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Energy storage will play a critical role in an efficient and renewable energy future; much more so than it does in today?s fossil-based energy economy. There are two principal reasons that energy storage will grow in importance with increased development of renewable energy:
Many important renewable energy sources are intermittent, and generate when weather dictates,
rather than when energy demand dictates.
Many transportation systems require energy to be carried with the vehicle.

Energy can be stored in many forms: as mechanical energy in rotating, compressed, or elevated substances; as thermal or electrical energy waiting to be released from chemical bonds; or as electrical charge ready to travel from positive to negative poles on demand. Storage media that can take and release energy in the form of electricity have the most universal value, because electricity can efficiently be converted either to mechanical or heat energy, whereas other energy conversion processes are less efficient. Electricity is also the output of three of the most promising renewable energy technologies: wind turbines, solar thermal, and photovoltaics. Storing this electricity in a medium that naturally accepts electricity is favored, because converting the energy to another type usually has a substantial efficiency penalty.
Still, some applications can benefit from mechanical or thermal technologies. Examples are when the application already includes mechanical devices or heat engines that can take advantage of the compatible energy form; lower environmental i mpacts that are associated with mechanical and thermal technologies; or low cost resulting from simpler technologies or efficiencies of scale. In this chapter, the technologies are grouped into five categories: direct electric, electrochemical, mechanical, direct thermal, and thermochemical.Table 15.1 is a summary of all of the technologies covered. Each is listed with indicators of appropriate applications that are further explained in Section 15.1.3.

Three Types of Storage

Compressed air energy storage (CAES)

• Large tank is buried underground
• During times of low demand electricity compresses air
• During times of peak demand compressed air is heated and released
  

Flywheels

• Captures energy in a rotating Mass
• Flywheel is charged using electric motor
• Electric generator extracts energy
  

Energy held in Spinning Rotor (Steel or Carbon composite)
Steel rotors can spin at several thousand rpm
Carbon composite spin up to 60k rpm Kinetic Energy 1/2mv2

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