*1.1. Role of EES*

Two properties of electricity cause challenges with its usage while also creating market demand for EES. To begin with, power is consumed at the same moment it is created. To fulfill the shifting demand, the appropriate amount of power must constantly be given. The second trait is that power plants are often positioned distant from where electricity is utilized [5].


#### 1.1.1. Optimization: High Generation Cost during Peak Hours

Generation costs vary across periods. Power suppliers should supplement base-load power plants with less cost-effective but more adaptable kinds of production, such as oiland gas-fired generators, during peak hours. Costly methods of generating can be shut down during off-peak hours. This surplus can be held in EES and used to lower generating costs. In contrast, EES can reduce energy cost for customers because it can store energy purchased at cheap off-peak prices and use it during prime times in place of costly power. During off-peak time, users can recharge batteries and may also sell to utilities or to other users during peak time [5–7].


#### 1.1.2. Continuous and Flexible Supply: Need of the Hour

The main issue for utilities is delivering a consistent and adaptable power supply for consumers, which is a critical quality of energy. If the proper amount of electricity is not accessible when consumers need it, power quality will suffer and service will be disrupted in the worst-case situation. To meet fluctuating power consumption, sufficient amounts of energy should be produced on a regular basis, based on an accurate assessment of demand variations [1,2]. Power generators require two extra functions in addition to the basic generating function. Firstly, producing facilities must have a "kilowatt function" that permits them to generate enough power (kW) as per requirement. Secondly, generating facilities must feature a frequency control mechanism that adjusts the output to fit minuteby-minute variations. To take care of the fluctuating power consumption, adequate amounts of energy should be generated and be available, based on an accurate estimate of demand fluctuations. Such issues are intended to be addressed by EES. When produced electricity is in low supply, pumped hydro has been routinely employed to deliver a huge amount of power [7].

#### 1.1.3. Distance between Generation and Consumer: A Deciding Factor

Consumers' premises are usually located far from power-producing facilities, increasing the likelihood of a power failure [8]. Natural calamities and causes due to human factors trigger system failures that interrupt power supply and have the potential to affect broad areas [9]. When power failures occur, EES will support consumers by continuing to provide electricity. Semiconductor and LCD manufacturing are two industries where voltage sag for just a few milliseconds has an impact on product quality, employ EES [10].

#### 1.1.4. Power Grid Congestion: A Point of Concern

The power flow in transmission networks is determined by the demand and supply. Power congestion can occur during the process of balancing supply and demand [11]. Utility companies strive to avoid future bottlenecks by moving generating output or establishing additional transmission connections. EES, when installed at appropriate places such as substations at the extremities of heavily loaded lines, can help to reduce congestion [12]. This method also helps utilities delay or cease power network reinforcement.

#### 1.1.5. Transmission by Cable: Point of Difficulty

Because power transmission usually entails the use of cables, supplying power to mobile applications and isolated areas is difficult. EES technology, with its mobility and charging functionality, can be beneficial to address this issue. It may be difficult to charge an EV in remote places without access to a power grid, but EES can aid in the creation of a green transportation system that does not rely on traditional IC engines [13].

#### *1.2. Emerging Needs for EES*

Two key market needs for EES as a critical and evolving technology are: (i) the use of more renewable energy and lower consumption of conventional fuel and (ii) a future smart grid [1] (shown in Figure 1).
