The average efficiency is about

The average efficiency is about 52.6%. Results concerning the economic evaluation are reported in Table 2 and Table 3. In Table 2, the potential hourly requests of PLX647 (DemandMGP,) the demand accepted (DemandMGP,accepted) and that refused (DemandMGP,refused) as well as Pd are reported. Only 17.5% of the demand is accepted (about 839 h/y) and 98.9% of cases the duty is accomplished. As a consequence, the working hours of the generation section of the HPP-CAES is about 830 per year. This value is consistent with the results found by RSE [31]. Almost 2 M€ would be lost in a year if the HPP-CAES were operated under these conditions, as shown in Table 3. In fact, it is not convenient to operate only in MGP because often the cost of electricity produced 'PLX647' by the HPP-CAES is higher than the selling price on MGP. In this scenario, the energy bought from the grid (Grid energy) is equal to zero (HPP-CAES operates in island mode) and the Simple Pay-Back Period (SPBP) is never reached. As shown in Fig. 2, this is not due to wrong size of the compressors, but to the large variability of the wind production. The average wind speed is 6.0 m/s and the standard deviation is 3.0 m/s, with significant number of hours without power production because of the too small wind velocity (about 6600 h). For this reason a different option is considered in next section.