Technology

Concrete has a very high carbon footprint. The manufacturing of cement liberates a lot of CO2 that had previously been in the ground as minerals. Different reaction from burning fossil fuels but indistinguishable as far as the atmosphere is concerned. Not to mention cement plant furnaces are usually fossil fuel powered.

Since the blocks are not (as) structural, less cement and more gravel/old concrete fragments can be used which would mitigate this to some extent, but I'd still imagine this is pretty similar to electric cars where it takes a significant portion of its service life to even reach net zero from the carbon released from building the thing.

Concrete is quite possibly the worst material ecologically speaking for this application. Presumably it was chosen for its physical properties, but still. I do wonder if just using straight stone quarried directly into the final block shapes could mitigate some of this, since concrete is basically just stone whose shape and composition we can control, and either way the same mass of material needs to be mined out of the ground so I imagine they'd break even on mining ecological footprint. In fact, if you just need something heavy to store gravitational potential energy, and water is not viable for whatever reason, why not use crushed cars or something?

I get why China wants to explore this, they're just putting some chips into all emerging technologies just in case, but I'm still of the "just use the excess energy instead of trying to store it" camp. Storing energy at grid scale is just not practical with our current technology, 100MWh is basically nothing at the level of entire countries' electricity demands, a single city goes through that in a very short amount of time. For example, data centres could be required to only use excess renewable energy for non-real-time computation like training AI, or scientific computation, since they're a load that can ramp up or down almost instantly. Or you can make hydrogen from water or even hydrocarbons from the air with the excess energy, for things like airplanes and rockets that don't yet have a viable path out of burning stuff to work. Or, since the majority of the world's electricity is still fossil fuel based, just build infrastructure to send that energy to where it's needed because there will always be somewhere that could ramp down a fossil fuel plant and use renewable instead. I think only after all electricity is renewable can storage really become viable, and technology will have long advanced by then and all the primitive storage schemes we come up with now will be obsolete anyway, so while I do support experimenting with storage methods (which this seems to be), actually committing to building out a ton of storage right now seems premature and a poor investment of our limited effort and resources.