## 1 Introduction
### The Importance of Energy Storage Systems  
The need for energy storage systems in electricity networks is becoming increasingly important as more and more generating capacity uses VRE sources [18; 19; 20]. The ideal ratio between the necessary and unnecessary amount of VRE power depends on balancing and storage resources [21; 22; 23], transmission [24; 25; 26; 27], and the characteristics of the load and the climate [28; 29; 30]. The key to addressing the variability and uncertainty of VRE integration is the increasing of the overall flexibility in the power systems. Energy storage technologies can provide a variety of flexibility services, including the provision of operating reserves and shifting energy over time to better match generation and load [31; 32; 33; 34; 35; 36; 37; 38]. This technology can provide both downward and upward flexibility, storing energy either when there is a generation surplus or lower demand and discharging in the opposite case [39; 40; 41]. With respect to solutions for the load balancing of demand and supply in the electricity system large-scale storage options, such as molten-salt thermal storage, pumped hydro storage (PHS), battery, flywheel, or compressed air storage systems are of main interest (Figure 2) [42; 43; 44; 45; 46]. However, the specific details of the features of these energy storage technologies are not included in this manuscript. Figure 2: Utility-scale energy storage technologies by typical discharge time and power capacity ranges based on [39]. The global stationary, grid-connected energy storage capacity was about 159 GW in 2017, most of which was provided by PHS with 153 GW. Around 0.5 GW new utility-scale energy storage capacity was added in 2017 of an estimated 5.9 GW in operation. Most of the growth was in electrochemical (battery) storage, which increased by 0.4 GW to a total of 2.3 GW [10; 47; 48; 49]. Nowadays, the battery storage systems are attractive because they are compact, mostly safe, easy to deploy, and already economical in many countries, and they provide a virtually instant response to network disturbances [38; 42; 50]. Current forecasts and analyses [42] show that the total energy capacity of stationary storage technologies will grow from an estimated 4.67 in 2017 to 6.62-15.89 TWh by 2030 if the share of VRE in the energy system is to be doubled by 2030. According to optimistic estimates and different scenarios [42], the non-PHS will grow from an estimated 162 GWh in 2017 to 5.8-8.4 TWh in 2030. The large increase in storage will be driven by the rapid growth of utility-scale and behind-the-meter applications; therefore, the total battery capacity in stationary applications will grow from an estimated 11 GWh in 2017 to 100-421 GWh by 2030. From the Global Energy Storage Database (DOE) [51] and the International Renewable Energy Agency (IRENA) (2017) [42] data, it can be concluded that 20-80 GWh of global electromechanical storage energy capacity will be built by 2030, of which 20%-30% will be in the EU. The economics of stationary battery electricity storage applications could improve significantly in the 10 next years. As an increasing number of countries begin to introduce market reforms to support higher shares of VRE, more transparent and new markets for ancillary services are emerging, often at a very granular level. This market situation will open up new opportunities for electrochemical storage deployment, given that battery storage will increasingly offer competitive services to these markets. Besides, frequency regulation, electric energy time shift, or renewable capacity firming from battery storage technologies will also expand. The cost reduction potential for new and emerging electricity storage technologies (such as vanadium redox flow battery (VRFB)) is significant [17; 42; 52]. In 2020, the average price of a utility-scale lithium-ion-based energy storage system (without tax, customs, shipping, authorization, installation costs, etc.) was 300-400 $/kWh, while in the case of VRFB technology, the price decreased to 200-295 $/kWh.