# Optimizing Sustainable Energy Transitions in Small Isolated Grids Using Multi-Criteria Approaches
## 2 The Electric System of El Hierro
### The Generation System
#### 2.2.2 The Hydro-Wind Power Plant  
The various facilities associated with the Gorona del Viento hydro-wind energy station are sited in the municipality of Valverde [47]. The selection of the wind generation with storage combination is not accidental, but it responds to the good characteristics of the island. On the one hand, north-northeast trade winds are predominant (Figure 5a), which are fairly constant throughout the year (Figure 5b). This is coupled with the favorable topography of the island, which allows the construction of reservoirs close to the island, as opposed to higher elevations.  
Regarding a detailed description of the Gorona del Viento plant, the system comprises an 11.5 MW wind farm and an 11.3 MW turbine power plant that generates power by harnessing the elevation difference between the facility's two water storage reservoirs. A pressurized hydraulic circuit connects both tanks through a 6 MW pumping station and 225 MWh of storage capacity [47; 48].  
Figure 4: Breakdown of annual electricity generation on the island of El Hierro.  
The operation of the hydro-wind power system in El Hierro depends on the available generation capacity. During windy periods, the wind turbines supply electricity to the whole island, and excess energy is used to pump seawater between the lower and upper tanks. While during low wind periods, stored water from the upper reservoir is released to drive turbines and generate electricity. In recent years, wind generation has ranged from 30 to 40 GWh annually, with around 20 GWh absorbed by the pumping process. Performance figures for the system over recent years are available on the company's website [25].  
Since the hydro-wind integration into the island's grid in 2016, the system has faced noteworthy energy losses, particularly due to the storage system's inability to absorb peak wind generation. With wind turbines with a capacity of 11.5 MW and the pumping system at 6 MW, excess energy often occurs, especially at night or during the winter months when demand is lower. This leads to storage reaching full capacity and causing energy wastage of about 30-40% of renewable generation. Figure 4 shows the system's performance from  
Figure 5: Characterization of the existing winds on the El Hierro Island for the year 2023: (**a**) wind rose; (**b**) box and whisker hourly graphs.  
2014, emphasizing the important generation rise during the initial two years because of the aforementioned aspects.  
The renewable energy contribution to the island's electricity generation varies throughout the year, with the highest contribution in the summer months, reaching 80% in July and around 20% in April. Over the years, renewable energy contributions have fluctuated between 60-80% from May to August and 20-40% in other months. Figure 6 illustrates El Hierro's monthly electricity generation in 2018 and 2023; the values are broken down by renewable and diesel generation.  
This shift to renewable energy has significantly reduced GHG emissions. In 2018, around 50% of emissions were avoided compared to pre-renewable energy operations (Figure 6a), though reductions were more modest in less windy years, such as 2023, where the reduction was around 25% (Figure 6b). The hydro-wind plant avoided approximately 18 kt CO\({}_{2}\) in 2018 and 10 kt CO\({}_{2}\) in 2023 [25].  
Figure 6: Breakdown between renewables and diesel of electricity generation for the El Hierro Island: (**a**) 2018; (**b**) 2023.