I posted a few comments in the past on my guess. A liquid filled loop with a (super)hydrophobic inner surface. Energy stored as kinetic, solar accelerated.
I have been looking, and haven't been able to find much info on reduced energy loss by using hydrophobic surfaces. It's a new enough tech that all the good info is behind academic paywalls.
Also, I would be concerned that with that much fluid moving through it that fast, a possible failure mode could be "temporarily turns into a horizontal launch loop". (which would be awesome ;)
Always a winning bullet point.