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What is energy storage?
What are the current uses of energy storage technologies?
What are the different energy storage technologies?
Different energy storage technologies coexist because their characteristics make them attractive to different applications. From a user point of view there are both technical and commercial criteria for selecting the most suitable technology.
What are the various aspects of the technologies and their applications?
Batteries and advanced batteries
Rechargeable batteries or accumulators are the oldest form of electricity storage and widely used. Batteries store electric energy in a chemical form. Their performance is linked in a complex manner to the materials used, the manufacturing processes and the operating conditions. Consequently, progress in battery technology is slow and the transfer of laboratory results into commercial applications is sometimes risky. Lithium ion and nickel-metal-hydride (NiMH) batteries are the only new battery technologies which have achieved significant market penetration in the last decade. Batteries can respond to changes in power demand within microseconds. Only super-capacitors equal such a response time. Batteries usually have very low standby losses and can have high energy efficiency, depending on the application and the details of the operation. Most batteries contain toxic materials, hence the ecological impact from uncontrolled disposal of batteries must always be considered.
Super-capacitors store electrical energy in the electric field between two electrodes. Ultra-capacitor, super-capacitor and electric double layer capacitor (EDLC) are also called electro-chemical capacitors working with chemical reactions or not like true capacitors. The fundamental design and electrical properties are those of conventional capacitors used throughout the electrical and electronics industry. EDLC uses electric double layer capacitance on both positive and negative electrodes.
Reversible fuel cell systems and redox flow batteries
Fuel cells convert hydrogen from a storage tank and oxygen from the air to water and generate a current from the electrochemical process. The electrochemical reaction itself is reversible. The fuel cells' energy capacity is determined by the size of the storage tanks for the active materials, and the power by the area of the electrodes and design of the reactor. Standby losses are low because the active materials are kept physically separate. Redox flow batteries are systems using materials other than hydrogen and oxygen. Their energy efficiency is higher than those of reversible fuel cells, but still below the energy efficiency of most batteries.
SMES (Super-conducting magnetic energy storage systems)
SMES store energy in the magnetic field of a coil made from special alloys. By cooling the conducting wires to - 269°C the resistance of the material to electrical current disappears, allowing it to conduct very high currents without electrical losses. When looking at the complete system, however, it is clear that there is considerable energy requirement for refrigeration. Also, the current has to flow through non-super-conducting components and solid-state switches, which cause resistive losses. Despite this, the overall efficiency in commercial applications is very high.
The energy is stored as kinetic energy in a rotating mass. The amount of energy stored increases with the square of the rotational speed, which is limited by the tensile strength of the material used.
Thermal storage (heat and cold)
Conventional heat and cold storage systems simply store excess energy in a large tank using the working medium at the temperature required for later use. Virtually every cooling and heating system has such storage tanks.
Compressed gas storage
Compressed air tanks are widely used in industry to provide a constant source of compressed air with uniform pressure in the range of 8-10 bar. There is renewed interest in compressed air storage for covering the demand of peak electricity or for small wind/hybrid applications, where the energy-to-power ratio of batteries is unsuitable, either because the energy content is very high but the power requirement low, or the energy through-put is very high compared to the energy content.
Pumped hydro storage
Pumped hydro storage is a conventional energy storage technology utilised by the electrical industry. Water in a basin at the top of a mountain is used to drive a generator in a reservoir at a lower level. When surplus energy is available, the water is pumped back up again. The power output and the cost efficiency of pumped hydro storage depends on the difference in height.