Why are there no flywheels for home-energy storage?

If you want to store energy from your home energy production (turbine, solar panel, etc.) you pretty much have two choices: lead-acid or lithium-ion batteries or rarely a heat-pump. Why not a flywheel for home use? People say because when they fail, it’s catastrophic but you could bury it in the ground like a septic tank. The other issue may be amount of power flywheels transfer. It seems that most applications are for a lot of energy over a very short time, but then again those are industrial uses. Is there an issue with flywheels not being efficient at transferring small amounts of power back and forth over the long term?

Flywheels are expensive because of the mechanical stresses involved. They require some sophisticated precision engineering to reduce the chance of failure, and to mitigate the consequences of failure (more on this in a second).

Additionally flywheels have low energy density. A flywheel has an energy density of 5-8 Wh/L whereas a Li-ion battery will have 250-670 Wh/L. So a flywheel system needs to be about 50 times bigger than a comparable Li-ion system.

Now back to the failure mitigation scenario. Should a flywheel fail at peak charge, it will release all its energy into the structure either like a bomb or a wrecking ball, depending on how it fails. So for safety reasons you need a container strong enough to dampen a bomb explosion. And because of the energy density issue, the size of that container needs to be more that 50x bigger than a comparable Li-ion system.

You mentioned underground excavation. That’s expensive even for new construction, given that you’re talking about an entire rec room of unusable space (plus waterproofing and drainage). For existing construction it will usually be unrealistic. Plus you’ll probably want to replace the flywheel every few years due to accumulation of micro-cracks that will raise the risk of failure, so now your little bomb shelter needs to be designed in a way that you can move a baby grand piano in and out of it every few years.

Flywheels are well-suited to certain industrial and aerospace contexts, but for a residential home, most certainly not.

This is a great post, but this sentence even more so. I love the analogies here!

I think it was an article on flywheels in Scientific American decades ago that mentioned - since flywheels store power at the square of rotational velocity, a lighter flywheel spinning much faster stores comparatively more energy. The article suggested that the best strategy for flywheels was using high-strength fibers wound around an axle, rather than the traditional flat disk for less demanding applications. Of course, housed in a vacuum chamber and using magnetic bearings. (Which points to another fail point, breach of the vacuum chamber.)

A Tesla Powerwall is IIRC about 14kWh, a decent sized house might use 2 (not counting need for vehicle charging) so that’s a good starting point for back of envelope calculations. My gut feeling is the ability to run large drain appliances may be the issue.

But the earlier point is correct - whatever the construction method or shielding, a catastrophic failure that leads to a chain reaction releasing 14kWh or more of energy in one swell foop is not good. To isolate individual cells would mean multiple holes in the ground with a decent amount of earth between then - basically a back yard that looks like a miniature cold war missile silo farm.

All that before even getting into what it takes to convert it [back] to clean, usable mains voltage electricity. Whoever has to foot the bill for this will be paying for a lifetime of maintenance on and periodic replacement of a generator.

Certainly paints a picture don’t it. I bought Kurzgesagt’s book on the immune system some months ago and it said eosinophils are like an angry chimp with a machine gun – they certainly attack the invader but also cause a lot of collateral damage.

I worked in NASA’s high energy flywheel facility so I can echo pretty much everything that has been written. The carbon fiber over titanium hub was the most efficient design and our 10" diameter wheels ran at 60,000 rpm in vacuum. The fiber wheels do not really explode. If they come apart they start turning to fuzz and stop the wheel from friction. They are a close fit in the vacuum chamber.

But those were small systems designed to provide computer backup power for a short time until your backup generators come online. All of our test beds were in underground pits. Everything attached to the lid and could easily be lifted out. I did design an above ground system using a 24" water wall backed up by a bomb blanket.

The drive motor was also the alternator. It was attached at one end of the shaft. 99% of the time it is just sitting there, occasionally giving a kick to the flywheel.

Flywheels are really ideal when you need an ability to store and deliver high power throughput but a moderate amount of stored energy. The bigger you make flywheels and the faster they spin (to store more energy per unit) the greater a challenge they are from a mechanical and materials standpoint, and while you can get millions of cycles out of them they are quite lossy over an extended period of time versus batteries, fuel cells, pumped hydro, and even molten salt (which if properly insulated can be quite a durable energy storage medium for days or even weeks). Unless you need a massive burst of power to energize your home defense laser system, you are better off with a LiFeO4 system.