# Renewables and Advanced Storage in Power Systems: The Iberian Case
## 1 Introduction  
Many countries are setting targets for renewable shares to achieve drastic cuts in greenhouse gas emissions. In them, storage will play a crucial role, as widely recognized in scientific literature and official reports such as [1, 2, 3]. In particular, the European Union has set a goal of 32% renewable energy share for 2030 [4] in the Clean Energy Package, and higher targets are being discussed. Concerning storage, although not an official target, reference [2] sets a need of 108 GW for storage installed power in the European Union in 2030, while today, the total installed power is approximately 40 GW, mostly coming from hydro pumping power plants. Storage in power systems includes technologies that have been used for many years, such as pumping hydropower plants or batteries, and others that are in an experimental or precompetitive stage, such as compressed air, flywheels, hydrogen, or supercapacitors. Their applications can be classified into those that can handle large amounts of energy (energy arbitrage, seasonal storage, congestion management, or operational reserves) and others aimed at power quality issues or providing short-term reserves. Each technology has its own niche, although some compete with each other for the same use [5]. In a general way, storage may provide substantial contributions to system adequacy, flexibility, and overall resilience in decarbonized power systems [5, 6]. Despite these benefits, the current electricity markets do not foster new storage integration, and often storage meets regulatory barriers [2]. The economic viability and business models for storage are not clear: it is generally agreed that market revenues do not cover investment costs. This is related to theproblem of missing money, which also affects generation assets [7; 8], particularly with a high renewable energy share [9]. Therefore, technological improvements and lower costs of storage may be of great help to the new decarbonized power systems. Stakeholders of storage are companies willing to invest in storage facilities, either existing generating companies, consumers, or dedicated companies. European regulation [4] gives a special role to storage facilities, but excludes, with some exceptions, transmission and distribution companies from the property or management of storage facilities. Of course, this might be an obstacle to the optimal sizing and management of system storage. To perform a prospective study of the future power systems, it is necessary to use power system planning methods. Power system planning with renewable energy targets is a well-established research field, and many references deal with this topic. A short review of those recent references more related to the present work is given below. Interested readers may find a more comprehensive review in [10]. European power system planning is the subject of publications from [PERSON] et al. (2017) [11], [PERSON] et al. (2017) [12] and [PERSON] et al. (2019) [13]. In them, the whole European system with the interconnection capacity between different countries is modelled, and optimal power mixes are found. Storage is addressed in reference [14], where the importance of different storage technologies to achieve greenhouse gas (GHG) targets are underlined. The Spanish system is studied by [PERSON] et al. (2017) [15], and the inadequacies of its current regulatory framework are discussed. Regulatory changes are also proposed to promote the needed storage capacity. Another interesting study is reference [16], where the effects of substituting the French nuclear power fleet for renewables are estimated. Storage is given a cost, and the externalities of renewables in power system operation are considered. Reference [17] presents an agent-based planning model applied to Great Britain, which remarks on the convenience of counting on storage coming from batteries. In publications [18; 19], optimal storage capacities for different technologies and their location for a given generation mix are calculated together with sensitivity studies.