Energy Storage in Grid-Connected Photovoltaic Plants

Nowadays, photovoltaic (PV) plants are receiving a very great attention due to their intrinsic ability to directly transform solar energy in electrical energy. Nevertheless, electricity generated from photovoltaic plants can rarely provide immediate response to load demand, as these sources do not deliver a regular supply immediately compatible with consumption needs. In stand-alone PV plants, energy storage (typically based on electrochemical batteries), together with the help of additional generation systems (such as those powered by fuel engines), is on the basis of regularization of PV generation and of full satisfaction of load consumptions. In grid-connected PV plants – theoretically energy storage is not necessary or useful, due to the availability of the distribution grid that should work as an ideal container of the electrical energy (theoretically, it can work both as an ideal generator and, also, as an ideal load). However, in this last years, an important attention has been devoted to the use of energy storage also in grid-connected PV plants, with the main aim of overcoming some important power quality problems of real distribution grids and for making PV plants more and more useful and attractive. In fact, avoiding more extensive details, energy-storage seems able to improve reliability, availability and energy generation efficiency of grid-connected PV plants, still poor because a lot of problems including: the high variability of solar energy availability, the high energy generation losses (i.e. caused by non-uniform solar irradiation conditions of PV fields) and the disturbances introduced by non-linear behaviour of power electronic apparatus (widely, PWM inverters) currently utilized for interconnecting PV plants with distribution grids. Most of these recent studies are substantially focused on sizing, operating and analysing of electrochemical energy storage systems to be used as an “energy buffer” for reducing, within acceptable limits, aforementioned problems and for improving some important grid auxiliary services as: • load balancing and peak shaving; • compensation of disturbances (outages, voltage drops and dips, PF correction, ...); • economic optimization of generated/consumed energy flows; • optimal integration among different kind of renewable energy resources (PV, wind, fuel-cells, ...).