Smoke and mirrors...
"The company that took the proposal to IMechE, Highview Power Storage, says scale is the key. A large cryo-generator could be co-located with a large-scale conventional industrial plant or power station, to harvest waste head otherwise released to the atmosphere. This heat could be used to speed up the warming of the stored air, boosting the power output. Also, when the cooling process has been completed, excess low-temperature air could be passed through gravel-filled tanks, to help prime the cooling process on the next cycle. This, the company says, could yield efficiency of as much as 70 percent (if its calculations are accurate)."
This is quite obviously bollocks, designed to attract funding / interest - the first part of this supposed 'efficiency' increase is nothing of the sort - they are simply proposing to put more energy into the system in the form of captured heat from a power plant. And I VERY much doubt that the second part - pre-cooling gravel could bring anywhere near the efficiency improvement proposed.
Even accepting the above; 70% MY ARSE!!!