**1. Introduction**

The economic success and technical competitiveness of a country are significantly influenced by its access to energy. World energy outlook [1] reported that the total energy demand for the countries like India and China is expected to increase from 55% in 2010 to 65% in 2035. Due to factors such as population growth, fast urbanisation, and economic development, demand for easily accessible energy sources like fossil fuels is rising at an almost exponential rate. Despite the development of resources and the expansion of energy supplies, enough energy has not been produced to fulfil the rising demand. Additionally, the demand for energy in various sectors like residential and commercial, industrial, and utility fluctuates on a time basis. In order to meet such variation, it is essential to look for suitable means of storage of energy.

The storage of energy offers an alternate approach to meeting peak demand for electricity. The most expensive and challenging times to provide are during peak hours. Therefore, energy storage systems offer a tremendous amount of untapped potential to improve the efficiency of equipment that converts energy. It also has the potential to be utilised for enabling large-scale fuel replacements in economies all around the world. Energy storage results in cutting down the energy wastage. This is a cheaper, easier, and faster way to address several energy issues. In a nutshell, energy storage not only acts as a link between demand and availability but also boosts the system's overall performance and dependability. Consequently, the development of energy storage systems that are efficient and low-cost is an equally vital subject of study as the development of new sources of energy.

#### **2. Energy storage systems**

The act of temporarily 'storing' energy for the purpose of using it at a later time is referred to as energy storage (ES). Power and energy services rely on ES devices, which take in excess energy and hold it for later use. Energy may be stored in a variety of forms, including mechanical, electrical, chemical, and thermal energy. The ES systems are generally categorised by output: electricity and thermal.

### **2.1 Electricity storage systems**

Electricity storage systems include hydropower, compressed air energy, flywheels, batteries, supercapacitors, chemical-hydrogen storage, etc.

