South Asian Journal of Engineering and Technology Vol.2, No.22 (2016) 112–117
ISSN No: 2454-9614
Osmotic Power Plant High Efficiency Renewable Energy System D.Govarthan, R.Kathiresan, K.Eswaramoorthy*
Department of EEE, SASURIE College of Engineering, Vijayamangalam, Tiruppur, Tamilnadu, India.
*Corresponding Author: K. Eswaramoorthy E-mail: [email protected]
Received: 13/11/2015, Revised: 18/12/2015 and Accepted: 16/04/2016
Abstract
The need of new energy sources has led to a number of alternatives. One of those alternatives is energy created by transportation of solutions, osmotic energy or salinity gradient energy. In the osmotic process two solutions with different salt-concentrations are involved (often freshwater and salt-water). A semi permeable membrane, which is an organic filter, separates the solutions. The membrane only lets small molecules like water- molecules pass. The water aspires to decrease the salt-concentration on the membrane side that contains more salt. The water therefore streams through the membrane and creates a pressure on the other side. This pressure can be utilized in order to gain energy, by using a turbine and a generator.
1. Introduction 1.1 Osmosis Principle Diffusion of molecules through a semi permeable membrane from a place of higher concentration to a place of lower concentration until the concentration on both sides is equal. Osmosis is a process by which water moves through a membrane which blocks other particles, which is used to purify water. For osmotic power it works in reverse, with osmosis drawing fresh water through the membrane to mix with salty water, thereby increasing its pressure which can be harnessed to drive electricity turbines. The main thing with osmotic energy is transportation of solutions (often pure water and salt-water), separated by a special filter, a membrane. In the osmotic process it is not possible to use an ordinary filter.which needs a "Semi permeable membrane". A semi permeable membrane is an organic filter with extremely small holes. The membrane will only allow small molecules, like water molecules, to pass through. The thin layers of material cause this effect thus called as osmotic energy process .
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Fig 2.1.a: Osmosis Process- Step 1 Fig 2.1.a shows a simple test rig in which the left side contains pure water. The right side contains a solvent with water and salt NaCl. Both pure and solvent water are separated by a semi permeable membrane.
Fig 2.1.b: Osmosis Process -Step 2 When the process gets started the pure water on the left side aspires to decrease the salt-concentration on the right side of the membrane. The amount of water on the right side will now increase and create an "Osmotic head pressure". This pressure can be used to force a water- turbine to rotate. The amount of freshwater that will pass through the membrane depends on the salt-concentration in the salt-water, before the osmotic process begins. For instance, if the salt-concentration from the beginning is 3.5%, the osmotic pressure will be about 28 bars. The problem with the test rig is that the salt-concentration in the salt-water will decrease and the process will slow down. The only way to fix this is to continuously, empty and refill both the left and the right side. This must be done very quickly to avoid run-interference. Another problem is that the membrane may wear out because of all silt and other contamination that get stuck in the membrane. Recently in Reverse Osmosis, where a pressure is created larger than the osmotic head pressure in which the salt water is pushed through the membrane. From this process fresh water is gained out of salt-water. 1.2 Overall view of osmotic power plant The core process is like desalination which is in in reverse. In desalination fresh water is sepatared from from salt water, but in osmotic power the fresh water is combined with salt water. Which is called a pressure exchanger.
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The pressure exchanger works similarly to a heat exchanger, essentially transferring the increased pressure from the salty outflow from the osmosis membrane to the fresh-water diluted output so it can drive a turbine. Without the pressure exchanger, the efficiency of the process would be too low to create full-scale osmotic energy generators. The pressure exchanger transfers pressure from a high-pressure stream to a low-pressure stream with 98 percent efficiency. On the other hand Osmotic power plants, discharge fresh water diluted with salt water exactly with the same proportions .Statkraft plans to build plants where fresh water is already dumping into the sea but the output of desalination plants is twice as salty as seawater, .This doubles the energy generation capability, which is proportional to saltiness. 2.Types of Osmosis: a. Pressure Retarded osmosis (PRO) b. Reverse electro dialysis (RED)
2.1Pressure Retarded Osmosis: Salinity gradient power is a specific renewable energy alternative that creates power by using naturally occurring processes. Salinity gradient energy s based on using the resources of osmotic pressure.The energy which is proposed to use salinity gradient technology relies on the evaporation to separate water from salt. Osmotic pressure is the "chemical potential of concentrated and dilute solutions of salt". Salinity gradient energy is based on using the resources of “osmotic pressure difference between fresh water and sea water”. As a result of the osmotic pressure, the water from solution B will diffuse through the membrane in order to dilute the solution. Thus, the pressure drives the turbines which in turn produces the electrical energy. 2.2 Reversed Electro Dialysis (RED) Reversed electro dialysis (RED) is the salinity gradient energy retrieved from the difference in the salt concentration between sea water and river water In reversed electro dialysis (RED) a salt solution and fresh water are let through a stack of alternating cathode and anode exchange membranes. The chemical potential difference between salt and fresh water generates a voltage over each membrane and the total potential of the system is the sum of the potential differences over all membranes. It is important to note that the process works through difference in ion concentration instead of an electric field, which has implications for the type of membrane needed. In RED, as in a fuel cell, the cells are stacked. A module with a capacity of 250 kW has the size of a shipping container.
2.3 Osmotic Pressure
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South Asian Journal of Engineering and Technology Vol.2, No.22 (2016) 112–117
Osmotic Pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semi permeable membrane. Jacobus Henrico van’t Hoff first proposed a formula for calculating the osmotic pressure, but this was later improved upon by Harmon Northrop Morse. On a related note, osmotic potential is the opposite of water potential, which is the degree to which a solvent tends to stay in a liquid.
2.4 Potential Osmotic Pressure Potential osmotic pressure is the maximum osmotic pressure that could develop in a solution if it were separated from distilled water by a selectively permeable membrane. It is the number of solute particles in a unit volume of the solution that directly determines its potential osmotic pressure. If one waits for equilibrium, osmotic pressure reaches potential osmotic pressure.
Fig 2.4.a: Osmotic Pressure 2.5 Morse Equation The osmotic pressure Π of a dilute solution can be approximated using the Morse equation which is named after Harmon Northrop Morse. Π = iMRT Where, i -> is the dimensionless van’t Hoff factor M -> is the molarity R -> 0.08206 L · atm · mol-1 · K-1 is the gas constant T-> is the thermodynamic (absolute) temperature This equation gives the pressure on one side of the membrane; the total pressure on the membrane is given by the difference between the pressures on the two sides. Note the similarity of the above formula to the ideal gas law and also that osmotic pressure is not dependent on particle charge. This equation was derived by van’t Hoff.
3. Applications
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Osmotic pressure is necessary for many plant functions. It is the resulting turgor pressure on the cell wall that allows herbaceous plants to stand upright, and how plants regulate the aperture of their stomata. In animal cells which lack a cell wall however, excessive osmotic pressure can result in cytolysis. Cell wall - A rigid layer of polysaccharides enclosing the membrane of plant and prokaryotic cells; maintains the shape of the cell and serves as a protective barrier. Cytolysis- Pathological breakdown of cells by the destruction of their outer membrane. Plasmolysis – The study of parasitic protozoan of the genus Plasmodium that causes malaria in humans. Turgor pressure- The normal rigid state of fullness of a cell or blood vessel or capillary resulting from pressure of the contents against the wall or membrane. For the calculation of molecular weight by using colligative properties, osmotic pressure is the most preferred property. Osmotic pressure is an important factor affecting cells. Osmoregulation is the homeostasis mechanism of an organism to reach balance in osmotic pressure. Hypertonicity is the presence of a solution that causes cells to shrink. Hypotonicity is the presence of a solution that causes cells to swell. Isotonic is the presence of a solution that produces no change in cell volume.
Fig 2.6.a: Different Cell Structure When a biological cell is in a hypotonic environment, the cell interior accumulates water, water flows across the cell membrane into the cell, causing it to expand. In plant cells, the cell wall restricts the expansion, resulting in pressure on the cell wall from within called turgor pressure. That needs to be developed .there are currently two hurdes to over come which includes membrane water part and sunlight. If we could develop the membrane to use salt-water as fresh water and brine with a higher salt- concentration as the concentrated solution, then it would be more feasible to use salinity for power. Or, the vapour pressure technique could be further developed. However, the biggest hurdle that needs to be overcome is the cost. Salinity power is not economically feasible compared to fossil fuels Currently, more effort is being put into developing salt-gradient solar ponds for energy (where osmosis is used). Therefore in the world of salt, there is more potential in using salt from the solar ponds as opposed from the ocean. The salt percentage will be much higher, which will increase the osmotic head pressure and more energy can be extracted.
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4.Conclusion: The conclusion we have reached during this project is that osmotic energy is not something we can use in the nearest future. The disadvantages, the obstacles, are too big to be overcome at the moment. The cleaning of the membranes and the cost are good examples of such obstacles. However in the future if the technology is further developed and the costs will decrease, osmotic energy might be an alternative to the energy sources we use today.
References:
[1] http://naring.regeringen.se/ [2] http://world-wide-water.com/index2.html [3] http://taipan.nmsu.edu/aght/soils/soil_physics/tutorials/wp/wp_comp.html [4] http://www-ib.berkeley.edu/IB/instruction/IB150/material/lectures/lecture22/ [5] http://edie.cprost.sfu.ca/~rhlogan/osmotic.html [6] http://www.seas.ucla.edu/~sechurl/CP/sld001.htm [7] www.ece.utep.edu [8] www.statkraft.de [9] www.wikipedi.com [10] www.exergy.se
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