Transitioning from fossil fuel classical generators to intermittent, non-synchronous sources like solar and wind presents a series of technical challenges to be overcome on large scale. A specific issue is related with the concept of inertia of the electrical system: the less the number of generators with rotating masses connected to the grid, the less the value of total inertia of the system. Solar driven generating units such as PV present no mechanical inertia, therefore their increase in the electricity generation mix decreases the total inertia of the system, which lower the overall reliability of the system. Logically, it is of fundamental importance to ensure that PV power plants are more and more capable to provide ancillary services to improve the stability of the grid, especially in terms of frequency. The need for faster frequency regulation and voltage control in the electrical system can be ensured effectively by energy storage systems. In the purpose of this study it is addressed the possibility of large battery systems to overcome the variability of the solar resource, and the forecasting error, resulting in higher profit for a PV plant operator. The methodology consists in the formulation and the resolution of a Non-Linear Programming (NLP) problem, implemented in GAMS, applied to a 9.4 MW PV power plant. The output of the simulation determines the parameters that characterize the optimal Hybrid Storage System, in order to increase the profit during one typical day of solar radiation (01 April), while participating actively in the PFC. The result of the investigation determines that the most profitable hybrid storage system to be coupled with the PVPP is formed by a 883 kWh Lithium Ion Battery and a 32 kWh High Speed Flywheel. The analysis is finally complemented with a realistic simulation in Simulink environment in which is developed and implemented a prototype of EM
