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This subsection discussed different types of ESS (Energy Storage Systems) technology available that can be utilized in the microgrid. The categorised ESS technology along with its corresponding advantages and disadvantages is enumerated [6, 7].

 1. Mechanical storage: Various technologies that falls under this category are:a. Pumped Hydro Storage (PHS): Pumped Hydro Storage has been extensively implemented for a very long period and is considered as a developed technology of energy storage system for power grid applications. PHS projects store water in the reservoir/pond placed at higher altitudes in times of energy availability with aim to save the energy and then electrical energy can be produced by transforming the penitential energy to electrical energy during the release of stored water through turbines. Significant benefits of pumped hydro system are:Huge power and energy ratingProlonged lifespanHigh efficiencyLess discharge lossesThe geographical dependence, large site area and long gestation periods are the major obstructions in this type of energy storage system. Several modifications have also been suggested and established for PHS.b. Compressed Air Energy Storage (CAES): CAES is a strategy to store energy during low energy demand in by means of compressed air in underground air chambers and utilise the same to meet higher demand. There are two methods of CAES to store the air: A diabatic and Diabatic. Adiabatic storage has much higher efficiency because it preserves the heat from compression and re-uses this when the air is expanded to generate the power. Adiabatic storage continues to store the energy produced by compression and returns in to the air as it is expanded to generate power. Whereas in diabetic storage, the heat generated during compression is released to the atmosphere through heat intercoolers. The advantages of CAES are:Huge power and energy ratingLong lifespanMinor discharge losses.Similar to PHS, prime complication in CAES is geographical dependence, extensive investments and commonly appropriate for only grid-level applications.c. Flywheel energy storage System (FESS): Like PHS, a prime complication in FESS can supply instantaneous support of active power for the microgrid. It contains a disk with a certain amount of mass charged by accelerating it to very high speed and storing energy by keeping it rotated at high speed when there is excess electricity. The flywheel is decelerated to generate power during the demand period. The disk of flywheel ESS is placed in the rotor’s perpendicular position to avoid the effect of gravity. Flywheel energy storage loses some energy due to friction; hence, minimizing friction can enhance its efficiency. The friction can be reduced by making a vacuum environment for the flywheel to spin in, ensuring no air friction, or having a permanent magnet or electromagnetic bearing to make the rotor float. It has several qualities, such as high-power density, high conversion efficiency, short response time, low maintenance costs, no greenhouse emission, no toxic by-product, and long-life span. During the last several decades, FESU has been used as an uninterruptible power supply in short-duration power inconsistency. Flywheels are also used in transportation and space applications for power delivery and, significantly, to stabilize or drive satellites (gyroscopic effect) [8]. The drawbacks of flywheels are a small capacity and high-power loss, ranging from 3% to 20% per hour. CAES is geographically dependent, extensive investments and commonly appropriate for only grid-level applications.

 2. Electrical storage: Several electrical energy storage technologies have been developed and a few of them are briefly explained as follows:a. Superconducting Magnetic Energy Storage (SMES): under this technology, a large quantity of energy from the grid is stored within the magnetic field of a superconducting coil and discharged within a fraction of a cycle to reinstate a abrupt loss or dip in power. The energy is stored in the magnetic field established by direct current flow in a cryogenically refrigerated superconducting coil below its critical temperature. The stored energy can be supplied to the electrical network by discharging the coil using an inverter/ rectifier to transform DC power back to AC power. The inverter/rectifier incurs about 2–3% energy loss in each transformation. The advantages of the SC are:Instantaneous power transferabilitySuitable for small-scale storage systems.Several modifications of the strategic injection of surges of power in the grid through SMES have been proposed.b. Supercapacitor: A supercapacitor (SC), also called an ultra-capacitor, is developed by placing an electrolyte solution between two conductors. The capacitance value of SC is much higher than other capacitors (Appx. 201000 times). The high-power density and intense energy conversion efficiency of SC make it more suitable for automobiles, buses, trains, cranes, and elevators, where regenerative braking and burst-mode power delivery is required. The voltage per cell in an SC is low; hence several capacitors are arranged in parallel or in series to form a practical supercapacitor. The differing internal factors of each capacitor may cause a voltage imbalance in supercapacitors affecting its operational reliability. The benefits of SC are:Infinite cycle stabilityExceptionally high-power densityRapid charge and dischargeHigh reliabilityNo maintenanceProlonged lifetimeWide temperature range for stable operation.

 3. Electro-chemical storage: An electrochemical energy storage system or Batteries energy storage system is a set of series and parallel connected low-voltage/power battery cells to attain a required electrical characteristic. Each fundamental cell contains electrolyte (liquid, paste or solid) along with anode and cathode, which experience reversible chemical reactions under charging and discharging mode of operation. The power rating of the battery depends on the reaction surface area of the electrolyte and electrodes [9]. Some of the benefits of BESS are:Quick response time.Ease of commercial availability.Minor self-discharge loss.Established technology.High power and energy densities.

 The types of BESS are as follows:a. Lead-acid Battery: The lead-acid battery is the most extensively used rechargeable battery for 100 years. Each cell of the battery consists of a lead dioxide (PbO2) anode, lead sponge cathode and solution of sulphuric acid as an electrolyte. Lead-acid battery cells of 2V are attached in series to achieve a high voltage. Due to the chemical reaction, electricity is generated between the anode and the cathode while discharging. If the voltage is applied across electrodes, the battery gets charged with a reverse reaction. This property makes the Lead-acid battery rechargeable and suitable for many applications. Benefits of lead-acid batteries are low cost, easy installation, high efficiencies, longer holding of stored charge, easy to recycle, and good surge capability. It also suffers from some disadvantages such as maintenance requirement for the liquid electrolyte and electrode corrosion, lower specific power, premature failure under partial discharging operation and risk to the environment due to poisonous metal.b. Lithium-Ion Batteries: It has enormous usage in portable electronic devices and the transportation sector due to the lightweight decent power density and quick response. Each cell of a lithium ion battery has 3.7 V made of a layer of lithium compound anode, a layer of graphite cathode behavior, and the third layer of insulator material is placed in-between. The operation of a lithium-ion battery resembles the capacitor. Lithium-ion has a very low self-discharge rate, high discharge/charge rate and high efficiency. The disadvantage is that lithium is a thermal runaway, instability at high operating temperatures, expensive.c. Nickel-based batteries: Nickel-based battery was introduced with nickel-cadmium (Ni-Cd). The cell of a nickel-based battery consists of nickel anode, cadmium cathode and alkaline electrolytes. Some of the advantages are fast discharging cycles, low cost/cycle, longer life, suitability for renewable applications. Nickel-cadmium battery suffers from toxicity problem caused by cadmium; hence nickel-metal-hydride (NiMH) battery was introduced to solve this problem. Nickel-metal-hydride (NiMH) battery has advantages such as mild toxins content and wide temperature range. It has few evident limitations like the need of a complex charging algorithm, sensitive to overcharge, heating during fast charging and high-load discharge and high self-discharge. A nickel-iron (Ni-Fe) battery was introduced to substitute the cadmium with iron. This battery (Ni-Fe) uses a nickel (III) hydroxide cathode, an iron anode with potassium hydroxide as an electrolyte that generates a nominal cell voltage of 1.20V. Nickel-iron (Ni-Fe) battery has a large service life, rugged construction, lower self-discharge and less affected by over- and undercharging. Like nickel-cadmium, a nickel-zinc battery was introduced that uses a nickel electrode and an alkaline electrolyte, but the Ni-Zn battery provides 1.65V/cell. Several advantages such as low cost, high power output, no heavy toxic materials and decent temperature operating range have made this attractive. At the same time, it also has some limitations as a brief life cycle and high self-discharge.d. Redox-Flow Battery: The redox (reduction-oxidation) cell is a rechargeable fuel cell that reversibly converts chemical energy to electricity by streaming an electrolyte with dissolved electroactive elements through an electrochemical cell. The advantages of a redox-flow battery include a longer lifespan of approximately 40 years and extendible capability by just increasing the number of tanks with additional electrolytes.

The pumped hydro storage is nearly 95.5% of the entire worldwide capacity of ESS, as shown in Figure 2.1 [5].

Figure 2.1 Distribution of worldwide operational ESS.