# Simulation and Optimisation of Utility-Scale PV-Wind Systems with Pumped Hydro Storage
## 1 Introduction
In many mountainous countries, and specifically in Spain, there is considerable potential for reusing traditional hydroelectric power plants through the aggregation of pumping systems, and there is also the possibility of expanding existing pumping plants by incorporating new groups with the same hydraulic infrastructure [15]. Further, using low-head applications can make PHS usable in regions where it is not feasible traditionally [5]. Underground pumped-storage hydropower (UPSH, also called sub-surface pumped hydro storage, SSPHS) can avoid the environmental problems of installing new massive PHS plants using underground deposits in caverns or in abandoned mines (use of water from mine drainage for generating electrical energy through a reversible purification plant, thereby allowing the regeneration of mining environments). Although this technology is still in the development phase [16], there are already projects for mining basins in Germany, the United Kingdom, Spain, China [17] and the USA [18]. Other nonconventional technologies are also being evaluated, e.g., the use of canals or streams at different heights or deposits on the seabed [19]. In addition, there are innovative projects that involve seawater pumping, in which the lower reservoir is the sea [20; 21], with existing projects in Japan, Ireland and Hawaii [18]. Renewable generation can be installed near existing PHS plants, operating in a similar way to a traditional power plant (energy management and arbitrage) and exploiting the existing grid connection infrastructure. Floating photovoltaic generation can also be added to existing PHS reservoirs [22]. [PERSON] et al. [7] presented a technological review of renewable power storage using PHS. [PERSON] et al. [23] reviewed the developments in relevant international electricity markets, and [PERSON] et al. [24] reviewed studies on hybrid renewable systems, including those that used PHS. [PERSON] et al. recently investigated the drivers of and barriers to PHS [25]. [PERSON] et al. presented an extensive review of solar and wind power generation systems with PHS [19]. The literature review indicates that most previous optimisation articles on PV-wind-PHS systems are related to off-grid systems. In addition, the literature review indicated that in PV-wind-PHS optimisation studies, the head dependence of water-power conversions and the variability of the friction losses and pump-turbine efficiency are neglected in studies on PV-wind-PHS optimisation, assuming fixed constant values. [PERSON] et al. performed a full review to address uncertainty in the optimisation of PHS [26], concluding that only a few studies considered uncertainties in PHS sizing, while even fewer studies focused on long-term planning. The majority of studies on grid-connected PV-wind-PHS optimisation attempted to optimise the daily operation (short-term). Thus far, few studies have optimised the size of the PV-wind-PHS system; however, they did not consider energy arbitrage, and only considered the grid to inject surplus energy (which cannot be used by the load that is not stored) or to import the energy required by the load that cannot be covered by the system. [PERSON] and [PERSON] [27] optimised the day-ahead operation of a wind-PHS system with two different objectives: maximisation of the market profit or generation of constant firm output power. [PERSON] and [PERSON] [1] presented a stochastic model for evaluating the effect of an optimally managed PHS power plant using certain price ranges for pumping and others for generation. They optimised energy management, but not component size, thereby considering the PHS global efficiency for a fixed value of 80%. [PERSON] et al. [28] optimised the size of a PV-wind-PHS system for supplying the load of an urban community in Libia, injecting surplus energy into the grid (which cannot be used by the load or stored) and consuming the energy that cannot be supplied by the system from the grid.