LCA Tab. 4: Quantitative indicators Lifetime Wind plant lifetime The lifetime of the wind plant was assumed to be 20 years and was considered the baseline scenario. Based on its professional experience, Valorem indicated that this figure might reach up to 40 years. Assuming that all the other variables remain fixed, increasing the lifetime of the wind plant will obviously result in lower emissions per kWh as the impacts associated with wind turbine manufacturing are amortised over a longer period of time. However, the required maintenance and The first results (for a wind plant lifetime period of 20 years) are compared to a 40year wind plant in Table 3: This assessment shows that the results decreased by 9-26% for all indicators. For five of the nine indicators studied, the overall results decreased by up to 20%. Tab. 3: Influence of lifetime on environmental impacts 20 years 40 years MJ 1.849E-01 1.458E-01 21 Abiotic depletion kg Sb eq 8.502E-05 6.684E-05 21 Acidification kg SO2 eq 5.354E-05 4.489E-05 16 Eutrophication kg PO4 eq 4.014E-05 3.657E-05 9 Global warming potential kg CO2 eq 1.177E-02 8.874E-03 25 Photochemical oxidation kg C2H2 eq 3.985E-06 3.213E-06 19 Agricultural land occupation 2 ma 1.935E-04 1.496E-04 21 Urban land occupation m2a 1.447E-04 1.185E-04 18 Natural land transformation m2 1.647E-06 1.211E-06 26 Unit Cumulative energy demand Energy intensity kWh used /kWh produced 0.051 Grams of CO2 /kWh produced 11.77 kg C2H2 eq 0.81 Energy intensity m2a 0.040 m2a 8.87 Quantitative indicators When assessing the environmental performance of wind plants, an interesting aspect to consider is the point in time after which the environmental burden of the wind plant manufacture is outweighed by the environmental benefits of the renewable energy generated. An energy balance was calculated to show the relationship between the energy requirement for the whole life cycle of the wind plant and its power output. The energy indicator calculated as explained before is called the energy payback time. Another indicator widely used in practice to compare the environmental performance of wind plants is CO2 intensity. This indicator is calculated as the equivalent amount of CO2 emitted per kWh of electricity produced by the wind turbine throughout its life cycle. Energy intensity, defined as the ratio of the amount of energy consumed and produced throughout the life cycle of the wind turbine, was also calculated. The results for these indicators are shown in Table 4. Conclusion The main outcome of this study is an accurate and non-biased environmental No90 June - July 2014 / Feature Wind energy Lifetime Change % Impact category 1.03 CO2 intensity part replacements will be correlated with the lifetime of the wind plant (a longer lifetime involves more maintenance). Indeed, it was considered that the lifetime period of all the parts was twice as long, except the moving parts that still have a 20-year lifetime period. Years Energy pay back time Fig. 5: Only a few life cycle assessment studies are available for wind plants Value CO2 intensity 40 years Unit Energy payback time 20 years Indicator jec composites magazine 39