Lead-acid battery technology is based on positive and negative electrodes submerged into electrolyte which is a combination of sulfuric acid and water, lead dioxide is used as a positive electrode and lead is used as a negative electrode [5] . This type of secondary cell is widely used in vehicles and other applications requiring high values of load current. Its main benefits are low capital costs, maturity of technology, and efficient recycling. Nominal voltage of this technology cell is around 2 volts. Merits of a lead-acid battery technology include low-cost of manufacture, high specific power low cost per watt-hour, capable of high discharge currents and good performance at low and high temperatures. Disadvantages include low specific energy; poor weight-to-energy ratio, slow charging (Fully saturated charge takes 14 - 16 hours) and the need for storage in charged condition to prevent sulfating. Lead-acid battery also suffers from Limited cycle life of which repeated deep-cycling reduces battery life [6] . Lead-Acid battery cross section is shown in Figure 1.

Cross section of the Nickel-cadmium battery is shown in Figure 2.

Nickel-cadmium (Ni-Cd) technology is type of battery technology that has in use for quite a long time. It finds application in systems that require operation in difficult environmental conditions and long battery life. Nickel cadmium technology is based on cathode made from nickel oxide hydroxide and anode made from metallic cadmium while electrolyte used for Ni-Cd batteries is potassium hydroxide [7] . Core advantages of Ni-Cd technology are ability to resist electrical and physical stress, low maintenance cost, a high number of lifetime cycles and suitability for long term storage and. Demerits of this battery technology includes high cost in comparison to the lead-acid technology, limited energy density, toxic and caustic elements in batteries and the memory effect.

Lithium-ion (Li-ion) battery technology is one of the most advanced battery technologies widely used today. As a result of its light weight and good energy density, cellphones, smartphones, tablets, laptops, all gadgets are powered with the Li-ion battery. The Li-ion battery consists of a positive electrode-anode, a negative electrode-cathode, separator, electrolyte, and two current collectors [8] . The anode consists of lithiated metal oxide, whereas, the cathode consists of graphitic carbon with a layering structure. The electrolyte is made up of lithium salt, which is dissolved in organic carbonates [9] . Lithium-ion battery working principle is given in Figure 3.

Advantages of lithium-ion batteries include high specific energy and high load capabilities with power cells long cycle and extended shelf-life; maintenance-free high capacity, low internal resistance, good columbic efficiency simple charge algorithm and reasonably short charge times while disadvantages are need for protection circuit to prevent thermal runaway if stressed, degradation at high temperature and when stored at high voltage, Impossibility of rapid charge at freezing temperatures [10] .

Nickel-metal hydride technology is based on the negative electrode made from hydrogen-absorbing alloys which have the possibility to absorb releasing hydrogen and the positive electrode made from nickel oxy-hydroxide. There is a separator which separates positive and negative electrodes to prevent shorting between electrodes. Electrolyte used in this technology is potassium hydroxide (KOH) [11] . There is a current collector made of metal which minimizes the internal battery resistance. To release gases produced during the overcharging or shorting there is a self-sealing safety vent. Nickel-metal hydride battery cross section is shown in Figure 4.

Some advantages of NiMH technology are long battery lifetime, high number of lifetime cycles, good performance at the high temperatures, high energy density, good ability of recycling and the high tolerance to battery overcharging and over discharging. Disadvantages are high cost in comparison to the lead-acid

technology and bad performance at the low working temperatures [12] .

Sodium-sulfur (NaS) battery is one of the most suitable battery technology that has being put to use in energy storage systems because of the high energy density and good maximum depth of discharge characteristic which is theoretically 100%. This battery technology is based on the use of sodium as anode and sulfur as cathode, with the electrolyte being of beta alumina ceramics. A defined advantage of this technology that the electrolyte also serves as a separator between the positive and negative terminal. As such, sodium sulphur battery technology has a low internal cell resistance because of the use a ceramic electrolyte which invariably increases its power to weight ratio and while also reducing the quantity of heat liberated internally during the charging process [13] .

Detailed characteristics of sodium-sulfur battery technology are shown in Figure 5. Advantages of sodium-sulfur technology as highlighted in Table 1 include high number of lifetime cycles, high energy density, capability of pulse power and good resistivity to self-discharging. Disadvantages are high cost and high temperature required for battery operation [14] .

Yet another promising type of battery technology is the Vanadium-redox flow battery (VRFB). This battery technology has very good characteristics. That includes very fast response time, long lifetime, and long storage time which makes it suitable for long-term energy storage. VRFB battery technology possesses a unique characteristic in that its Power and energy are independent of the other. As such, power of this battery type depends on the number and size of the cells while its energy depends on the available electrolyte, respectively tank size [15] . Cross section diagram of Vanadium-redox flow battery is shown in Figure 6.

From Table 1, it becomes obvious why lithium ion battery technology is still the batter technology of choice when it comes to implementing energy storage system among other battery technology. This is not only due to its matured technology but primarily because of its higher specific power, energy density and efficiency when compared to other battery technology [17] . Although there are other batteries with promising specific power and energy density, these are still in the research and development stage.

Super-capacitors (SCs) also known as ultra-capacitors or electric double layer capacitors [18] consists of at least two electrical conductors, often in form of a metal foil, which are separated by an insulating layer made of plastic, ceramics or glass as shown in Figure 7. When a capacitor is charged, the energy is stored

in the dielectric material in form of electrostatic field. SCs stores energy in the two series capacitors of the electric double layer which is formed between each of the electrodes and the electrolyte ions. They are capable of storing large energy density and can respond to any change in power demand in tens to hundreds of milliseconds [19] .

A superconducting magnetic energy storage system is a system that stores energy within the magnetic field of a coil contained of superconducting wire with very little energy loss. Schematic diagram of a super magnetic energy storage (SMES) system is shown in Figure 8. It occurs by inducing DC current into coil

made of superconducting cables of nearly zero resistance (usually made of niobiumtitane (NbTi)) filaments that operate at very low temperature of about −270˚C [20] . SMES consists of three parts namely superconducting coil/magnet, power conditioning system and cryogenically cooled refrigeration [21] . The DC current increases during charging while the reverse is the case during the discharging operation. Although the system requires considerable quantity of energy to attain cryogenic condition and the current has to flow through non-superconducting material and solid state switches which cause resistive losses, the overall efficiency in commercial applications in the range of MW is very high [21] .

First deployed in 1944 in Switzerland, pumped hydro storage (PHS) is one of the most popular storage techniques due to its simplicity and large storage capacity. PHS is a mature and robust technology with high efficiency of 76% - 85%, low capital cost per unit energy, long storage period, practically unlimited life cycle, and a very long life of 50 years or more [25] .

The energy produced from a given quantity of water depends on the head, which is the distance between the turbines and the upper reservoir. The larger the head, the greater the energy generated for a given volume of water will be [26] . Schematic diagram of a pumped hydro storage system is shown in Figure 10.

Gravity Energy Storage technology is a variation of the PHS technology that uses a very large piston that is suspended in a deep water filled shaft with sliding seals which helps to prevent leakage around the piston. The system operates as a

closed loop which means that the shaft is only filled with water once, mainly at the start of the operation. During the charging mode, the off peak electricity is used to drive the motor/generator which spins the pump to force water down the return pipe and into the shaft thus lifting the piston. The piston is held at the high position until when power is needed during the discharging mode. In the discharging mode, the piston drops forcing water down the storage shaft up the return pipe and through the turbine which spins a motor/generator to produce electricity [27] . Image illustration Gravity Power Module is shown in Figure 11.

The compressed air energy storage (CAES) is a technology where compressed and pressured air is utilized to store energy. The CAES plant consists of two major components—a storage vessel, and a compressor/expander [28] . The storage vessel is used to store air at high pressure without any pressure loss. The air is compressed using the compressor and, when needed, expanded by the expander. Surplus electricity during the off-peak time, i.e., when demand and energy rate is low, is used to compress the air at high pressure and store it in the storage chamber. During the peak time, this stored energy is retrieved by expanding the compressed air through the expander [29] . Process Diagram of CAES is shown in Figure 12.

In recent times, liquid air energy storage (LAES), which is similar to the CAES technology, has gained much attention. In this type of storage, a liquid instead of air is compressed; this is more advantageous than the CAES system in terms of

space demands. Using liquid instead of air increases the energy storage density of the system [30] . Schematic diagram of liquid air energy storage system is shown in Figure 13.

The flywheel energy storage (FES) comprised of steel was first developed by John A. Howell in 1983 for military applications [31] . FES possesses high energy and power density, high energy efficiency, the flywheel uses the kinetic energy, i.e., rotational energy of a massive rotating cylinder to store the energy in the form of mechanical energy [31] . Through magnetically levitated bearings, this large cylinder is supported over a stator and electric motor/generator is coupled with the flywheel as shown in Figure 14.

The power density of an energy storage technology is defined as the rated power output divided by the volume of the device. Its unit is W/kg or W/litre. This is

slightly different from the energy density which is defined as the actual energy stored divided by the volume of the storage device (Wh/kg or Wh/litre). [34] . As shown in Table 2, HES and Zn-Air battery have very high energy density but suffer from low round trip efficiency. Other conventional batteries, CAES and LAES follow with medium energy density while PHES, SMES, flywheel and super capacitors have lower energy density. Amongst the batteries, Li-on, NaS and NaNiCl₂ have higher energy density than the others. Comparison of energy and power density for all selected energy storage systems is shown in Figure 15.

Comparison of life time for all selected energy storage systems is shown in Table 3. Every energy storage technology has a definite life span that also plays a

significant part in deciding whether the technology will be adopted for any given application or not. All things been equal, energy storage technologies with long life span are usually preferred from an investment point of view over those with short life span. it can be affirmed that mechanical energy storage technologies which are based on conventional mechanical engineering such as PHES, CAES, flywheel, gravity energy storage and hydrogen energy storage systems usually have long life time as their life time is mainly determined by the life time of the mechanical components [34] Battery based systems usually have short life time owing to chemical deterioration with the operating time.

The cost of an energy storage technology is one of the most important factors for commercial deployment of a given energy storage technology. For detailed analysis, the cost of any given energy storage technology should include both the capital and operating costs. The operating cost covers the cost of operation, maintenance, disposal and replacement. The auxiliary components used by some energy storage technologies add to the total capital cost of the system. As a result of this, some energy storage systems tends to be only economically feasible above a minimum energy content and power output [34] .

Table 3 shows that CAES has a lower capital cost per kWh than the PHES but also suffers from low round trip efficiency. Flywheel, SMES, super-capacitors have very high power density and capital cost per kWh but low capital cost per kWh per cycle which makes them suitable for applications that require high power output for short duration.

Comparison of round-trip efficiency for all selected ESTs is shown in Figure 16. The round trip efficiency is the ratio of the electricity output from the storage device to the electricity input to the device during one charge/discharge cycle. It

accounts for the losses which occur as a result of storing and withdrawing energy from the energy storage device [34] . As shown in Table 2, SMES, flywheel, super-capacitors and Li-ion battery have very high efficiency (>90%). These are followed by PHES, CAES, batteries (50% - 90%) and then HES, Zn-air battery technologies which have low round trip efficiency (<50%).

This denote how fast the energy storage system can release the store energy or respond to quick demand of stored power. Depending on the energy system requirement, some applications may require a very fast release of the stored energy to meet the system energy demand [34] . From Table 3, super capacitors, SMES and flywheel have very fast response time in the order of milliseconds. This is followed by batteries with response time in order of seconds, then PHES and CAES in minutes.

Primarily, different energy storage technologies can be used to achieve intermittent balancing of the electricity supply [36] . With such seasonal variations occurring for days, weeks or months in which case efficiency of the storage plays a crucial role or momentarily such as seconds to minutes as the case with load following, frequency regulation and voltage support in which case the response time particular ESS storage technology becomes key requirement.

This involves using electrical energy storage technologies to store electricity during periods of low demand and subsequently sell it during periods of high demand. This type of storage requires technologies which can achieve long storage duration (hours to days), together with high round trip efficiency. Figure 16 shows that technology such as batteries, CAES, LAES, GES, and PHS can be used to achieve this type of application.

During emergency system power collapse, ancillary electricity supply resources may be needed to be restart such failed mechanisms without pulling electricity from the grid. This would require an electrical energy storage technology which should be used to quickly restart such failed power or machine infrastructure devoid of any energy intensive auxiliary equipment. Electrical energy storage technologies such as batteries and super capacitors are best suited for this.

As the name implied, it covers standalone energy storage in which the device can easily be moved around from one location to another. It usually occurs for off-grid applications. Some typical examples are electric vehicles which uses electrical energy stored in batteries.

Due to dynamic changes do occur, on electric power systems creating a dynamic response or changes in active and reactive power and thus influencing the magnitude and profile of the voltage in the networks, the need to mitigate this effect, energy storage systems becomes necessary. ESS can be used to control constant maintain and dynamic voltage behaviors.

Energy storage technology can be used as reserves to deal with network constraints arising when the electricity demand exceeds the supply, which can happen when load forecasts are inaccurate, or in case unexpected plant outage occurs.

This is basically a non-isolated Bi-DC/DC structure that is based on the buck/boost converter topology, which as presented in Figure 17. The topology is very advantageous in applications where access to the DC bus neutral-point is needed. The buck-boost converter design has a low number of power electronic devices, which in turn doubles down on its lower costs of implementation and higher reliability than other converters. Its major drawback is that it cannot be

used in applications where isolation is required. Another area weakness is that its voltage step-up ratio is relatively low, making it unsuitable for low-voltage ESSs.

This power switching topology is one of the most widely used DC-AC converters basically consists of two-level inverters as shown in Figure 18, which can be single-phase and three-phase. This type of bidirectional two-level DC-AC inverter has a compact design and high reliability.

Bidirectional AC-AC converter is a matrix converter, of other types of converter topology already discussed. Presented in Figure 19. Matrix converter utilizes an intermediate DC link that may consist of the Bidirectional DC/DC Converters and Bidirectional two-level DC-AC converter which have their merits. However, more bi-directional switches are involved, which makes the system more complex and costly. Furthermore, parallel DC and AC busses are usually adopted since most ESSs are commonly combined to achieve a high power rating, large scale or high reliability for utility applications.