Comment by burnte | Hacker News Reader

Comment by burnte | Hacker News Reader

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> You also need uninterruptible power supply to maintain safety and control systems when grid power is unavailable since you've still gotten a huge spinning mass that you can't slow down without somewhere to send the energy

Why? If it's safe in it's vault when powered, why is it unsafe when in it's vault unpowered? Hell, you could use one flywheel to power the rest, and as they lose too much velocity the next one becomes the generator, until either they're all spun down, or power comes back, then you have at least some of the flywheels ready to go. Now, let's say you just don't do anything, they're spinning away slowly slowing, then power comes back and they don't have to spin back up from a dead stop. Why is that not true?

The rotor spins in a vacuum to reduce standby friction losses, and it requires power to maintain that vacuum. The bearings also need active cooling. Without these support systems you won't have a catastrophic failure, but you'll be damaging your equipment and reducing its lifespan.

As for using the flywheel(s) themselves as the source of backup power, that was our original design and definitely feasible at a conceptual level, but there's a lot of engineering in getting that to work properly while maintaining grid code compliance. You need your grid-tie inverter (which also provides the 60 Hz AC used by the support systems) to disconnect from the grid and transition to island mode /without interruption/ very, very fast (since utilities have standards on how fast generators need to disconnect during a system fault) and basically it required us to write our own firmware for the VFDs we were using which in turn invalidated their safety certifications. So definitely a solvable problem but we just didn't get there.

I would think that a momentary loss of power wouldn't be a big deal, even with active cooling and a vacuum pump. As long as your disconnect is fast, if it takes 500-1000ms for the pump and cooling to come back online from flywheel power, that seems like a much easier solution than worrying about five nines. The wear in that second can't be significant.

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