Process design

The basic process of an osmotic power plant is shown in Fig. 5.

A pressure exchanger is used in order to maintain a high pressure at the feed side of the membrane. The pressure of the brackish water leaving the system is used to pressurize the incoming seawater. The flows of the brackish water leaving the system and the seawater entering the system should be equal. The amount of water permeating through the membrane is used to generate electricity via a turbine. It should be noted that the pressure exchange should work very efficiently at low pressures (14.8 bar half the osmotic pressure) in order not to lose too much energy. Statkraft a Norwegian energy company found a very elegant solution for this problem by placing the osmotic power plant below sea level at such a depth that the

Water selective membrane

Water selective membrane

Figure 5 Basic principle of PRO water transport from freshwater toward a pressurized saltwater solution. Q is the flow of water (m3/s), the subscripts f, s, and p stand for freshwater, seawater, and permeated water, respectively.

hydrostatic pressure equals the optimal operation pressure [Eq. (18)] for PRO [17,18].

The water selective membrane consists of a dense selective top layer (which is permeable for water and not for salts) and a porous support backing this thin layer. The selective top layer is facing the pressurized seawater.

The flux of water occurs due to an osmotic pressure difference between the freshwater and the saltwater and is retarded by the higher pressure ofthe saltwater. This can be described by the following relationship:

where J ho is the water flux in m3/(m2 s); A a specific membrane constant, An the osmotic pressure, and Ap the pressure difference between both solutions.

The amount of energy produced per square meter of membrane (E) is obtained by multiplying the water flux with the hydrostatic pressure difference:

E = jh2QDp

The maximal power density is obtained when dE/dP — 0, resulting in which is the optimal pressure of the concentrated salt solution at the feed side of the membrane giving the highest power output. For PRO on river water and seawater this would mean an optimal pressure at the seawater side of 14.8 bar.

The maximal obtainable amount of power can be derived by substitution of DPmax in Eq. (17) resulting in

This equation clearly shows the effect of the osmotic pressure and membrane properties (A) on the energy production of PRO.

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