What I don't get, why the complicated design to assemble the blocks into stacks? This seems to add a lot of non-essential complexity: Now you need cranes, a hooking/unhooking mechanism (that seems to even require cameras and image recognition!), the ability to move blocks horizontally and lots of computing power to coordinate the whole thing.

Even more, the up-front cost to recover energy is higher because you need to lift the blocks from the stacks before you can drop them.

Why not simply use a fixed hook/rope/motor/generator assembly per concrete block? Or, if there are too many blocks for that, use a hydraulic system? (But then again, what is the advantage of using many small blocks instead of a few large ones?)

Nothing against the company - the idea is awesome - but it seems weird that almost all the advertised innovations are solutions to problems that wouldn't exist without the "put the blocks into stacks" design:

>The innovation in Energy Vault’s plant is not the hardware. Cranes and motors have been around for decades, and companies like ABB and Siemens have optimized them for maximum efficiency. The round-trip efficiency of the system, which is the amount of energy recovered for every unit of energy used to lift the blocks, is about 85%—comparable to lithium-ion batteries which offer upto 90%.

Pedretti’s main work as the chief technology officer has been figuring out how to design software to automate contextually relevant operations, like hooking and unhooking concrete blocks, and to counteract pendulum-like movements during the lifting and lowering of those blocks.

(Not counting the described innovation of finding a new mixture that includes waste and reduces the amount of cement needed.)