Electricity generation through the use of salinity gradients between salt and fresh water is a relatively new concept. While discovered and discussed in the 1970s, research has been slow and most of it only recently. Two practical methods concerning membrane technology are currently being researched: the reverse electrodialysis (RED) method and pressure retarded osmosis (PRO). Both technologies are dependent on the semi permeable membrane. A semi-permeable membrane is selective in its permeability, i.e. only specific substances can pass through the membrane.