Earlier quoted context omitted.
Pretty inefficient, yes. Thermodynamics is a harsh mistress. But if the other choice is "throttle down the wind farm, because the grid doesn't need that much power" - then a really cheap/simple/safe (but inefficient) storage tech could prove pretty useful.
One can get higher round trip efficiency (practically, perhaps 65%) using pumped thermal storage. Here, one uses some thermal cycle in reverse to separate "cold" and "hot", then reverse that to discharge. This also reduces the maximum temperature needed to maybe 500 C, below the creep limit for cheap steel. The cold end would be maybe -100 C, stored in something like liquid hexane.
For limited & short-term use, the plant with vastly-more-expensive storage masses might make sense.
But as soon as you were faced with NIMBYs or environmentalists (hexane's MSDS is far closer to hydrogen fluoride's MSDS than it is to sand's), or if you are working in a less-prosperous part of the world...sand is great stuff.
