Solar desalination

Solar desalination is a technique to produce water with a low salt concentration from sea-water or brine using solar energy. There are two common methods of solar desalination. Either using the direct heat from the sun or using electricity generated by solar cells to power a membrane process.[1]

Water desalination
Methods

Methods
The status of renewable-powered desalination technologies. [2]

In the direct method, a solar collector is coupled with a distilling mechanism and the process is carried out in one simple cycle.[3] Solar stills of this type are described in survival guides, provided in marine survival kits, and employed in many small desalination and distillation plants. Water production by direct method solar distillation is proportional to the area of the solar surface and incidence angle and has an average estimated value of 3–4 litres per square metre (0.074–0.098 US gal/sq ft).[4] Because of this proportionality and the relatively high cost of property and material for construction direct method distillation tends to favor plants with production capacities less than 200 m3/d (53,000 US gal/d).[4]

Indirect solar desalination employs two separate systems; a solar collection array, consisting of photovoltaic and/or fluid based thermal collectors, and a separate conventional desalination plant.[3] Production by indirect method is dependent on the efficiency of the plant and the cost per unit produced is generally reduced by an increase in scale. Many different plant arrangements have been theoretically analyzed, experimentally tested and in some cases installed. They include but are not limited to multiple-effect humidification (MEH), multi-stage flash distillation (MSF), multiple-effect distillation (MED), multiple-effect boiling (MEB), humidification–dehumidification (HDH), reverse osmosis (RO), and freeze-effect distillation.[5]

Indirect solar desalination systems using photovoltaic (PV) panels and reverse osmosis (RO) have been commercially available and in use since 2009. Output by 2013 is up to 1,600 litres (420 US gal) per hour per system, and 200 litres (53 US gal) per day per square metre of PV panel.[6][7] Municipal-scale systems are planned.[8] Utirik Atoll in the Pacific Ocean has been supplied with fresh water this way since 2010.[9]

Indirect solar desalination by a form of humidification/dehumidification is in use in the seawater greenhouse.

History

Methods of solar distillation have been employed by humankind for thousands of years. From early Greek mariners to Persian alchemists, this basic technology has been utilized to produce both freshwater and medicinal distillates. Solar stills were in fact the first method used on a large scale to process contaminated water and convert it to a potable form.[4]

In 1870 the first US patent was granted for a solar distillation device to Norman Wheeler and Walton Evans.[10] Two years later in Las Salinas, Chile, Charles Wilson, a Swedish engineer, began building a direct method solar powered distillation plant to supply freshwater to workers at a saltpeter and silver mine. It operated continuously for 40 years and produced an average of 22.7 m3 of distilled water a day using the effluent from mining operations as its feed water.[11]

Solar desalination of seawater and brackish groundwater in the modern United States extends back to the early 1950s when Congress passed the Conversion of Saline Water Act, which led to the establishment of the Office of Saline Water (OSW) in 1955. The OSW's main function was to administer funds for research and development of desalination projects.[12] One of the five demonstration plants constructed was located in Daytona Beach, Florida and devoted to exploring methods of solar distillation. Many of the projects were aimed at solving water scarcity issues in remote desert and coastal communities.[11] In the 1960s and 1970s several modern solar distillations plants were constructed on the Greek isles with capacities ranging from 2000 to 8500 m3/day.[4] In 1984 a MED plant was constructed in Abu-Dhabi with a capacity of 120 m3/day and is still in operation.[11] In Italy, an open source design called "the Eliodomestico" by Gabriele Diamanti was developed for personal use at the building materials price of $50.[13]

Of the estimated 22 million m3 of freshwater being produced a day through desalination processes worldwide, less than 1% is made using solar energy.[4] The prevailing methods of desalination, MSF and RO, are energy intensive and rely heavily on fossil fuels.[5] Because of inexpensive methods of freshwater delivery and abundant low cost energy resources, solar distillation has, up to this point, been viewed as cost prohibitive and impractical.[4] It is estimated that desalination plants powered by conventional fuels consume the equivalent of 203 million tons of fuel a year.[4] With the approach (or passage) of peak oil production, fossil fuel prices will continue to increase as those resources decline; as a result solar energy will become a more attractive alternative for achieving the world's desalination needs.[14]

Types of solar desalination

There are two primary means of achieving desalination using solar energy, through a phase change by thermal input, or in a single phase through mechanical separation.[15] Phase change (or multi-phase) can be accomplished by either direct or indirect solar distillation. Single phase desalination is predominantly accomplished in a solar-powered desalination unit, which uses photovoltaic cells that produce electricity to drive pumps, although there are experimental methods being researched using solar thermal collection to provide this mechanical energy.[14]

Multi-phase desalination. Direct methods

Direct methods for multi-phase solar desalination are those which use the sun thermal energy collected to heat the sea water and produce the vaporization needed for this 2 phases separation. Such methods are relatively simple and require little space so they are normally used on small production systems. However, they have a low production rate due to low operating temperature and pressure, so they are useful on places where the demand of freshwater is below 200 m3/day.[16]

Single-effect solar still

This is a simple device that works using the same natural process of the natural rainfall production. A transparent cover encloses a pan where saline water is placed. The latter traps solar energy within the enclosure, heating up the seawater and evaporating it. Condensation is produced on the inner face of the sloping transparent cover, and all the salts, inorganic and organic components and microbes are left behind.

The direct method that a solar still uses has a low productivity, achieving values of 4-5 L/m2/day and efficiency of 30-40%.[17] Several methods have been studied to improve this technology. The basin type is the most commonly used, but there are other kind of improvements:

Efficiency can be improved by up to 45% by using a double slope or with an additional condenser.[18]

In a wick still, feed water flows slowly through a porous radiation-absorbing pad. This requires less volume of water to be heated and it easier to change the angle towards the sun which speeds up its use and higher temperatures can be achieved.

A diffusion still It is composed of two parts, one hot storage tank coupled to the solar collector, and the distillation unit. Heating is produced by the thermal diffusion between those two units.

Increasing the internal temperature by using another external energy source can improve productivity. This is the only active method commented, those above are all passive devices.

Multi-phase desalination. Indirect methods

Multi-stage flash distillation (MSF)
Main article: Multi-stage flash distillation

Multi-stage flash distillation is one of the predominant conventional phase-change methods of achieving desalination. It accounts for roughly 45% of the total world desalination capacity and 93% of all thermal methods.[4]

In Margarita de Savoya, Italy there is a 50–60 m3/day MSF plant with a salinity gradient solar pond providing its thermal energy and storage capacity. In El Paso, Texas there is a similar project in operation that produces 19 m3/day. In Kuwait a MSF facility has been built using parabolic trough collectors to provide the necessary solar thermal energy to produce 100 m3 of fresh water a day.[5] And in Northern China there is an experimental, automatic, unmanned operation that uses 80 m2 of vacuum tube solar collectors coupled with a 1 kW wind turbine (to drive several small pumps) to produce 0.8 m3/day.[19]

Production data shows that MSF solar distillation has an output capacity of 6-60 L/m2/day versus the 3-4 L/m2/day standard output of a solar still.[5] MSF experience very poor efficiency during start up or low energy periods. In order to achieve the highest efficiency MSF requires carefully controlled pressure drops across each stage and a steady energy input. As a result, solar applications require some form of thermal energy storage to deal with cloud interference, varying solar patterns, night time operation, and seasonal changes in ambient air temperature. As thermal energy storage capacity increases a more continuous process can be achieved and production rates approach maximum efficiency.[20]

Freezing

Although it has only been used on demonstration projects, this indirect method based on crystallization of the saline water has the advantage of the low energy required. Since the latent heat of fusion of water is 6,01 kJ/mole and the latent heat of vaporization at 100 °C is 40,66 kJ/mole, it should be cheaper in terms of energy cost. Furthermore, the corrosion risk is lower too. There is although a disadvantage related with the difficulties of mechanically moving mixtures of ice and liquid. It has not been commercialized yet due to cost and difficulties with refrigeration systems.

The most studied way of using this process is the refrigeration freezing. A refrigeration cycle is used to cool the water stream to form ice, and after that those crystals are separated and melted to obtain fresh water. There are some recent examples of this solar powered processes: the unit constructed in Saudi Arabia by Chicago Bridege and Iron Inc. in the late 1980s, which was shut down for it’s inefficiency.

Nevertheless, there is a recent study for the saline groundwater [21] concluding that a plant capable of producing 1 million gal/day would produce water at a cost of $1.30/1000 gallons. Being this true, it would be a cost-competitive device with the reverse osmosis ones.

Problems with thermal systems

There are two inherent design problems facing any thermal solar desalination project. Firstly, the system's efficiency is governed by preferably high heat and mass transfer rates during evaporation and condensation. The surfaces have to be properly designed within the contradictory objectives of heat transfer efficiency, economy, and reliability.[1]

Secondly, the heat of condensation is valuable because it takes large amounts of solar energy to evaporate water and generate saturated, vapor-laden hot air. This energy is, by definition, transferred to the condenser's surface during condensation. With most forms of solar stills, this heat of condensation is ejected from the system as waste heat. The challenge still existing in the field today, is to achieve the optimum temperature difference between the solar-generated vapor and the seawater-cooled condenser, maximal reuse of the energy of condensation, and minimizing the asset investment.

Solutions for thermal systems

Efficient desalination systems use heat recovery to allow the same heat input to provide several times the water than a simple evaporative process such as solar stills.[1]

One solution to the barrier presented by the high level of solar energy required in solar desalination efforts is to reduce the pressure within the reservoir. This can be accomplished using a vacuum pump, and significantly decreases the temperature of heat energy required for desalination. For example, water at a pressure of 0.1 atmospheres boils at 50 °C (122 °F) rather than 100 °C (212 °F).[22]

Solar humidification–dehumidification
Main article: Solar humidification

The solar humidification–dehumidification (HDH) process (also called the multiple-effect humidification–dehumidification process, solar multistage condensation evaporation cycle (SMCEC) or multiple-effect humidification (MEH)[23]) is a technique that mimics the natural water cycle on a shorter time frame by evaporating and condensing water to separate it from other substances. The driving force in this process is thermal solar energy to produce water vapor which is later condensed in a separate chamber. In sophisticated systems, waste heat is minimized by collecting the heat from the condensing water vapor and pre-heating the incoming water source.[24] This system is effective for small- to mid- scale desalination systems in remote locations because of the relative inexpensiveness of solar thermal collectors.

Single-phase solar desalination

In indirect, or single phase, solar-powered desalination, two different technological systems are combined: a solar energy collection system (e.g. through the use of photovoltaic panels) and a proven desalination system such as reverse osmosis (RO), are combined. The main single-phase processes, or membrane processes, consist of Reverse Osmosis (RO) and Electrodialysis (ED). Single phase solar desalination is predominantly accomplished by the use of photovoltaic cells that produce electricity to drive pumps used for reverse osmosis desalination. Nowadays there are over 15,000 desalination plants around the world, of these plants almost 70% of them use RO method, which makes RO processes responsible for 44% of desalination production capacity internationally.[25] However, alternative experimental methods are being researched, which use solar thermal collection to provide mechanical energy to drive the reverse osmosis process.

Solar-powered reverse osmosis

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.[26]

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[27] and a thorough optimisation strategy[28] 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.

This type of systems (PV-RO) convert solar radiation into direct-current (DC) electricity, which will feed the RO unit. While the intermittent nature of sunlight and its variable intensity throughout the day makes PV efficiency prediction difficult and desalination during night time challenging, several solutions exist. For example, batteries, which provide the energy required for desalination in non-sunlight hours can be used to store solar energy in daytime. 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.[29]

Nevertheless, it is valid to pointed out some pros and cons regarding the use of batteries in a PV-RO system. In one hand, as mentioned above, the use of batteries is a solution that targets the uniformity operation of the system, maintaining the desired set point along the variation of sunlight during the day, as a buffer. Studies have indicates that intermittent operations can increase biofouling.[30]

Still, the use of batteries has some drawbacks as the price. Batteries are expensive and they increase the investment amount and maintenance of a PV-RO plant, due to the periodic maintenance required by batteries. Also, when electrical energy from the PV is converted to chemical energy in the battery and send to the pumps of the RO system, energy is lost. Hence, the use of batteries could decrease the efficiency of the plant.[30]

Reported average cost of seawater desalination with RO is 0.56 USD/m3, while, using renewable energy sources, that cost could increase up to 16 USD/m3.[25] Although the costs for using renewable energy sources are greater, the perspectives of using them in desalination systems are increasing, due to the environmental concerns and future availability of fossil fuels. Furthermore, economic analysis of PV-RO small scale units shows that it could be a solution for remote areas supply of fresh water, competing with the conventional desalination methods.

Solar-powered Electrodialysis

Both Electrodialysis (ED) and Reverse Electrodialysis (RED) are based on the principle of selective ions transport through ion exchange membranes (IEMs) due either to the influence of concentration difference (RED) or electrical potential (ED).

In Electrodialysis (ED) an electrical force is applied to the electrodes; the cations travel toward the cathode and anions travel toward the anode. The exchange membranes only allow the passage of its permeable type (cation or anion), hence with this arrangement, diluted and concentrated salt solutions are placed in the space between the membranes (channels). The configuration of this stack could be either horizontally or vertically, and the feed water passes in parallel through all the cells, providing a continuous flow of permeate and brine. Although this is a well known process Electrodialysis is not commercially suited for seawater desalination, it can be used only for brackish water (TDS < 1000 ppm) [25] and due to the complexity for modeling ion transport phenomena in the channels, the process perform could be affected considering the non-ideal behavior presented by the exchange membranes.[31]

The basic ED process could be modified and turn into RED, which operates in almost the same way of ED, except by the fact that the polarity of the electrodes changes periodically reversing the flow through the membranes. Due to that the deposition of colloidal substances is very difficult which makes this a self-cleaning process, almost eliminating the need for chemical pre-treatment, also making this type of treatment economically attractive process for brackish water.[32]

The use ED systems is not new, it has been used since 1954 and RED was developed in the 70's. Today these processes are used over 1100 plants worldwide and also PV-ED process. The main advantages for using PV technology in desalination plants its due the possibility to build small-scale plants, which are suitable for remote areas and without fossil fuel availability, there are some interesting reported examples of PV-ED use, one of them it's in Japan, on Oshima Island (Nagasaki), operating since 1986 with 390 PV panels producing 10 m3/day with a Total Dissolved Solids (TDS) about 400 ppm.[32]

See also

References
  1. J H Lienhard, G P Thiel, D M Warsinger, L D Banchik (2016). "Low Carbon Desalination: Status and Research, Development, and Demonstration Needs". Report of a Workshop Conducted at the Massachusetts Institute of Technology in Association With the Global Clean Water Desalination Alliance, MIT Abdul Latif Jameel World Water and Food Security Lab, Cambridge, Massachusetts.CS1 maint: multiple names: authors list (link)
  2. McLellan, Benjamin; Ogata, Seiichi; Mohammadi-Ivatloo, Behnam; Tezuka, Tetsuo (2020). "An Integrated Planning Framework for Sustainable Water and Energy Supply". Sustainability. 12 (10): 4295. doi:10.3390/su12104295. Missing |author1= (help)
  3. García-Rodríguez, Lourdes; Palmero-Marrero, Ana I.; Gómez-Camacho, Carlos (2002). "Comparison of solar thermal technologies for applications in seawater desalination". Desalination. 142 (2): 135–42. doi:10.1016/S0011-9164(01)00432-5.
  4. Kalogirou, S. (2009). Solar energy engineering: Processes and systems. Burlington, MA: Elsevier/Academic Press.
  5. Qiblawey, Hazim Mohameed; Banat, Fawzi (2008). "Solar thermal desalination technologies". Desalination. 220: 633–44. doi:10.1016/j.desal.2007.01.059.
  6. "Trunk size solar desalination unit"
  7. "Container size solar desalination unit"
  8. "Al-Khafji plant"Arab News item 2013
  9. "Utrik RO unit a big success"Marshall Islands Journal Jan 17th 2014
  10. Wheeler, N., Evans, W., (1870) Improvements in Evaporating and Distilling by Solar Heat. http://www.google.com/patents/US102633
  11. Delyannis, E. (2003). Historic background of desalination and renewable energies, Solar Energy, 75(5), 357-366.
  12. National Archives, https://www.archives.gov/research/guide-fed-records/groups/380.html
  13. http://www.civil.northwestern.edu/EHE/HTML_KAG/Kimweb/files/SolarStill%20Project.pdf
  14. Attia, Ahmed A.A. (2012). "Thermal analysis for system uses solar energy as a pressure source for reverse osmosis (RO) water desalination". Solar Energy. 86 (9): 2486–93. Bibcode:2012SoEn...86.2486A. doi:10.1016/j.solener.2012.05.018.
  15. Li, Chennan; Goswami, Yogi; Stefanakos, Elias (2013). "Solar assisted sea water desalination: A review". Renewable and Sustainable Energy Reviews. 19: 136–63. doi:10.1016/j.rser.2012.04.059.
  16. García-Rodríguez, Lourdes (May 2002). "Seawater desalination driven by renewable energies: a review". Desalination. 143 (2): 103–113. doi:10.1016/s0011-9164(02)00232-1. ISSN 0011-9164.
  17. Mink, György; Aboabboud, Mohamed M.; Karmazsin, Étienne (April 1998). "Air-blown solar still with heat recycling". Solar Energy. 62 (4): 309–317. doi:10.1016/s0038-092x(97)00121-7. ISSN 0038-092X.
  18. Fath, Hassan E.S. (September 1998). "Solar distillation: a promising alternative for water provision with free energy, simple technology and a clean environment". Desalination. 116 (1): 45–56. doi:10.1016/s0011-9164(98)00056-3. ISSN 0011-9164.
  19. Chen, Zhili; Xie, Guo; Chen, Ziqian; Zheng, Hongfei; Zhuang, Chunlong (2012). "Field test of a solar seawater desalination unit with triple-effect falling film regeneration in northern China". Solar Energy. 86: 31–9. Bibcode:2012SoEn...86...31C. doi:10.1016/j.solener.2011.08.037.
  20. Gude, Veera Gnaneswar; Nirmalakhandan, Nagamany; Deng, Shuguang; Maganti, Anand (2012). "Low temperature desalination using solar collectors augmented by thermal energy storage". Applied Energy. 91: 466–74. doi:10.1016/j.apenergy.2011.10.018.
  21. "Task 21 - Evaluation of Artificial Freeze Crystallization and Natural Freeze-Thaw Processes for the Treatment of Contaminated Groundwater at the Strachan Gas Plant in Alberta, Canada - Sour Gas Remediation Technology R{ampersand}D". 1997-03-01. Cite journal requires |journal= (help)
  22. "Archived copy". Archived from the original on 2008-12-21. Retrieved 2008-11-05.CS1 maint: archived copy as title (link) Large scale Solar Desalination using Multi Effect Humidification
  23. The MEH-method (in German with english abstract): Solar Desalination using the MEH method, Diss. Technical University of Munich
  24. A scheme for large scale desalination of sea water by solar energy
  25. Esmaeilion, Farbod (March 2020). "Hybrid renewable energy systems for desalination". Applied Water Science. 10 (3). doi:10.1007/s13201-020-1168-5. ISSN 2190-5487.
  26. 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.
  27. 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:10.1016/S0196-8904(02)00247-9.
  28. 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:10.1016/S0011-9164(01)00078-9.
  29. Low temperature desalination using solar collectors augmented by thermal energy storage
  30. Lienhard, John; Antar, Mohamed A.; Bilton, Amy; Blanco, Julian; Zaragoza, Guillermo (2012). "SOLAR DESALINATION". Annual Review of Heat Transfer. 15 (15): 277–347. doi:10.1615/annualrevheattransfer.2012004659. ISSN 1049-0787.
  31. Tedesco, M.; Hamelers, H.V.M.; Biesheuvel, P.M. (June 2017). "Nernst-Planck transport theory for (reverse) electrodialysis: II. Effect of water transport through ion-exchange membranes". Journal of Membrane Science. 531: 172–182. arXiv:1610.02833. doi:10.1016/j.memsci.2017.02.031. ISSN 0376-7388.
  32. Al-Karaghouli, Ali; Renne, David; Kazmerski, Lawrence L. (February 2010). "Technical and economic assessment of photovoltaic-driven desalination systems". Renewable Energy. 35 (2): 323–328. doi:10.1016/j.renene.2009.05.018. ISSN 0960-1481.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.