Development of a multi-criteria decision-making tool for combined offshore wind and wave energy site selection

The combined exploitation of offshore wind and wave energy offers several advantages, including increased and more stable power output, as well as shared infrastructure and maintenance costs, enhancing both economic feasibility and operational efficiency. However, optimal site selection for integrated wind-wave farms is a complex task, involving various factors, such as resource availability and complementarity, technology survivability in extreme conditions, logistics for operation and maintenance tasks (temporal weather windows, proximity to grid connections and ports), and seabed characteristics (depth, slope, and geology). On these grounds, this paper introduces the Composite Suitability Index (CSI), a novel Multi-criteria Decision-making (MCDM) framework specifically designed for the optimal site selection of integrated wind-wave energy farms. The CSI incorporates novel decision factors, including wind and wave resource variability and complementarity as well as temporal weather windows for operation and maintenance tasks. The coast of mainland Portugal is used as a case study to demonstrate the applicability and validity of the tool. The results highlight that the CSI effectively identifies optimal sites for offshore wind - wave energy integration. These findings are validated by their alignment with the concession zones designated by the Portuguese government for the exploitation of marine renewable energy. Furthermore, the hot spot analysis incorporated into the methodology provides detailed spatial assessments, identifying high, moderate, and low potential areas while excluding those with environmental or logistical constraints. In sum, this study provides a robust site-selection tool for integrated offshore wind-wave farms, with broad applicability to coastal regions worldwide.