I always thought it was interesting that more of these systems didn't exploit phase changes and the cold side of the Carnot cycle. For example, instead of compr

I always thought it was interesting that more of these systems didn't exploit phase changes and the cold side of the Carnot cycle. For example, instead of compressing air, you can liquify it, and then when you want to recover the energy you use it as a heat sink for a heat engine (for example, solar thermal) - in theory, this is just as efficient as adding energy to the 'hot' side of the cycle, and you can expand it out through an engine as well.