1.4.4.Direct electric storage
The energy can be stored directly in ultracapacitors and superconducting magnetic energy storage units [1, 2].
A capacitor stores energy in the electric field between two electrodes in the form of thin rolled plates with a dielectric between them. The ultracapacitor, also known as supercapacitor, electrochemical capacitor, or electric double layer capacitor (EDLC) is a capacitor which uses a thin electrolyte instead of a dielectric and electrodes made of porous carbon in order to increase the energy density. The specific energy obtained is about 15Wh/kg.
Ultracapacitors are useful for power-quality management and for regulating voltage in automotive systems during regular driving conditions. They can also work in tandem with batteries and fuel cells to relieve peak power needs.
Supercapacitors generally are very durable (10–12 years life span), have very high cycle life (over 500 000 cycles) and efficiency (95%) and 5% per day self-discharge.
The energy storage in superconducting magnetic energy storage (SMES) system is achieved in the magnetic field created by inducing direct current into a coil made of superconducting material which operates at very low temperature (-270°C). The cryogenically cooled, superconducting material has very low electrical resistance. The energy loss of only 0.1% is duet of energy required by cooling system. SMES systems can operate at (97–98)% efficiency and are useful for providing reactive power on demand. A SMES system can provide full power within 100 ms. A SMES system with capacity of 18000 GJ requires a coil with diameter of 300-1000 m. These systems are capable of discharging the near totality of the stored energy, as opposed to batteries.
1.4.5.Electrochemical energy storage
The technologies which store energy as electrochemical energy are the following:
rechargeable batteries (Lead–acid, Lithium-ion, Nickel–cadmium, Nickel–metal hydride, Zebra, Sodium–sulfur);
flow batteries (Vanadium redox, Polysulfide bromide, Zinc bromide);
electrolytic hydrogen.
The storage battery is known as a secondary cell because its electrochemical reactions are electrically reversible.
There is a wide range of technologies used in fabrication of rechargeable batteries which can provide high energy densities (up to 200 Wh/kg for Lithium-ion) [1]. The main disadvantage is their relatively low durability for large-amplitude cycling (a few 100 to a few 1000 cycles). Relative positioning of different storage battery types is shown in Fig. 5.
Figure 5. positioning of the different storage batteries according to their energy density and power [1].
Gaseous hydrogen (H2) can be manufactured using electrolysis, an electric current applied to water separates it into O2 and H2. The resulting hydrogen can be stored and later combusted to provide heat or work, or to power a fuel cell.
Unlike secondary batteries, the flow batteries store and release electrical energy in two liquid electrolytes by means of reversible electrochemical reaction that occur in a set of cells connected in series and/or parallel (Fig. 6). A cell has two compartments separated by an ion exchange membrane. The aqueous electrolytic solutions are contained in separate tanks and during the normal operation are pumped through the electrochemical cell where the reactions occur, with ion or proton exchange through the membrane and electron exchange through the external electric circuit. during the charge process, the electrolyte A is oxidized at the anode, while the electrolyte B is reduced at the cathode.
Figure 6. Description of flow battery energy storage system [4].
the main advantages of the flow batteries are: the capacity can be increased by simply increasing the quantity of liquid electrolytes; the battery can be fully discharged without side effects; have little loss of electrolyte over time and have one of lowest self-discharge rates. The installation cost becomes lower with the increase of system size. The power capacity depends on the cell number and the size of the electrodes [4]. There are three types of flow batteries on commercialization: vanadium redox, polysulfide bromide, and zinc bromide. The overall electricity storage efficiency is about 75%. They are useful in utility-scale power shaping and smoothing application and might be adaptable for distributed-generation use [2].