Water, for as long as life has existed, has been the universal indicator of sustainable life. Most biologists agree that without water, life would be impossible to sustain, and so in a way, water is life (Alpert, 2005). But now, water can be energy too.
New studies indicate that a new, feasible way to produce electricity is through a new form of “blue energy,” that is, energy generated from galvanic cells, or batteries, between wastewater and seawater in coastal areas (Thennakoon et al., 2023). By putting simple electrodes into areas where seawater and freshwater meet, an electric current can be produced. It seems like alchemy, but it’s nothing more than simple chemistry.
Galvanic cells are extremely simple. All you need are two pieces of metal, an electrolytic solution (a solution with a dissolved salt that conducts electricity), and a wire (Bhatt et al., 2016). A galvanic cell generates electricity through redox reactions, or a reaction where electrons are exchanged between two compounds. Here, these two compounds are the solutions and their electrodes. The two pieces of metal are called the anode and the cathode. The anode undergoes oxidation, where it loses electrons, and the cathode undergoes reduction, where it gains electrons (Bhatt et al., 2016). Charged particles in the solution move through the electrodes, and this electron flow through an external circuit creates an electric current (Bhatt et al., 2016).
Researchers found that locations where saltwater and freshwater meet have untapped electrical potential. The ocean serves as the electrolytic solution and all that’s needed are the anode and cathode. Treated wastewater has a concentration of 30mM of NaCl, while seawater has a concentration of 600mM, theoretically being able to produce 0.65kWh per cubic meter of freshwater. Globally, this translates to almost 18 gigawatts of electricity (Ye et al., 2019). In other words, wastewater energy can produce enough electricity to power 6 million homes in the United States (Lewis et al., 2024).
But where do those electrodes come from? Though originally too expensive to produce the battery, researchers from Stanford University have been able to create a working, low-cost system for producing electricity using electrodes made from Prussian Blue, a common pigment, and Polypyrrole, a conductive polymer.
The Stanford researcher’s experimental battery cell works as follows: As freshwater flows out, Na+ ions and Cl- ions are released into the water, producing an electric current between the two electrodes. Then, as seawater flows into the cell, the chemical equilibrium shifts, forcing Na+ into the Polypyrrole anode and Cl- into the Prussian Blue cathode, reversing current flow and harnessing the seawater, producing more electrical current. Then, freshwater refills into the cell as seawater flows out, restarting the cycle again (Ye et al., 2019). This elegant solution takes an initial amount of electricity to start, but overall, produces electricity from ions in the seawater, all while using wastewater that wouldn’t be used otherwise.
Salinity gradient energy, as it is known, is highly versatile and has a low initial cost to produce. Prussian Blue is less than $1 per kilogram, and polypyrrole is less than $3, while producing 63mW/m2 of electrode exposure each cycle, making it sustainable and an economical solution to our world’s ever-changing electricity needs.
This economical solution is also extendable to all coastal locations across the world. In the European Union alone, 281 potential sites for these salinity gradient energy plants have been surveyed as feasible use cases, potentially reclaiming 3.7 million cubic meters of water per day, and reducing carbon emissions by 150,000 tons per year (Sampedro et al., 2023). With commercial investment and adoption, salinity gradient batteries can become cheaper, more efficient, and more than enough to power wastewater treatment plants. As the global warming crisis continues to threaten millions of people, this technology serves as just one of many tools that governments can use to mitigate the climate crisis, making salinity gradient batteries a promising technology that calls for increased awareness and further research.
References
- Alpert, P. (2005, November). Sharing the Secrets of Life Without Water. Integrative and Comparative Biology, 45(5), 683–684. 10.1093/icb/45.5.683
- Thennakoon, T.; Hewage, H.; Sandunika, D.; Panagoda, L.; Senarathna, W.; Sulaksha, L.; Weerarathna, D.; Jayathma, W.; Gamage, D.; Perera, M. (2023). Harnessing the Power of Ocean Energy: A Comprehensive Review of Power Generation Technologies and Future Perspectives. Journal of Research Technology & Engineering, 4(3), 73-102.
- Bhatt, A., Forsyth, M., Withers, R., & Wang, G. (2016, February 25). How a battery works - Curious. Australian Academy of Science. Retrieved August 5, 2024, from https://www.science.org.au/curious/technology-future/batteries
- Barrett, K., Navarro, G., Koressel, J., & Kohn, J. (2023, August 29). The Cell Potential. Chemistry LibreTexts. Retrieved August 5, 2024, from https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Voltaic_Cells/The_Cell_Potential
- Ye, M., Pasta, M., Xie, X., Dubrawski, K. L., Xu, J., Liu, C., Cui, Y., & Criddle, C. S. (2019, July 8). Charge-Free Mixing Entropy Battery Enabled by Low-Cost Electrode Materials. ACS Omega, 4(7), 11785-11790. ACS Publications. 10.1021/acsomega.9b00863
- Lewis, M., Lambert, F., & Johnson, P. (2024, May 1). The US just proposed 18 GW of new offshore wind sales. Electrek. Retrieved August 5, 2024, from https://electrek.co/2024/05/01/us-offshore-wind-sales/
- Sampedro, T., Gómez-Coma, L., Ortiz, I., & Ibañez, R. (2023, September 25). Unlocking energy potential: Decarbonizing water reclamation plants with salinity gradient energy recovery. Science of the Total Environment, 906. Elsevier. 10.1016/j.scitotenv.2023.167154