I took graduate level courses in storage with these technologies at scale. Neat that this knowledge is useful again.

Pumped and compressed require specific geologic formations. Most of the sites for pumped have already been developed in NA. There's room for growth for compressed, but compressed also suffers from losses when the air that's pumped into the crust cools. Hopefully, there are undeveloped compressed sites near regions with energy demands.

Flywheels are a neat idea and still just that: an idea. It's yet to been demonstrated they can reliably do more than grid frequency moderation. The reason it's not very attractive to investors is that we don't have materials to match the energy density of other technologies.

Green hydrogen is also just an idea at the present. Nobody's pursues this because of losses incurred generating hydrogen from water. I want this one to work!

Finally, batteries. Do you think there are enough metals on the planet to build enough batteries for current and future demand?