> We don't need to do [AC/DC conversion] all that often, and the flywheel is lossy too. And you can get 96% efficient...
Sure, even looking at that page 10-15% does seem a regular rule of thumb though. Many devices in the home (computers, displays, etc.) require DC. Photovoltaics produce DC, but not necessarily at a level suitable for variable, direct load by devices. So in a sustainable source powered situation, you have to pay for this loss somewhere: either in storage or inversion or both. It would seem that the 1979 author was refering to the flywheel's potential toremove the 'double tax' for storage and inversion. The listed 'motor generator' benefits (load and spike isolation, etc.) would also seem to apply.
> this one is high profile and recent
Beacon are often referenced. It seems they were revived and the major facility they built is still in operation. They cited a combination of DOE policy and 'the current political environment' as their cause of temporary bankruptcy.