Traditional perceptions that “emissions from the manufacturing stage dominate overall lifecycle GHG emissions” and that GHG emissions from the operational phase are “almost negligible in relation to the total” or “negligible” may be due for reconsideration for offshore wind power; and furthermore, such conclusions may not always be extendable to non-GHG pollutants (e.g., NO_x, SO₂ and particulates). Future LCA research on offshore wind power should strive to develop detailed inventories for installation and maintenance life cycle phases while ensuring transparency in the reporting of materials.

—Arvesen et al.

While offshore wind power technology is based on onshore wind technology, there are important differences, they note. For a given capacity installed, resource requirements are generally larger for offshore wind power systems than for land-based systems due to more complicated installation and maintenance activities; and that heavier and longer cables may be required for electricity transmission. On the other hand, ocean-based systems typically benefit from better wind conditions.

The NTNU team raised three areas of concern with existing LCAs for offshore wind:

• Assessments of offshore activities associated with constructing and operating wind farms in LCAs are rather tentative, and appear to lack detailed representations of different vessels involved; appear to be based on simplified theoretical calculation that are yet to be verified; and/or do not reveal key assumptions.

  If offshore installation and maintenance costs are to some considerable degree attributable to the burning of fuel oil in vessels, associated pollution is important. In the general case, global emissions of greenhouse gases, nitrogen and sulfur compounds, and particulates from shipping activities are significant and growing.

• Maintenance reports of operating wind farms have traditionally not been made public. As a result, LCA analysts have had little basis for making substantiated assumptions regarding the rate at which wind turbine parts need to be replaced. The NTNU team says there is a need to verify assumptions about replacement rates in LCAs against operational experiences, and to begin to explore how to best extrapolate current knowledge to the future operation of modern, large-scale wind turbines.

• Published LCAs of wind power predominantly employ process-LCA methodologies known to suffer from systematic underestimation of impacts, the team pointed out.

The NTNU team used a hybrid LCA methodology, in which monetary inventories are used to model the effects of operations that are omitted in process-LCA. The hybrid LCA model for their study combined physical inventories from the Ecoinvent database and monetary inventories from environmentally extended input-output (IO) tables for the year 2000 developed for the EXIOPOL project.

In their study, the NTNU team preformed impact assessments over 9 scenarios for: climate change (g CO₂-Eq/kWh); freshwater ecotoxicity (g 1,4-DCB-Eq/MWh); freshwater eutrophication (g P-Eq/MWh); human toxicity (g 1,4-DCB-Eq/kWh); marin eutrophication (g N-Eq/MWh); metal depletion (g FE-Eq/kWh); particulate matter (g PM10-Eq/MWh); photochemical oxidant formation (g NMVOC/MWh); terrestrial acidification (g SO₂-Eq/MWh); terrestrial exotoxicity (g 1,4-DCB-Eq/MWh); and water depletion (m³/GWh). Among the findings of their study were:

• Installation and maintenance phases in many cases give significant contributions to overall indicator values: respectively 15% and 14% for climate change; 17% and 14% for marine eutrophication; 19% and 12% for particulate matter; 25% and 21% for photochemical oxidant formation; and 22% and 15% for acidification (replacement parts not included).

  Half of GHG emissions from installation arise due to transport and dumping of rock for stone bed and scour protection; roughly 20% is contributed by jack-up vessel, crane vessel, and tugboat operations taken together.

  Installation of cables and onshore truck transport causes 6−7% each of GHG emissions due to installation. The main culprit behind emissions due to maintenance is the “Support vessel, maintenance of wind turbines” process, which is responsible for 85% of total GHG emissions.

• The contribution of the supply of spare parts to total indicator values is typically of the order 5−10%, and 13% at the most (freshwater ecotoxicity). Large (small) parts cause 99% (1%) of totals due to spare parts. Mining operations to acquire iron, nickel, copper, and manganese together constitute 98% of metal depletion burden. As for toxic releases from waste handling, disposal of tailings in relation with mineral resource extraction and disposal of smelter slag are dominant pollution sources. Eutrophying emissions to freshwater stem in large part from disposal of tailings and spoil in connection with mineral resource extraction.

• The monetary subsystem generates 52% (climate change), 44% (acidification), 38% (photochemical oxidants), 26% (particulate matter), 19% (marine eutrophication), and 16% (terrestrial ecotoxicity) of total emissions, of which 70−80% is due to activities in the Europe region.

• Under optimistic assumptions about implementation of NO_x abatement, total emissions of particulates, acidifying and eutrophying substances and smog-forming gases are reduced markedly, in the latter case by 24%. Under the given assumptions, implementation of sulfur abatement does not produce corresponding benefits as NO_x abatement, though there is a 5% reduction in acidification impact potential.

• Including additional dismantling activities increases total indicator values by 3% or less.

• Lowering the assumed lifespan by five years increases total impact potentials by 18−22% .

Resources

• Anders Arvesen, Christine, Birkeland, and Edgar G. Hertwich (2013) The Importance of Ships and Spare Parts in LCAs of Offshore Wind Power. Environmental Science & Technology doi: 10.1021/es304509r