# Optimizing Sustainable Energy Transitions in Small Isolated Grids Using Multi-Criteria Approaches
## 5 Conclusions  
This research investigates key aspects of global economy decarbonization, focusing on isolated regions and islands. The transition to renewable energy sources like wind and solar poses challenging issues due to their intermittency, unpredictability and variability. Isolated systems face additional difficulties as they must ensure self-sufficiency, traditionally relying on fossil fuels for stability despite advancements in renewable technologies. This study highlights the necessity of substantial energy storage to support renewables and applies a multi-criteria optimization method to minimize the entire system costs and GHG emissions. EnergyPLAN, a widely used energy system analysis tool, is employed to simulate different energy scenarios, while EPLANopt enhances the process by optimizing system configurations. El Hierro, an isolated island with an existing renewable energy system, serves as a case study to evaluate performance under various conditions. The island currently operates a hybrid hydro-wind system that meets approximately 50% of its electricity demand. However, even with optimized configurations, a small share of diesel generation remains necessary to prevent excessive over-dimensioning of renewables and storage. This study underscores the role of dispatchable energy sources in maintaining grid stability. Using real-world data from El Hierro, this analysis validates the modeling tools while incorporating economic and environmental considerations. This study tests system resilience by modeling conditions from 2023, including the worst wind and solar resource year in the past decade. The proposed optimized system, developed with EPLANopt, achieves significant cost and emission reductions, lowering GHG emissions to approximately 6 kt and costs to 6.5 M EUR, a 75% and 30% reduction, respectively, compared to the current system. The main findings obtained from the studies conducted include the following general conclusions:* Diesel-based systems are cost-effective but environmentally harmful, and they remain essential in small-scale networks until renewable infrastructure is fully developed. * Fossil fuels may act as transitional backups in renewable systems due to the unpredictability of wind and solar energy. * Islands and remote regions face greater reliance on fossil fuels or large storage due to lack of grid connections. * The most used sustainable renewable backup options, biomass and geothermal, are often unavailable, leading to more fossil fuel dependence. * There is a need to promote other renewable backup energies, like solar thermal energy, micro-hydropower, tidal and wave energy, to avoid using fossil-fuel-based systems. * Relying on a single renewable backup source is difficult due to variability, geographic limits, and infrastructure constraints. Using multiple backup systems can fill gaps left by others. * Large-scale energy storage is fundamental but economically challenging due to high costs. * Oversizing renewable systems can reduce emissions but create inefficiencies, making storage vital for balancing supply and demand. * GHG penalties are necessary to phase out fossil-fuel-based systems, but significant taxes are required to make renewable solutions economically viable. * Policies should balance emissions reduction and storage investments to ensure an efficient and sustainable energy transition. Future research could build upon this study by conducting predictive investigations of demand progress under various scenarios, including Business-As-Usual (BAU) trends, energy efficiency improvements, and disruptive changes such as the adoption of Electric Vehicles (EV), full electrification of major energy consumers, and the integration of hydrogen as an energy vector. Additionally, a more detailed economic analysis of future technology costs and energy prices would enhance the accuracy of projections.