Issue link: http://resourceworld.uberflip.com/i/1012424
66 www.resourceworld.com A U G U S T / S E P T E M B E R 2 0 1 8 Developments in Green Technology by Jane Bratun GREEN TECHNOLOGIES WATER-BASED BATTERY STORES SOLAR AND WIND ENERGY As reported on the Stanford University website, researchers in Stanford, California have developed a water-based battery that could provide an inexpensive way to store wind or solar energy. This energy could then be fed back into the electric grid and redistributed when demand is high. The prototype manganese-hydrogen battery stands three inches tall and gen- erates 20 milliwatt hours of electricity, which is equivalent to the energy levels of LED flashlights that hang on a key ring. The researchers are confident they can scale up this table-top technology to an industrial-grade system that could charge and recharge up to 10,000 times, creating a grid-scale battery with a useful lifespan in excess of a decade. Yi Cui, professor of materials science at Stanford, said manganese-hydrogen battery technology could be one of the missing pieces in the nation's energy puz- zle – a way to store unpredictable wind or solar energy so as to lessen the need to burn reliable but carbon-emitting fossil fuels when the renewable sources aren't available. "What we've done is thrown a special salt into water, dropped in an elec- trode, and created a reversible chemical reaction that stores electrons in the form of hydrogen gas," Cui said. The researchers coaxed a reversible electron exchange between water and man- ganese sulfate, an inexpensive, abundant industrial salt used to make dry cell batter- ies, fertilizers, paper, and other products. To mimic how a wind or solar source might feed power into the battery the researchers attached a power source to the prototype. The electrons flowing in reacted with the manganese sulfate dissolved in the water to leave particles of manganese dioxide clinging to the electrodes. Excess electrons bubbled off as hydrogen gas, storing that energy for future use. Engineers know how to re-create elec- tricity from the energy stored in hydrogen gas, so the important next step was to prove that the water-based battery can be recharged. The researchers did this by re-attaching their power source to the depleted prototype, this time with the goal of inducing the manganese dioxide par- ticles clinging to the electrode to combine with water, replenishing the manganese sulfate salt. After this salt was restored, incoming electrons became surplus, and excess power could bubble off as hydrogen gas in a process that can be repeated. Cui estimated that, given the water- based battery expected lifespan, it would cost a penny to store enough electricity to power a 100-watt lightbulb for 12 hours. "We believe this prototype technology will be able to meet Department of Energy goals for utility-scale electrical storage practicality," Cui said. The U.S. Department of Energy (DOE), which funded this project, recommends that batteries for grid-scale storage should store and then discharge at least 20 kilo- watts of power over a period of an hour, be capable of at least 5,000 recharges, and have a useful lifespan of 10 years or more. To make it practical, such a battery system should cost $2,000 or less, or $100 per kilo- watt hour. According to DOE estimates, about 70% of US electricity is generated by coal or nat- ural gas plants, which account for 40% of carbon dioxide emissions. Shifting to wind and solar generation is one way to reduce those emissions. But that creates challenges involving the variability of the power sup- ply, for example, the sun shines by day and sometimes the wind doesn't blow. But another less understood but impor- tant form of variability comes from demand surges on the grid – that network of high- tension wires that distributes electricity over regions and ultimately to homes. On a hot day, when people come home from work and crank up the air conditioning, utilities must have load-balancing strate- gies to meet peak demand: some way to boost power generation within minutes to avoid brownouts or blackouts that might otherwise bring down the grid. Utilities often accomplish this by firing up on-demand or "dispatchable" power plants that may lie idle much of the day but can come online within minutes – pro- ducing quick energy but boosting carbon emissions. Some utilities have developed short- term load balancing that does not rely on