It's funny you mentioned that in this thread about super-caps.... I brought this up years ago. Compressing air underground via excess power from windmills, to later be released through turbines for power generation. https://io9.gizmodo.com/how-to-turn-caves-into-giant-batteries-5893480 Efficient? I dunno. But there's lots of ways to skin a cat. Those old Citroens, aside from being frog cars, were actually pretty cool. Remember the rear suspension? Same air.
I thought Tesla is trying to have a low carbon footprint. But this is something new I read today. As far as I know, Tesla announced that they have an acquisition of ultracapacitors and Maxwell for more than $200 million. But still it is not clear if it will use that in its own product or not. But whatever it is, they haven’t mentioned the cost of super capacitors as per my knowledge. I think this would not interest me until and unless I have invested in Tesla .
There is a company from an asian country which stock has produced 100%, supplier of Tesla. I'll get the full details and make a post on that.