Solar Still Basics
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Solar Still Basics
Solar Still Basics
Solar Still Basics
Item#: solstilbas

Product Description


Solar Still Background | Still Operation
Still Water Production | Distillation Purification Capabilities


The basic principles of solar water distillation are simple yet effective, as distillation replicates the way nature makes rain. The sun's energy heats water to the point of evaporation. As the water evaporates, water vapor rises, condensing on the glass surface for collection. This process removes impurities such as salts and heavy metals as well as eliminates microbiological organisms. The end result is water cleaner than the purest rainwater. The SolAqua still is a passive solar distiller that only needs sunshine to operate. There are no moving parts to wear out.

The distilled water from a SolAqua still does not acquire the "flat" taste of commercially distilled water since the water is not boiled (which lowers pH). Solar stills use natural evaporation and condensation, which is the rainwater process. This allows for natural pH buffering that produces excellent taste as compared to steam distillation. Solar stills can easily provide enough water for family drinking and cooking needs.

Solar distillers can be used to effectively remove many impurities ranging from salts to microorganisms and are even used to make drinking water from seawater. SolAqua stills have been well received by many users, both rural and urban, from around the globe. SolAqua solar distillers can be successfully used anywhere the sun shines.

The SolAqua solar stills are simple and have no moving parts. They are made of quality materials designed to stand-up to the harsh conditions produced by water and sunlight. Operation is simple: water should be added (either manually or automatically) once a day through the still's supply fill port. Excess water will drain out of the overflow port and this will keep salts from building up in the basin. Purified drinking water is collected from the output collection port.

Supply Fill Port: Water should be added to the still via this port. Water can be added either manually or automatically. Normally, water is added once a day (in the summer it's normally best to fill in the late evening and in the winter, in the early morning). Care should be taken to add the water at a slow enough flow rate to prevent splashing onto the interior of the still glazing or overflowing into the collection trough.

Overflow Port: Once the still basin has filled, excess water will flow out of this port. SolAqua recommends three times daily distilled water production to be allowed to overflow from the still on a daily basis to prevent salt build-up in the basin. If your still produced 2 gallons of product water then you should add 6 gallons of fresh feedwater through the fill port. If flushed like this on a daily basis, the overflow water can be used for other uses as appropriate for your feedwater (for example, landscape watering).

Distilled Output Collection Port: Purified drinking water is collected from this port, typically with a glass collection container. Stills that are mounted on the roof can have the distillate output piped directly to an interior collection container. For a newly installed still, allow the collection trough to be self-cleaned by producing water for a couple of days before using the distillate output.

Solar Still Background
Solar distillation is a tried and true technology. The first known use of stills dates back to 1551 when it was used by Arab alchemists. Other scientists and naturalists used stills over the coming centuries including Della Porta (1589), Lavoisier (1862), and Mauchot (1869).

The first "conventional" solar still plant was built in 1872 by the Swedish engineer Charles Wilson in the mining community of Las Salinas in what is now northern Chile (Region II). This still was a large basin-type still used for supplying fresh water using brackish feedwater to a nitrate mining community. The plant used wooden bays which had blackened bottoms using logwood dye and alum. The total area of the distillation plant was 4,700 square meters. On a typical summer day this plant produced 4.9 kg of distilled water per square meter of still surface, or more than 23,000 liters per day. This first stills plant was in operation for 40 years!

Over the past century, literally hundreds of solar still plants and thousands of individual stills have been built around the world. SolAqua stills have built upon years of still research and development, use NSF and FDA approved materials, and are the state of the art for commercial solar still distillation.

Still Operation
A solar still operates on the same principle as rainwater: evaporation and condensation. The water from the oceans evaporates, only to cool, condense, and return to earth as rain. When the water evaporates, it removes only pure water and leaves all contaminants behind. Solar stills mimic this natural process.

A SolAqua single basin solar still has a top cover made of glass, with an interior surface made of a waterproof membrane. This interior surface uses a blackened material to improve absorption of the sun's rays. Water to be cleaned is poured into the still to partially fill the basin. The glass cover allows the solar radiation (short-wave) to pass into the still, which is mostly absorbed by the blackened base. The water begins to heat up and the moisture content of the air trapped between the water surface and the glass cover increases. The base also radiates energy in the infra-red region (long-wave) which is reflected back into the still by the glass cover, trapping the solar energy inside the still (the "greenhouse" effect). The heated water vapor evaporates from the basin and condenses on the inside of the glass cover. In this process, the salts and microbes that were in the original water are left behind. Condensed water trickles down the inclined glass cover to an interior collection trough and out to a storage bottle.

The still is filled each morning or evening, and the total water production for the day is collected at that time. The still will continue to produce distillate after sundown until the water temperature cools down. Feedwater should be added each day that roughly exceeds the distillate production to provide proper flushing of the basin water and to clean out excess salts left behind during the evaporation process.

The intensity of solar energy falling on the still is the single most important parameter affecting production. The daily distilled water output (M e in kg/m2 day) is the amount of energy utilized in vaporizing water in the still (Q e in J/m2 day) over the latent heat of vaporization of water (L in J/kg). Solar still efficiency (n) is the amount of energy utilized in vaporizing water in the still over the amount of incident solar energy on the still (Q t in J/m2 day). These can be expressed as:

Solar still production: M e = Q e / L
Solar still efficiency: n = Q e / Q t

Typical efficiencies for single basin solar stills approach 60 percent. General operation is simple and requires facing the still towards solar noon, putting water in the still every morning to fill and flush the basin, and recovering distillate from the collection reservoir (for example, glass bottles). Stills are modular and for greater water production requirements, several stills can be connected together in series and parallel as desired.

As water evaporates from the solar still basin, salts and other contaminants are left behind. Over time, these salts can build to the point of saturation if the still is not properly maintained and flushed on a regular basis. Properly operating a still requires about three times as much make-up water as the distillate produced each day. If the still produced 3 gallons of water, 9 gallons of make-up water should be added, of which 6 gallons leaves the still as excess. The excess water flushes the still basin through the overflow to prevent salt buildup. If this is done on a daily basis, the flushed water is of approximately the same quality as the original feedwater that was added to the still. The excess water is of suitable quality that it can be used to water landscaping, wash pots and pans, etc. No sediment or sludge will buildup if the still is properly operated and flushed daily.

Still Water Production
Solar still production is a function of solar energy (insolation) and ambient temperature. A SolAqua solar still produces about 0.8 liters of purified water per sun-hour (i.e. 1 kWh). Thus, production rates in the Southwest U.S. can average about 3 liters per day in the winter (4 kWh/m2/day) to over 6 liters per day during the summer (8 kWh/m2/day

Distillation Purification Capabilities
Solar stills have proven to be highly effective in cleaning up water supplies to provide safe drinking water. The effectiveness of distillation for producing safe drinking water is well established and recognized. Most commercial stills and water purification systems require electrical or other fossil-fueled power sources. Solar distillation technology produces the same safe quality drinking water as other distillation technologies; only the energy source is different: the sun.

Distillation is the only stand alone point-of-use (POU) technology with National Sanitation Foundation (NSF) international certification for arsenic removal, under Standard 62. Solar distillation removes all salts as well as biological contaminants (for example,cryptosporidium, E. coli, etc.). There are many studies in the literature, such as tests conducted on solar stills at New Mexico State University and Sandia National Laboratories, that clearly verify solar stills' effectiveness in eliminating microbial contamination and salts.

If you have any questions about what a solar still can effectively clean, please contact SolAqua for additional information.