## 1 Introduction  
In the recent years, a considerable change in the energy mix took place. The main driver is the penetration of non-predictable renewable energy sources (NPRESs) into the electricity grid pushed by the more and more urgent environmental requirements. Since the Kyoto conference of 1997, when over 160 representatives of different countries established the objectives to shrink greenhouse gas (GHG) emissions, the increase of the use of renewable energy sources (RESs) was considered one of the key mechanisms to reach the overall goals. In 2010, the European Union (EU) promoted its solution in the field of environment and sustainability, as a natural consequence of the expiration of the 2012 Kyoto protocol. Three environmental objectives were established: a 20% reduction in GHG emissions compared to the 1990 level; a 20% rise in the use of energy from RES; and a 20% increase in energy efficiency. Finally, the EU members with the \"2030 Framework for Climate and Energy\" [1] established a 40% reduction of the GHG emissions from 1990 levels, a 27%increase of energy saving, and a 27% growth of EU energy consumption produced from RESs by 2030. A second step, the \"EU Low-carbon Economy Roadmap\" [2] requires a 60% drop of GHG emissions compared to 1990 levels by 2040 and an 80% reduction by 2050. This important and mandatory growth of RES employment demands strong innovations in the electrical system, mainly due to the intermittent and unpredictable nature of many of these renewable sources. Changes are required on the grid because it must be strengthened in terms of both infrastructure and management points of view. In particular, the distribution network needs to be made capable of managing bidirectional electric flows. From the management viewpoint, there is the need for new grid strategies and more effective operation plans based on optimized tools to forecast electricity demand and supply. The latter is becoming a challenging task, and conventional generators have to bear the burden of providing electrical system security, as production and demand must continually match. Consequently, even the reduction of emissions is lower than the ratio between energy produced from RESs and fossil fuels [3]. In the development of electrical networks, storage systems are an increasingly interesting tool, since they provide flexibility to the electrical network and guarantee higher energy efficiency and better system management [4; 5]. They can be used to separate the generation of electricity over time, space, and shape. From an economic point of view, the installation of storage systems allows to stock the energy produced over time at low cost and release it later during a peak of demand to avoid expensive and less efficient forms of purchase or production [6; 7]. From the environmental viewpoint, which is the focus of this work, storage systems make possible to entirely exploit the RES production, even if it exceeds the current demand or the grid capacity, and to supply it to the users when their demand overcomes the production. This operation can reduce the use of energy supplied by fossil fuels. But, despite this operating mode clearly decreases the emissions of pollutants during the operation of an energy system, a question remains regarding the overall environmental sustainability of the final energy production, which can be a cornerstone for the widespread use of storage systems. The International Renewable Energy Agency (IRENA) [8] underlines that the environmental concern can be a constraint when storage is matched to a real energy system and that an improvement of environmental compatibility is a key point for the development of storage technologies. Similarly, the European Energy Research Alliance (EERA) and the European Association for Storage of Energy (EASE) in a joint statement [9] confirm that, among other challenges, improvements of the environmental sustainability are mandatory for the development of energy storage technologies.