Modern world is seeking alternative ways to solve the problems of climate change, uncertainty in the future availability of fossil fuels and growing concerns over energy security. Energy storage is currently a widely discussed topic due to its increased influence over renewable energy technologies, which is going to be the solution of these problems and it is the future. But renewable energy sources have many intermittency issues like availability of sunlight or wind throughout a day and it is hugely unpredictable. Energy storage technologies can be employed to store the useful energy from renewable resources according to their availability and can be made use in future. They have the added benefits of improving stability, power quality, and reliability of supply. Various energy storage technologies are discussed below.
Figure 1: Pumped Hydro Storage System (PHS)
Pumped Hydro Storage (PHS)
Pumped hydroelectric energy storage system utilizes the energy in the form of potential energy of water that is pumped from a lower level reservoir to a higher level reservoir. During off peak time, pumps are used to raise the water from the lower reservoir to the upper one. When there is a high power demand, the stored water is released through hydro turbines to produce electric power. Whenever necessary the reversible turbine/generator assemblies act as a pump or turbine. It is one of the most cost-effective mechanism to store large amount of energy .Presence of geographical areas having sufficient water availability and a large variation of heights are the most decisive factor.
Since the 1920s, pumped storage hydroelectric projects have been providing energy storage capacity and transmission grid ancillary benefits in the United States (US) and Europe. There are about 40 pumped-storage projects operating in the US which provide more than 20 GW, or nearly two percent, of the capacity of the electrical supply system. It was over 100 GW in the year of 2009, the world’s total pumped hydroelectric storage generating capacity. By 2012 it was over 104 GW, with Europe itself providing around 44 GW. India is having a plan to build 10 GW of Pumped Hydro Storage to improve renewable power storage.
Battery Energy Storage System (BESS)
In battery energy storage system, energy is stored in the form of electrochemical energy in low-voltage/power battery modules connected in parallel and series to achieve a desired electrical characteristic and is one of the cost effective energy storage method. There are many battery technologies under consideration for large-scale energy storage. Lead-acid batteries represent a low cost technology used for bulk storage with the low energy density and limited cycle life as the chief disadvantages. Mobile applications use sealed lead-acid battery technologies in account for safety and ease of maintenance.
Other battery technologies having higher energy density capabilities than lead-acid batteries are not currently a cost effective method for high power applications. This includes nickel–metal hydride batteries, nickel–cadmium batteries, and lithium-ion batteries. The last two technologies are both being considered for electric vehicle applications where high energy density can offset higher cost to some level.
There are concerns regarding rapid, deep discharges which lead to early replacements since heating result reduced battery lifetime. Also batteries cannot operate at high power levels for long period of time and there are many disposal hazards as many of them involves hazardous materials.
Compressed Air Energy Storage (CAES)
It is similar to pumped hydrogen storage in regards of the pump-turbine and motor- alternator situation. During off peak period electrically powered compressors are used to force air into underground voids such as caverns or in aquifers at a very high pressure. Then the compressed air is released and used to drive an expansion turbine and subsequent generator by mixing it with fuel and heating it. This is a long term energy storage technology. The same can be used for smoothing of the intermittent power output from renewable energy sources. Since air is a compressible fluid, more energy can be stored by increasing the pressure or by pushing back more of the interstitial water in the underground aquifers.
Figure 2: Compressed Air Energy Storage (CAES) system
Flow Battery Energy Storage System
A flow battery is a type of rechargeable battery where reversible electrochemical reactions occur in a set of cells connected in series, parallel or both, in order to achieve the desired voltage level. It is provided by two chemical components dissolved in liquids and most commonly separated by a membrane. The aqueous solutions are pumped through the electrochemical cell during normal operation. Most commercially available flow batteries are: Vanadium Redox Battery (VRB), Zinc Bromine Battery (ZBB) and Polysulphide Bromide Battery (PSB).Since it operates based on redox reaction, it also called redox flow batteries. Their biggest advantage is that they can be instantly recharged by replacing their electrolyte liquid and simultaneously recovering the spent material for re-energization.
Figure 3: Polysulphide Bromide Flow Battery
Hydrogen-based Energy Storage System (HESS)
Electricity can be converted into hydrogen by the process of electrolysis. Then hydrogen can be then stored and re-electrified. The efficiency of such a system today is as low as 30 to 40% but could increase up to 50% if we are able to develop more efficient technologies. When compared to batteries (small scale) or pumped hydro and CAES (large scale) they have much higher storage capacity so the interest is growing on using hydrogen despite its low efficiency.
Regenerative Fuel Cell (RFC) is the technology used to store hydrogen. It is composed of a water electrolyzer system, a fuel cell system, a hydrogen storage and a power conversion system. This technology carries out the electrochemical transformations in order to store energy in the form of hydrogen and inject it as electricity into the grid, whenever required. There are many types of electrolyzers from Alkaline electrolyzers to Polymer Electrolyte Membrane (PEM) electrolyzers.
PEM (Proton Exchange Membrane) electrolyzers are more flexible and it can be used for small decentralized solutions. Both technologies have a conversion efficiency of about 65%-70% (lower heating value). High temperature electrolyzers with efficiencies up to 90% will be a better alternative to both alkaline and PEM electrolyzers.
Hydrogen can be stored in many forms such as gas in metal tanks, in metal hydrides or nanotubes. Man made underground salt caverns of up to 500,000 m3 at 200 bar (2,900 psi), corresponding to a storage capacity of 167 GWh hydrogen (100 GWh electricity) can be used to store very large amounts of hydrogen.
Figure 4: Storage capacity of different storage systems
Flywheel Energy Storage System (FESS)
A flywheel consists of a rotating mechanical device that is used to store rotational energy. A flywheel has a spinning mass in its center that is driven by a motor and when energy is required, the spinning force drives a device similar to a turbine to produce electricity, slowing the rate of rotation. A flywheel is recharged by using the motor to increase its rotational speed by accelerating the flywheel. Energy stored is dependent on the square of the rotating speed and its inertia. Axial-flux and the radial-flux permanent magnet machines are used for flywheel’s systems.
Flywheel technology is very beneficial by capturing energy from intermittent energy sources and delivers a continuous supply of uninterrupted power to the grid. They respond almost instantaneously delivering frequency regulation and electricity quality improvements. Now-a-days the traditional flywheels made of steel are being replaced by carbon fiber materials, stored in vacuums to reduce friction and employ magnetic bearings, enabling them to revolve at speeds up to 60,000 RPM.
Figure 5: Main components of a flywheel energy storage system
Superconducting Magnetic Energy Storage (SMES)
Superconducting Magnetic Energy Storage (SMES) is based on the fact that in a superconductor a current will continue to flow even after the voltage across it has been removed. Superconductor coil when it is cooled below its superconducting critical temperature it has negligible resistance. Hence, current will continue to flow even after a voltage source is disconnected. In the superconducting coil, energy is stored in the form of a magnetic field generated by the current. It can be released by discharging the coil. Niobiumtitane (NbTi) filaments which has a critical temperature of around 9K are usually used to make coils. Efficiencies of SMES systems are very high because the only conversion involved is the conversion from AC to DC. The energy required for cooling systems to maintain superconductor coils in its cryogenic state is much smaller than the energy stored in the system.
SMES has the ability to inject or absorb large amounts of energy in a very short time. The magnetic energy stored in a conducting coil is given by:
E = ½ L I2
SMES is useful for high power short duration applications due to its very high cycling capacity and high efficiency over short time periods.
Figure 6: Superconducting magnetic energy storage system
Super Capacitor Energy Storage System
Super capacitors are also called an electric double-layer capacitor or ultra capacitor. It is a type of high power high energy density capacitor. In a super capacitor, there are two metal electrodes coated with a high surface area type of activated carbon and separated by a thin porous insulator all in an electrolyte. Depending on application, the power requirement or peak current demand, the operating voltage and the allowable temperature range the electrolyte varies. Electrolyte can be either aqueous or non-aqueous.
The energy stored by a capacitor, E (Joules), is given by:
E = ½ C V2 , Where E is the electrostatic energy stored, C is the capacitance and V is the voltage difference across the capacitor plates.
The capacitance C is given by:
C = εr ε0 A/d
According to the above formula, a very large surface area of the porous electrode enables much higher capacitance than conventional capacitors.
The product of the equivalent resistance of the electrolyte and the capacity of the super capacitor determine its charge and discharge time constants. Since the equivalent resistance is very small (less than 1 milliohm), short time constants can be achieved. Also power densities which are almost 10 times higher than that of batteries can be achieved. They have long life, more than 5×10e4–10e5 cycles with almost no maintenance and energy efficiency of about 75–80%.
They have high self-discharge rates leading to significant drop in voltage while discharging. Thus, it requires more complex electronic control. Due to the difficult access to the porous surface of the electrode by ions, its specific energy and energy density are low, 2–5 W h/kg and 10,000 W h/m3 .Their major drawbacks is high cost, estimated about five times than that of Lead-Acid battery cost.
Many of the above technologies have huge capital costs and a low anticipated return on investment. With increased development in this area, it can be expected that it will revolutionize our future energy infrastructure.
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