3.1.2. Compressed Air Energy Storage (CAES)

CAES has been used in a range of industrial applications since the eighteenth century. Electricity is used to compress air and store it in a subsurface construction or an aboveground system of containers or lines. Subsurface storage options include tunnels, aquifers, and abandoned mines. Diabatic technology is well proven; the plants are highly reliable and can operate without external power [20] (shown in Figure 3). CAES has a large capacity, but it has drawbacks such as low round-trip performance (less than 50%) and geographical constraints.

**Figure 3.** Compressed air energy storage system schematic.

### 3.1.3. Flywheel Energy Storage (FES)

In flywheel energy storage, kinetic energy is stored in an accelerated rotor which is a massive rotating cylinder. Electricity is supplied to the flywheel using a transmission mechanism and with rise in the speed, amount of stored energy increases [17]. Flywheels are commonly utilized for power quality in industrial and other applications. Flywheels have advantages of exceptional cycle stability and long life, low maintenance, greater power density and the use of environmentally friendly materials. However, it has demerits such as high self-discharge and poor current efficiency [21]. Efforts are focused on improving the management of flywheels as power storage devices for usage in cars and industries for long operation hours (shown in Figure 4).

**Figure 4.** Flywheel energy storage system schematic.

#### *3.2. Electrochemical Storage Systems*

Electrochemical energy storage devices have the ability to make a major contribution to the deployment of sustainable energy. Electrochemical energy storage is based on systems with high energy density (batteries) or power density (electrochemical capacitors). High energy and high power densities in the same material are increasingly required in current and near-future applications [17,22]. These are categorized in two types: secondary batteries and flow batteries. The secondary batteries have again classified into following types: lead– acid, NiCd/NiMH, Li-ion, metal–air, sodium–sulfur and sodium–nickel chloride [22].

#### 3.2.1. Secondary Batteries

A secondary battery, or charge accumulator, is a cell or set of cells with reversible cell processes. This implies that the original chemical conditions inside the cell can be restored by allowing current to flow into it, i.e., charging from outside [22].

#### Lead–Acid Battery (LA)

Lead–acid batteries are the most widely used form of battery in the world, dating back to roughly 1890. Service life is typically 6–15 years, with a service life of 1500 cycles at a % depth of discharge and a cycle efficiency of 80–90% [22–24]. The downsides are lower energy density and the use of lead, a dangerous element that is prohibited or restricted in some locations. Advantages include a good cost/performance ratio, simple recyclability, and a simple charging method. The current focus of lead–acid battery development is to improve their efficiency for micro-hybrid electric vehicles.

#### Nickel–Cadmium and Nickel–Metal Hydride Battery (NiCd, NiMH)

Before the commercial launch of nickel–metal hydride (NiMH) batteries in 1995, nickel– cadmium (NiCd) batteries had been in use since around 1915. NiMH batteries contain all of the advantages of NiCd batteries, such as greater power density, marginally better energy density, and a larger number of cycles, with the exception of a 10-fold lower maximum nominal capacity. They are far more robust and secure than lithium-ion batteries. However, due to the toxicity of cadmium, they have been limited for consumer use since 2006. NiMH batteries are currently about the same cost as Li-ion battery packs [22].
