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.
True. But the govt. report on this idea seems confident of 50% RTE, or 55% if they used a more-complex turbine system.
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.
seems to me that this would result in heat-loss as the air is heated quickly then ejected from the mass. perhaps the bubbled gas doesn't hold a significant amount of heat though? (but if it did, it could be used to extract the heat without pumping the sand...)
I was going to chime in to second this. In a former life I worked on power towers and we had designs for air receivers that would potentially work really well with this type of system:
- High temperatures
- Intermittent solar input not a problem
- tall central structure (?? maybe a plus given the paper's tall storage vessels)
But high temperature air receivers have their own problems, mostly around receiver material properties (thermal cycling / stress) and heat loss. It's really hard to focus a lot of light from the sun into a tiny aperture, because the sun isn't really a point source, and no mirror is perfectly shaped.
And highly concentrating mirrors only work with direct sunlight, while PV works with diffuse sunlight scattered off clouds, dust, or the air itself. Bifacial PV cells even capture light hitting the back of the panel.
