, S.G.; Eibling, J.A.; and Lof, George. Manual on Solar Distillation of Saline Water. Springfield, Virginia: National Technical Information Service, April 1970.
Dunham, Daniel C. Fresh Water 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 Performance of Solar Stills." Solar Energy and the Arab World (1983): 331-35.
McCracken, Horace: Only a small amount of McCracken's work has been published, but the data are available. Inquiries will be welcomed:
McCracken Solar Co.
P.O. Box 1008
Alturas, California 96101