Osmotic power plant

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. 

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. 

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). 

Typical layout of a PRO-based power plant is represented in Figure 9.5. Seawater from the offshore intake is first moved to a sedimentation basin. Then, an electric pump supplies a proper head to overcome the pressure drop of the filtration system and avoid cavitation in the main pump. A complex filtration system is required both on seawater and freshwater to prevent membrane fouling that negatively affects its permeation. After filtration, pressure of the seawater stream is increased to some bar (stream S2) before it is addressed to the permeate side of the osmotic membrane module. 

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. 

Removing these chlorides from raw materials is very difficult. Although deionized water can eliminate inorganic chlorides effectively, it caimot remove the organic ones. The power plants always adopt the reverse osmotic treatment to remove organic chlorides. However, it is uneconomical to those aimnonia plants because of the large quantities of water needed. 

The plasma membranes of growing plant cells select to incorporate sugars, amino adds, ions and other low molecular weight compounds from the apoplastic space, and then, the cells have a certain level of osmotic pressure. The difference between their osmotic pressure and their wall pressure (=turgor pressure) is due to motive power (suction force) to suck water from the apoplastic space (Figure 1). The plant hormone auxin, which decreases the wall pressure in a growing plant cell, therefore induces cell elongation or expansion. 

The circulatory systems in plants and animals conform to physical principles. Each system has a means to propel the fluid, because, from Newton s laws of motion, fluid will not move by itself. Power is required to move the fluid. In plants, this power is supplied by a combination of water evaporation in the leaves, an osmotic gradient from root to leaf, and capillary action. In animals, this power is usually supplied by one or more hearts through which the blood flows. 

Ramon et al [17] compiled a list of commercial and prototype osmotic membranes and evaluated the resulting theoretical power densities according to their characteristics. The analysis showed that prototype lab-cast thin-film composite membranes based on a selective polyamide active layer can achieve water permeability in the range of 5-7 m/s-Pa, which result in theoretical power density in the range of 5-6 W/m with seawater feed and over 15 W/m in plant fed with I.IM brine discharged from RO-based desalinization plants. Theoretical power densities achievable with commercial membranes are below 3 W/m with seawater feed. 

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