Solar still for pure water


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A simple and economical way to get pure water on a small scale using solar insolation

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Solar still for pure water

  2. 2. SOLAR STILL - WHAT’S IT • A solar still uses the heat of the sun directly to purify water by vaporization - condensation. • A solar still is simply a shallow basin with a transparent glass cover. The sun heats the water in the basin, causing evaporation. Vapour rises, condenses on the cover and runs down into a collection trough for pure water. • Left behind is a fraction of input water with salts, minerals, and other impurities, like germs. 2
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  4. 4. This presentation 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 simple and economical. 4
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  7. 7. How’s -its operation: 1. The sun's energy - short electromagnetic waves - passes through a clear glazing surface such as glass. Upon striking a darkened surface, this light changes wavelength, becomes long waves of heat- added to the water in a shallow basin below the glazing. As the water heats up, it begins to evaporate. 2. The warmed vapor rises to a cooler area. Almost all impurities are left behind in the basin. 7
  8. 8. How’s -its operation: … continued 3. The vapor condenses onto the underside of the cooler glazing and accumulates into water droplets or sheets of water. 4. The combination of gravity and the tilted glazing surface allows the water to run down the cover and into a collection trough, where it is channeled into storage. 8
  9. 9. SOLAR DISTILLATION: ENERGY REQUIREMENTS • In this process, water is evaporated, thus separating water vapor from dissolved matter, the vapor is then condensed as pure water. • At least 2260 kJ/kg is required to evaporate water. • To pump a kg of water through 20m head requires only 0.2 kJ/kg. • Only where there is no local source of fresh water that can be easily pumped or lifted, distillation is therefore normally considered. 9
  10. 10. WHEN USE SOLAR DISTILLATION ? • Solar stills should normally only be considered for removal of dissolved salts from water. • If there is no fresh water then the main alternatives are desalination, transportation and rainwater collection. • Unlike other techniques of desalination, solar stills are more attractive, the smaller the required output. 10
  11. 11. WHY USE A SOLAR STILL? • Solar distillation can be a cost-effective means of providing clean water -- on a small scale, for drinking, cooking, washing and bathing--four basic human needs. • It can improve health standards by removing low concentration inorganic impurities from questionable water supplies. 11
  12. 12. •The solar still is also used to purify water for some business, industry, laboratory, and green-house applications. • It also appears able to purify polluted water. WHY USE A SOLAR STILL ? 12
  13. 13. Design objectives for an efficient solar still • For high efficiency the solar still should maintain: • a high feed (un-distilled) water temperature • a large temperature difference between feed water and condensing surface • low vapor leakage. 13
  14. 14. Efficiency range: In most units, less than half the calories of radiant energy falling on the still are used for the heat of vaporization necessary to produce the distilled water. All commercial stills sold to date have had an efficiency range of 30 to 45 percent. (The maximum efficiency is just over 60 percent.) 14
  15. 15. Efficiency = (Energy required for the vaporization of the distillate that is recovered) / (Energy in the sun's radiation that falls on the still.) Efficiency is calculated in the following manner: 15
  16. 16. TO ACHIEVE HIGH EFFICIENCY-1: A high feed water temperature can be achieved if: • A high proportion of incoming radiation is absorbed by the feed water as heat. • Hence low absorption glazing and a good radiation absorbing surface are required • heat losses from the floor and walls are kept low • the water is shallow so there is not so much to heat. 16
  17. 17. TO ACHIEVE HIGH EFFICIENCY-2: A large temperature difference can be achieved if: • the condensing surface absorbs little or none of the incoming radiation • condensing water dissipates heat which must be removed rapidly from the condensing surface • by, for example, a second flow of water or air, or by condensing at night. 17
  18. 18. EFFICIENCY VS. COST OF STILL • Provided the costs don't rise significantly, an efficiency increase of a few percent is worth working for. • Improvements are principally to be sought in materials and methods of construction. 18
  19. 19. DESIGN TYPES AND THEIR PERFORMANCE-1 • Single-basin stills have been much studied and their behavior is well understood. Efficiencies of 25% are typical. • Daily output is a function of solar irradiation and is greatest in the early evening when the feed water is still hot but when outside temperatures are falling. 19
  20. 20. DESIGN TYPES AND THEIR PERFORMANCE-2 • Material selection is very important. The cover can be either glass or plastic. Glass is considered to be best for most long-term applications, whereas a plastic (such as polyethylene) can be used for short-term use. 20
  21. 21. DESIGN TYPES AND THEIR PERFORMANCE-3 • Sand concrete or waterproofed concrete are considered best for the basin of a long-life still if it is to be manufactured on-site, • but for factory-manufactured stills, prefabricated ferro-concrete is a suitable material. 21
  22. 22. OUTPUT OF A SOLAR STILL • Q = [E x G x A] / 2.3 where: • Q = daily output of distilled water (litres /day) • E = overall efficiency • G = daily global solar irradiation (MJ/m²) • A = aperture area of the still i. e, the plan areas for a simple basin still 22
  23. 23. OUTPUT PER SQUARE METRE OF AREA IS: • The average, daily, global solar irradiation is typically 18.0 MJ/m² (5 kWh/m²). • A simple basin still operates at an overall efficiency of about 30%. • daily output = [0.30 x 18.0 x 1] / 2.3 = 2.3 litres (per square metre) 23
  24. 24. DESIGN VARIATIONS • concentrating collector stills • multiple tray tilted stills • tilted wick solar stills • and basin stills • 95 percent of all functioning stills are of the basin type 24
  25. 25. FOUR MAJOR COMPONENTS - BASIN STILL 1. a basin; 2. a support structure; 3. a transparent glazing cover; and 4. a distillate trough (water channel) 25
  26. 26. ANCILLARY COMPONENTS 1. insulation (usually under the basin); 2. sealants; 3. piping and valves; 4. facilities for storage; 5. an external cover to protect the other components from the weather and to make the still esthetically pleasing; and 6. a reflector to concentrate sunlight. 26
  27. 27. PHYSICAL DIMENSIONS • 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. It is generally best to design an installation with many small modular units to supply the water. 27
  28. 28. COMMUNITY AND RESIDENTIAL SIZE STILLS • Most community size stills are 1/2 to 2 1/2 meters wide, with lengths ranging up to around 100 meters. • Their lengths usually run along an east-west axis to maximize the transmission of sunlight through the equatorial facing 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. 28
  29. 29. DEPTH OF WATER: THE SHALLOWER THE DEPTH, THE BETTER. • 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 can be flushed out by the refilling operation. • Of course, if the basin is too shallow, it will dry out and salts will be deposited, which is not good. 29
  30. 30. TWO GENERAL TYPES OF BASINS • material that maintains its own shape and provides the waterproof containment by itself / with the aid of a surface material applied directly to it • uses one set of materials (such as wood or brick) to define the basin's shape; Into this is placed a second material that easily conforms to the shape of the structural materials and serves as a waterproof liner. 30
  31. 31. One alternative is ordinary aluminum coated with silicone rubber. The durability of basins made with this material increased into the 10- to 15-year range. For the hundreds of stills one company sold using this material, the coating was all done by hand. With production roll coating equipment, the basin's durability could probably be increased even more. 31
  32. 32. Glazing Cover is a critical component of a solar still. It is mounted above the basin and must be able to transmit light in the visible spectrum yet keep the heat generated by that light from escaping the basin. Exposure to ultraviolet radiation requires a material that can withstand the degradation effects or that is inexpensive enough to be replaced periodically. 32
  33. 33. 33 Since it may encounter temperatures approaching 95 [degrees] Celsius , it must also be able to support its weight at those temperatures and not undergo excessive expansion, which could destroy the airtight seals.
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