Basin Type Solar Still

Reasonable amounts of fresh water can be produced via inexpensive and sturdy solar stills in places that are exposed to solar radiation and have a brackish water. A solar still distills water, using the heat of the Sun to evaporate, cool then collect the water. There are many types of solar still, including large scale concentrated solar stills, and condensation traps (better known as moisture traps amongst survivalists). In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic or glass. The pure water vapour condenses on the cool inside surface and drips down, where it is collected and removed.

SOLAR STILL 

Solar Still is such kind of system for the purification of salt water in which a shallow pool of salt water evaporates within a greenhouse like structure, causing the formation on the ceiling of droplets of fresh water, which drip down into collection pools. A small device originally designed for army and navy fliers forced down in the sea that converts salt water or contaminated water into drinking water by vaporization by the sun's rays and condensation .

HISTORY & DEVELOPMENT OF SOLAR STILL

Phoenician sailors which travelled along the Mediterranean Sea were already making use of the solar thermal radiation to convert seawater into fresh drinking water. The classic Greece philosopher Aristotle described the water cycle in nature and remarked that when salt water turns into vapour and the vapour condenses, it does so in the form of sweet water. In fact, boiling seawater and condensing the vapours on sponges was one of the first means of desalination (Delyannis, 2003). Another was the use of glass jars exposed directly to the sun radiation, which gave rise to the concept of the alembic. During Medieval times, Arab alchemists desalinated seawater using vessels heated with solar radiation concentrated by concave mirrors. In 1589, Della Porta mentions several methods of desalination in his book “Magiae Naturalis”, including his solar distillation unit and a method to obtain freshwater from the air (Delyannis, 2003). But only in 1870, the first patent on solar distillation was granted in the USA to Wheeler and Evans, describing the basic operation of the solar still based on extensive experimental work.

In 1872, the first large installation of solar passive desalination was erected near Las Salinas, in northern Chile, designed by the Swedish engineer Charles Wilson. The installation was built to supply freshwater to the settlement associated to a saltpeter mine. It was fed with effluents of high salinity (140 g kg-1) and produced about 22.7 m3 of freshwater per day, operating since 40 years. It was built of wood blackened with logwood dye and alum to absorb sunlight, using a glass cover. It consisted of 64 bays, with a total surface area of 4450 m2 covering 7896 m2 of land. Although some optimizing devices were designed in the early 20th century to increase the yield of solar distillation, basically solar concentrators (Delyannis and Delyannis, 1984), it was not until the World War II that research on solar distillation was strongly promoted to supply drinking water to troops in remote isolated places.

Maria Telkes, from the Massachusetts Institute of Technology (MIT) developed a portable air inflated plastic solar still to be used in emergency life rafts. It consisted of an inflatable device made of transparent plastic with a felt pad at the bottom and an attached container for collecting the distillate. Floating alongside the raft, the felt pad would saturate with seawater. Solar radiation passing through the transparent plastic would heat the pad creating water vapour which would condense on the inside of the plastic cover and end up sliding down to the container (Telkes, 1945). She continued working in different designs at MIT, including the multi-effect concept (Telkes, 1953).

In 1951, the Seawater Conversion Laboratory at the University of California started its inves­tigations which led to building a solar distillation station at the Engineering Field Station in Richmond, California (Howe and Tleimat, 1974). At about the same time (1952), the United States Government set up the Office of Saline Water, which opened and financed a solar dis­tillation program at the Daytona Beach Test Station in Florida. Many types of configuration of solar stills were analyzed in Daytona, including basin-type, multiple-effect and active solar stills (Talbert et al., 1970).

In Australia, a solar distillation program was developed at the Melbourne facilities of the Commonwealth Scientific and Industrial Research Organization (CSIRO) (Wilson, 1957). A prototype of bay-type still covered with glass was designed and some were built in the desert, the largest being in Coober Pedy. In Greece, the Technical University of Athens designed some solar stills for the islands. They were asymmetric glass-covered greenhouse type with aluminum frames. One of them, in the island of Patmos, was the largest ever built so far, with capacity 8640 m3 day-1 (Delyannis, 1968). In other Greek islands smaller stills covered with plastic material (Tedlar) were built from designs by Frank E. Edlin tried out in Daytona Beach (Eckstrom, 1965). In the former USSR several institutions researched on solar stills and a plant was built in Ashkabad, Turkmenistan (Baum and Bairamov, 1966). There was research and experimentation on solar stills in other countries like Spain (Barasoain and Fontan, 1960), Portugal (Madeira and Cape Verde Islands), India, Cyprus, Chile, Mexico, Egypt, Algeria and Tunisia.

The information from these experiments was summarized by Eibling et al. (1971), analysing the results from 27 of the largest basin-type solar stills operating up to that moment. From the

10 years of experience, they observed that the average productivities were mostly a function of total solar radiation. They estimated the productivity of a solar still to be about 3.26 Lm-2 per day, which corresponds to about 1.2 m3 m-2 per year. They stated that large durable-type, glass – covered solar stills would produce water on a consistent, dependable basis for a cost between 0.8 and 1.1 US$ m-3 in most situations, this range representing the best estimates for planning purposes, at least for the next several years.

By the early 1970s, the state-of-the-art of solar stills was declared understood from the stand­points of thermodynamic and geometric effects. Some modified types of solar stills had shown to improve the productivity (tilted wicks; inclined trays; solar stills with forced convection; stills with external condensation or multiple-effect solar stills with latent heat reuse), sometimes achiev­ing twice that of a simple-basin. However, at that time, none of those improvements could be justified on an economic basis. Also, long-term testing of the materials was needed and long life with minimum maintenance had to be demonstrated.

In the 40 years since then, a very large amount of experimental solar stills have been constructed and their performance sufficiently demonstrated. Research on solar stills has continued and only in the 21st century more than 200 scientific papers have been published so far dealing with solar stills. Although the basic knowledge and state-of-the art has not changed dramatically in the last 40 years, there are some interesting new designs and modified solar stills with increased efficiency, as well as validated models of their thermodynamics and performance.

CONSTRUCTION & WORKING OF BASIN TYPE SOLAR STILL

CONSTRUCTION

It is a shallow basin with blackened surface called basin linear .The saline water is supplied to the basin by a filler. A overflow pipe allows the excess water to flow out from the basin. The top of the basin is covered with a sloping air tight transparent cover that encloses the space above the basin. The cover is made of glass or plastic. The cover is roof like and the slope is provided towards a collection trough.

                                    Figure : Basin type solar still

WORKING

Solar Radiation pass through the glass cover & these radiation are absorbed & converted into heat by basin linear. The saline water is then heated and the water vapour is produced inside the solar still. The water vapour comes in contact with cooler interior surface of the transparent cover and it gets condensed .The condensed water vapours  flow down the sloping roof and it is collected in a tray or through as distilled water.

Performance of Basin type Solar Still

The performance of solar still is represented in terms of litres of water produced day per square meter of basin area.(l/m^2/day).The efficiency of basin type solar still is 35% to 50%. The efficiency ƞ

  Efficiency ƞ = (m∗Δh)/H

Where

m = mass of distill water produced

Δh = Enthalpy Change

H = Intensity of solar radiation.

ADVANTAGES

• Free of charge sun energy (during sunlight it eliminates 500 Watt electric consumption per one hour of sunlight)
• There are no moving parts; it is therefore reliable and almost maintenance free (cleaning is required though)
• Water taste is claimed to be better since the device act as a Solar Water Vapouriser and it doesn’t boil the water (resembling rain water)
• Neutral pH is claimed (like rainwater), not like the not neutral pH of steamed distilled water 

DISADVANTAGES

• Solar distillers don’t kill bacteria and they don’t break down harmful chemicals because they don’t boil the water
• The large area tilted glass cover might be an attraction to bugs and insects
• Low production capacity, not enough for the drinking water needs of the average family