Flow batteries – large scale energy storage

The ability to store renewable power will be crucial to the success of integrating solar and wind energy into our grids. Flow batteries are a promising technology with good potential for scaling up to utility size.

Energy storage solutions are not only essential for dealing with peaks and lows in power supply as a result of the intermittency of solar and wind energy, but also for developing a stable smart grid. There are many types of batteries and there are other forms of storage to like P2G (Power to Gas), compressed air and cryogenic storage. Research into flow batteries, originally developed by NASA, has accelerated as they hold great promise for both small and utility scale applications.

External tanks

So how do flow batteries work? Basically, they store energy in external tanks filled with a charged liquid. And because the energy is stored in tanks, the technology can be easily scaled up. They can also be idled for a long time without losing charge and charged or discharged quickly thousands of times. In conventional flow batteries there are two tanks and an interaction chamber with a membrane. Each tank contains a different liquid – a positively and a negatively charged solution – which is pumped into the interaction chamber where dissolved molecules undergo chemical reactions that store or give up energy by releasing or taking up electrons.

Vanadium

The cost and efficiency of such batteries depends to a large extent on the chemicals in the solutions and the material for the membrane, so the race is on to find the best ones. One type of flow battery under development in several countries is the Vanadium Redox Battery, which uses the rare metal Vanadium. They are very effective but Vanadium is expensive. Other options, using more low-cost reactants, are Hydrogen/Bromium flow batteries and Zinc/Iron flow batteries. The American company ZAI has developed Zinc/Iron batteries of up to 80kW, which it says is modular and easily scalable. The Fraunhofer institute in Germany has developed a (vanadium-) flow battery of 25kW by using new membrane materials and by redesigning battery management and design. The next step up will be 100kW batteries.

Single chamber

At Harvard University in the United States a research project has begun into using small, inexpensive organic molecules in aqueous electrolytes for flow batteries. Other research, at Stanford University, is revolutionizing the concept of the flow battery even further. It is pioneering a single chamber concept for the flow battery, using a lithium polysulfide solution and not a membrane but a piece of lithium metal with a special coating. The interaction that would otherwise take place at the membrane now occurs at the metal piece. The Stanford team, which has so far produced only a very small version of the battery, says its concept is cheaper and less bulky than ‘normal’ flow batteries.

Peak hours

Flow batteries that come in different sizes, from micro-grid units to utility-scale units, could make a significant contribution to grid stability. First of all, it would facilitate integrating a larger share of intermittent renewable power in the grid. It would also alter the economics of renewable power, as large generators could store surplus power and sell it at peak hours when prices are high. Smaller, distributed generators, from households to community sized projects, would no longer be forced to sell to the grid. Instead they could use their stored power when electricity prices peak and so lower their energy bills. This would simultaneously have the effect of levelling peaks in supply and demand.
 

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