This is a different application of the same principle. What is different about this and a mine is this starts at a energy reserve of zero. The mine has an energy reserve which os the equivalent of the size of the mine. The criticism of the concrete is removed by using sand. The mine will eventually fill up, but its life can be extended by using excess energy from the grid to remove material that was placed in the mine. This could put some coal miners back to work filling up the old mines. My only criticism is that coal plants would produce more energy reliably and cheaper. Second would be nuclear. However, the problem that needs to be addressed is political and not technical. China has propagandized the west into believing that coal is bad. While in turn buying these now worthless mines and shipping the coal to China to feed their hundreds of new coal plants. Edit:https://www.newscientist.com/article/2317274-china-is-building-more-than-half-of-the-worlds-new-coal-power-plants/