15-03-2011, 12:13 PM
Presented By:
Nisha Sinha
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Technical seminar on ultracapacitor
Energy Storage
• In the past 2 classes we have discussed battery technologies and how their characteristics may or may not be suitable for microgrids.
• 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:
• In ideal capacitors:
• Equivalent model of real standard capacitors:
Flywheels
• Kinetic energy:
where I is the moment of inertia and ω is the angular velocity of a rotating disc.
• For a cylinder the moment of inertia is
• So the energy is increased if ω increases or if I increases.
• I can be increased by locating as much mass on the outside of the disc as possible.
• But as the speed increases and more mass is located outside of the disc, mechanical limitations are more important.
• 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
• If and if an useful energy (Eu) proportional to the difference between the disk energy at its maximum and minimum allowed speed is compared with the maximum allowed energy (Emax) then