*3.1. Optimum Sizes*

Figures 8–10 present the results of the Monte-Carlo simulation. Figure 8 shows that diesel-favored systems need less energy storage capacity, while solar PV-favored systems will need larger Li-ion BESS capacities. For these hybrid systems, larger Li-ion BESS sizes indicate higher RE shares. Bimodal distributions indicate the possibility of either RE-favored generation or diesel-favored generation. Figure 8 also presents that low energy and water demand on an island such as in Camasusu would result in diesel-favored generation systems, as the installation costs for diesel-favored generations are lower than that of RE. At low energy demands, the large installation cost required makes RE less economical than diesel-generation. In contrast, larger islands with more energy and water demands such as in Lubang prefer RE-favored systems. On these islands, the large installation cost of RE is offset by the decreased fuel consumption.

**Figure 8.** Distribution of optimum sizes of energy components on Camasusu Island (**top**) and Lubang Island (**bottom**). Systems in Camasusu have small Li-ion BESS installation sizes, except for RO, where the Li-ion BESS installation is large. Systems in Lubang have large Li-ion BESS installations.

**Figure 9.** Distribution of optimum sizes of water storage (**left** axis) and desalination unit (**right** axis) on Camasusu Island (**top**) and Lubang Island (**bottom**). There is high uncertainty in the optimum water storage sizes in Camasusu, except for RO.

Figure 9 shows how diesel-dominated systems compensate for the lack of Li-ion BESS capacity. As these systems do not have energy storage, they use the desalination plant as an energy sink to produce and store water that will be utilized daily. The same figure presents that if the desalination plant is used as an energy sink, the resulting storage sizes for water have large uncertainty or spread. The RO system in Camasusu is an exception to the trends observed earlier. RO has a high turndown capability that allows RE intermittence as seen in the spread between solar PV and Li-ion BESS installation in Figure 8. In cases where RE is heavily preferred over diesel-generators, RO systems are suggested for small islands.

**Figure 10.** Distribution of optimum LCOE (**left** axis) and levelized cost of water (LCOW) (**right** axis) on Camasusu Island (**top**) and Lubang Island (**bottom**). The metrics of the MED and MVC systems in Camasusu exhibit a bimodal distribution.

As for the optimum metrics shown in Figure 10, RO desalination provides the lowest water generation costs from its low energy intensity and fuel independence. Electricity generation costs are comparable for MED, MSF, and RO because the corresponding desalination unit has low electricity consumption, although MED may achieve a lower cost. The larger Lubang Island has lower generation costs than Camasusu Island, further demonstrating the effectiveness of RE systems on larger islands. Figure 10 also reflects the importance of desalination turndown on the stability of generation costs. RO systems have a more distinct generation cost distribution because shortage in generation due to intermittencies is allowed by larger turndown; thus, additional generation capacity and the associated costs are unnecessary. In other systems, however, turndown is limited and therefore, intermittency is met with additional generation capacity. The amount of additional generation is unpredictable, and this reflects as uncertainties in generation costs.
