That reminds me of a more 'out there' energy storage mechanism I've looked at a couple times: Hyperloop If you've got a large Hyperloop network with hundreds o

That reminds me of a more 'out there' energy storage mechanism I've looked at a couple times: Hyperloop If you've got a large Hyperloop network with hundreds or thousands of ~40,000 kg vehicles shooting through it at 700 mph, each of those is 400 kWh of stored energy. Of course, it'd be crazy hard to actually use because demand for travel on the network isn't gonna be constant, but it's an entertaining thought.