The confluence of sea water and fresh water with its inherent difference in salinity is providing researchers with a new approach to energy harvesting. The techniques known as Reverse Electro Dialysis (discussed in this article) and Pressure Retarded Osmosis (which uses thin film composite membranes) generate a potential difference by the transportation of ions through Ion selective membranes.
Reverse Electro Dialysis (RED) is not very dissimilar a process to Proton Exchange Membrane (PEM) in Fuel Cell technology. In each RED cell, salt and fresh water pass through channels separated by membranes. The Cat-ion exchange membranes allow sodium ions (Na+) to pass through to the freshwater channel, while An-ion exchange membranes permit Chlorine ions (Cl -) through. A potential difference is created by the ensuing redox half reactions. The by-product is brackish water.
Each of these cells are then stacked together in the hope of generating electricity of useable quantity. There are still some challenges to be overcome such as improvements in stack design to eliminate short circuits by ionic salt bridges, before REDs are scalable to their theoretical power generation capacity (1) which Dutch Company Redstack BV estimates as a whopping 1MW of electricity generated from a flow of 1000 Litres per second; a very easily achievable figure for any river aspiring to empty into the salty seas.
Professor Logan's team at Penn State sees an even brighter future and more noble goals for similar osmotic technologies; such as being able to generate power while cleaning waste water full of murk, serving in-land populations especially in developing countries. Though they believe this must be aided by microbial activity (2) the team at Penn State sees potential in these microbial aided osmotic technologies for the limitless generation of hydrogen gas (3), (4).
Yet until the design and membrane challenges are fully addressed, researchers have suggested other uses for RED systems, Dr. Sadegian a postdoctoral researcher at the University of California - Santa Cruz demonstrated miniaturized cells aimed at the production of just about enough power to run tiny devices (5), while researchers at Peking University in China have demonstrated devices employing single Ionic Nano-pores as membranes and Nano-fluidic channels (6). The Peking team foresees a 2 order of magnitude increase in electricity generation once the short circuit and design challenges are overcome (7).
Microprocessor manufacturers are constantly reducing their power consumption and availability requirements, with Nanowatt-XLP® devices from Microchip® and Texas Instruments® Ultralow Power processors; allowing designers to create devices that draw the minutest power. Perhaps very soon we would all walk about carrying devices powered by brackish murky waters.
If you would like to know more about emerging Energy Harvesting technologies, Market Size Forecast attend the upcoming Energy Harvesting & Storage Europe 2012, 15-16 May, 2012 in Berlin, Germany or the USA event Energy Harvesting & Storage USA 2012, 7-8 November 2012 in Washington, DC, USA.
1. Veerman, J., M. Saakes, SJ Metz, and GJ Harmsen. "Reverse Electrodialysis: A Validated Process Model for Design and Optimization." Chemical Engineering Journal 166, no. 1 (2011): 256-268.
2. Cusick, Roland D, Younggy Kim, and Bruce E Logan. "Energy Capture from Thermolytic Solutions in Microbial Reverse-Electrodialysis Cells." Science (March 1, 2012). http://www.sciencemag.org/content/early/2012/02/29/science.1219330.
3. Kim, Y., and B.E. Logan. "Hydrogen Production from Inexhaustible Supplies of Fresh and Salt Water Using Microbial Reverse-electrodialysis Electrolysis Cells." Proceedings of the National Academy of Sciences 108, no. 39 (2011): 16176-16181.
4. Logan, B. "'Inexhaustible' Source of Hydrogen May Be Unlocked by Salt Water." Membrane Technology 2011, no. 12 (2011): 9.
5. Sadeghian, R.B., O. Pantchenko, D. Tate, and A. Shakouri. "Miniaturized Concentration Cells for Small-scale Energy Harvesting Based on Reverse Electrodialysis." Applied Physics Letters 99 (2011): 173702.
6. Guo, Wei, Liuxuan Cao, Junchao Xia, Fu‐Qiang Nie, Wen Ma, Jianming Xue, Yanlin Song, Daoben Zhu, Yugang Wang, and Lei Jiang. "Energy Harvesting with Single‐Ion‐Selective Nanopores: A Concentration‐Gradient‐Driven Nanofluidic Power Source." Advanced Functional Materials 20, no. 8 (April 23, 2010): 1339-1344.
7. Cao, L., W. Guo, W. Ma, L. Wang, F. Xia, S. Wang, Y. Wang, L. Jiang, and D. Zhu. "Towards Understanding the Nanofluidic Reverse Electrodialysis System: Well Matched Charge Selectivity and Ionic Composition." Energy Environ. Sci. (2011).
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