# Renewables and Advanced Storage in Power Systems: The Iberian Case
## 2 Storage and Its Applications  
Storage in power systems comprises technologies with very different properties, costs, and maturity. The main technologies are hydro pumping, batteries (lead-acid, NaS, Li-ion, redox flow, etc. ), compressed air, flywheels, supercapacitors, and hydrogen. They may be classified into four groups from the point of view of their possible applications in power systems. Pumped hydro and compressed air, to begin with, may handle large amounts of energy, which allows them to participate in electricity markets for energy and reserve, even with a seasonal timescale, but they cannot be used in small size applications. Hydrogen and Vanadium redox flow batteries combine the possibility of large storage capacity with scalability that allows them to be used by small consumers to optimize energy purchases in combination with distributed generation, such as photovoltaics. Flywheels and supercapacitors are better fitted for short-term applications such as power quality or short-term reserves due to their quick response and short storage capacity. Finally, most of the batteries are in an intermediate place: they can be used by individual consumers or stacked in large units, and can participate in energy and reserve markets, but not on a seasonal scale, and they can also contribute to power system quality. Batteries have experienced in the last year big advances that have dramatically reduced their costs, so they are seen as complementary to intermittent renewables, such as wind or photovoltaics. A thorough survey of the techno-economic and regulatory status of storage at the international stage is in reference [5], where the deployment and regulatory status in many countries are checked, and an analysis of the main expectations of different technologies is given. Reference [6] makes a useful and extensive survey of different references for a prospective study of storage costs up to 2050. It concludes that batteries are the most competitive short-term storage technology in this time horizon. Among them, lithium-ion technologies seem to be the most efficient for most applications, such as energy balancing, ancillary services (including reserves) and congestion management. According to reference [6], Lithium-ion batteries may range up to 35 MW, with 5 h of discharge, a life of 3500 cycles and a response time lower than 10 s. Their efficiency is up to 86%, and advanced manufacturing, operation and maintenance cost reductions may reach 60% by 2030. Applications of the different storage technologies for different sectors of the electricity market are detailed in many references [26]. Official reports such as [1,2] provide a general survey and a prospective view of storage needs and deployment. Reference [2] centres on the contribution of storage to the security of supply, describes the European state of deployment of storage and its potential and provides a set of recommendations to overcome the detected barriers, such as the lack of a viable business case for storage. Notably, seasonal storage does not seem to be a pressing need in the European Union until 2040, according to the study [9]. The need for storage for energy transition is also underlined in reference [27], among other issues, such as the proper way to incentivise renewables and/or the role of demand response. The interaction between storage, renewable energy curtailment and system flexibility is studied in reference [28]. Forecasts for storage advance indicate higher performance and lower costs. For the Li-ion batteries, the most promising technology [6], the investment costs reduce to 23% in 2030 and to 14% in 2050 with respect to the 2015 values. This is the main component of the Levelized Cost of Storage (LCOS), and this reduction leads to a decrease in LCOS from 250 USD/MWh in 2015 to 190-150 USD/MWh [6]. Comments about the need for energy capacity will be made later in this study when discussing the results of the optimization problem.