Citywide compressed air energy systems have been built since 1870. Cities such as Paris, Birmingham, Offenbach, Dresden in Germany and Buenos Aires in Argentina installed such systems. Victor Popp constructed the first systems to power clocks by sending a pulse of air every minute to change the pointer. They quickly evolved to deliver power to homes and industry. As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa in 50 km of air pipes for motors in light and heavy industry. Usage was measured in metres. The systems were the main source of house-delivered energy in these days and also powered the machines of dentists, seamstresses, printing facilities and bakeries. The application of elastic energy storage in the form of compressed air storage for feeding gas turbines has long been proposed for power utilities; a compressed air storage system with an underground air storage cavern was patented by Stal Laval in 1949. Since that time, only two commercial plants have been commissioned; Huntorf CAES, Germany, and Mcintosh CAES, Alabama, USA.The compressed air energy storage (CAES) concept involves a thermodynamic process in which the major energy flows are of work and heat, with virtually no energy stored in the compressed air itself. The performance of a CAES plant depends on the precise details of both the compression process and the expansion process.