Water desalination |
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Methods |
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Solar desalination is a desalination technique powered by solar energy. The two common methods are direct (thermal) and indirect (photovoltaic). [1]
Solar distillation has been used for thousands of years. Early Greek mariners and Persian alchemists produced both freshwater and medicinal distillates. Solar stills were the first method used on a large scale to convert contaminated water into a potable form. [2]
In 1870 the first US patent was granted for a solar distillation device to Norman Wheeler and Walton Evans. [3] Two years later in Las Salinas, Chile, Swedish engineer Charles Wilson began building a solar distillation plant to supply freshwater to workers at a saltpeter and silver mine. It operated continuously for 40 years and distilled an average of 22.7 m3 of water a day using the effluent from mining operations as its feed water. [4]
Solar desalination in the United States began in 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. OSW's main function was to administer funds for desalination research and development projects. [5] One of five demonstration plants was located in Daytona Beach, Florida. Many of the projects were aimed at solving water scarcity issues in remote desert and coastal communities. [4] In the 1960s and 1970s several distillation plants were constructed on the Greek isles with capacities ranging from 2000 to 8500 m3/day. [2] In 1984 a plant was constructed in Abu-Dhabi with a capacity of 120 m3/day that is still in operation. [4] In Italy, an open source design called "the Eliodomestico" by Gabriele Diamanti was developed for personal costing $50. [6]
Of the estimated 22 million m3 daily freshwater produced through desalination worldwide, less than 1% uses solar energy. [2] The prevailing methods of desalination, MSF and RO, are energy-intensive and rely heavily on fossil fuels. [8] Because of inexpensive methods of freshwater delivery and abundant low-cost energy resources, solar distillation has been viewed as cost-prohibitive and impractical. [2] It is estimated that desalination plants powered by conventional fuels consume the equivalent of 203 million tons of fuel a year. [2]
Solar desalination is a technique that harnesses solar energy to convert saline water into fresh water, making it suitable for human consumption and irrigation. The process can be categorized based on the type of solar energy source utilized. In direct solar desalination, saline water absorbs solar energy and evaporates, leaving behind salt and other impurities. An example of this is solar stills, where an enclosed environment allows for the collection and condensation of pure water vapor. On the other hand, indirect solar desalination involves the use of solar collectors that capture and transfer solar energy to saline water. This energy is then used to power desalination processes such as Humidification-Dehumidification (HDH) and diffusion-driven methods.
In the direct (distillation) method, a solar collector is coupled with a distilling mechanism. [9] 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 is proportional to the area of the solar surface and solar incidence angle and has an average estimated value of 3–4 litres per square metre (0.074–0.098 US gal/sq ft). [2] Because of this proportionality and the relatively high cost of property and material for construction, distillation tends to favor plants with production capacities less than 200 m3/d (53,000 US gal/d). [2]
This uses the same process as rainfall. A transparent cover encloses a pan where saline water is placed. The latter traps solar energy, evaporating the seawater. The vapor condenses on the inner face of a sloping transparent cover, leaving behind salts, inorganic and organic components and microbes.
The direct method achieves values of 4-5 L/m2/day and efficiency of 30-40%. [10] Efficiency can be improved to 45% by using a double slope or an additional condenser. [11]
In a wick still, feed water flows slowly through a porous radiation-absorbing pad. This requires less water to be heated and is easier to change the angle towards the sun which saves time and achieves higher temperatures. [12]
A diffusion still is composed of a hot storage tank coupled to a solar collector and the distillation unit. Heating is produced by the thermal diffusion between them. [13]
Increasing the internal temperature using an external energy source can improve productivity.[ citation needed ]
Direct methods use thermal energy to vaporize the seawater as part of a 2-phase separation. Such methods are relatively simple and require little space so they are normally used on small systems. However, they have a low production rate due to low operating temperature and pressure, so they are appropriate for systems that yield 200 m3/day. [14]
Indirect desalination employs a solar collection array, consisting of photovoltaic and/or fluid-based thermal collectors, and a separate conventional desalination plant. [9] Many arrangements have been analyzed, experimentally tested and deployed. Categories include 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. [8]
Large solar desalination plants typically use indirect methods. [15] Indirect solar desalination processes are categorized into single-phase processes (membrane based) and phase change processes (non-membrane based). [16] Single-phase desalination use photovoltaics to produce electricity that drive pumps. [17] Phase-change (or multi-phase) solar desalination is not membrane-based. [18]
Indirect solar desalination systems using photovoltaic (PV) panels and reverse osmosis (RO) have been in use since 2009. Output by 2013 reached 1,600 litres (420 US gal) per hour per system, and 200 litres (53 US gal) per day per square metre of PV panel. [19] [20] Utirik Atoll in the Pacific Ocean has been supplied with fresh water this way since 2010. [21]
Single-phase desalination processes include reverse osmosis and membrane distillation, where membranes filter water from contaminants. [16] [18] As of 2014 reverse osmosis (RO) made up about 52% of indirect methods. [15] [22] [23] Pumps push salt water through RO modules at high pressure. [16] [22] RO systems depend on pressure differences. A pressure of 55–65 bar is required to purify seawater. An average of 5 kWh/m3 of energy is typically required to run a large-scale RO plant. [22] Membrane distillation (MD) utilizes pressure difference from two sides of a microporous hydrophobic membrane. [22] [24] Fresh water can be extracted through four MD methods: Direct Contact (DCMD), Air Gap (AGMD), Sweeping Gas (SGMD) and Vacuum (VMD). [22] [24] An estimated water cost of $15/m3 and $18/m3 support medium-scale solar-MD plants. [22] [25] Energy consumption ranges from 200 to 300 kWh/m3. [26]
Phase-change (or multi-phase) solar desalination [18] [23] [27] includes multi-stage flash, multi-effect distillation (MED), and thermal vapor compression (VC). [18] It is accomplished by using phase change materials (PCMs) to maximize latent heat storage and high temperatures. [28] MSF phase change temperatures range 80–120 °C, 40–100 °C for VC, and 50–90 °C for the MED method. [18] [27] Multi-stage flash (MSF) requires seawater to travel through a series of vacuumed reactors held at successively lower pressures. [23] Heat is added to capture the latent heat of the vapor. As seawater flows through the reactors, steam is collected and is condensed to produce fresh water. [23] In Multi-effect distillation (MED), seawater flows through successively low pressure vessels and reuses latent heat to evaporate seawater for condensation. [23] MED desalination requires less energy than MSF due to higher efficiency in thermodynamic transfer rates. [23] [27]
The multi-stage flash (MSF) method is a widely used technology for desalination, particularly in large-scale seawater desalination plants. It is based on the principle of utilizing the evaporation and condensation process to separate saltwater from freshwater. [29]
In the MSF desalination process, seawater is heated and subjected to a series of flashings or rapid depressurizations in multiple stages. Each stage consists of a series of heat exchangers and flash chambers. The process typically involves the following steps:
The multi-stage flash (MSF) method, known for its high energy efficiency through the utilization of latent heat of vaporization during the flashing process, accounted for approximately 45% of the world's desalination capacity and a dominant 93% of thermal systems as recorded in 2009. [2]
In Margherita di Savoia, Italy a 50–60 m3/day MSF plant uses a salinity gradient solar pond. In El Paso, Texas a similar project produces 19 m3/day. In Kuwait a MSF facility uses parabolic trough collectors to provide solar thermal energy to produce 100 m3 of fresh water a day. [8] And in Northern China an experimental, automatic, unmanned operation 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. [30]
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. [8] MSF experience poor efficiency during start-up or low energy periods. Achieving highest efficiency requires controlled pressure drops across each stage and steady energy input. As a result, solar applications require some form of thermal energy storage to deal with cloud interference, varying solar patterns, nocturnal operation, and seasonal temperature changes. As thermal energy storage capacity increases a more continuous process can be achieved and production rates approach maximum efficiency. [31]
Indirect solar desalination by a form of humidification/dehumidification is in use in the seawater greenhouse. [32]
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 however a disadvantage related with the difficulties of mechanically moving mixtures of ice and liquid. The process has not been commercialized yet due to cost and difficulties with refrigeration systems. [33]
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 Bridge and Iron Inc. in the late 1980s, which was shut down for its inefficiency. [34]
Nevertheless, there is a recent study for the saline groundwater [35] 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.
Inherent design problems face thermal solar desalination projects. First, the system's efficiency is governed by competing heat and mass transfer rates during evaporation and condensation. [1]
Second, 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 solar stills, this heat is emitted as waste heat.[ citation needed ]
Heat recovery allows the same heat input to be reused, providing several times the water. [1]
One solution is to reduce the pressure within the reservoir. This can be accomplished using a vacuum pump, and significantly decreases the required heat energy. For example, water at a pressure of 0.1 atmospheres boils at 50 °C (122 °F) rather than 100 °C (212 °F). [36]
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) [37] mimics the natural water cycle on a shorter time frame by distilling water. Thermal energy produces water vapor that is 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. [38]
In indirect, or single phase, solar-powered desalination, two systems are combined: a solar energy collection system (e.g. photovoltaic panels) and a desalination system such as reverse osmosis (RO). The main single-phase processes, generally membrane processes, consist of RO and electrodialysis (ED). Single phase desalination is predominantly accomplished with photovoltaics that produce electricity to drive RO pumps. Over 15,000 desalination plants operate around the world. Nearly 70% use RO, yielding 44% of desalination. [39] Alternative methods that use solar thermal collection to provide mechanical energy to drive RO are in development.
RO is the most common desalination process due to its efficiency compared to thermal desalination systems, despite the need for water pre-treatment. [40] Economic and reliability considerations are the main challenges to improving PV powered RO desalination systems. However, plummeting PV panel costs make solar-powered desalination more feasible. [41]
Solar-powered RO desalination is common in demonstration plants due to the modularity and scalability of both PV and RO systems. An economic analysis [42] that explored an optimisation strategy [43] of PV-powered RO reported favorable results.
PV converts solar radiation into direct-current (DC) electricity, which powers the RO unit. The intermittent nature of sunlight and its variable intensity throughout the day complicates PV efficiency prediction and limits night-time desalination. Batteries can store solar energy for later use. Similarly, thermal energy storage systems ensure constant performance after sunset and on cloudy days. [44]
Batteries allow continuous operation. Studies have indicated that intermittent operations can increase biofouling. [45]
Batteries remain expensive and require ongoing maintenance. Also, storing and retrieving energy from the battery lowers efficiency. [45]
Reported average cost of RO desalination is US$0.56/m3. Using renewable energy, that cost could increase up to US$16/m3. [39] Although renewable energy costs are greater, their use is increasing.
Both electrodialysis (ED) and reverse electrodialysis (RED) use selective ion transport through ion exchange membranes (IEMs) due either to the influence of concentration difference (RED) or electrical potential (ED). [46]
In 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 can be either horizontal or vertical. 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, because it can be used only for brackish water (TDS < 1000 ppm). [39] Due to the complexity for modeling ion transport phenomena in the channels, performance could be affected, considering the non-ideal behavior presented by the exchange membranes. [47]
The basic ED process could be modified and turned into RED, in which the polarity of the electrodes changes periodically, reversing the flow through the membranes. This limits the deposition of colloidal substances, which makes this a self-cleaning process, almost eliminating the need for chemical pre-treatment, making it economically attractive for brackish water. [48]
The use ED systems began in 1954, while RED was developed in the 1970s. These processes are used in over 1100 plants worldwide. The main advantages of PV in desalination plants is due to its suitability for small-scale plants. One example is in Japan, on Oshima Island (Nagasaki), which has operated since 1986 with 390 PV panels producing 10 m3/day with dissolved solids (TDS) about 400 ppm. [48]
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.
Geothermal desalination refers to the process of using geothermal energy to power the process of converting salt water to fresh water. The process is considered economically efficient, and while overall environmental impact is uncertain, it has potential to be more environmentally friendly compared to conventional desalination options. Geothermal desalination plants have already been successful in various regions, and there is potential for further development to allow the process to be used in an increased number of water scarce regions.
Ultrafiltration (UF) is a variety of membrane filtration in which forces such as pressure or concentration gradients lead to a separation through a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained in the so-called retentate, while water and low molecular weight solutes pass through the membrane in the permeate (filtrate). This separation process is used in industry and research for purifying and concentrating macromolecular (103–106 Da) solutions, especially protein solutions.
Forward osmosis (FO) is an osmotic process that, like reverse osmosis (RO), uses a semi-permeable membrane to effect separation of water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient, such that a "draw" solution of high concentration, is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. In contrast, the reverse osmosis process uses hydraulic pressure as the driving force for separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. Hence significantly more energy is required for reverse osmosis compared to forward osmosis.
Multi-stage flash distillation (MSF) is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers. Current MSF facilities may have as many as 30 stages.
Physical plant, mechanical plant or industrial plant refers to the necessary infrastructure used in operation and maintenance of a given facility. The operation of these facilities, or the department of an organization which does so, is called "plant operations" or facility management. Industrial plant should not be confused with "manufacturing plant" in the sense of "a factory". This is a holistic look at the architecture, design, equipment, and other peripheral systems linked with a plant required to operate or maintain it.
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell. The cell consists of a feed (dilute) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. In almost all practical electrodialysis processes, multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation-exchange membranes forming the multiple electrodialysis cells. Electrodialysis processes are different from distillation techniques and other membrane based processes in that dissolved species are moved away from the feed stream, whereas other processes move away the water from the remaining substances. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electrodialysis offers the practical advantage of much higher feed recovery in many applications.
Osmotic power, salinity gradient power or blue energy is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) and pressure retarded osmosis (PRO). Both processes rely on osmosis with membranes. The key waste product is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water.
Thin-film composite membranes are semipermeable membranes manufactured to provide selectivity with high permeability. Most TFC's are used in water purification or water desalination systems. They also have use in chemical applications such as gas separations, dehumidification, batteries and fuel cells. A TFC membrane can be considered a molecular sieve constructed in the form of a film from two or more layered materials. The additional layers provide structural strength and a low-defect surface to support a selective layer that is thin enough to be selective but not so thick that it causes low permeability.
Nanofiltration is a membrane filtration process that uses nanometer sized pores through which particles smaller than about 1–10 nanometers pass through the membrane. Nanofiltration membranes have pore sizes of about 1–10 nanometers, smaller than those used in microfiltration and ultrafiltration, but a slightly bigger than those in reverse osmosis. Membranes used are predominantly polymer thin films. It is used to soften, disinfect, and remove impurities from water, and to purify or separate chemicals such as pharmaceuticals.
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.
Membrane fouling is a process whereby a solution or a particle is deposited on a membrane surface or in membrane pores in a processes such as in a membrane bioreactor, reverse osmosis, forward osmosis, membrane distillation, ultrafiltration, microfiltration, or nanofiltration so that the membrane's performance is degraded. It is a major obstacle to the widespread use of this technology. Membrane fouling can cause severe flux decline and affect the quality of the water produced. Severe fouling may require intense chemical cleaning or membrane replacement. This increases the operating costs of a treatment plant. There are various types of foulants: colloidal, biological, organic and scaling.
A seawater greenhouse is a greenhouse structure that enables the growth of crops and the production of fresh water in arid regions which constitute about one third of the earth's land area. This in response to 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.
Reverse osmosis (RO) is a water purification process that uses a semi-permeable membrane to separate water molecules from other substances. RO applies pressure to overcome osmotic pressure that favors even distributions. RO can remove dissolved or suspended chemical species as well as biological substances, and is used in industrial processes and the production of potable water. RO retains the solute on the pressurized side of the membrane and the purified solvent passes to the other side. It relies on the relative sizes of the various molecules to decide what passes through. "Selective" membranes reject large molecules, while accepting smaller molecules.
Pressure retarded osmosis (PRO) is a technique to separate a solvent from a solution that is more concentrated and also pressurized. A semipermeable membrane allows the solvent to pass to the concentrated solution side by osmosis. The technique can be used to generate power from the salinity gradient energy resulting from the difference in the salt concentration between sea and river water.
Australia is the driest habitable continent on Earth and its installed desalination capacity has been increasing. Until a few decades ago, Australia met its demands for water by drawing freshwater from dams and water catchments. As a result of the water supply crisis during the severe 1997–2009 drought, state governments began building desalination plants that purify seawater using reverse osmosis technology. Approximately one percent of the world's drinkable water originates from desalination plants.
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.
Zero Liquid Discharge(ZLD) is a classification of water treatment processes intended to reduce wastewater efficiently and produce clean water that is suitable for reuse (e.g., irrigation). ZLD systems employ wastewater treatment technologies and desalination to purify and recycle virtually all wastewater received.
There are approximately 16,000 operational desalination plants, located across 177 countries, which generate an estimated 95 million m3/day of fresh water. Micro desalination plants operate near almost every natural gas or fracking facility in the United States. Furthermore, micro desalination facilities exist in textile, leather, food industries, etc.
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.
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