## 1 Introduction
### Hybrid Wind-Powered Pumped-Hydro Energy Storage
Research paper [15] analyzes the configuration and operation principles of hybrid wind-hydropower plants with pumped storage integrated into autonomous electric grid systems to support high wind energy penetration. This research used a computer algorithm for the detailed simulation and analysis of a plant installed in the autonomous power system of the Greek island of Crete. The purpose of the simulation was to study the effect of some main design parameters on the plant energy results such as the reservoir capacity and the wind farm power and to find possible optimum options of the system. The results showed that a hybrid plant with the new operation strategy is able to effectively substitute a significant portion of both installed power and electricity production of the conventional fossil-fuel plants on the island. In [16], [PERSON] and [PERSON] propose the installation of an appropriately administered wind-powered pumped-hydro storage system in the Canary Islands to overcome the stability and safety restrictions of the island's electrical system. The results obtained from the application of an optimum-sized economic model of such a system indicate that penetration of renewable sourced energy can be increased by 1.93% (52.55 GW h/year) at a competitive cost for the unit energy supplied. Investment, operating, and maintenance costs are taken into account, as well as those costs involving health and environmental damage associated with energy production and use (externalities). For the Greek Island of Ikaria, the first of its kind, a wind-powered pumped-hydro storage station was presented in [17]. The project is designed to exploit the hydroelectric potential of an existing water reservoir at high elevation, along with the energy produced by a new wind farm, via pumped-storage operation. In this paper, the plant is described and a suitable operating policy is introduced, both for the wind-powered pumped-hydro storage systems and the overall generation system of Ikaria, taking into consideration the existing regulatory framework for such installations. According to the authors, the plant achieves smooth integration, without adverse effects on the operation of the conventional generation and other Renewable Energy Systems (RES) stations on the island. The studied project results in a substantial increase of renewable energies penetration in the energy equilibrium of the island, which may exceed 50% on an annual basis. Additionally, the plant exploits in a very efficient manner the wind and hydro potential and can provide firm capacity to the island's electrical system, substituting expensive conventional peak units. [PERSON] et al. provide a complete sensitivity analysis based on economic perspectives on wind-powered pumped-hydro storage systems for remote islands in [18]. It was concluded that under certain conditions, wind-powered pumped-hydro storage systems may considerably contribute to the reduction of the current high energy production costs, by taking advantage of the wind potential in areas with considerable wind potential. Furthermore, this system is said to constitute a solution to the problem of the restricted wind energy contribution due to its high intermittency. Finally, the socio-economic benefits of such systems were found to be significant starting with the contribution to regional development by creating local job opportunities and the reduction of harmful emissions related with the fossil-fuel combustion. In Ireland [19], a literature explored how large-scale storage provided aid for the integration on intermittent renewable energy systems. A similar case study of using a wind power plant in combination with pumped-hydro storage was the primary area of the investigation. In total three key features of the pumped-hydro storage systems were investigated: its operation, size, and cost. From the results it was evident that pumped-hydro storage systems can increase the wind penetration feasible on the Irish energy system and reduce its operating costs.