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Electrical energy storage (apart from pumped storage hydropower) is still a peripheral part of the power generation infrastructure. However, the advancing use of renewable energy is changing the perception of storage and has lead to significant increase in interest towards energy storage. The developments over the last ten years have brought a range of new storage technologies to the brink of commercialization. Though only few of the new storage technologies have made it to the commercialization stage for the large-scale, grid-connected electricity sector, falling costs and increasing efficiencies among storage technologies could imply a much larger application of these technologies for the electricity generation sector.

There are many possible techniques for energy storage, found in practically all forms of energy: mechanical, chemical, and thermal. The storage technologies that answer to specific technical and economic criteria, which vary considerably as a function of the applications and needs, will obviously be of different types.

The storage techniques can be divided into four categories, according to their applications:

1. Low-power application in isolated areas, essentially to feed transducers and emergency terminals.
2. Medium-power application in isolated areas (individual electrical systems, town supply).
3. Network connection application with peak levelling.
4. Power-quality control applications.

The first two categories are for small-scale systems where the energy could be stored as kinetic energy (flywheel), chemical energy, compressed air, hydrogen (fuel cells), or in supercapacitors or superconductors. Categories three and four are for large-scale systems where the energy could be stored as gravitational energy (hydraulic systems), thermal energy (sensible, latent), chemical energy (accumulators, flow batteries), or compressed air (or coupled with liquid or natural gas storage).

• Pumped hydro storage
• Thermal energy storage
• Compressed air energy storage
• Flow batteries for energy storage
• Fuel cells
• Chemical storage
• Flywheel energy storage
• Superconducting magnetic energy storage
• Energy storage in supercapacitors

Pumped Hydro Storage (PHS)

Pumped storage hydroelectricity is a type of hydroelectric power storage used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps. During periods of high electrical demand, the stored water is released through turbines.

The main advantage of this technology is that it is readily available. It uses the power of water, a highly concentrated renewable energy source. The Pumped Hydro Storage technology is currently the most used for high-power applications. Pumped hydroelectric systems have conversion efficiency, from the point of view of a power network, of about 65-80%, depending on equipment characteristics. The main shortcoming of this technology is the need for a site with different water elevations.

Thermal Energy Storage (TES)

Thermal energy storage technologies store heat in an insulated repository for later use in space heating, domestic or process hot water, or to generate electricity. There are two types of TES systems, depending on whether they use sensible or latent heat.

Latent-fusion-heat TES makes use of the liquid–solid transition of a material at constant temperature. During accumulation, the bulk material will shift from the solid state to liquid and, during retrieval, will transfer back to solid. The heat transfers between the thermal accumulator and the exterior environment are made through a heat-transfer fluid.

Sensible heat thermal storage is achieved by heating a bulk material (sodium, molten salt, pressurized water, etc.) that does not change states during the accumulation phase; the heat is then recovered to produce water vapour, which drives a turbo-alternator system.

Compressed Air Energy Storage (CAES)

Compressed Air Energy Storage (CAES) refers to the compression of air to be used later as energy source. At utility scale, it can be stored during periods of low energy demand (off-peak), and for use in meeting periods of higher demand (peak load).

Compressed air energy storage (CAES) can be used to store off-peak electricity from wind farms or other sources to pump air underground. The high pressure air acts like a huge battery that can be released on demand to turn a gas turbine and make electricity. CAES relies on relatively mature technology with several high-power projects in place. A power plant with a standard gas turbine uses nearly two-thirds of the available power to compress the combustion air. It therefore seems possible, by separating the processes in time, to use electrical power during off-peak hours (storage hours) in order to compress the air, and then to produce power during peak hours (retrieval hours) by expanding the air in a combustion chamber before feeding it into the turbines. However, a good portion of the input energy is lost in this process, making CAES one of the least efficient storage technologies available.

Energy Storage Using Flow Batteries (FBES)

A flow battery is a form of rechargeable battery in which electrolyte containing one or more dissolved electro active species flows through an electrochemical cell that converts chemical energy directly to electricity. An additional electrolyte is stored externally, generally in tanks, and is usually pumped through the cell (or cells) of the reactor, although gravity feed systems are also known. Flow batteries can be rapidly "recharged" by replacing the electrolyte liquid (in a similar way to refilling fuel tanks for internal combustion engines) while simultaneously recovering the spent material for re-energization. These batteries overcome the limitations of standard electrochemical accumulators (lead–acid or nickel–cadmium for example) in which the electrochemical reactions create solid compounds that are stored directly on the electrodes on which they form, limiting their capacity. Various types of electrolytes have been developed using bromine as a central element: with zinc (ZnBr), sodium (NaBr), vanadium (VBr) and, more recently, sodium polysulfide.

Fuel Cells-Hydrogen Energy Storage (FC- HES)

Fuel cells are a means of restoring spent energy to produce hydrogen through water electrolysis. The storage system proposed includes three key components: electrolysis which consumes off-peak electricity to produce hydrogen, the fuel cell which uses that hydrogen and oxygen from air to generate peak-hour electricity, and a hydrogen buffer tank to ensure adequate resources in periods of need.

There are many types of fuel cells, such as: Alkaline Fuel Cell (AFC), Polymer Exchange Membrane Fuel Cell (PEMFC), Direct Methanol Fuel Cell (DMFC), Phosphoric Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC). The basic differences between these types of batteries are the electrolyte used, their operating temperature, their design, and their field of application. Moreover, each type has specific fuel requirements. There are several hydrogen storage modes, such as: compressed, liquefied, metal hydride, etc.

Fuel cells can be used in decentralized production (particularly low-power stations- residential, emergency, etc.), mid-power cogeneration (a few 100 kW), and centralized electricity production without heat upgrading. They can also represent a solution for isolated areas where the installation of power lines is too difficult or expensive (mountain locations, etc.).

Chemical Storage

Chemical storage is achieved through accumulators and batteries. These systems have the double function of storage and release of electricity by alternating between the charge– discharge phases. They can transform the chemical energy generated by electrochemical reactions into electrical energy and vice versa, without harmful emissions or noise, and require little maintenance. There is a wide range of technologies used in the fabrication of accumulators: lead–acid, nickel–cadmium, nickel–metal hydride, nickel–iron, zinc–air, iron–air, sodium–sulphur, lithium–ion, lithium–polymer, etc. Their main assets are their energy densities (up to 150 and 2000Wh/kg for lithium) and technological maturity. Their main inconvenience however is their relatively low durability for large-amplitude cycling (a few 100 to a few 1000 cycles).

Some of the applications of chemical energy storage are listed below.

• Lead acid batteries
• Lithium ion batteries
• Sodium Sulphur (NaS) batteries

Flywheel Energy Storage (FES)

Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel.

Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are being developed. Advanced FES systems have rotors made of high strength carbon-composite filaments, suspended by magnetic bearings, and spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure.

Flywheels are today used in specific cases of road and rail transportation, where they eliminate many of the disadvantages of existing battery power systems, such as low capacity, long charge times, heavy weight, and short usable lifetimes. They have also been used as uninterrupted power supply systems in data centres. They also find applications in laboratories where circuit-breakers and similar devices are tested (that is, where the enormous transient loads produced by deliberately forcing such devices to demonstrate their ability to interrupt simulated short circuits would have unacceptable effects on the local grid if these tests were done directly off building power). In addition, they find applications in providing pulsed power and in niche areas in motor sports.

Superconducting Magnetic Energy Storage (SMES)

Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature.

Superconducting magnetic energy storage is achieved by inducing DC current into a coil made of superconducting cables of nearly zero resistance, generally made of niobium-titanium (NbTi) filaments, that operates at very low temperatures. The current increases when charging and decreases during discharge and has to be converted for AC or DC voltage applications.

One advantage of this storage system is its great instantaneous efficiency, near 95% for a charge–discharge cycle. Moreover, these systems are capable of discharging the near totality of the stored energy, as opposed to batteries. Their major shortcoming is the refrigeration system which, while not a problem in itself is quite costly and makes operation more complicated.

Energy Storage in Supercapacitors

These components have both the characteristics of capacitors and electrochemical batteries, except that there is no chemical reaction, which greatly increases cycling capacity. Energy storage in supercapacitors is done in the form of an electric field between two electrodes. This is the same principle as capacitors except that the insulating material is replaced by electrolyte ionic conductor in which ion movement is made along a conducting electrode with a very large specific surface (carbon percolants grains or polymer conductors).

The energy/volume obtained is superior to that of capacitors (5 Wh/kg or even 15 Wh/ kg), at very high cost but with better discharge time constancy due to the slow displacement of ions in the electrolyte (power of 800–2000 W/kg). Supercapacitors generally are very durable, that is to say 8–10 years, 95% efficiency, and 5% per day self-discharge, which means that the stored energy must be used quickly.

Comparison of Technical Parameters of Energy Storage Technologies

Status of Electricity Storage Technology Development

Renewable Energy and its Impact on Energy Storage

• The fast growth of the Indian renewable energy sector is likely to spur significant growth in the use of energy storage systems as well.
• Most growth for energy storage is expected to emanate from the solar energy sector, within which, adoption will be highest for the off-grid power generation segment.
• Batteries will continue to remain the storage system of choice for off-grid renewable energy storage.
• Renewable energy segments that offer high market potential for energy storage technologies are:
• Off-grid Solar PV market – rooftop solar

• Village electrification
• Commercial demand for replacing DG sets
• Solar-based products – lanterns, lamps and more
• Indian cellular tower industry 

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