*4.2. Exergy Storage Terminology—The Gas Storage Experience*

The technology behind current and future storages for electrical energy based on compressed air, H2, or SNG storages is derived largely from tried and tested storage technologies developed for the storage of natural gas [4]. A terminology has emerged to define operations and refer to the volumes of gas in an underground gas storage facility, which we adopt here when defining the exergy stored and explain below.

Underground gas storages generally operate by compressing the storage gas during injection and decompressing the gas again during withdrawal. The total gas storage capacity or volume is the maximum volume of natural gas that can be stored at the storage facility. This is governed by several physical factors such as the reservoir volume, engineering, and operational procedures including minimum and maximum pressure ranges and injection rates, which are determined from rock mechanical studies. The total storage volume comprises two elements:


The working gas volume represents the 'static', one-fill gas capacity and does not reflect multiple filling cycles. Thus, it is representative of a seasonal storage, similar to most traditional aquifer and depleted field storages. Of course, gas storages may be cycled many times during a year, which gives rise to what is described as a 'dynamic working gas volume' [39], which is greater than the static one-fill working gas volume.

Thus, exergy storage estimates are here referred to as the 'working exergy' (that available for work) and the 'cushion exergy' (that portion that must remain in the salt cavern/storage). The exergy tool was set up to calculate the static 'working exergy' (available) volume (see below). After introducing the static one fill 'working exergy' storage, we describe how, through a series of filters, attempts are made to derive realistic static 'working exergy' storage estimates from the total theoretical storage calculated (Figures 2 and 3a,b). These are based on cavern sizes and percentages of the total number of caverns, including that based upon the number of gas storage caverns in any particular basin (Figures 4–8).

However, as with gas storage caverns, the static 'working exergy' storage capacity is increased by multiple cavern-filling cycles. Therefore, also described and calculated are 'dynamic working exergy storage' capacity estimates, which are based upon multiple cavern cycles per year. The yearly cycle numbers are derived from different injection and withdrawal rates, which are informed by both CAES and UGS experience (S2, S3).
