I think you might be wrong but my thermal is very rusty. Where you are wrong is that you might be forgetting the over 100% efficiency in heat transfer when using a heat pump.

http://energystorage.org/energy-storage/technologies/pumped-...

https://www.economist.com/news/technology-quarterly/21603184...

http://wordpress.ei.columbia.edu/lenfest/files/2012/11/Abstr...

I don't follow. The second law of thermodynamics doesn't say anything about the specific efficiency limits of an energy conversion or storage system. Ideal reversible systems can maintain entropy if fully isolated (e.g. insulated). In real systems, perfect isolation is impractical, of course, but the resulting loss of energy is a function of the shortomings of the manufacturers of the system (e.g. use of low R factor insulation, high resistance electrical conductors, or poorly shaped pump impellers, etc), not some simple "2nd law limit".

Same observation here ... seems to be a bit nebulous ...

Pumped hydro is about 3x as expensive, and batteries I think are still more expensive than pumped hydro, so it's absurdly cheap.

First, not saying I believe that these guys are legit, but "thermodynamics" is a noun. Thermodynamic energy is a type of energy, like chemical energy is a type of energy. There is also nuclear, stored, potential, light, kinetic, and I'm sure I'm missing a few.

These are all basic physics concepts. These guys are claiming to be able to store the energy as thermal energy, and then get it back out at 70% efficiency without much degradation inside of 25 years.

All these ideas are plausible from a physics point of view, but I don't buy this simply because it requires an efficient way of converting heat to energy. If this existed, it would already be a holy grail for other reasons. Being able to turn the waste-heat from an engine back into more energy efficiently would be incredible. Right now, to get energy from heat, you typically need the heated material to be near something that's very cold, and you use the difference in temperature to get energy out of it. Or you boil water and spin turbines. It sounds like MAYBE that's what these guys are doing, if what they are claiming to be able to do is legitimate.

Indeed. I fixed the title.