4 ms·

Power does not need to be stored as DC. Flywheels store rotational energy, and starting with a spinning flywheel it's pretty simple to get AC out of it.

eigenvector 12y ago
True, but your electrical output would be variable-frequency AC at the drive terminal. This isn't particularly useful unless you use a back-to-back converter (i.e. DC link) to get constant-frequency AC.
- derekp7 12y ago
  Would it have to be variable? If it goes through a CVT, then you can keep the output whatever speed you want.
  - eigenvector 12y ago
    I'm no mechanical engineer but at first glance the conversion efficiency of using a CVT with a synchronous AC drive (vs. direct coupling the motor rotor and the flywheel rotor and using a variable-frequency induction motor + back-to-back converter) would seem to be lower. Modern VFDs have very high part-load efficiency and we can easily maintain a constant power characteristic through the full speed range by operating in the field weakening mode. One way or another you're losing energy in frequency conversion, it's just a question of whether you do that mechanically (with CVT) or electrically (with back-to-back converters). Also keep in mind that for grid storage devices, we're usually talking about 500+ kW on each flywheel which, at low speed, is A LOT of torque.
drakaal 12y ago
That would not be efficient. Yes you could also store the energy by pumping water up hill and letter it turn a turbine later, but you'd lose a LOT of power. You could store the energy in springs ala the da Vinci cart, but again not efficient. All of these methods require a conversion, where as a Battery, Capacitor, or Leyden Jar will store DC as DC.
- dannypgh 12y ago
  Converting electric power to and from a battery requires an electrochemical reaction to occur. That is a conversion just the same - I don't really buy this "no conversion required" argument, just because you started and ended with direct current. Flywheels aren't some hypothetical means of storing power, either - they've seen a lot of exciting R&D over the last 15 years, and there are operators of grid-scale flywheels. e.g. http://beaconpower.com/ http://beaconpower.com/ http://en.wikipedia.org/wiki/Flywheel_energy_storage http://en.wikipedia.org/wiki/Flywheel_energy_storage

eigenvector 12y ago

True, but your electrical output would be variable-frequency AC at the drive terminal. This isn't particularly useful unless you use a back-to-back converter (i.e. DC link) to get constant-frequency AC.

derekp7 12y ago
Would it have to be variable? If it goes through a CVT, then you can keep the output whatever speed you want.
- eigenvector 12y ago
  I'm no mechanical engineer but at first glance the conversion efficiency of using a CVT with a synchronous AC drive (vs. direct coupling the motor rotor and the flywheel rotor and using a variable-frequency induction motor + back-to-back converter) would seem to be lower. Modern VFDs have very high part-load efficiency and we can easily maintain a constant power characteristic through the full speed range by operating in the field weakening mode. One way or another you're losing energy in frequency conversion, it's just a question of whether you do that mechanically (with CVT) or electrically (with back-to-back converters). Also keep in mind that for grid storage devices, we're usually talking about 500+ kW on each flywheel which, at low speed, is A LOT of torque.

drakaal 12y ago

That would not be efficient. Yes you could also store the energy by pumping water up hill and letter it turn a turbine later, but you'd lose a LOT of power. You could store the energy in springs ala the da Vinci cart, but again not efficient. All of these methods require a conversion, where as a Battery, Capacitor, or Leyden Jar will store DC as DC.

dannypgh 12y ago
Converting electric power to and from a battery requires an electrochemical reaction to occur. That is a conversion just the same - I don't really buy this "no conversion required" argument, just because you started and ended with direct current. Flywheels aren't some hypothetical means of storing power, either - they've seen a lot of exciting R&D over the last 15 years, and there are operators of grid-scale flywheels. e.g. http://beaconpower.com/ http://beaconpower.com/ http://en.wikipedia.org/wiki/Flywheel_energy_storage http://en.wikipedia.org/wiki/Flywheel_energy_storage