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Solar-powered desalination unit

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A solar-powered desalination unit produces potable water from saline water through direct or indirect methods of desalination powered by sunlight. Countries such as Australia, Italy and Egypt have adopted this system as an alternative source of water for the population.[1] 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. [2]

Direct solar desalination produces distillate directly in the solar collector. An example would be a solar still which traps the Sun's energy to obtain freshwater through the process of evaporation and condensation. Indirect solar desalination incorporates solar energy collection systems with conventional desalination systems such as multi-stage flash distillation, multiple effect evaporation, freeze separation or reverse osmosis to produce freshwater.[1]

This challenge can be overcome by storing solar power in, for example batteries, which provide the energy required for desalination in non-sunlight hours. Apart from the use of conventional batteries, alternative methods for solar energy storage exist. For example, thermal energy storage systems solve this storage problem and ensure constant performance even during non-sunlight hours and cloudy days, improving overall efficiency.[3] Alternatively, stored gravitational power can be harnessed to provide energy to a solar-powered reverse osmosis unit during non-sunlight hours. In this set-up pioneered by Elemental Water Makers, solar power is used to power a pump that displaces saline water up to an elevated water buffer during sunlight-hours. During non-sunlight hours, the water pressure required for the reverse osmosis process is provided by the saline water stored in the elevated water buffer, thus enabling continuous fresh water production. By incorporating an energy recovery device, effluent water pressure is recycled, thereby reducing the required water buffer elevation to 90 meters.[4] This innovative desalination method using only energy provided by the sun and earth, was awarded the first prize in the MBR Global Water Award 2017.[5]

Contents

Direct solar desalinationEdit

Solar stillsEdit

One type of solar desalination unit is a solar still, it is also similar to a condensation trap. A solar still is a simple way of distilling water, using the heat of the Sun to drive evaporation from humid soil, and ambient air to cool a condenser film. Two basic types of solar stills are box and pit stills. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic. The pure water vapor condenses on the cool inside plastic surface and drips down from the weighted low point, where it is collected and removed. The box type is more sophisticated. The basic principles of solar water distillation are simple, yet effective, as distillation replicates the way nature makes rain. The sun's energy 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, and eliminates microbiological organisms. The end result is water cleaner than the purest rainwater.

Indirect solar desalinationEdit

Indirect solar desalination systems comprise two sub-systems: a solar collection system and a desalination system. The solar collection system is used, either to collect heat using solar collectors and supply it via a heat exchanger to a thermal desalination process, or to convert electromagnetic solar radiation to electricity using photovoltaic cells to power an electricity-driven desalination process.

Solar-powered reverse osmosisEdit

Osmosis is a natural phenomenon in which water passes through a membrane from a lower to a higher concentration solution. The flow of water can be reversed if a pressure larger than the osmotic pressure is applied on the higher concentration side. In Reverse osmosis desalination systems, seawater pressure is raised above the natural osmotic pressure, forcing pure water through membrane pores to the fresh water side. Reverse osmosis (RO) is the most common desalination process in terms of installed capacity due to its superior energy efficiency compared to thermal desalination systems, despite requiring extensive water pre-treatment. Furthermore, part of the consumed mechanical energy can be reclaimed from the concentrated brine effluent with an energy recovery device.[6]

Solar-powered RO desalination is common in demonstration plants due to the modularity and scalability of both photovoltaic (PV) and RO systems. A detailed economic analysis [7] and a thorough optimisation strategy [8] of PV powered RO desalination were carried out with favorable results reported. Economic and reliability considerations are the main challenges to improving PV powered RO desalination systems. However, the quickly dropping PV panel costs are making solar-powered desalination ever more feasible.

A solar powered desalination unit designed for remote communities has been tested in the Northern Territory of Australia. The "reverse-osmosis solar installation" (ROSI) uses membrane filtration to provide a reliable and clean drinking water stream from sources such as brackish groundwater. Solar energy overcomes the usually high-energy operating costs as well as greenhouse emissions of conventional reverse osmosis systems. ROSI can also remove trace contaminants such as arsenic and uranium that may cause certain health problems, and minerals such as calcium carbonate which causes water hardness.[9]

Project leader Dr Andrea Schaefer from the University of Wollongong's Faculty of Engineering said ROSI has the potential to bring clean water to remote communities throughout Australia that do not have access to a town water supply and/or the electricity grid.[9]

Groundwater (which may contain dissolved salts or other contaminants) or surface water (which may have high turbidity or contain microorganisms) is pumped into a tank with an ultrafiltration membrane, which removes viruses and bacteria. This water is fit for cleaning and bathing. Ten percent of that water undergoes nanofiltration and reverse osmosis in the second stage of purification, which removes salts and trace contaminants, producing drinking water. A photovoltaic solar array tracks the Sun and powers the pumps needed to process the water, using the plentiful sunlight available in remote regions of Australia not served by the power grid.[10]

See alsoEdit

ReferencesEdit

  1. ^ a b Solar thermal desalination technologies
  2. ^ Mohammad Abutayeh; Chennan Li, D; Yogi Goswami; Elias K. Stefanakos (January 2014). Kucera, Jane, ed. "Solar Desalination". Desalination: 551–582. doi:10.1002/9781118904855.ch13.
  3. ^ Low temperature desalination using solar collectors augmented by thermal energy storage
  4. ^ "Solutions: Solar Powered Desalination without Batteries". Elemental Water Makers. Retrieved 12 December 2018.
  5. ^ "HH Sheikh Maktoum bin Mohammed bin Rashid Al Maktoum honours 10 winners from 8 countries at Mohammed bin Rashid Al Maktoum Global Water Award". www.suqia.ae. Retrieved 12 December 2018.
  6. ^ Mohammad Abutayeh; Chennan Li, D; Yogi Goswami; Elias K. Stefanakos (January 2014). Kucera, Jane, ed. "Solar Desalination". Desalination: 551–582. doi:10.1002/9781118904855.ch13.
  7. ^ Fiorenza, G.; Sharma, V.K.; Braccio, G. (August 2003). "Techno-economic evaluation of a solar powered water desalination plant". Energy Conversion and Management. 44 (14): 2217–2240. doi:https://doi.org/10.1016/S0196-8904(02)00247-9 Check |doi= value (help).
  8. ^ Laborde, H.M.; França, K.B.; Neff, H.; Lima, A.M.N. (February 2001). "Optimization strategy for a small-scale reverse osmosis water desalination system based on solar energy". Desalination. 133 (1): 1–12. doi:https://doi.org/10.1016/S0011-9164(01)00078-9 Check |doi= value (help).
  9. ^ a b "Award-winning Solar Powered Desalination Unit aims to solve Central Australian water problems". University of Wollongong. 4 November 2005. Retrieved 2017-07-19.
  10. ^ PV-Powered Desalination in Australia: Technology Development and Applications

External linksEdit