Yes, we used these in data centers. They were not considered a battery replacement though, what they did was for areas of the world that take frequent and quic

Yes, we used these in data centers. They were not considered a battery replacement though, what they did was for areas of the world that take frequent and quick power loss events they allow the UPS system to ride through a small power loss without hitting the batteries (prolonging life) or generators (saving wear/fuel). The flywheel only lasts a couple minutes under full load. Any power loss longer than that transfers to battery and triggers a generator run to prepare to take the load long term. In that case why have batteries at all? Because generators and transfer switches aren't perfect. On typical data centers the batteries are sized for a fifteen minute full load capacity. That's fifteen minutes where the on site people can potentially fix something. That's one reason, the other is with flywheel you must start the genset earlier so it can take power before the flywheel dies, leading to more wear and fuel usage. Starting motors creates a lot of wear, drastically increase that and your gensets don't last long at all. Why not have a lot more batteries than fifteen minutes? The batteries are sized for the computer load only, not the full facility HVAC load. In general in a server room that's at capacity you have fifteen minutes run time without HVAC before you start shutting servers down on high temperature. So as you can see flywheels are an expensive addition that only make sense in certain isolated areas with bad power.