A solar still is a device that uses the sun’s energy to purify water by distillation. It is a simple and effective way to obtain clean drinking water from contaminated sources such as seawater, brackish water, or even dirty rainwater. Solar stills are commonly used in areas where access to cle Contact online >>
A solar still is a device that uses the sun’s energy to purify water by distillation. It is a simple and effective way to obtain clean drinking water from contaminated sources such as seawater, brackish water, or even dirty rainwater. Solar stills are commonly used in areas where access to clean water is limited, such as remote locations or during emergencies.
A solar still works by harnessing the heat from the sun to evaporate water and then condense the vapor to produce clean drinking water. The process begins with the contaminated water being poured into a basin or container placed inside the still. The sun’s rays heat the water, causing it to evaporate and rise to the top of the still. The vapor then condenses on a sloped surface, such as a clear plastic sheet, and drips down into a collection container as purified water.
There are several benefits to using a solar still to purify water. One of the main advantages is that it requires no electricity or fuel to operate, making it a cost-effective and environmentally friendly solution. Solar stills are also easy to use and maintain, making them ideal for use in remote or off-grid locations. Additionally, solar stills can effectively remove contaminants such as salt, bacteria, and other impurities from water, providing a reliable source of clean drinking water.
There are several different types of solar stills, each with its own design and method of operation. Some common types include single slope stills, double slope stills, and multiple effect stills. Single slope stills have a sloped surface that collects condensed water, while double slope stills have two sloped surfaces to increase the efficiency of water collection. Multiple effect stills use a series of chambers to further purify water through multiple distillation stages.
Building a solar still is a relatively simple process that can be done using common materials found around the home. To build a basic solar still, you will need a clear plastic sheet, a container for the contaminated water, and a collection container for the purified water. Start by placing the contaminated water in the container and covering it with the plastic sheet, making sure to seal the edges to prevent vapor from escaping. Place the still in direct sunlight and wait for the water to evaporate and condense on the plastic sheet, dripping down into the collection container.
Solar stills have a wide range of applications, from providing clean drinking water in remote areas to purifying water during emergencies or natural disasters. They can also be used for desalination to convert seawater into freshwater for agricultural or industrial purposes. Solar stills are a versatile and sustainable solution for water purification that can help improve access to clean water in communities around the world.
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This paper is one of a series published by Volunteers in Technical Assistance to provide an introduction to specific state-of-the-art technologies of interest to people in developing countries. The papers are intended to be used as guidelines to help people choose technologies that are suitable to their situations. They are not intended to provide construction or implementation details. People are urged to contact VITA or a similar organization for further information and technical assistance if they find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated almost entirely by VITA Volunteer technical experts on a purely voluntary basis. Some 500 volunteers were involved in the production of the first 100 titles issued, contributing approximately 5,000 hours of their time. VITA staff included Maria Giannuzzi as editor, Suzanne Brooks handling typesetting and layout, and Margaret Crouch as project manager.
VITA is a private, nonprofit organization that supports people working on technical problems in developing countries. VITA offers information and assistance aimed at helping individuals and groups to select and implement technologies appropriate to their situations. VITA maintains an international Inquiry Service, a specialized documentation center, and a computerized roster of volunteer 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 the technology presented in this paper, contact VITA at 1815 North Lynn Street, Suite 200, Arlington, Virginia 22209 USA.
Ninety-seven percent of the earth's water mass lies in its oceans. Of the remaining 3 percent, 5/6 is brackish, leaving a mere .5 percent as fresh water. As a result, many people do not have access to adequate and inexpensive supplies of potable water. This leads to population concentration around existing water supplies, marginal health conditions, and a generally low standard of living.
Solar distillation uses the heat of the sun directly in a simple piece of equipment to purify water. The equipment, commonly called a solar still, consists primarily of a shallow basin with a transparent glass cover. The sun heats the water in the basin, causing evaporation. Moisture rises, condenses on the cover and runs 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 still that is both effective and long-lasting, it can produce purified water at a reasonable cost if it is built, operated, and maintained properly.
This paper focuses mainly on small-scale basin-type solar stills as suppliers of potable water for families and other small users. Of all the solar still designs developed thus far, the basin-type continues to be the most economical.
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.
P.I. Cooper, in his efforts to document the development and use of solar stills, reports that 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 modern solar still was built in Las Salinas, Chile, in 1872, by Charles Wilson. It consisted of 64 water basins (a total of 4,459 square meters) made of blackened wood with sloping glass covers. This installation was used to supply water (20,000 liters per day) to animals working mining operations. After this area was opened to the outside by railroad, the installation was allowed to deteriorate but was still in operation as late as 1912--40 years after its initial construction. This design has formed the basis for the majority of stills built since that time.
During the 1950s, interest in solar distillation was revived, and in virtually all cases, the objective was to develop large centralized distillation plants. In California, the goal was to develop plants capable of producing 1 million gallons, or 3,775 cubic meters of water per day. However, after about 10 years, researchers around the world concluded that large solar distillation plants were much too expensive to compete with fuel-fired ones. So research shifted to smaller solar distillation plants.
Despite the growing discouragement over community-size plants, McCracken Solar Company in California continued its efforts to market solar stills for residential use. Worldwide interest in small residential-units is growing, and now that the price of oil is ten times what it was in the 1960s, interest in the larger units may be revived.
Although solar distillation at present cannot compete with oil-fired desalination in large central plants, it will surely become a viable technology within the next 100 years, when oil supplies will have approached exhaustion. When that day arrives, the primary question will be, "Which method of solar distillation is best?" Meanwhile, almost anyone hauling drinking water any distance would be economically better off using a solar still.
Solar distillation could benefit developing countries in several ways:
The energy from the sun used to distill water is free. But the cost of building a still makes the cost of the distilled water rather high, at least for large-scale uses such as agriculture and flushing away wastes in industry and homes. Consequently, the solar still is used principally to purify water for drinking and for some business, industry, laboratory, and green-house applications. It also appears able to purify polluted water.
For field agriculture, the solar still is not very promising. It takes about one meter depth of irrigation water per year to produce crops in dry climates, whereas the solar still can evaporate about two meters' depth. Thus, one square meter of solar still would irrigate two square meters of land. Unquestionably, the cost of building the still would make water more valuable than the crops being produced. This may not be true, however, for agriculture in controlled environments, i.e., greenhouses. A well-designed hydroponically-operated greenhouse should be able to produce 8 to 10 times as much food, per unit volume of water consumed, as field crops.
Since salt is a very cheap industrial material, and a solar still cannot produce anymore than an open pond, combining the recovery of salt with the distilling of water is not attractive economically. Where a family is using a solar still to provide water valued at $1 per day, the amount of salt they need might cost them half a cent.
Although it seems possible that potable water can be recovered from sewage, if contaminants such as odorous gases are present in sewage water fed to the still, some portion of those gases will evaporate and condense with the distilled water. In all probability they could be filtered out with activated carbon, but to date, however, no one has had any experience with this.
If the "contaminant" is alcohol, it can be separated from the water. But it would take two or three passes through the still to attain a high enough concentration of alcohol to be used as a fuel. Considering the current availability of fossil fuels, producing alcohol in this way is not yet economical. However, when fossil fuel supplies run low and the price rises, solar distillation could play a significant role.
Whether or not solar distillation can actually purify polluted water is not yet known. Laboratory tests have shown, however, that a solar still can eliminate bacteria. If after additional research, a quantity of clean water can be recovered from polluted water, this capability may become economically more important than the purification of sea water. It may also be used to remove toxic substances such as pesticides.
Preliminary laboratory tests show that a modified version of the still--now commercially available--can do a very good job of removing such substances from feed water. Trichloroethylene (TCE), for example, has been removed by a factor of 5,000 to 1; ethylene dibromide (EDB) by 100 to 1; nitrates by 50 to 1; and others within those ranges. Of course, more work must be done to quantify these numbers, not to mention the unending list of chemicals that need to be tested.
Elimination of Algae. While algae will grow in some deep basin stills where the water temperature seldom gets very high, in the shallow basin still it is usually killed by the high temperature.
Distillation operates by the escape of moving molecules from the water surface into the gases above it. Sensible heat--the kind you can measure with a thermometer--is caused by the movement of molecules, zig-zagging about constantly, except that they are not all moving at the same speed. Add energy and they move faster, and the fastest-moving ones may escape the surface to become vapor.
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