Salinity gradient power generation is achieved by creating a rechargeable ion battery via the mixing entropy of salt and freshwater. Mixing entropy batteries can produce electricity through the ion exchange of sodium and chlorine in saltwater when it mixes with freshwater. This mixing entropy battery works by electrochemical processes related to the charge required for the two solutions—freshwater and seawater—to reach equilibrium.
Researchers at Stanford have incorporated Na2-xMn5O10 nanorods into the cathode to increase the surface area and capture sodium ions. These Nanorods increase the cathode surface area by 100 times over other materials, are low cost, electrochemically stable, and environmentally benign. In the research prototype, the anode is made of silver and attracts the chlorine ions. Commercial applications will need to find a substitute for silver electrodes due to cost considerations. The resultant battery is capable of extracting 74% of the entropic energy in the saltwater/freshwater solution. The battery can be either a closed loop battery for portable applications or a flow battery for stationary, utility scale applications.
The limiting factor on the amount of energy these devices can extract is the flowrate of the freshwater source. The voltage of the system is dependent on the salinity difference of the two water sources. Additionally, researchers are studying the possibility of using sewage—mainly storm runoff and grey water—as a source to reduce the impacts on freshwater. There are potentially two terrawatts of extractable energy per annum—13% of global annual consumption—in all of the world’s rivers if this technology were to prove scalable.
- . Batteries for Efficient Energy Extraction from a Water Salinity Difference. Nano Letters. 2011 ;11(4):1810 - 1813.
- . Stanford Researchers Use River Water and Salty Ocean Water to Generate Electricity. [Internet]. Submitted . Available from: http://news.stanford.edu/news/2011/march/saline-rechargeable-battery-032811.html
The function of the nanorods is to improve the energy production capabilities of mixing entropy batteries by increasing the surface area and allowing for a greater net energy output.
The potential benefit of this technology is reduced emissions from power generation through the use of constantly renewable resources.
These nanostructures are embedded in closed systems and are made of environmentally benign materials. Any environmental health risks associated with this technology are in the fabrication process of the sodium manganese oxide nanorod cathodes. These risks would be heavily dependent on the morphology of the nanostructures and the toxicity of manganese to biological organisms.