> When electricity is required, the liquid CO2 is run through an evaporator to turn it back to a pressurised gas, which is then warmed up back to 290-300°C causing the stored heat. The gas is then introduced into an expansion turbine, where it rapidly expands at atmospheric pressure to drive a power-generating rotor, with the uncompressed CO2 then stored in a flexible dome — hence the company name — at ambient temperature and pressure for later re-use.

It's a closed loop system, so it doesn't seem particularly dirty to me, presuming the energy to run it will come from the renewable source that it's storing. The materials used to make it (steel, quartzite, PVC) don't seem too troubling.

Seems a bit cleaner than chemical batteries at first glance.