Energy Storage

Published : 01-01-2015 by : Raj Janorkar

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In the past 2 classes we have discussed battery technologies and how their characteristics may or may not be suitable for micro grids.

Batteries are suitable for applications where we need an energy delivery profile. For example, to feed a load during the night when the only source is PV modules.

However, batteries are not suitable for applications with power delivery profiles. For example, to assist a slow load-following fuel cell in delivering power to a constantly and fast changing load.

For this last application, two technologies seem to be more appropriate:

  • Ultracapacitors (electric energy)
  • Flywheels (mechanical energy)

Other energy storage technologies not discussed in here are superconducting magnetic energy storage (SMES – magnetic energy) and compressed air (or some other gas - mechanical energy)

  • Ultracapacitors

Capacitors store energy in its electric field.

In ideal capacitors, the magnitude that relates the charge generating the electric field and the voltage difference between two opposing metallic plates with an area A and at a distance d, is the capacitance:

  • Flywheels

However, high speed is not the only mechanical constraint

If instead of holding output voltage constant, output power is held constant, then the torque needs to increase (because P = Tω) as the speed decreases. Hence, there is also a minimum speed at which no more power can be extracted. 

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