Using Saltwater for Power
In many cases, electricity is generated by driving turbines with one fluid or another, such as steam or water. What can really set power plants apart is what puts the energy into the fluid that the turbines extract. One new method may use salt for that purpose, and researchers at MIT have found that such a system is not as simple as believed.
If you have two fluids with different solute concentrations separated by a semi-permeable membrane, such as having saltwater on one side and fresh water on the other, the fluids will move to try to equalize the concentrations on both sides of the membrane. The motion of the fluids is called osmosis, and pressure retarded osmosis (PRO) is a process some have been investigating for producing electricity. The idea would be to put pressurized salt water on one side of a membrane and fresh water on the other, and use the movement of the fresh water through the membrane to turn a turbine. What the MIT researchers have discovered is that the efficiency of a PRO system is more complicated than previously thought. According to their new model, the optimal membrane size is not the maximum membrane size, as a membrane half the area could produce 95% of the maximum output power.
Potentially PRO systems could be used to power desalination plants and water treatment plants, by putting saltwater or brine on one side of the membrane, and fresh or waste water on the other. To completely power some treatment plants may require some of the largest membranes in the world, but new configurations are being developed to fit the millions of square meters in relatively small packages.