Blue energy, osmotic power plant or salinity gradient power is the energy retrieved from the difference in the salt concentration between seawater and river water. Two practical methods for this are Reverse Electrodialysis  (RED), or Pressure Retarded Osmosis  (PRO).
Both processes rely on osmosis with ion specific membranes. The key waste product is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water.The technologies have been confirmed in laboratory conditions. They are being developed into commercial use in the Netherlands (RED) and Norway (PRO). The cost of the membrane has been an obstacle. A new, cheap membrane, based on an electrically modified polyethylene plastic, made it fit for potential commercial use
PRO (Pressure Retarded Osmosis)In PRO, the water potential between fresh water and sea water corresponds to a pressure of 26 bars. This pressure is equivalent to a column of water (hydraulic head) 270 meters high.  However, the optimal working pressure is only half of this, 11 to 15 bar. 
In the Netherlands, for example, more than 3,300 m³ fresh water runs into the sea per second on average. The membrane halves the pressure differences which results in a water column of approx. 135 meters. The energy potential is therefore e=mgΔh=3.3*10^6 kg/s*10 m/s^2*135 meters ca.= 4.5*10^9 Joule, Power=4.5*10^9 watt.RED (Reverse ElectroDialysis)
In reverse electrodialysis (RED) a salt solution and fresh water are let though 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 remember that the process works though difference in ion concentration instead of an electric field, this has implications for the needed properties for a suitable membrane
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.