Energy storage is the equipment that keeps electricity for later: it stores power when there is too much and releases it when there is too little. Demand does not follow the weather. A solar park produces most at noon, but a city uses most power at night. Storage fills the gap.
The first kind is the battery. A battery energy storage system, or BESS, is a set of batteries in a shipping-container-sized box, together with software. It charges when power is cheap and plentiful and discharges when power is scarce. It is built from cells, modules and packs. Two variants matter: sodium-ion batteries, which use cheaper materials than lithium batteries, and mobile battery storage, which replaces diesel generators on construction sites.
The second kind is hydrogen. An electrolyser uses spare electricity to split water into hydrogen and oxygen. The hydrogen can be stored in tanks and used later. A fuel cell turns it back into electricity.
The two kinds suit different lengths of time. Say a grid operator needs to cover a gap of four hours every evening, and also a three-day spell of cloudy monsoon weather when solar output is low. A battery is the right choice for the evening gap, because it can charge and discharge every day and returns most of the energy. Hydrogen is considered for the three-day gap, because tanks hold energy for days at a low cost. But a lot of energy is lost on the way.
Storage supports grid balancing, the rule that the power put into the grid must equal the power taken out every second. When it stores solar or wind power for later use, it also lowers a company's emissions, which are counted as Scope 1, 2 and 3 emissions. India now invites bids for large storage projects, and the software that decides when to charge and discharge is a growing business.