I would like somebody to explain his "replace batteries every 2-3 years" comment. I think this is true of small scale Lead-Acid based UPS, but Neoen scale Li-Ion stacks come with a battery management system which I believe offers better than 80% effective battery energy density over a longer life: 5+ years. Certainly the assumptive economics behind grid-scale battery models from Tesla is, a far longer working life than this. And, the evidence of full cycle life on Tesla car batteries and even Prius, is better than 2-3 years of useful life.<p>This may be because they over-provision so "rated" energy is met by replacement from a 110% sold capacity-type model.<p>I think the rest of the article is great. Fascinating.<p>Grid scale compressed air storage is a thing, and may have low energy efficiency, but so does PHES. The thing is, they can be huge, and they can sustain power for long periods. The heat loss story, I am confused by because usually this gets better with scale: you can exploit a lot of lower grade energy in heat/cool cycles, either to retain it in system eg heat transfer to another form of heat energy storage, or, for adjunct purpose like building heating or cooling.<p>Fan, not expert, experts can put me right!