**4. Energy storage**

The surplus energy is stored in different forms and can be used later as needed. There are many different types of energy storage, including batteries and thermal and mechanical systems. The first electrochemical battery, storing and releasing a charge through a chemical reaction, was invented in 1800 by Italian chemist Alessandro Volta [5]. The modern high-energy density lithium-ion batteries were commercialized in the 1970s [6]. Now, the batteries have been widely deployed in applications ranging from electric vehicles and consumer electronics to grid-scale energy storage projects.

Thermal energy storage systems store heat or cold. There are three main types of thermal energy systems: sensible heat, latent heat, and thermo-chemical heat storage. A water tank is often used to store sensible heat, while phase change materials are for latent heat storage. Thermo-chemical energy storage systems store heat or cold through reversible chemical reactions. The main types of mechanical energy storage systems are pumped hydro, flywheel, and compressed air. The pumped hydro storage uses two reservoirs with one at a higher elevation than the other and currently is the most widely used large-scale mechanical energy storage system [7].

Energy storage plays a pivotal role in bridging the gap between energy supply and demand, enabling the efficient utilization of various forms of energy. Energy storage technologies find applications across diverse sectors, including transportation, residential, commercial, industrial, and utility-scale energy systems. Many challenges to wide deployment of energy storage systems remain to be addressed. These include cost competitiveness, technological limitations, regulatory barriers, grid integration issues, and environmental considerations.
