“Virtual batteries” could lead to cheaper, cleaner power

small info about virtual battery....... 

 The question is, can we develop algorithms that schedule consumption of these loads in such a way that satisfies all their constraints and makes them appear to the power system as a battery, which can store a certain amount of energy and absorb and release it at a certain rate?” 

 Similarly, an electric car parked in an office building needs to recharge its battery, but the charge rate can be fast or slow, and the charging might take place at any time within, say, a four-hour window. Slowing the charging rates or deferring the charge times for a group of cars reduces demand on the grid (equivalent to a release of energy from the grid battery). The charge rate of this virtual battery is limited by the available capacity of the cars’ own batteries and by their individual maximum charge rates. 

 Suppose, for instance, that you have two batteries, one that can be charged or discharged quickly, the other slowly. Now suppose that you’re treating these two real batteries as a single virtual battery, and the virtual battery is half full. How do you distribute the virtual battery’s charge across the two real batteries? If you want to maximize the charge rate of the half-full virtual battery, you need to keep the faster-charging real battery more depleted than the slower-charging one; that way, it can handle the bulk of any incoming charge. 

The opposite is true, however, if you want to maximize the discharge rate; then, you need to keep the faster-charging battery fuller than the other, so it can handle the bulk of any discharges. To see how charge rates trade off against battery capacity, suppose that both of the real batteries are empty. To maximize the charge rate of the virtual battery, you need to use both real batteries; any two batteries can absorb charge faster than either of them can in isolation. But the faster-charging real battery will fill up before the slower-charging one does. 

 So at the maximum charge rate, the capacity of the virtual battery is the capacity of the faster real battery, plus however much charge the slower battery can absorb by the time the faster battery fills. The remaining capacity of the slow battery must go unused. Lowering the aggregate charge rate, however, allows the slower battery to absorb more charge by the time the faster battery is full, increasing aggregate capacity.

https://news.mit.edu/2017/virtual-batteries-cheaper-cleaner-power-0324