## 4 Discussion  
Even though real measures for the case studies are not available to test the effectiveness of the method proposed, the first approach of its reliability can be made from the data published by other authors. [PERSON] and [PERSON] [30] published external costs at berth (EUR/h) related to 2016 for several Spanish ports. Despite the authors' advice about the nature of this indicator, it is  
Figure 3: Emissions per year of Ro-Pax vessels in port. R1: Vigo (Spain)–St. Nazaire (France) R2: Las Palmas (Gran Canaria island)–Huelva (Spain) R3: Las Palmas (Gran Canaria island)–Sta. Cruz de Tenerife (Tenerife island). Figure 2: Emissions per year of feeder vessels in port. R1: Vigo (Spain)-St. Nazaire (France) R2: Las Palmas (Gran Canaria island)–Huelva (Spain) R3: Las Palmas (Gran Canaria island–Sta. Cruz de Tenerife (Tenerife island). a homogeneous value (an aggregated value that includes whole fleet activity and all the operational stages in port), that collects the same pollutants as this paper, excepting the CH\({}_{4}\), and therefore this value can be taken as a reference point to test the reliability of the results obtained by the proposed method. For this aim, two assumptions are considered: firstly, most of the 2016 fleet operated with conventional on-board electricity supply in Spanish ports and secondly, since time invested in berthing is significantly higher than in maneuvering, the former has been predominant in the [PERSON] and [PERSON] (2020) indicators. [PERSON] and [PERSON] (2020) found that in 2016, the Las Palmas port had an external cost at berth of 215 EUR/h. Tables 6 and 7 show that for Las Palmas port the values obtained for the berthing stage per trip (3.42 h for feeders and 2.3 h for Ro-Pax) are 178.92 EUR/h for feeder vessels (611.92 EUR/3.42 h see Table 6) and 257.61 EUR/h for Ro-Pax vessels (592.52 EUR/2.3 h see Table 7). Therefore, despite the simplifications assumed, the values obtained are close to those previously published by other authors, which suggest an acceptable level of reliability in the calculation model. On the other hand, the results achieved in this paper confirm the need for increasing the energy environmental sources in electricity insular generation to enhance the desired effects from the application of Directive 2014/94/EU. To this aim, self-generated local renewable energy in port ([PERSON] et al., 2018) has proved to be one effective solution (in Gothenburg, through local surplus wind generated power, among others). Nevertheless, due to the absence of a continental grid connection and the uncertainty about on-shore renewable resource availability, a challenge arises for the islands. Off-shore wind energy can reduce that uncertainty since the further away from the coast, the higher and more constant the wind energy source is. In fact, floating off-shore wind energy is currently able to overcome the inherent difficulties of traditional \"bottom-fixed\" foundations for larger depths (such a case of the Canary Islands) by avoiding interferences with port activity ([PERSON] et al., 2019). According to the FLOATNT project ([PERSON] et al., 2019), two 600 MW floating off-shore wind farms (a turbine of 12 MW is supported by each floating substructure) could provide 2.8 TWh per year ([PERSON] et al., 2019). This is Gran Canaria island's required renewable power to reach almost the same share of air pollutant sources in its energy mix as in France (see Table 4). In other words, in such a scenario CI performance in the Las Palmas de Gran Canaria port would be equivalent to that of St. Nazaire. As has been shown in this paper, in the meantime, LNG use could be a more effective option in insular ports. Despite the fact that its mitigation potential has proved to be lower than CI (methane effect), LNG provides an additional benefit by also acting during the maneuvering stage. However, LNG feasibility is again conditioned by the availability of LNG refueling points in ports (Directive 2014/94/EU) and dual engines installed in vessels.