# Optimizing Sustainable Energy Transitions in Small Isolated Grids Using Multi-Criteria Approaches
## 3 Materials and Methods
### Definition of Scenarios
The relatively high costs are due to the exploitation of the resource being considered only on rooftops (minimizing visual pollution, given the island's prominent tourist nature), with a maximal usable surface on the island of 1.2 km\({}^{2}\), of which a maximum exploitable surface of 0.8 km\({}^{2}\) was considered, representing 83.5 MW of peak solar PV generation capacity [57]. These figures are well above the required installed power values, the current peak demand of which is below 10 MW. Therefore, the island's capacity is more than sufficient for solar photovoltaic installations for self-consumption. Importantly, as part of the transition to a fully sustainable energy system, several projects are already underway to advance the decarbonization of the island. The ultimate goal is to achieve a completely carbon-free economy within a few years. In the first phase, the plans include installing 5 MW of solar photovoltaic capacity along with 5 MW of battery storage, aiming to boost renewable electricity generation from the current level of about 50% up to nearly 80%. In a subsequent phase, an additional 7 MW of solar generation and a second 5 MW battery storage group are scheduled to be added, with the objective of reaching 100% decarbonization. However, the characteristics of the storage systems using mega-batteries were analyzed, reaching the conclusion that the landscape pollution, the associated emissions in the life cycle and the costs of this system are higher than those that would involve the expansion of the current reversible pumping system existing on the island. Therefore, only the use of the latter was considered. However, in terms of the performance of both systems and characteristics in terms of technical issues, both systems would be equally valid and yield similar results. Consequently, for additional storage needs, the aforementioned Canary Islands Government study shows that any future expansion of the island's storage capacity by pumping would require, as the best option, an increase in the size of the reservoirs used in the current system and an improvement in pumping and/or turbination powers [48]. Another possibility would be the installation of an additional reverse pumping station; anyway, to reach a completely renewable system, at least the storage capacity and probably the generation capacity must be augmented. A summary of the technology costs used is shown in Table 2, in which the investment costs are normalized versus installed capacity, while fixed O&M costs are given as an investment cost percentage. For diesel generation, a fuel cost of 150 EUR/MWhe was considered. Although the table provides the initial investment costs for diesel technology, these costs were not considered in the final calculations. This is because, given the years this system has been in operation, it is assumed that its investment has already been amortized. Conversely, for the other two cases, the amortization costs of the Gorona delViento system were included, as its operation has not yet reached 10 full years, meaning this period has not yet elapsed. This approach is considered appropriate to ensure the general applicability of this document. When extrapolating to other cases, it is common to encounter fossil fuel power plants that are intended to be either fully or partially replaced by renewable technologies. Alternatively, as is widely recognized, taking advantage of renewable technologies, such as the ones used here, i.e., wind, solar PV and reverse pumping, significantly reduces GHG emissions with respect to fossil-fuel-based technologies since renewable energy technologies produce far fewer emissions than fossil fuels because they do not rely on burning fuel, which is the main source of CO\({}_{2}\) and other pollutants. While manufacturing and installing renewables do generate some emissions, these are much lower than those from fossil fuel extraction and use.