4 ms·

But how much cycle inefficiency would you be willing to take for that advantage? I could totally see that storage setup be a huge success at places where you ha

by usrusr 4y ago

But how much cycle inefficiency would you be willing to take for that advantage? I could totally see that storage setup be a huge success at places where you have continuous or forced-schedule intermittent low intensity heating and cooling demands (data center within insulated pipe distance of a spa resort or something like that), and want to make the best out of their opposite heat demands using intermittent electricity, but for general purpose electricity where the recovery would have to go through one of the famous thermal cycles? I really doubt it.

pfdietz 4y ago
Um, you are aware that CAES also has these inefficiencies. Adiabatic CAES does compression with the heat transferred to a store. The compressed air is a store of "low entropy". Pumped thermal energy storage stores that low entropy as cold rather than as compressed gas. But in both cases, energy is recovered by exploiting a compressed gas (preexisting in one case, manufactured by compression of a cooled gas in the other) heated and reexpanded. I don't understand why you're apparently ok with one and not the other.
- usrusr 4y ago
  Aren't you completely forgetting the non-thermal part of that firm of energy storage? CAES even works completely without heat recovery/reuse. Not particularly well, the adiabatic/store is certainly added for a reason, but a carnot cycle (theoretical best) would take a non-trivial amount of delta to reach even that, even if you had perfectly lossless on the charging side. You'd have to be well within the domain of molten salt to achieve a delta where the theoretical carnot maximum minus real life losses could be interesting. Heat/cold storage can be very useful if you have direct uses for the heat/cold, but if you want to get back to general purpose electricity, it's not good at all.
  - pfdietz 4y ago
    What non-thermal part? When a compressed gas expands and does work, its thermal energy is converted to work, and the temperature declines. There is no "energy of compression" in the gas aside from its thermal energy content. The internal energy of an ideal gas is a function only of temperature, not of pressure. Adiabatic CAES ideally stores ALL the energy invested during the compression into the thermal store. The compressed gas is just there as an entropy sink, so that this stored thermal energy can be converted back to work efficiently. But that's just what the "cold" in a PTES system is, too. Ideally, the efficiency of the two systems is the same.
    - usrusr 4y ago
      A compressed piston would allow energy recovery even after allowing the entire heat generated by the compression to dissipate down to ambient levels. The pressure gets slightly lower from the distillation, but it remains considerably above ambient. The fraction of the work that went into the pressure delta post-dissipation can be recovered 1:1 and then you'd still have a temperature delta left to hypothetically salvage, in part, using some carnot approximation. Your purely thermal approach would certainly try to salvage the work that went towards pressure right in the heat pump (decompressing through a piston or turbine mechanically connected to its counterpart on the compression side), but all recovery would still be constrained by carnot, which isn't the case for the "pressure part" of the work if you store that instead of immediately reusing it to gain more temperature delta. Thermal storage is the right tool for the job when all you have to begin with its a temperature gradient (e.g. in a mirror collector spat plant) and you want to time-shift conversion to electricity, or when you have a thermal use case and you want to time-shift electricity use from thermal utilization, but not for round trip electricity/electricity. Some quick googling yields a presentation comparing A-CAES and PTES by Xue Haobai at 2019 Offshore Energy and Storage Summit that puts the low end of round trip efficiencies of A-CAES to where the best of the best in PTES/Carnot batteries would max out. I can't tell you exactly where your model goes wrong, but I really believe that you must be missing something.

by usrusr 4y ago

pfdietz 4y ago

Um, you are aware that CAES also has these inefficiencies. Adiabatic CAES does compression with the heat transferred to a store. The compressed air is a store of "low entropy". Pumped thermal energy storage stores that low entropy as cold rather than as compressed gas. But in both cases, energy is recovered by exploiting a compressed gas (preexisting in one case, manufactured by compression of a cooled gas in the other) heated and reexpanded. I don't understand why you're apparently ok with one and not the other.

usrusr 4y ago
Aren't you completely forgetting the non-thermal part of that firm of energy storage? CAES even works completely without heat recovery/reuse. Not particularly well, the adiabatic/store is certainly added for a reason, but a carnot cycle (theoretical best) would take a non-trivial amount of delta to reach even that, even if you had perfectly lossless on the charging side. You'd have to be well within the domain of molten salt to achieve a delta where the theoretical carnot maximum minus real life losses could be interesting. Heat/cold storage can be very useful if you have direct uses for the heat/cold, but if you want to get back to general purpose electricity, it's not good at all.
- pfdietz 4y ago
  What non-thermal part? When a compressed gas expands and does work, its thermal energy is converted to work, and the temperature declines. There is no "energy of compression" in the gas aside from its thermal energy content. The internal energy of an ideal gas is a function only of temperature, not of pressure. Adiabatic CAES ideally stores ALL the energy invested during the compression into the thermal store. The compressed gas is just there as an entropy sink, so that this stored thermal energy can be converted back to work efficiently. But that's just what the "cold" in a PTES system is, too. Ideally, the efficiency of the two systems is the same.
  - usrusr 4y ago
    A compressed piston would allow energy recovery even after allowing the entire heat generated by the compression to dissipate down to ambient levels. The pressure gets slightly lower from the distillation, but it remains considerably above ambient. The fraction of the work that went into the pressure delta post-dissipation can be recovered 1:1 and then you'd still have a temperature delta left to hypothetically salvage, in part, using some carnot approximation. Your purely thermal approach would certainly try to salvage the work that went towards pressure right in the heat pump (decompressing through a piston or turbine mechanically connected to its counterpart on the compression side), but all recovery would still be constrained by carnot, which isn't the case for the "pressure part" of the work if you store that instead of immediately reusing it to gain more temperature delta. Thermal storage is the right tool for the job when all you have to begin with its a temperature gradient (e.g. in a mirror collector spat plant) and you want to time-shift conversion to electricity, or when you have a thermal use case and you want to time-shift electricity use from thermal utilization, but not for round trip electricity/electricity. Some quick googling yields a presentation comparing A-CAES and PTES by Xue Haobai at 2019 Offshore Energy and Storage Summit that puts the low end of round trip efficiencies of A-CAES to where the best of the best in PTES/Carnot batteries would max out. I can't tell you exactly where your model goes wrong, but I really believe that you must be missing something.