FLOATING OFFSHORE WIND FARM IN THE MEDITERRANEAN SEA: A LIFE CYCLE ASSESSMENT

Abstract

The employment of renewable sources for power generation is fundamental to meet the increasing energy demand and at the same time tackle anthropogenic climate change. The use of wind power is encouraged considering that relies on a free and abundant energy source. In the last years, the market is moving toward offshore installations, firstly due to the lack of space for land-based wind turbines but also to limit the visual impact of wind farms and increase their social acceptability. Recently, floating offshore solutions are of growing interest since they give access to additional wind resources, further from shore and in deeper waters, where more favourable wind conditions for power generation can be found i.e. higher wind velocity with lower turbulence and lower variability.

The present work aims to assess the environmental performance of a large floating offshore wind farm, consisting of multi-megawatt wind turbines and intended to be deployed off the west coast of Sicily (Italy). The selected case study is representative of a floating wind farm whose project has been submitted to the Environmental Impact Assessment procedure and currently is still undergoing the permitting process. Nevertheless, this project has been chosen as case study since it would become the largest floating wind farm in Europe, consisting of 190 aerogenerators with 14.7 MW rated power, for a total of 2.8 GW.

The environmental performance is evaluated by means of the Life Cycle Assessment (LCA) methodology, which is aimed at assessing the potential environmental impacts of the object of the study with a holistic perspective.

The selected functional unit is the delivery of 1 GWh of electricity to the onshore grid, including in the system boundaries also the electrical system for the transmission of the generated electricity to the onshore national grid. The adopted perspective for the LCA is cradle-to-grave i.e. the life cycle of the wind plant includes acquisition and processing of raw materials, transport of components, assembly and installation by means of specialised vessels, maintenance during the operational phase, disassembly and end-of-life. The chosen impact assessment methods are the EPD (2018) which involves eight impact categories, and the Cumulative Energy Demand (CED), related to direct and indirect energy consumption. Based on the estimated impacts, carbon and energy payback time (CPBT and EPBT) have been evaluated since they are found to be significant indicators for the effective communication of the results related to the sustainability of renewable energy plants.

In the modelled system the NREL (National Renewable Energy Laboratory) reference wind turbine and semi-submersible platform, moored by catenary lines, are assumed as representative of the employed aerogenerators, since their design data are publicly available. For the other main components and life cycle stages of the wind farm, several modelling assumptions were necessary to include them in the system boundaries.

One major conclusion of the study is that the supply of raw materials is the hotspot in all the analysed impact categories, often covering altogether more than half of the total potential impacts. Focussing on the contributions of the single components, the wind turbine structure is the most significant one, except for the category Photochemical ozone creation potential where the floating structure is the most important contributor and for Abiotic depletion of elements where power cables are the hotspot. Among the employed materials, steel is the most impacting, also due to the large amounts that are needed. Due to the relevance of wind power to decarbonisation, the estimated potential impact on global warming, also called carbon intensity, is highlighted and is equal to 31.3 t CO_2eq/GWh. The performance of the wind farm, expressed by CPBT and EPBT, results in 2 years and 3 years respectively: in view of an expected 30-year lifetime, it can be concluded that the development of big offshore wind farms is convenient form the environmental point of view.

From the comparison of the estimated environmental performance of the case study to other LCA studies retrieved in the literature, it can be observed that onshore and offshore bottom-fixed installations are generally less impacting, since for floating offshore deployments there are additional materials and infrastructure requirements. Among the different floating solutions, the best environmental performances are achieved by structures that allow to limit the total weight, while the overall impacts of the semi-submersible platform, modelled for the present study, are significantly affected by the heavier structure.

Considering the estimated carbon intensity, the comparison with other energy sources highlights that the emission factor of wind power is much lower than the ones of fossil fuels and comparable with other low carbon and renewable technologies, despite the complex infrastructure put in place.