Osmotic power

As a river empties into the sea, it is possible to extract hydro power by converting gravitational potential energy into kinetic energy— mechanical energy and electrical energy. At the same time, the difference in salt concentration between the river and the sea results in a difference in chemical potential or free energy (AG) which can be exploited by means of the osmotic pressure. 

The osmotic pressure of a solution is the pressure exerted by the solution on a semipermeable membrane which separates the solution from pure solvent. The semipermeable membrane allows only the solvent to pass freely through it while opposing the transport of solute. This is illustrated in Fig. 1.14. The solvent tends to dilute the solution and passes through the membrane until equilibrium is established, at which point a pressure differential, it, exists between solution and solvent. This pressure differential is called the osmotic pressure. 

At equilibrium, the osmotic pressure, 7i, of a solution relative to the pure solvent is given by a relation which resembles the Ideal Gas Law (PV — nRT) where 

The average molar concentration of salt in seawater is about 0.5 M and since it may be assumed that the salt is primarily NaCl, then the total molar concentration of solute is 1.0 M (i.e., 0.5 M Na- + 0.5 M Cl ). Hence, 

This osmotic pressure represents a hydrostatic head of water of about 700 ft (30 ft/atm x 23 atm) or over 200 m. A schematic diagram of osmotic power is illustrated in Fig. 1.15 where seawater is pumped into a pressure chamber at a constant rate depending on the flow of the river. The river passes through the membrane, diluting the seawater and creating the hydrostatic head which can then turn a water wheel and generate electricity. The membrane area must be enormous to accommodate the 

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The whole membrane module is composed of six racks, each of them consisting of eleven pressure vessels, operated in parallel, designed to house one spiral wound membrane each. The pressure vessels were originally equipped with cellulose acetate membranes specifically developed for PRO. During the first period, the plant operation has been optimized and the performance has been monitored, resulting in a power density lower than 0.5 W/m. Next, thin film composite membranes developed for PRO were installed in early 2011. So far the measured power density has reached nearly 1 W/m, which is a major improvement compared to the cellulose acetate membranes originally installed. Based on this preliminary experience, Statkraft plans to build a full-scale 25 MW osmotic power plant by 2015 [20]. 

The potential values presented in Table 9.2 are based on the consideration of all river systems worldwide. In practice, only a part of the rivers offers suitable conditions for the operation of osmotic power plants, taking practical and economic considerations into account. Salinity distribution depends on the characteristics of the river estuary that can determine the four configurations shown in Figure 9.16. 

Table 9A Estimated osmotic power potential of some hypersaline lakes [36]... 

Van Der Zwana, S Pothofa, W.M.L, Blankertc, B. and Bara, I.J. (2012) Feasibility of osmotic power from a hydrodynamic analysis at module and plant scale. Journal of Membrane Science, 389, 324-333. 

Gerstandta, K., Peinemanna, K.V., Skilhagenb, S.E. et al. (2008) Membrane processes in energy supply for an osmotic power plant. Desalination, 224, 64-70. 

Stenzel, P. and Wagner, H.J. (2010) Osmotic power plants potential analysis and site criteria, Proceedings of 3rd International Conference on Ocean Energy, Bilbao, Spain. 

Kelada, M. (2010) Global Potential of Hypersalinity Osmotic Power, MIK Technology. 

Heifer F, Lemckert C, Anissimov YG. Osmotic power with pressure retarded osmosis theory, performance and trends — a review. J Membr Sci 2014 453 337-58. 

The former small unit infusion pumps are pressured by human blood pressure using a blood pressure cuff. However, this mechanism leads to serious problems, such as sudden change of the flow rate because the infusion flow rate is only dependent on the patient s blood pressure. Thus, recently, small volume pumps have been usually operated by an automatically controlled motor or a small embedded system. In addition, for the portable infusion pump, other possible pump power sources have been developed, such as osmotic pressure and spring-powered systems. However, with osmotic power needs it is necessary to change a salt solution bag after finishing infusion and with spring power it is necessary to overcome the limitation of the flow rate for use in various conditions. 

Only with the developments in membrane technology as a result of work in reverse osmosis will osmotic power become a significant factor in world energy supply. 

It has been estimated by the Scripps Institution of Oceanography that world power needs in the year 2000 will be about 33 million megawatts (33 TW). The seas can provide all this and more wave energy 2.5 TW, tidal power 2.7 TW, current power 5 TW, osmotic power 1,400 TW, OTEC 40,000 TW. The Weitzman Institute of Science has studied this topic and has shown that with a suitable membrane, it would be a feasible source of energy. 

She, Q., Jin, X. Tang, C.Y. (2012b). Osmotic power production from salinity gradient resource by pressure retarded osmosis effects of operating conditions and reverse solute diffusion. Journal of Membrane Science, 401 02, 262-273. 

Reuters News Agency, Norway Opens Worlds First Osmotic Power PlcUit, http //www.reuters.com/article/2009/11/24/us-norway-osmotic-idUS-TRE5A-N20Q20091124), 2009, accessed 3-26-14... 

Statkraft Halts Osmotic Power Investments, Statkrcdt AS News Archive, December 20, 2013, http //www.statkr ift.com/medi i/news/News- U chive/ 2013/Statkrcdt-hcilts-osmotic-power-investments/, accessed April 8,2014. 

Another way to carry out mixing of two saline solutions would be what is practiced in the concept known as osmotic power plant. Suppose you have a saline water (1) at a pressure (Pi) less than its osmotic pressure ( rj. Let the aqueous solution (2) on the other side of the reverse osmosis membrane (having a lower osmotic pressure 1C2 < Tti) be present at atmospheric pressure, Pg. If the osmotic pressure of this solution 2 is less than the pressure of the saline water 1 (i.e. 2 < P ), then water from solution 2 will go to solution 1 and will dilute it However, this water will increase the volume of the saline solution present at pressure Pi. Therefore one could use this extra energy through an appropriate hydraulic device/arrangement (Loeb, 1976). 

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