Influence of functional unit on the life cycle assessment of traction batteries

Abstract

This paper describes the influence of the choice of the functional unit on the results of an environmental assessment of different battery technologies for electric and hybrid vehicles. Battery, hybrid and fuel cell electric vehicles are considered as being environmentally friendly. However, the batteries they use are sometimes said to be environmentally unfriendly. At the current state of technology different battery types can be envisaged: lead-acid, nickel-cadmium, nickel-metal hydride, lithium-ion and sodium-nickel chloride. The environmental impacts described in this paper are based on a life cycle assessment (LCA) approach. One of the first critical stages of LCA is the definition of an appropriate and specific functional unit for electric and hybrid vehicle application. Most of the known LCA studies concerning batteries were performed while choosing different functional units, although this choice can influence the final results. An adequate functional unit, allowing to compare battery technologies in their real life vehicle application should be chosen. The results of the LCA are important as they will be used as a decision support for the end-of-life vehicles directive 2000/53/EC (Official Journal of the European Communities L269/24 2000). As a consequence, a thorough analysis is required to define an appropriate functional unit for the assessment of batteries for electric vehicles. This paper discusses this issue and will mainly focus on traction batteries for electric vehicles. An overview of the different parameters to be considered in the definition of a functional unit to compare battery technologies for battery electric vehicle application is described and discussed. An LCA study is performed for the most relevant potential functional units. SimaPro 6 is used as a software tool and Eco-indicator 99 as an impact assessment method. The influence of the different selected functional units on the results (Eco-indicator Points) is discussed. The environmental impact of the different electric vehicle battery technologies is described. A sensitivity analysis illustrates the robustness of the obtained results. Five main parameters are considered in each investigated functional unit: an equal depth of discharge is assumed, a relative number of batteries required during the life of the vehicle is calculated, the energy losses in the battery and the additional vehicle consumption due to the battery mass is included and the same lifetime distance target is taken into account. On the basis of the energy content, battery mass, number of cycles and vehicle autonomy three suitable functional units are defined: ‘battery packs with an identical mass’, ‘battery packs with an identical energy content’ and ‘battery packs with an identical one-charge range’. The results show that the differences in the results between these three functional units are small and imply less variation on the results than the other uncertainties inherent to LCA studies. On the other hand, the results obtained using other, less adequate, functional units can be quite different. When performing an LCA study, it’s important to choose an appropriate functional unit. Most of the time, this choice is unambiguous. However, sometimes this choice is more complicated when different correlated parameters have to be considered, as it is the case for traction batteries. When using a realistic functional unit, the result is not influenced significantly by the choice of one out of the three suitable functional units. Additionally, the life cycle assessment allowed concluding that three electric vehicle battery technologies have a comparable environmental impact: lead-acid, nickel-cadmium and nickel-metal hydride. Lithium-ion and sodium-nickel chloride have lower environmental impacts than the three previously cited technologies when used in a typical battery electric vehicle application. The article describes the need to consider all relevant parameters for the choice of a functional unit for an electric vehicle battery, as this choice can influence the conclusions. A more standardised method to define the functional unit could avoid these differences and could make it possible to compare the results of different traction battery LCA studies more easily.