Storing Green Energy

It has long been recognised that the vagaries of  the waves, the sun, and the wind,  mean that renewable "green" cannot be relied upon for 100% all-year-round electricity generation.  While batteries can be used to store energy generated while the waves are high, the wind is blowing and the sun is shining, they have been inefficient and expensive. However  energy Researchers from the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory and Stanford University,  have designed a low-cost, long-life battery that could enable solar and wind energy to become major suppliers to a country's electrical grid, according to this month's edition of Energy and Environmental Science.

Storing Green Energy

Yi Cui is a Stanford associate professor of materials science and engineering and a member of the Stanford Institute for Materials and Energy Sciences, a SLAC/Stanford joint institute. He said:"We believe our new battery may be the best yet designed to regulate the natural fluctuations of these alternative energies."

It is generally accepted that as more of a grid's energy comes from renewables, it is vital that the fluctuations in the sources of renewable energy do not compromise energy user's energy requirements. Once the green energy contribution rises to a fifth or more of a grid's power, then a storage system is vital to smooth out the peaks and troughs of energy production and demand for energy. In this way, when wind or solar power generation is not occurring, but demand outstrips production, then the excess electricity required has to be drawn from battery storage.

 The new flow battery developed by Cui's group is a "flow" type battery. Flow batteries are favoured because it's relatively simple to scale their tanks, pumps and pipes to the sizes needed to handle large capacities of energy. The new battery has a simplified, less expensive design that presents a potentially viable solution for large-scale production. Today's flow batteries pump two different liquids through an interaction chamber where dissolved molecules undergo chemical reactions that store or give up energy. The chamber contains a membrane that only allows ions not involved in reactions to pass between the liquids while keeping the active ions physically separated. So far so good, but this battery design has a couple of major drawbacks. firstly, the  liquids contain rare materials such as vanadium and this is expensive. It is also required in very large quantities for grid storage. Secondly,  the membrane requires frequent maintenance (and is also not cheap).

However the new Stanford/SLAC battery  uses only one stream of molecules and requires no membrane. Its molecules mostly consist of the relatively inexpensive elements lithium and sulfur, which interact with a piece of lithium metal coated with a barrier that permits electrons to pass without any degrading the metal. When discharging, the molecules, called lithium polysulfides, absorb lithium ions and when charging, they lose them back into the liquid. The entire molecular stream is dissolved in an organic solvent, which doesn't have the corrosion issues of water-based flow batteries, and is cheaper.

To demonstrate their new battery, the researchers created a miniature system using glassware. Adding a lithium polysulfide solution to the flask immediately produced electricity that lit up an LED. A commercial version of the new battery would be scaled up to store many megawatt-hours of energy. The next step is to make a laboratory-scale system to optimize its energy storage process and identify potential engineering issues, and at the same time to start discussions with potential hosts for a full-scale field-demonstration unit.

Exciting times ahead for wind energy storage it would seem!


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