A REVIEW ON SOLAR POWERED MEMBRANE DISTILLATION

Transcription

1 International Journal of Recent Innovation in Engineering and Research Scientific Journal Impact Factor by SJIF e- ISSN: A REVIEW ON SOLAR POWERED MEMBRANE DISTILLATION Sophy Mathew 1 and Atheena A 2 1 UG Scholar, Department of EEE, Sree Buddha College of Engineering, Kerala 2 Assistant Professor (EEE), Sree Buddha College of Engineering, Kerala Abstract-Global problem on fresh water supply is increasing due to the rise in population growth.the water challenges faced by the people can overcome by the use of new technologies in the field of desalination of brackish water. This paper focuses on the technologies applied to the field of desalination of brackish water to produce freshwater supplementing drinking water supplies. Vacuum membrane distillations using solar energy provide better quality of water in comparison to other treatment. Desalination of sea water accounts for a worldwide water production of 5000 million m3/year. Increasing demand for water in the domestic sector has shifted attention to the role of desalination in alleviating water shortages. Experience in the Gulf States demonstrates that desalination technology has developed to a level where it can serve as a reliable source of water at a price comparable to water from other conventional sources. It is considered as a strategic option for satisfying current and future domestic water supply requirements, in comparison to the development of other water resources. Keywords- Desalination, Thermal desalination, membrane desalination, membrane distillation, vacuum membrane distillation. I. INTRODUCTION Water is the basic source of life for the human survival, and the principal material base to guarantee the substantial development of a country s economy. With the increase in global population, the gap between the supply and demand for water is widening and is reaching such alarming levels that in some part of the world, it is posing a threat to human existence. The stress on the water resources will increase due to economic development of countries like China and India. Out of the total water available, about 97.5% is saline and 2.5% is fresh water of which 70% found in the form of icecaps and the remaining 30% as underground aquifers. A small fraction of the freshwater available (less than 1% of total freshwater) in rivers, lakes and reservoirs is readily accessible for direct human use. The water challenge faced by the world can be overcome by the use of new developments in the field of desalination of water. Desalination technology along with utilization of renewable energy can provide unlimited fresh water in remote area affected by saline water [1] Water scarcity is a growing problem for large regions of the world. Scarcity results when the local fresh water demand is similar to that of local fresh water supply. The primary drivers of increasing water scarcity are population growth and the higher consumption associated with rising standards of living. A lack of infrastructure for water storage and distribution is also a factor in the developing world. Global climate change is expected to affect existing water resources as well, potentially altering the distribution of wet and arid regions and raising the salinity of some coastal aquifers. Among these factors, consumption in the developed world can be moderated relatively quickly by government policies aimed at reducing per capita water use, and new supplies can be established through technology; however population growth can be moderated only over very long time scales and infrastructure may not be developed quickly. All of these pressures are moving water-scarce regions toward purification of water supplies that are otherwise too saline for human consumption rights Reserved Page 95

2 II. DESALINATION Desalination is the process that removes salt and minerals from sea water to produce fresh water suitable for human consumption and irrigation. One by-product of desalination is salt. It is used on many sea going ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Due to its energy consumption, desalinating sea water is generally more costly than fresh water from rivers or groundwater, water recycling and water conservation. Factors that determine the costs for desalination include capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal. Desalination stills control pressure, temperature and brine concentrations to optimize efficiency. Nuclear-powered desalination might be economical on a large scale. In nature the sun causes water to evaporate from surface sources such as lakes, oceans, and streams. The water vapor eventually comes in contact with cooler air, where it re-condenses to form dew or rain. This process can be copied artificially more rapidly than nature, using man made heating and cooling processes[1]. III. DESALINATION TECHNOLOGIES A. THERMAL DESALINATION Desalination plants separates sea and brackish water into two flows consisting of a freshwater stream (permeate in reverse osmosis, condensate in thermal processes) with a low salt content and a stream with a high salt concentration. (Brine or concentrate) Every desalination technology requires energy for this separation process, which is supplied to the system by thermal or mechanical means (generally as electrical power). The thermal desalination process technologies is based on evaporation and the subsequent condensation of the steam. Fig.1: Thermal desalination B. MEMBRANE DESALINATION It is a membrane separation process in which water from a pressurized saline solution is separated from the solutes by flowing through a membrane. For this separation no heating or phase change is necessary. The major energy required for desalting is for pressurizing the feed water of the membranes. Pre-treatment is very important because the membrane surfaces must remain clean. Therefore, all suspended solids must be first removed, and the water pre-treated so that salt precipitation or microbial growth does not occur on the membranes. Pre-treatment may involve conventional methods such as a chemical feed followed by coagulation/flocculation/sedimentation, and sand filtration, or pretreatment may involve membrane processes such as microfiltration (MF) and ultrafiltration (UF). High pressure pumps supply the pressure needed to enable the water to pass through the membranes and reject the salts. This pressure ranges from 54 to 80 atmospheres. As a portion of the water passes through the membranes, in the creating such problems as precipitation of supersaturated salts and increased osmotic pressure across the membranes. The amount of the feedwater discharged to waste is between 20 and 70 percent of feed flow and depends on the salt content of the feedwater. Available Online at : Page 96

3 Fig.2: Membrane desalination C. MEMBRANE DISTILATION (MD) Membrane Distillation (MD) is a water desalination membrane process currently in limited commercial use. MD is a hybrid process of Reverse Osmosis and distillation in which a hydrophobic synthetic membrane is used to permit the flow of water vapor through the membrane pores, but not the solution itself. The driving force for MD is the difference in vapor pressure of the liquid across the membrane. MD was a membrane separation process combining with membrane technology and evaporation, and the membrane used was hydrophobic micro-porous membrane which could not be wetted by pending solutions. One side of membrane exposed to warm pending solutions directly (hot procedure side), the other side exposed to cold water solutions directly or indirectly (cold procedure side). Volatile components in hot procedure side evaporated in membrane surface, enter cold procedure side through membrane, and condensed into liquid phrase. Other components block off in hot procedure side by hydrophobic membrane and thus realize the objective of compounds separation and purification. MD was the process that transferred heat and quality simultaneously, and mass transfer impetus was steam differential imposed by components permeating through the both membrane sides. Fig. 3: Membrane Distillation IV. DESIGN OF THE PILOT PLANT The design of solar collector field will depend upon efficiency of heat exchanger, the weather condition, recovery of energy from distillate, etc. The choice of module of the membrane depends on physical and chemical membrane characteristics, pass number in module, costs, fouling of membrane, etc. The plate heat exchanger is composed of a set of corrugated metal plates. The number of plates is determined by the flow, fluid properties, pressure and temperature. The main components of desalination plant are: Membrane module Available Online at : Page 97

4 Field of solar collector (7 rows and 5 collectors in series) Field of solar photovoltaic cell (16 modules) Flow pumps and circulator, Peristaltic pump, Plate heat exchanger, Tank of fresh water production. V. EXPERIMENTAL SET UP Volume: 02 Issue: 03 March 2017 (IJRIER) The schematic diagram of the experimental system was shown in fig 8 where the two subsystems, the solar heat collection (SHC) unit and the VMD unit, are interconnected by a titanium plate heat exchanger where feed seawater for the VMD is heated by the hot water from the SHC. The SHC subsystem mainly consists of a solar collector, a solar thermal storage water tank for water replenishment. Tap water is the working fluid used in the SHC, heated in the solar collector, and then returns to the hot water tank. The heated water is given into a heat exchanger, releasing heat to the feed seawater, and then returning to the water tank. In the VMD subsystem, hot seawater from the heat exchanger enters the VMD module.. Hot seawater cross-flows over the outside surface of the membrane fibers. The water vapor is condensed in the condenser and then collected in the distillate tank. The brine flows back to the feed tank where seawater or distilled water at ambient temperature is added to keep a constant water level [1]. Fig 4: Flow sheet of pilot plant VI. CONCLUSION Vacuum membrane distillation using solar energy proved to be a better technology for the desalination of brackish water on small scale. From the tests, it appears that the most sensitive parameter is the feed rate of the membrane. In the current state, an increase in feed rate of the membrane causes a decrease in the exchange area. We must search a position of the membrane that allows a high flow rate without losing the exchange area. The treated water from the vacuum technology has significant benefit over other techniques in terms of the quality of water. This technology using solar energy can be advantageous for the people living in remote areas affected from problem of saline water. It can fulfill their needs at a low cost in comparison to other technology. Available Online at : Page 98

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