*4.2. Energy Storage*

A crucial issue for both the deeper penetration of renewables with fluctuating performance, such as PV and wind and resolving peak and off-peak demands, is energy storage. Recently numerous interesting ideas have been published. Researchers from the Massachusetts Institute of Technology presented a noteworthy study [156] that analysed economic and environmental variability of the reuse of e-car batteries as stationary batteries for power storage. Similar contributions to circular economies should be the subject of future design development and implementation. The rapid development of advanced batteries is also taking place [157], for example, the development of nanographene batteries. Improving and reducing the cost of batteries represents a major research challenge that has attracted significant investment.
