Editing Compressed-air energy storage (section)

=== Diabatic ===
Diabatic storage dissipates much of the heat of compression with [[intercooler]]s (thus approaching isothermal compression) into the atmosphere as waste, essentially wasting the energy used to perform the work of compression. Upon removal from storage, the temperature of this compressed air is ''the one indicator'' of the amount of stored energy that remains in this air. Consequently, if the air temperature is too low for the [[energy recovery]] process, then the air must be substantially re-heated prior to expansion in the [[turbine]] to power a [[Electrical generator|generator]]. This reheating can be accomplished with a [[Natural gas|natural-gas]]-fired burner for [[utility]]-grade storage or with a heated metal mass. As recovery is often most needed when renewable sources are quiescent, the fuel must be burned to make up for the ''wasted'' heat. This degrades the efficiency of the storage-recovery cycle. While this approach is relatively simple, the burning of fuel adds to the cost of the recovered electrical energy and compromises the ecological benefits associated with most [[renewable energy]] sources. Nevertheless, this is thus far{{As of when|date=May 2024}} the only system that has been implemented commercially.

The [[McIntosh, Alabama]], CAES plant requires 2.5&nbsp;MJ of electricity and 1.2&nbsp;MJ [[lower heating value]] (LHV) of gas for each MJ of energy output, corresponding to an energy recovery efficiency of about 27%.<ref>{{cite report | title=History of First U.S. Compressed-Air Energy Storage (CAES) Plant (110 MW 26h) |volume=2: Construction | website=EPRI Home | url=https://www.epri.com/research/products/TR-101751-V2 |date=May 7, 1994 |df=mdy-all }}</ref> A [[General Electric]] 7FA 2x1 [[combined cycle]] plant, one of the most efficient natural gas plants in operation, uses 1.85&nbsp;MJ (LHV) of gas per MJ generated,<ref>{{cite web|url=http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG-WI/pnw_5pp_02.pdf |title=Natural Gas Combined-cycle Gas Turbine Power Plants |date=August 8, 2002 |access-date=2008-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20080411124518/http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG-WI/pnw_5pp_02.pdf |archive-date=April 11, 2008 |df=mdy }}</ref> a 54% [[thermal efficiency]].

To improve the efficiency of Diabatic CAES systems, modern designs incorporate heat recovery units that capture waste heat during compression, thereby reducing energy losses and enhancing overall performance.