Energy Storage Devices
M. Karthigai Pandian, K. Saravanakumar, J. Dhanaselvam, and T. Chinnadurai
Sri Krishna College of Technology
Introduction to Energy Storage Devices
The process of harvesting energy in an effective manner and storing it is very much essential for this society. Renewable energy resources such as photovoltaic cells, biogas, hydroelectric and tidal power have recently been proposed to overcome the drawbacks of fossil fuels such as pollution and global warming. Battery banks and fuel cells are employed in modern-day electric vehicles (HEVs and EVs) where they are expected to provide an energy supply of a few kilowatts, with the driving time lasting from a few minutes to a few hours . Similarly, portable electronic devices such as pocket radio, mobile phones and laptops are bound to have an energy capacity in terms of ampere-hours (Ah). These are some of the practical examples that present batteries as the main source for ESDs, but there are various methods and other forms of devices that can be employed in practical applications. The basic classification of ESDs is shown in Figure 8.1. Modern researchers have been actively
FIGURE 8.1 Energy Storage Devices - Basic Classification.
investigating various opportunities such as molecular physics, flexible nanomaterials and fiber-shaped devices that could actively replace the existing storage devices.
Energy Storage Devices in Existence
To obtain the best performance of electrical and electronic devices, normally the batteries are combined with supercapacitor (SC) packs and employed. Basically, for a huge amount of energy to be stored in capacitors, the basic requirement is that it should have a very huge capacitance or the voltage applied across its terminals should be very huge. In contrast, SCs are bound to have very low voltage abilities . They can be further classified into double-layer capacitors and pseuodcapacitors based on the mode of storage as shown in Figure 8.2. A combination of both these modes leads us to hybrid devices.
Flywheels can be touted as the examples of kinetic ESDs . A simple diagram of flywheel’s structure is shown in Figure 8.3. Based on the inertia of the rotating mass and the rotor’s speed, the energy is stored in the rotating mass. The basic classification of flywheels depends on their speed. They are generally classified as low-speed and high-speed devices. They find their applications in wind turbines, locomotive propulsion systems and in the direction control systems of satellites.
In fuel cells, an electrochemical process is used to convert the fuel’s chemical energy into electrical energy directly . The schematic diagram of a basic fuel cell is shown in Figure 8.4. Many types of fuel cells are available with varying energy
FIGURE 8.2 Classification of SuperCapactitors.
FIGURE 8.3 A Flywheel.
FIGURE 8.4 A Fuel Cell.
storage capabilities. Some of them are Alkaline Fuel Cells (AFCs), Solid Oxide Fuel Cells (SOFCs) and Phosphoric Acid Fuel Cells (PAFCs). Due to their ability to work at very low temperatures and efficient starting characteristics, fuel cells are majorly used in electric vehicle technologies. In compressed air energy storage systems, energy is stored in the form of compressed air for future use. Another well-known technology employed in energy storage is superconductive magnetic energy storage (SMES). It employs a superconducting coil and the direct current (DC) flowing through it is used to create a huge DC magnetic field and this is further used to store the energy .
Numerous research studies have been carried out to enhance the characteristics and efficiency of ESDs. Some of the new technologies under consideration are Molecular Solar Thermal Energy Devices (MOSTs), flexible storage devices using nanomaterials and fiber-shaped ESDs.
Molecular Solar Thermal Energy Storage Devices
Energy storage is the process of capturing energy from one source and the same can be utilized for future purpose. Storing solar energy in the form of heat and chemical energy is generally called molecular solar thermal storage, where chemical bonds are used to store the solar energy . In this process, a catalyst is used to recycle an isomer and convert it into heat where ionized chemical compounds are transformed from chemical isomerization to metastable isomers. There are various methods available for thermal energy storage where the molten-salt technique is the simplest one, but it experiences thermal losses due to insulation problems . Another method is the solar-driven conversion type in which dicyclopentadiene is converted into cyclopen- tadiene, which is thermodynamically the most favorable at increased temperature.