Assessing the environmental sustainability of alternative design concepts for transmission systems of offshore renewable energy

Limited attention was dedicated to date to the life cycle assessment (LCA) of renewable electricity generation offshore, although the selection and design of the transmission infrastructure can play a relevant role in the overall sustainability of offshore energy projects. In the present study, the LCA of alternative design concepts of offshore transmission systems, based on an innovative modelling of the related life cycle inventory, is proposed. It addresses the comparison of High Voltage Alternating Current (HVAC) and High Voltage Direct Current (HVDC) systems to determine their environmental impacts in delivering electricity from offshore wind farms for a wide combination of power ratings and distances to the shore. The LCA model includes manufacturing, installation, operation and maintenance of the subsystems of the transmission line, as well as physical modelling of the associated electricity losses. Cables are found to be the main contributors to environmental impact, followed by the offshore substation and by losses. As HVDC systems generate less losses but require more complex substations, for each power rating a break-even distance is present over which HVDC systems become more environmentally favourable. This is in the order of 100 km for large transmission systems (500 MW and beyond) and increases up to 150 km for lower power capacities (200 MW). A sensitivity analysis tested the influence of several assumptions, identifying capacity factor and infrastructure lifetime as critical parameters. The study provides insights for the selection and design of transmission systems to minimize environmental impacts, supporting decision-making in offshore renewable energy projects.