[bjoli]

90% roundtrip efficiency is pretty darn cool. I am not an engineer, but is it the high temperature difference that makes that possible?

[IshKebab]

I think they're talking about thermal efficiency (since this is a thermal battery, not an electrical battery). 90% of the heat they put in, they get out again. Probably all you need is good insulation.

[smokel]

That's only in heat transfer, that does not include generating electricity from it.