When a river flows into the sea, the mixing of fresh and salt water unleashes energy. A team of researchers from Stanford University has estimated that the electric power generated from this mixing could provide about 13 percent of the world’s current energy needs. The team has developed a system based on the principle of entropy that generates such energy.
Power from Osmosis
Scientists have conventionally used the method of osmosis to harvest the energy produced from the interaction of fresh and seawater. Without using any external energy input, a thin membrane allows water molecules to pass through it but not salt molecules. This creates a difference in pressure between the side of the membrane with higher salinity and the one with lower. The increased pressure can then be used to generate electricity, for example, by turning a turbine. The Statkraft facility in Norway is a plant that generates power by the process of Pressure Retarded Osmosis (PRO).
Renewable Energy from Entropy
The technical challenge for osmotic plants is to develop a membrane that draws through enough water to create an effective pressure to run the turbine. Yi Cui’s team at Stanford University, California, approached the problem differently. The team succeeded in extracting energy using the change in entropy that occurs when freshwater flows into saline water. Entropy is the amount of energy in a system that is no longer available for doing mechanical work. The Stanford method extracts energy from the difference in concentration of freshwater and seawater. The team developed a system with a battery that drew energy from a crystal lattice made of manganese dioxide nanorods and silver electrodes. The nanorods reduce a large surface area into a small space. The energy is stored chemically in batteries. The advantage of this system is that unlike osmotic systems, it does not require membranes. The team at Stanford claims to have extracted energy from the interaction with 74 percent efficiency.
Salt and sustainability
Salt may well turn out to be an interesting ‘additive’ to the green future of the planet. In contrast with the process of PRO, Reverse Electrodialysis (RED) employs two membranes. Both do not let freshwater pass through them but one is permeable to sodium ions and the other to chloride ions from seawater. The electrical difference between the positively charged sodium ions and the negatively charged chloride ions makes for a chemical battery. Molten salt is being developed as a storage solution for solar power installations. One more development is a fuel cell that rolls into one, desalination, wastewater treatment, energy generation and hydrogen gas production.
The pros:
This is as green as energy generation can get. The only waste product is brackish water that can be made to flow back into the sea. When tapped into perennial rivers, plants that harvest energy from the mixing of fresh and saline water can function for longer hours than solar–energy farms and wind farms. These plants can generate electric power at a fairly constant rate.
The cons:
The biggest obstacle for the energy generation processes that rely on membranes is the development of good quality membranes. Another issue is the clogging of membranes and ‘bio-fouling’ resulting from slit, algae and impurities present in water. The method employed by the team of researchers at Stanford needs a lot of electrode surface area, which can be achieved only by miniaturizing the manganese dioxide electrodes. Still, if they can improve efficiency, salinity power will be a welcome addition to the growing green energy family.
Via: CleanTechnica
