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jandrese

For home use you also have to consider that you're basically making a bomb every time you store energy in the thing.

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Any kind of high-density energy storage is basically a bomb.

If you charge or discharge Lithium polymer or lithuium ion batteries "wrong" they explode too.

I wouldn't want to be near a flywheel that stores 360 Wh of energy when it breaks.

On the other hand, flywheels can be easily buried underground, and easily contained. The max power of a flywheel is known, so it's just a matter of putting enough concrete between humans and flywheel, a straightforward and inexpensive engineering problem. And unlike compressed air, the containment isn't under constant, wearing pressure.

You can bury the air tanks too and avoid most of the danger, but that's expensive and makes them harder to maintain. The big advantage of flywheels is that when they fail you know the energy will be discharged in a particular plane. You can orient them such that nothing vital is in that plane (like your house!). With compressed air all bets are off.

SCUBA tanks have a "burst-disk" or a weaker part of the tank that is designed to fail first, rather than the whole thing exploding and launching shrapnel. It's still violent and can throw some shrapnel, but it's much less. Also when SCUBA shops fill tanks they usually put them into a container that would somewhat contain that.

I imagine something similar could be done. It's not like there are zero safety mechanisms with compressed air.

A berm would split the difference for cost and maintenance.

Not necessarily, 1.3MJ (360 Wh; but screw that unit) of diesel isn't much of an explosive under normal circumstances. It really depends on the maximum power output under normal operation, and whether it can occur uncontrollably.

Energy density and release rate tend to have inverse relations, for a given energy mechanism.

TNT has about 1/10 the energy density of petrol or diesel. Capacitors have lower capacities than batteries.

People say that but searching youtube for exploding lithium ion batteries they seem to make fairly lame bombs. More some flames come out maybe a foot and then they burn eg https://youtu.be/SMy2_qNO2Y0?t=1m57s

I'm not sure they ever explode in a bomb like way?

Batteries don't explode in anywhere near the same fashion as an air tank, but they do include the added complication of adding a huge conflagration to the mix. So your house is burnt down instead of shattered and scattered all over the neighborhood.

360Wh of flywheel is tiny. You could easily bury it and never have to worry about the danger of a failure. Grid scale flywheels are the ones that are scary.

... and they too can be buried. Seems a bit more attractive than compressed air.

Not really. A good bomb has high velocity. Lots of high energy storage methods are slow reacting.

>For home use you also have to consider that you're basically making a bomb every time you store energy in the thing.

No different than people who have LP tanks outside their homes. Or fuel oil tanks buried in their yards. Or coal bins in their basements. Or natural gas lines coming their house. Or pretty much any other storage method except a dam for your water wheel, but even that can burst and flood your home.

few mechanical engineer~ youtubers said so many times; compressed gas seems less dangerous than water but compression is terribly dangerous

This doesn't make sense at all. High pressure water can be dangerous for sure, but water based energy storage doesn't use high pressure... It's all about pumping the water uphill and then letting the water flow back down through the pump to recover the energy. At worst your house gets flooded because the containment vessel failed.

Worst case with compressed air is a total disintegration of the tank, the room it's in, and the rest of the house along with the occupants.

Water is pressurized. The pressure is a function of gravity and depth, but that is still a lot of pressure.

my english is flawed, I was trying to say that. Water is not pressurized but feel scarier than air for most people, so they go care free around compressed gases.

Water is pressurized. However, it is not compressed. That's an important distinction.

I'm fuzzy and since water is incompressible, "pressure" may not have the same meaning on water than on other substance

Eh, at least air isn't as dangerous as e.g. water would be, the tank can be / is designed to fail gracefully and there's overpressure valves.

Huh? Compressed air is way way more dangerous than pumped water. A pumped water disaster is a flood. A compressed air disaster completely obliterates the containment vessel and surrounding room and the overpressure wave and shrapnel are also fatal. I'm not sure how you would design a high pressure air tank to fail gracefully. The air is like a gigantic spring, and once it starts to go it's going to tear the containment vessel to shreds.

There are things you can do to make it safer, like installing the tank at the bottom of a pool or buried under a few feet of dirt, but they add complications and expense.

Failed water lines can ruin your house. Failure of a compressed air vessel of sufficient size could turn your house into shrapnel flying fast enough to endanger your neighbors.

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