It sounds unbelievable.

> Energy Retention After 10 Years 100%

> Potential Storage Capacity 1 day 32 kWh, 10 years 116,800 kWh, 25 years 292,000 kWh

What's the down side? Is it expensive? Is it slow to release energy on demand?

Everyone still slightly worried about what happens if it cracks, and I think the cost of lithium batteries has simply won?

Neat! Hadn't heard of them. I think the company I remember was called Ballard, and their flywheels were intended to drive the vehicle mechanically rather than to generate electricity.