This is a smart solution - it's great that stories like this are getting attention. I'd like to share a similar project that I'm working on called Terrament. https://www.terramenthq.com/

Terrament is building underground pumped hydro energy storage (UPHS). Just like other pumped storage solutions, UPHS is extremely cost competitive (measured by LCOE, levelized cost of energy). It's extremely efficient (80-90% round trip). But unlike most pumped hydro, you can build UPHS anywhere. Just like this liquid air solution, you only need a couple acres of land. Many people assume this would use lots of water, but it doesn't because it's a closed-loop system.

I see that Heindl energy's solution was also mentioned above -- I love that design. Though, our analysis suggests that our design will give us better capacity.

Thanks everyone for your interest in this important field! This is not just a booming business, it's also critical for supporting the sustained growth renewable energy.