*1.1. Background*

The consequences of climate change due to energy production from traditional energy sources, the growing energy demand and the EU decarbonization targets lead to a need for replacement of conventional resources with renewable ones.

However, in the power sector, large-scale integration of intermittent output renewable sources is a great challenge, especially in non-interconnected or saturated grids. For this reason, the growth of large-scale wind and solar integration is a prerequisite for the simultaneous development of energy storage infrastructure.

Storage infrastructure may be a decisive factor for the size of wind and solar energy integration in the power system, as energy storage can act as a balancing component of the system. The energy storage technology that has met the biggest development, is applicable at large scale and has a considerable rate of efficiency is hydro-pumped storage.

**Citation:** Dianellou, A.; Christakopoulos, T.; Caralis, G.; Kotroni, V.; Lagouvardos, K.; Zervos, A. Is the Large-Scale Development of Wind-PV with Hydro-Pumped Storage Economically Feasible in Greece?. *Appl. Sci.* **2021**, *11*, 2368. https://doi.org/10.3390/app11052368

Academic Editor: Jorge Loredo and Javier Menéndez

Received: 17 December 2020 Accepted: 2 March 2021 Published: 7 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Hydro-pumped storage attracts the attention of the scientific community. In parallel, the role of storage solutions is drawing towards large-scale non-dispatchable renewable energy penetration [1]. The policy framework for large-scale electricity storage to use wind energy surplus has been comparatively analyzed in France and Germany for 2020 and 2030 [2]. Additionally, hydro-pumped storage has been widely examined as a solutionto reduce wind energy curtailment in the cases of Ireland [3], China [4,5], Greece [6–8] and in some North European Smart Islands [9]. A special research interest on the combination of hydro-pumped storage with wind energy was shown for autonomous islands, like Azores [10], Gran Canaria [11], El Hierro [12], Crete [13,14] and other Greek islands [7,13,15].

Concerning this paper's case study, the Greek energy supply system is characterized by the oddity that it is not a cohesive system. In particular, it consists of the mainland's electric grid and small non-interconnected power systems on the islands. The interconnections of Greece with the neighboring countries have a relatively small capacity, and the power supply of the system is mainly based on the production of lignite and natural gas power plants, with lignite power plants being gradually phased out and natural gas power gaining a larger production share.

Moreover, the potential of energy production from renewable sources is considered to be high due to the geographical location and the weather conditions that prevail across the country. The instability and the deficiency that could be caused to the Greek power system with the higher integration of renewable sources, inhibit their integration, as the system depends mainly on inland production, and security of supply cannot be compromised. For that reason, hydro-pumped storage or other energy storage systems should be integrated into the power system, in order to facilitate the share increase of power produced from renewable sources and in parallel diversify the energy mix of the country, introducing storage capacity.

A literature review of barriers related to large-scale market integration of Variable Renewable Energy Sources in European electricity markets design has been included in the discussion of the storage facilities' importance underlying the barriers of their high capital cost and the unsound business case due to the lack of scarcity price [16]. However, the role of storage is analyzed in large-scale wind and PV integration in Germany showing that integration level up to 50% could be achieved if flexible back-up power plants are used [1]. Different storage technologies (batteries, pumped hydro storage, adiabatic compressed air energy storage, thermal energy storage, and power-to-gas technology) and their role have been investigated in the transition path towards a 100% renewable energy power sector by 2050 in Europe [17]. The synergy between storage and balancing is analyzed in a fully or highly renewable pan-European power system, based on 8 years solar—wind demand data, with focus on the forms of hydro-pumped storage and hydrogen [18].

The sitting of hydro-pumped storage is a critical parameter from the social, environmental and energy points of view. A review of the existing types of pumped-hydro storage plants, highlights the advantages and disadvantages of each configuration and proposes innovative arrangements in order to increase the possibility to find suitable locations for building large-scale reservoirs for long-term energy and water storage [19]. A global analysis identifies 616,000 sites for closed-loop off-river hydro-pumped storage, based on high resolution global digital elevation models [20]. Off-river pumped hydro energy storage together is also proposed as an effectual solution to support 100% renewable energy in East Asia [21]. Finally, underwater pumped storage has been proposed as an alternative emerging technology with significant potential [22].
