# Optimizing Sustainable Energy Transitions in Small Isolated Grids Using Multi-Criteria Approaches
## 1 Introduction  
### Global Energy Transitional Scenario  
The \"World Energy Outlook 2023\" by the International Energy Agency (IEA) [1] highlights the persistent rise in global energy demand, briefly interrupted in 2020 by the CoronaVIrus Disease 2019 (COVID-19) pandemic but which resumed in 2021 [2]. Fossil fuels dominate energy production, supplying nearly two-thirds of electricity [3], though their share has gradually declined in recent years [4]. This reliance is unsustainable due to the medium-term depletion of fossil fuel reserves [5, 6] and their significant greenhouse gas (GHG) emissions [7, 8]. Renewable energy sources are essential to replace or supplement fossil fuels [9], particularly for electricity generation, to meet ambitious CO\({}_{2}\) reduction targets amid rising electricity demand [10]. Electricity is expected to exceed 30% of total energy consumption in many countries soon [11]. Challenges are exacerbated in remote areas, such as islands [12], which rely heavily on fossil fuels due to limited grid connectivity, leading to emissions, supply chain complexities, price fluctuations, and geopolitical risks [12]. Renewable energy adoption offers strategic energy independence and environmental benefits but presents challenges due to the variability in sources like solar and wind power [13; 14; 15; 16]. This variability necessitates oversized systems with large storage capacities to ensure energy availability during low-generation periods [17; 18]. Optimal sizing of generation and storage is usually driven by economic considerations to minimize costs while managing excess energy [19; 20]. To meet energy demands using renewable sources, energy systems need to be designed with surplus capacity and equipped with substantial storage solutions to manage excess energy and ensure reliability during periods of low generation [19]. Despite these measures, some surplus energy is unavoidable, though typically at a reduced scale. As a result, the optimal sizing of generation and storage systems is usually guided by economic considerations, striving to fulfill energy needs at the lowest possible cost [18]. Islands, however, encounter unique challenges in deploying large-scale renewable energy systems. These include lacking suitable sites for essential infrastructure such as pumping stations, Compressed Air Energy Storage (CAES), large-scale Battery Energy Storage Systems (BESSs), and expansive solar PV and wind installations [21]. All European Union (EU) countries are mandated to achieve economic decarbonization by mid-century. The Canary Archipelago is actively working towards this goal, aiming to decrease its dependence on fossil-fuel-based generation within a specific timeframe. The archipelago benefits from abundant natural resources, particularly wind and solar energy. The Canary Islands and Spanish national governments are optimistic and have set an accelerated target to complete the decarbonization process 10 years ahead of schedule (PTECan project) [22; 23]. The Canary Islands consist of seven islands located between 100 km and 300 km off the Moroccan coast and approximately 1500 km from mainland Europe. More than 2 million people live on these islands, according to official data from 2020 [24]. The largest islands, Gran Canaria and Tenerife, have populations of around 1 million inhabitants, each of them. Lanzarote, Fuerteventura and La Palma all have more than or close to 100,000 inhabitants. The smaller islands, La Gomera and El Hierro, have between 10 and 20 thousand inhabitants. In 2019, the total final energy demand for the Canary Islands was about 9.4 TWh, which remained relatively constant over the previous decade but decreased in 2020 and 2021 due to the COVID-19 pandemic [24]. Of this demand of almost 10 TWh, only 50 GWh is consumed in El Hierro, i.e., about 0.5%.