There are a number of excellent reports that exist

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There are a number of excellent reports that exist providing overviews of the technologies presented here from varying perspectives, including [2,3,5,7,8], all of which were drawn upon to compile this report. In the following section, a number of technologies are discussed, organize according to the form of energy that is stored: Mechanical Energy Storage Pumped Hydroelectric Storage: Potential energy of water at different elevations, Section 3.1 . Compressed Air Energy Storage: Kinetic energy stored in compressed air, Section 3.2 . Flywheel Energy Storage: Kinetic energy stored in a rotating disk, Section 3.3 . Electrical Energy Storage Electrochemical Capacitors (Supercapacitors): Electrostatic energy stored in an electric field (elec- trostatic energy), Section 3.4 . Superconducting Magnetic Energy Storage: Energy stored in a magnetic field (magnetic energy), Section 3.5 . Chemical Energy Storage Lead Acid Batteries: Conventional secondary battery, Section 3.6 . Nickel-electrode Batteries: Conventional secondary battery, Section 3.7 . Lithium-ion Batteries: Secondary battery, Section 3.8 . Sodium-sulfur Batteries: Molten salt battery, Section 3.9 . Sodium Nickel Chloride Batteries (ZEBRA): Molten salt battery, Section 3.10 . Zinc-bromine Batteries: Flow battery, Section 3.11 . Polysulfide-bromide Batteries: Flow battery, Section 3.12 . Vanadium Redox Batteries: Flow battery, Section 3.13 . 9
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Energy Storage Technologies Pumped Hydroelectric Storage 3.1 Pumped Hydroelectric Storage The oldest (1929) and most prominent energy storage technology to date has been pumped hydroelectric storage of which there are 20.36 GW of installed capacity in the United States alone [9] across 39 sites with capacities ranging from 50 MW to 2,100 MW [10]. Its simplicity of design, relatively low cost, and similarity in operation to hydroelectric power has made it the industry standard for storage for a century. These systems can quickly ramp up to full load: 10 seconds if the turbine spinning, and 1 minute from standstill [8]. However, they require very specific geographic features that limit unit siting. These systems have high capital cost but very low maintenance costs, and also face criticism due to their significant impact on local wildlife and ecosystems. New designs, however, may be opening the door for additional siting opportunities in the near future. How it Works As shown in Figure 3.1, PHS consists of two reservoirs with a height differential and a pipe (or penstock) connecting them. To store energy, electricity turns a motor which pumps water from the lower reservoir, up the pipe, to the upper reservoir. To produce energy, water is allowed to flow from the upper reservoir down the pipe through a turbine and into the lower reservoir. The turbine is connected to a generator and as the turbine turns so does the generator, producing electricity. Today, the motor and generator are typically one in the same, since a motor can also act as a generator (in one case it is turned and electricity is produced, in the other electricity is sent in, causing it to turn).
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