Solar distillation refers to the use of solar radiation to purify water with relartively simple equipment. The equipment, commonly called a solar still, consists primarily of a shallow basin with a transparent glass cover. Solar radiation 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 most microbiological organisms.
Distillation has long been considered a way of making salt water drinkable and purifying water in remote locations. As early as the fourth century B.C., Aristotle described a method to evaporate impure water and then condense it for potable use. Arabian alchemists were the earliest known people to use solar distillation to produce potable water in the sixteenth century. But the first documented reference for a device was made in 1742 by Nicolo Ghezzi of Italy, although it is not known whether he went beyond the conceptual stage and actually built it.
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. 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.
Mass production occurred for the first time during the Second World War when 200,000 inflatable plastic stills were made to be kept in life-rafts for the US Navy.
Energy requirements for water distillation
The energy required to evaporate water is the latent heat of vaporisation of water. This has a value of 2260 kilojoules per kilogram (kJ/kg). This means that to produce 1 litre (i.e. 1kg since the density of water is 1kg/litre) of pure water by distilling brackish water requires a heat input of 2260kJ. This does not allow for the efficiency of the heating method, which will be less than 100%, or for any recovery of latent heat that is rejected when the water vapour is condensed.
It should be noted that, although 2260kJ/kg is required to evaporate water, to pump a kg of water through 20m head requires only 0.2kJ/kg. Distillation is therefore normally considered only where there is no local source of fresh water that can be easily pumped or lifted.
Solar still technologies
The glass cover allows the solar radiation (short-wave) to pass into the still, which is mostly absorbed by a 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 heated water vapor evaporates from the basin and condenses on the inside of the glass cover. In this process, the salts, microbes and other impurities 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.
Ther are many different kinds of solar stills that fall into four general categories, (concentrating collector stills; multiple tray tilted stills; tilted wick solar stills; and basin stills. The majority of stills in use are of the basin type.
Concentrating solar stills use parabolic mirrors to focus sunlight onto an enclosed evaporation vessel. This concentrated sunlight provides extremely high temperatures which are used to evaporate the contaminated water. The vapor is transported to a separate chamber where it condenses, and is transported to storage. This type of still is capable of producing from .5 to .6 gallons per day per square foot of reflector area. This type of output far surpasses other types of stills on a per square foot basis. Despite this still's outstanding performance, it has many drawbacks; including the high cost of building and maintaining it and the need for strong, direct sunlight.
A multiple tray tilted still consists of a series of shallow horizontal black trays enclosed in an insulated container with a transparent top glazing cover. The vapor condenses onto the cover and flows down to the collection channel for eventual storage. This still can be used in higher latitudes because the whole unit can be tilted to allow the sun's rays to strike perpendicular to the glazing surface. The tilt feature, however, is less important at and near the equator where there is less change in the sun's position over the still. Even though efficiencies of up to 50 percent have been measured, the practicality of this design is diminished due to the complicated nature of construction involving many components and the high cost for multiple trays and mounting requirements.
A tilted wick solar still draws upon the capillary action of fibers to distribute feed water over the entire surface of the wick in a thin layer. The water is then exposed to sunlight. A tilted wick solar still allows a higher temperature to form on this thin layer than is prodiuced from a larger boddue to high costs due to mounting requirements and essential insulation; the need to frequently clean the cloth wick of built-up sediments, highlighting the need for an operable glazing cover; the need to replace the black wick material on a regular basis due to sun bleaching and physical deterioration by ultra-violet radiation; uneven wetting of the wick which that results in dry spots, leading to reduced efficiency; and the unnecessary aspect of the tilt feature except where it is required higher latitudes.
The basic design of the basin still has many variations, but the actual shape and concept have not changed substantially from the days of the Las Salinas, Chile stills built in 1872. The greatest changes have involved the use of new building materials that lower overall costs while providing an longer lifetime and better performance.
All basin stills have four major components:
- a basin;
- a support structure;
- a transparent glazing cover; and
- a distillate trough (water channel).
In addition to these, ancillary components may include:
- insulation (usually under the basin);
- piping and valves;
- facilities for storage;
- an external cover to protect the other components from the weather and to make the still esthetically pleasing; and
- a reflector to concentrate sunlight.
The actual dimensions of basin stills vary greatly, depending on the availability of materials, water requirements, ownership patterns, and land location and availability. If the only glazing available is one meter at its greatest dimension, the still's maximum inner width will be just under one meter. And the length of the still will be set according to what is needed to provide the amount of square meters to produce the required amount of water. Likewise, if an entire village were to own and use the still, the total installation would have to be quite large.
Most community size stills 0.5 to 2.5 meters wide, with lengths ranging up to around 100 meters. Their lengths usually run along an eastwest axis to maximize the transmission of sunlight through the equatorialfacing sloped glass. Residential, appliance type units generally use glass about 0.65 to 0.9 meter wide with lengths ranging from two to three meters. A water depth of 1.5 to 2.5 cm is most common.
The usual argument for greater depths is that the stored heat can be used at night to enhance production when the air temperatures are lower. Unfortunately, no deep basin has ever attained the 43 percent efficiency typical of a still of minimum water depth. The results to date are clear: the shallower the depth the better. Of course, if the basin is too shallow, it will dry out and salts will be deposited. Note that solar heat can evaporate about 0.5 cm of water on a clear day in summer. By setting the initial charge at about 1.5 cm depth, virtually all of the salts remain in the solution and are flushed out by the refilling operation.
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