Solar still

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Solar still built into a pit in the ground Evapo still.svg
Solar still built into a pit in the ground
"Watercone" solar still Watercone on earth.svg
"Watercone" solar still
Solar Seawater Still.svg

A solar still distills water with substances dissolved in it by using the heat of the Sun to evaporate water so that it may be cooled and collected, thereby purifying it. They are used in areas where drinking water is unavailable, so that clean water is obtained from dirty water or from plants by exposing them to sunlight.

Contents

Still types include large scale concentrated solar stills and condensation traps. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through a transparent collector. The pure water vapour condenses on the cool inside surface and drips into a tank.

Distillation replicates the way nature makes rain. The sun's energy heats water to the point of evaporation. As the water evaporates, its vapour rises, condensing into water again as it cools. This process leaves behind impurities, such as salts and heavy metals, and eliminates microbiological organisms. The end result is pure (potable) water.

History

Condensation traps have been in use since the pre-Incan peoples inhabited the Andes.

In 1952, the United States military developed a portable solar still for pilots stranded in the ocean. It featured an inflatable 610-millimetre (24 in) floating plastic ball, with a flexible tube in the side. An inner bag hangs from attachment points on the outer bag. Seawater is poured into the inner bag from an opening in the ball's neck. Fresh water is taken out using the side tube. Output ranged from 1.4 litres (1.5 US qt) to 2.4 litres (2.5 US qt) of fresh water per day. [1] Similar stills are included in some life raft survival kits, though manual reverse osmosis desalinators have mostly replaced them. [2]

Today, a method for gathering water in moisture traps is taught within the Argentinian Army for use by specialist units expected to conduct extended patrols of more than a week's duration in the Andes' arid border areas.

Methods

Solar Well Puits Solaire.jpg
Solar Well

Pit still

A collector is placed at the bottom of a pit. Branches are placed vertically in the pit. The branches are long enough to extend over the edge of the pit and form a funnel to direct the water into the collector. A lid is then built over this funnel, using more branches, leaves, grasses, etc. Water is collected each morning.

This method relies on the formation of dew or frost on the receptacle, funnel, and lid. Forming dew collects on and runs down the outside of the funnel and into the receptacle. This water would typically evaporate with the morning sun and thus vanish, but the lid traps the evaporating water and raises the humidity within the trap, reducing the amount of lost water. The shade produced by the lid also reduces the temperature within the trap, which further reduces the rate of water loss to evaporation.

A solar still can be constructed with two–four stones, plastic film or transparent glass, a central weight to make the funnel and a container for the condensate. [3] Better materials improve efficiency. A single sheet of plastic can replace the branches and leaves. Greater efficiency arises because the plastic is waterproof, preventing water vapour from escaping. The sheet is attached to the ground on all sides with stones or earth. Weighting the centre of the sheet forms the funnel. Condensate runs down it into the receptacle. One study of pit distillation found that angling the lid at 30 degrees angle captured the most water. The optimal water depth was about 25 millimetres (1 in). [4]

Transpiration

During photosynthesis plants release water through transpiration. Water can be obtained by enclosing a leafy tree branch in clear plastic, [5] capturing water vapour released by the tree. [6] The plastic allows photosynthesis to continue.

In a 2009 study, variations to the angle of plastic and increasing the internal temperature versus the outside temperature improved output volumes.

Unless relieved the vapour pressure around the branch can rise so high that the leaves can no longer transpire, requiring the water to be removed frequently.

Alternatively, clumps of grass or small bushes can be placed inside the bag. The foliage must be replaced at regular intervals, particularly if the foliage is uprooted.

Efficiency is greatest when the bag receives maximum sunshine. Soft, pulpy roots yield the greatest amount of liquid for the least amount of effort.

Wick

Wick basin solar still. Wick solar still.png
Wick basin solar still.

The wick type solar still is a vapour-tight glass-topped box with an angled roof. [7] Water is poured in from the top. It is heated by sunlight and evaporates. It condenses on the underside of the glass and runs into the connecting pipe at the bottom. Wicks separate the water into banks to increase surface area. The more wicks, the more heat reaches the water.

To aid in absorbing more heat, wicks can be blackened. Glass absorbs less heat than plastic at higher temperatures, although glass is not as flexible.

A plastic net can catch the water before it falls into the container and give it more time to heat.

Additives

When distilling brine or other polluted water, adding a dye can increase the amount of solar radiation absorbed.

Reverse still

A reverse still uses the temperature difference between solar-heated ambient air and the device to condense ambient water vapour. One such device produces water without external power. It features an inverted cone on top to deflect ambient heat in the air, and to keep sunlight off the upper surface of the box. This surface is a sheet of glass coated with multiple layers of a polymer and silver. [8]

It reflects sunlight to reduce surface heating. Residual heat that is not reflected is reemitted in a specific (infrared) wavelength so that it passes through the atmosphere into space. The box can be as much as 15 °C (27 °F) cooler than the ambient temperature. That stimulates condensation, which gathers on the ceiling. This ceiling is coated in a superhydrophobic material, so that the condensate forms into droplets and falls into a collector. A test system yielded 4.6 ml (0.16 US fl oz) of water per day, using a 10 cm (3.9 in) surface or approximately 1.3 L/m2 (0.28 gal/ft2) per day. [8]

Efficiency

Condensation traps are sources for extending or supplementing existing water sources or supplies. A trap measuring 40 cm (16 in) in diameter by 30 cm (12 in) deep yields around 100 to 150 mL (3.4 to 5.1 US fl oz) per day.

Urinating into the pit before adding the receptacle allows some of the urine's water content to be recovered.

A pit still may be too inefficient as a survival still, because of the energy/water required for construction. [9] In desert environments water needs can exceed 3.8 litres (1 US gal) per day for a person at rest, while still production may average only 240 millilitres (8 US fl oz). [9] [10] Several days of water collection may be required to equal the water lost during construction. [10]

Applications

Remote sites

Solar stills are used in cases where rain, piped, or well water is impractical, such as in remote homes or during power outages. [11] In subtropical hurricane target areas that can lose power for days, solar distillation can provide an alternative source of clean water.

Solar-powered desalination systems can be installed in remote locations where there is little or no infrastructure or energy grid. Solar is still affordable, eco-friendly, and considered an effective method amongst other conventional distillation techniques. Solar still is very effective, especially for supplying fresh water for islanders. This makes them ideal for use in rural areas or developing countries where access to clean water is limited. [12] [13]

Survival

Solar stills have been used by ocean-stranded pilots and included in life raft emergency kits. [1]

Using a condensation trap to distill urine will remove the urea and salt, recycling the body's water. [14]

Wastewater treatment

Solar stills have also been used for the treatment of municipal wastewater, [15] the dewatering of sewage sludge [16] as well as for olive mill wastewater management. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Solar energy</span> Radiant light and heat from the Sun, harnessed with technology

Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy, and solar architecture. It is an essential source of renewable energy, and its technologies are broadly characterized as either passive solar or active solar depending on how they capture and distribute solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power, and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light-dispersing properties, and designing spaces that naturally circulate air.

<span class="mw-page-title-main">Boiling</span> Rapid phase transition from liquid to gas or vapour

Boiling or ebullition is the rapid phase transition from liquid to gas or vapor; the reverse of boiling is condensation. Boiling occurs when a liquid is heated to its boiling point, so that the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Boiling and evaporation are the two main forms of liquid vapourization.

<span class="mw-page-title-main">Passive solar building design</span> Architectural engineering that uses the Suns heat without electric or mechanical systems

In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.

<span class="mw-page-title-main">Water vapor</span> Gaseous phase of water

Water vapor, water vapour or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor is transparent, like most constituents of the atmosphere. Under typical atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation. It is less dense than most of the other constituents of air and triggers convection currents that can lead to clouds and fog.

<span class="mw-page-title-main">Desalination</span> Removal of salts from water

Desalination is a process that takes away mineral components from saline water. More generally, desalination is the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. Saltwater is desalinated to produce water suitable for human consumption or irrigation. The by-product of the desalination process is brine. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rainfall-independent water resources.

<span class="mw-page-title-main">Solar thermal energy</span> Technology using sunlight for heat

Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in industry, and in the residential and commercial sectors.

<span class="mw-page-title-main">Heat pipe</span> Heat-transfer device that employs phase transition

A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces.

<span class="mw-page-title-main">Solar thermal collector</span> Device that collects heat

A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and solar towers or non water heating devices such as solar cooker, solar air heaters.

<span class="mw-page-title-main">Solar cooker</span> Device for cooking with the heat of sunlight

A solar cooker is a device which uses the energy of direct sunlight to heat, cook or pasteurize drink and other food materials. Many solar cookers currently in use are relatively inexpensive, low-tech devices, although some are as powerful or as expensive as traditional stoves, and advanced, large scale solar cookers can cook for hundreds of people. Because they use no fuel and cost nothing to operate, many nonprofit organizations are promoting their use worldwide in order to help reduce fuel costs and air pollution, and to help slow down deforestation and desertification.

Solar desalination is a desalination technique powered by solar energy. The two common methods are direct (thermal) and indirect (photovoltaic).

An atmospheric water generator (AWG), is a device that extracts water from humid ambient air, producing potable water. Water vapor in the air can be extracted either by condensation - cooling the air below its dew point, exposing the air to desiccants, using membranes that only pass water vapor, collecting fog, or pressurizing the air. AWGs are useful where potable water is difficult to obtain, because water is always present in ambient air.

A solar-powered desalination unit produces potable water from saline water through direct or indirect methods of desalination powered by sunlight. Solar energy is the most promising renewable energy source due to its ability to drive the more popular thermal desalination systems directly through solar collectors and to drive physical and chemical desalination systems indirectly through photovoltaic cells.

A seawater greenhouse is a greenhouse structure that enables the growth of crops and the production of fresh water in arid regions. Arid regions constitute about one third of the Earth's land area. Seawater greenhouse technology aims to mitigate issues such as global water scarcity, peak water and soil becoming salted. The system uses seawater and solar energy, and has a similar structure to the pad-and-fan greenhouse, but with additional evaporators and condensers. The seawater is pumped into the greenhouse to create a cool and humid environment, the optimal conditions for the cultivation of temperate crops. The freshwater is produced in a condensed state created by the solar desalination principle, which removes salt and impurities. Finally, the remaining humidified air is expelled from the greenhouse and used to improve growing conditions for outdoor plants.

Vapor-compression desalination (VC) refers to a distillation process where the evaporation of sea or saline water is obtained by the application of heat delivered by compressed vapor.

<span class="mw-page-title-main">Reflux</span> Condensation of vapors and their return to where they originated

Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reactions over a long period of time.

Membrane distillation (MD) is a thermally driven separation process in which separation is driven by phase change. A hydrophobic membrane presents a barrier for the liquid phase, allowing the vapour phase to pass through the membrane's pores. The driving force of the process is a partial vapour pressure difference commonly triggered by a temperature difference.

<span class="mw-page-title-main">Hygroscopic cycle</span> Thermodynamic cycle converting thermal energy into mechanical power

The Hygroscopic cycle is a thermodynamic cycle converting thermal energy into mechanical power by the means of a steam turbine. It is similar to the Rankine cycle using water as the motive fluid but with the novelty of introducing salts and their hygroscopic properties for the condensation. The salts are desorbed in the boiler or steam generator, where clean steam is released and superheated in order to be expanded and generate power through the steam turbine. Boiler blowdown with the concentrated hygroscopic compounds is used thermally to pre-heat the steam turbine condensate, and as reflux in the steam-absorber.

<span class="mw-page-title-main">Concentrated solar still</span> High-output water distillation and purification system using solar energy

A concentrated solar still is a system that uses the same quantity of solar heat input as a simple solar still but can produce a volume of freshwater that is many times greater. While a simple solar still is a way of distilling water by using the heat of the sun to drive evaporation from a water source and ambient air to cool a condenser film, a concentrated solar still uses a concentrated solar thermal collector to concentrate solar heat and deliver it to a multi-effect evaporation process for distillation, thus increasing the natural rate of evaporation. The concentrated solar still is capable of large-scale water production in areas with plentiful solar energy.

<span class="mw-page-title-main">Solar-assisted heat pump</span>

A solar-assisted heat pump (SAHP) is a machine that combines a heat pump and thermal solar panels and/or PV solar panels in a single integrated system. Typically these two technologies are used separately to produce hot water. In this system the solar thermal panel performs the function of the low temperature heat source and the heat produced is used to feed the heat pump's evaporator. The goal of this system is to get high COP and then produce energy in a more efficient and less expensive way.

The low-temperature distillation (LTD) technology is the first implementation of the direct spray distillation (DSD) process. The first large-scale units are now in operation for desalination. The process was first developed by scientists at the University of Applied Sciences in Switzerland, focusing on low-temperature distillation in vacuum conditions, from 2000 to 2005.

References

  1. 1 2 "Solar Still". Popular Mechanics. Hearst Magazines. February 1952. p. 113.
  2. ↑ "Manual Reverse Osmosis Desalinator - Notice of Intent to Award Sole Source, USAF". fbo.gov. 2012. Retrieved July 3, 2012.
  3. ↑ "Uncle John's Portable Solar Water Distiller ( for Survival )". Instructables.
  4. ↑ Khalifa, Abdul Jabbar N.; Hamood, Ahmad M. (November 2009). "Performance correlations for basin type solar stills". Desalination. 249 (1): 24–28. doi:10.1016/j.desal.2009.06.011.
  5. ↑ "Solar Still". Practical Survivor. Retrieved 2023-01-12.
  6. ↑ O'Meagher, Bert; Reid, Dennis; Harvey, Ross (2007). Aids to survival: a handbook on outback survival (PDF) (25th ed.). Maylands, W.A.: Western Australia Police Academy. p. 24. ISBN   978-0-646-36303-5 . Retrieved 7 February 2017.
  7. ↑ Manikandan, V.; Shanmugasundaram, K.; Shanmugan, S.; Janarthanan, B.; Chandrasekaran, J. (April 2013). "Wick type solar stills: A review". Renewable and Sustainable Energy Reviews. 20: 322–335. doi:10.1016/j.rser.2012.11.046.
  8. 1 2 Irving, Michael (June 24, 2021). ""Reverse solar still" keeps its cool to wring drinking water from air". New Atlas. Retrieved 2021-06-27.
  9. 1 2 Alloway, David (2000). Desert survival skills. University of Texas Press. pp. 63–65. ISBN   978-0-292-79226-5 . Retrieved 9 May 2013.
  10. 1 2 United States Air Force (1 April 2008). U.S. Air Force Survival Handbook. Skyhorse Publishing. p. 285. ISBN   978-1-60239-245-8 . Retrieved 9 May 2013.
  11. ↑ Anjaneyulu, L.; Kumar, E. Arun; Sankannavar, Ravi; Rao, K. Kesava (13 June 2012). "Defluoridation of drinking water and rainwater harvesting using a solar still". Industrial & Engineering Chemistry Research. 51 (23): 8040–8048. doi:10.1021/ie201692q.
  12. ↑ Özcan, Y., & Deniz, E. (2023). Solar thermal waste heat energy recovery in solar distillation systems by using thermoelectric generators. Engineering Science and Technology, an International Journal, 40, 101362. https://doi.org/10.1016/j.jestch.2023.101362
  13. ↑ Mohaisen, H. S., Esfahani, J. A., & Ayani, M. B. (2021). Improvement in the performance and cost of passive solar stills using a finned-wall/built-in condenser: An experimental study. Renewable Energy, 168, 170-180.
  14. ↑ Grantham, Donald F. (March 2, 2001). A Source of Wilderness Novice Survival Skills. Xlbris Corp. p. 119. ISBN   0738836826.
  15. ↑ R. Zarasvand Asadi, F. Suja, M.H. Ruslan, N.A. Jalil The application of a solar still in domestic and industrial wastewater treatment. (2013) Sol. Energy, 93, pp. 63-71, https://doi.org/10.1016/j.solener.2013.03.024
  16. ↑ D.A. Haralambopoulos, G. Biskos, C. Halvadakis, T.D. Lekkas (2022) Dewatering of wastewater sludge through a solar still. Renew. Energy, 26, pp. 247-256, https://doi.org/10.1016/S0960-1481(01)00114-8as
  17. ↑ Mastoras, Petros; Vakalis, Stergios; Fountoulakis, Michail S.; Gatidou, Georgia; Katsianou, Panagiota; Koulis, Georgios; Thomaidis, Nikolaos S.; Haralambopoulos, Dias; Stasinakis, Athanasios S. (2022-09-10). "Evaluation of the performance of a pilot-scale solar still for olive mill wastewater treatment". Journal of Cleaner Production. 365: 132695. doi:10.1016/j.jclepro.2022.132695. ISSN   0959-6526. S2CID   249722738.

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