**4. Discussion**

Upon analysis of the optimum size and metric distributions, RO systems have the lowest water generation cost. The low energy use and high turndown of RO allows for possible coupling with RE-favored generation. The average LCOW of RO systems is 5.11 USD/m3 and 4.70 USD/m3 for Camasusu and Lubang, respectively. Compared to the mainland price of 0.70 USD/m3, however, water generation costs are still higher by 630% and 572% for Camasusu and Lubang, respectively. As for electricity generation costs, MED systems have the lowest average LCOE of 0.348 USD/kWh in Camasusu, while RO systems have the lowest average LCOE of 0.296 USD/kWh in Lubang. These are 74% and 47% more expensive for Camasusu and Lubang, respectively, compared to the mainland price of 0.2 USD/kWh [62]. Overall, RO systems provide cost-effective water and electricity, especially for larger islands, where RE-favored generation is preferred.

For project developers, MED and MSF could be more attractive because these systems are less RE-dependent than RO due to the limited turndown. This allows easier entry into off-grid islands currently powered by diesel generators. There is also less uncertainty in optimum sizes, especially for MSF, implying that there is less risk of installing an undersized system. Seasonal weather patterns also do not strongly affect the stored water level. Lastly, these technologies have smaller water storage sizes that are more appropriate for off-grid islands. However, due to the large typical sizes of MED and MSF plants, these technologies are favorable for larger islands only [44].

Energy-desalination systems on small islands prefer diesel-favored generation, while larger islands prefer RE-favored generation, as shown by the power flows. Cost projection analysis, however, suggests that RE-favored generation will be more favorable in the future even for small islands. RO desalination is therefore a favorable investment as its high turndown allows it to accept RE intermittency, in addition to its low generation costs. Given that most of the off-grid islands are currently powered by diesel generators [1] and that PV-based installations are gradually being deployed [63], energy-RO systems for energy and water cogeneration may be feasible in the near future.

The sensitivity analysis shows that energy-desalination system configurations heavily depend on diesel and coal prices. If fuel prices are low, the fuel share in thermal desalination systems increases and uncertainty in water storage installation size increases. Energy and water generation costs are reduced especially for MED, MSF, and MVC, as they rely on fuel. If fuel prices are high, RE-favored generation becomes favorable. There is less uncertainty in the optimum sizes of both energy and water components because Li-ion BESS energy storage minimizes intermittencies. However, large Li-ion BESS installations for RE-dominated generation result in high costs of energy and water.

In addition to the fuel dependence of the optimum configuration, the sensitivity analysis also has implications on system resiliency on fuel price shocks. If fuel prices were to surge upwards, the generation costs of RO systems will be affected the least due to its low dependence on fuel. MVC systems will have the largest increase in generation costs due to its inefficient fuel usage. Small islands are more susceptible to fuel price surges, as these tend to have diesel-favored energy systems. In contrast, large islands are more likely to have RE-favored generation, which is optimal at higher fuel prices. This incentivizes the implementation of RE-RO systems, as they are favorable for small islands and are resistant to fuel price shocks.
