1. Introduction
The development of marine energies has become essential for the green energy sector, in order to mitigate the human influence on climate change, and move towards 100% clean energy, as stated in the 2030 Agenda for Sustainable Development released by the UN in 2015 [
1]. Pursuing this goal, both the European Parliament and the European Council have endorsed 60% emission reductions by 2030 [
2], and long-term climate neutrality for the EU by 2050.
Europe’s decarbonization plan to achieve the net-zero greenhouse gas emission target relies heavily on an offshore renewable energy strategy [
3]. The strategic roadmap is to deliver up to 100 GW of capacity by 2050, which represents approximately 10% of Europe’s electricity consumption [
3,
4]. The European Commission, through its member states, has made great efforts to achieve this goal by promoting projects that address ocean energy efficiency [
3,
4,
5,
6,
7], and the competitiveness of offshore wind [
8,
9,
10] and ocean energies [
11,
12]. In fact, the European Union is currently the world leader in offshore wind [
13] and intends to advance in the implementation of the actions of its Strategic Energy Technology (SET) plan [
14].
The current development of wind technologies has reduced the cost, maintenance, and equipment of wind turbines, and, at the same time, has increased wind turbine efficiency and availability [
8,
10,
15]. This makes wind power cheaper than photovoltaic power, while costing about half of atomic power [
16]. Therefore, an explosion in the offshore wind industry is expected, which may positively influence the development of other technologies with high synergy [
15,
17], such as wave energy conversion. In fact, one of the strategies of the EU SET plan is to increase competitiveness by exploiting wave energy at the same time as wind power.
Hybrid wind-wave farms can reduce installation and maintenance costs [
17,
18] and increase the availability of a location by compensating for the winds intermittent nature with good wave conditions. Furthermore, the wave farm can provide protection to the wind farm, and the combined wind-wave farm can provide coastal protection [
19,
20,
21]. Several authors have assessed the combined exploitation of wind and wave resources [
22] in Europe [
17,
23,
24,
25], and worldwide [
26,
27]. These studies determined the most suitable locations for wind exploitation, and the specific wave energy conversion (WEC) devices for those locations. Current wind and wave energy technology is intended to have a useful life of at least 20 years [
28,
29], therefore, when using this combined energy resource, it is important to take into account not only current climatic conditions but also the effect of climate change on wind and wave energy resources in the near future [
30].
Climate change is expected to affect future wind and wave yields [
31,
32,
33]. The global climate models (GCMs) from Phase 5 of the Coupled Model Intercomparison Project (CMIP5) have been tested and verified as reliable sources for future wind and wave variables [
34,
35]. The EURO-CORDEX initiative, the European branch of the Coordinated Regional Climate Downscaling Experiment (CORDEX) project [
36], used a wide range of regional climate models (RCMs) driven by several CMIP5 GCMs, under different greenhouse gas emission scenarios, to project wind and the significant height and period of waves, with a high spatial and temporal resolution. However, to assess the feasibility of operating a hybrid wind-wave farm in a specific location, it is necessary to take into account not only wave and wind resources for the near future but also environmental conditions and cost factors [
37,
38].
The NW Atlantic coast of the Iberian Peninsula, from 36.5° to 45.5° N and from 6° to 11° W (
Figure 1), covers the coast of Portugal and Galicia (Spain). The wind-wave climate is characterized by a high seasonal, and energetic, behavior [
39,
40], the North Atlantic Oscillation (NAO) being the greatest source of inter-annual variability in the wave climate in this region [
41]. However, the wave field is still dominated by the swell component (over 78%) during most of the year [
42], because of being located in a mid-latitude of a western region [
43]. These features makes this region one of the maximum oceanic energy resources in Europe [
44], and one of the highest marine energy potentials in the world [
45]. In particular, the west coast of the Iberian Peninsula is one of the most energetic coasts, in which different projects for the exploitation of wind and wave resources have been built or approved [
46,
47]. In fact, the first project to install a commercial-scale wave energy extraction in the world was in the Pilot Zone off the coast of Portugal [
47].
The aim of this study was to assess the present and near future wind and wave energy resources along the Atlantic coast of the Iberian Peninsula. A Delphi classification method [
48] that takes into account wind and wave resources, stability, risk, and installation and maintenance costs was applied for the analysis. The originality of the present study relies not only on the evaluation of the combined resources using a Delphi, as done in previous studies, but also in the consideration of other factors besides the wind/wave power resources, to assess the feasibility of a hybrid wind-wave energy farm at a particular location for future exploitation conditions. This methodology contributes to the knowledge and applicability for global-to-local scale studies, and predicts future wind-wave energy resource for researchers and ocean energy entrepreneurs.
The paper is organized as follows: in
Section 2, the data is presented, followed by the methodology applied to assess energy resources. Results and discussion can be found in
Section 3. Finally, conclusions are presented in the
Section 4.
3. Results and Discussion
The assessment of the potential energy resource for a hybrid wind-wave farm was carried out along the northwest coast of the Iberian Peninsula for the near future (2026–2045). First, spatial maps of all the wind and wave indices that make up the categories of resource richness, variability, risk, and cost were calculated.
The method used in this study is similar to the one developed by [
62], where the authors proposed a multi-criteria approach for wave energy converters. They used similar coefficients (resource, variability, downtime, risk, and cost) although their study was more focused on particular devices for three specific locations. In the case of this research, which is based on previous studies carried out for wind energy converters [
37,
49,
50,
61], the analysis is focused on a large area covering the Atlantic part of the Iberian Peninsula under near future conditions. Previous works also focused on particular locations [
25,
65], although as far as we know, none on them considered future wind and wave conditions.
The richness category accounts for the available resource in terms of W
ann, RLO, and DWNT for the wind resource (
Figure 2), and WP and DWNT for the wave resource (
Figure 3). W
ann index (
Figure 2a) shows the maximum value of 1 in the ocean, and decreases to less than half near land. This is consistent with the WPD values shown for this region in [
67]. The RLO index pattern is similar to the one of W
ann, with the lowest values (around 0.1) being detected along the coast, and the maximum values, of around 0.7, in the northwest region. The DWNT index shows a smaller difference (0.2) between the ocean and the coastal regions.
Wave richness resource shows some similarities to that of the wind, mostly due to the waves being generated as a result of the wind forcing on the water surface. WP index (
Figure 3a) shows its maximum (0.7) on the northwest region, as the RLO index. The lowest WP values are found near the coast, in particular in the Gulf of Cadiz (<0.2). Regarding wave DWNT index (
Figure 3b), it should be noted that it reaches the maximum value of 1 in most of the study area, except in the Gulf of Cadiz.
The indices that account for the wind power resource variability,
Cv and
Mv, are represented in
Figure 4a,b, respectively. In general, the highest
Cv values of 0.5 are in the ocean region south of Cape Finisterre, which is not optimal for energy production. However, the
Mv index is very good (>0.8) for the same region. These
Mv and
Cv indices suggest the presence of short events linked to seasonal fronts.
The behavior of the wave power resource variability shows a high resemblance with the one shown for the wind. A
Cv index (
Figure 5a) of around 0.4 is obtained for the northern part of the area, with higher values (~0.5) in the southern part.
Mv (
Figure 5b) shows values higher than 0.6 in the southwest region of the study area and in a small fringe along the north coast of Spain. For the rest of the area under study,
Mv shows values of 0.5. Once again, the lowest values of
Cv and
Mv indices are located in the Gulf of Cadiz.
Figure 6 represents the risk category that considers the EWS and EWH indices, which affect the safety related to the maneuverability of energy devices. The wind risk index (
Figure 6a) shows high values (>0.8) along a narrow fringe in the coastal region, moderate values in the southwest region (0.6), and low values (<0.3) in the northwest region. The wave risk index (
Figure 6b) shows close to 1 in most of the area, apart from the northwest region, where values of 0.5 are shown. It is noticeable that the lowest risk values (the worst risk conditions) in both wind and wave energy resources occur in the same region, the northwest section of the study area. This is precisely the region with the highest values of W
ann, RLO, and WP indices.
The cost category, represented by DC (
Figure 7a) and WD (
Figure 7b) is the same for both wind and wave resources, since they only depend on topographic constraints.
The classification indices calculated following Equation (5) for wind and Equation (6) for waves are shown in
Figure 8. The Wind index (C
wind, panel a) shows elevated values (>0.6 in most of the area). Only a very narrow fringe along the cost shows values lower than 0.5. High values can be observed in a wider fringe (~50 km wide) along the coast, the highest ones being (around 0.7) observed from Cape São Vicente to Cape Roca. The wave classification index (C
wave, panel b) shows the same pattern as the one observed for the wind index but with lower values (between 0.4 and 0.7) in the whole region. In this case, the coastal fringe with the highest values covers from Cape Roca to approximately Cape Peña.
The patterns observed in
Figure 8 are similar to the ones depicted in
Figure 9 and
Figure 10 obtained through the sensitivity analysis described in
Section 2.3.5. In particular,
Figure 9a and
Figure 10a calculated with Equation (15) are similar to
Figure 8a,b, respectively. The RMSE values (
Figure 9b and
Figure 10b) obtained from Equation (13) are at least one order of magnitude smaller than the average classification indices. This fact is corroborated by the distribution of the RMSE
j calculated from Equation (14) (
Figure 9c and
Figure 10c).
The combined index (CI) to classify the hybrid wind-wave energy resource (
Figure 11) shows strong similarities to the patterns observed in
Figure 8. In general, the CI is lower than C
wind (
Figure 8a) and higher than C
wave (
Figure 8b). The narrow coastal fringe with low values, and the wider coastal fringe with high values (from Cape Peñas to Cape São Vicente) can also be observed in this case. Once again, the lowest values, around 0.5, are observed in the Gulf of Cadiz.
4. Conclusions
The viability of exploiting the energy to be produced in hybrid wind-wave farms in the NW Iberian Peninsula for the near future (2026–2045) was investigated using a Delphi method to classify wind and wave energy resources. This classification was based on different indices that represent the resource richness, variability, risk, and cost of the energy resources.
The richness category shows the same patterns for both energy resources, with the highest values in the NW corner of the study area. The variability of the resource also shows a similar pattern for both wind and waves, with the highest values located south of Cape Finisterre. Both resource and variability factors are higher (better) for wind than for wave resource, the downtime factor being better for waves.
The best risk conditions occur along the coastal region, both for wind and waves. In the oceanic part, the conditions improve from north to south. In general, the risk index is worst for wind than for waves.
Finally, a macroscopic evaluation of the hybrid wind-wave resource shows that most of the NW Iberian Peninsula presents good conditions (CI > 0.6) for harvesting energy from wind and wave resources simultaneously. In particular, there are some optimal areas (CI > 0.7), such as the region near Cape Roca and the Galician coast, which had already been highlighted in previous studies on wind [
46] and wave [
30] resources.
The assessment carried out in this study must be considered as a decision support toolkit in the identification of the best locations for hybrid wind-wave farms, or in the management, adaptation, and resilience of projected and ongoing plans. This assessment only considers the resource from a physical point of view and can be complemented with the technical features of different devices, legal, and environmental constraints, the costs linked to the support of the electric grid connection, and other indices that consider the relation between the maximum production values of both wind and wave resources and the grid energy demand.