3 ms·
It's hard to generate a strict apples to apples comparison -- not only do the numbers depend on operating parameters, but pure adiabatic operation causes both v
by DaniFong 12y ago
It's hard to generate a strict apples to apples comparison -- not only do the numbers depend on operating parameters, but pure adiabatic operation causes both very high temperatures, imposing strict materials constraints and very difficult heat exchange and heat storage challenges, and very low temperatures, causing icing.
This means no working pure compressed air energy storage tech is in existence today -- a couple exist which add heat via combustion, one product used to exist which used an electric heater to keep a heat store warm. Both are thermodynamically non-ideal.
Based on pure theory, however, running two stages of compression to 200 bar (14.1:1 each), running perfectly adiabatically during compression and expansion, temperatures would reach to about 600 C during compression. If one didn't recover the heat of compression, and let the temperature of the compressed air drop to ambient, the temperature upon expansion would get to about -170 C. Pure thermal efficiency would be about 34%.
The results are so extreme / bad that one would choose a whole separate set of parameters and compromises. But that should show you the power of direct contact heat exchange.
- dredmorbius 12y agoThanks. I've seen some discussion of CAES using former natural gas wells for storage. I don't know the scale of operations there, but it could be large (meaning that relatively small amounts of energy would be stored or extracted at any given point in time. Rock also serves as a pretty good insulator (R5 as I understand, and there's a lot of it), which might preserve some of the heat of compression. In your example, is 200 bar a typical pressure for CAES? I see that as about 2900 psi for the imperial units types.
- DaniFong 12y ago200 bar is an inflection point in the regulatory landscape for above ground tanks. A sweet spot. There's another sweet spot around 300 - 400 bar due to the physics of air. Salt caverns melt at high enough temps (above 200 C) and injecting hot air into a methane reservoir is not terribly advisable...
- dredmorbius 12y agoTook me a moment to realize you were talking CAES in salt caverns, not salt caverns for thermal energy storage, though that hadn't occurred to me either (I don't think it's necessary). The constraints you raise on natural-formation CAES do raise some interesting challenges (or Hollywood movie plots).