Green Energy From Estuaries
An estuary is defined as being the mixing zone where fresh river water meets the salt water of the ocean. The penetration of salt water into the fresh river water can extend backwards from the coast for some distance. Researchers at Stanford University recently published details of a battery which uses changes in salinity to produce electricity (see Nano Lett., 2011, 11 (4), pp 1810-1813).
The device, which incorporates a Na(2-x)Mn5O10 nanorod electrode works on the basis of the difference in concentration between salt (sodium chloride) between the two environments. Initially, the electrodes are charged with sodium (cathode) and chlorine (anode) ions in a low salinity environment (this requires priming the cell with electricity). In the next stage, the fresh water empties from the cell and is replaced with seawater.
The ionic concentration of sodium and chlorine is at least sixty times higher than in freshwater and the sodium ions in it will migrate towards the cathode whilst chlorine ions migrate to the anode. The migration of ions causes the electrical potential in the battery to increase allowing more energy to be extracted from the system than was used to prime it. Once the energy in the system has been discharged, the seawater is replaced with freshwater and the process can be started again.
In laboratory experiments, Yi Cui and his team were able to demonstrate a 74% efficiency in converting the potential energy stored in the battery into electrical current, although they are optimistic that design modifications could boost this figure to 85%. The use of nanorods to form the electrodes serves to increase the effective surface area by a factor of approximately 100 over conventional materials.
The Stanford team calculated that their batteries could produce 13% of global electrical energy demand if all of the world's estuaries could be utilised. More realistically, they determined that a power station with a flow rate of 50 cubic metres per second could yield up to 100 MW - enough to supply 100 000 households with electricity.