When you think of renewable energy, the likes of wind turbines, hydroelectric dams, geothermal power plants, and solar panels probably come to mind. These are certainly the main sources of the world’s renewable energy, but as some groundbreaking work by Penn State University (PSU) highlights, there may soon be another.
It’s widely known that when freshwater and saltwater meet, the forces of nature act to balance out the salinity gradient. Over time, the concentration of salt molecules will be even across the board.
What you may not realize, however, is that this can also be used to generate an electric current.
If you place a barrier between the two bodies of water, one that allows water to move through but prevents the more massive salt molecules from traversing along, then a type of pressure build up occurs. Technically known as “osmotic pressure”, it increases the more imbalanced the salt gradient gets.
The more the salt is blocked from flowing through to the other side of the barrier, the higher the salt concentration will get in the initial chamber. This increases the osmotic pressure on the second freshwater chamber. This pressure can then be converted into energy that causes turbines to spin, and ultimately you get electricity.
“The goal of this technology is to generate electricity from where the rivers meet the ocean,” said team member Christopher Gorski, an assistant professor in environmental engineering at Penn State, in a statement.
This method, known as pressure retarded osmosis (PRO), is not ideal. The membranes that prevent the salt from flowing through tend to get blocked up by bacteria and detritus, which leads to the water itself being blocked from flowing through.
Another technology, reverse electrodialysis (RED), relies on the electrical gradient set up by the salt imbalance. In this case, instead of allowing water through the barrier, either positively charged sodium ions or negatively charged chloride ions are let through. This creates a positively charged section on one side and a negatively charged section on the other.
This imbalance also leads to the generation of energy, but the major shortfall here is that it doesn’t produce very much of it.
The PSU-led team, however, has come up with a third method that may beat both RED and PRO, hands down – although they have cheated a little.