Scope-dependent modelling of electricity supply in life cycle assessments

Electricity use or substitution is one of the key parameters with regard to life cycle assessment (LCA) results. At the same time, it is often used as an illustrative example to highlight the modelling differences between decision-oriented and descriptive LCA. Three basically different models exist in life cycle inventory analysis: the attributional, the consequential and the decisional model. This paper proposes criteria that help to classify typical LCA questions regarding real business cases and find the most appropriate life cycle inventory (LCI) model. The framework is applied to a case study of an LCA of electricity use and supply within the international operations of an environmental service company with headquarters in France.

Individual decision with comparatively small consequences can be modelled under ceteris paribus (other things being equal) conditions. Decision situations with medium to large potential consequences should be modelled under the conditions of mutatis mutandis (the necessary changes being made). The key question is how to distinguish between small, medium and large consequences. We recommend using the relative economic size to classify objects of investigation and the LCA goals related to them into three groups to which the most appropriate LCI models are assigned.

The attributional approach is sensible for environmental reporting and product labelling and declaration where the relative economic size of the object of investigation is small. The decisional approach is sensible for LCAs of product and process development, as well as site and supplier evaluation carried out by private companies in case the relative economic size of the object of investigation is medium. The consequential approach is of relevance for policy support of governments and international organisations as well as for strategic decisions of companies, where the relative economic size of the object of investigation is large. The consequential approach is also sensible in product or service comparisons by companies, if they offer products or services that are in line or help to comply with large-scale government policy measures (like for instance promoting renewable fuels). The French attributional and decisional electricity supply mix causes greenhouse gas emissions of 98 and 225 g CO₂-eq./kWh, respectively, whereas the European attributional and decisional electricity supply mix causes greenhouse gas emissions of 554 and 473 g CO₂-eq./kWh, respectively. The volumes of high radioactive waste generated with the French and EU-27 electricity mixes amount to 11 and 3.5 mm³/kWh for the attributional mixes as well as 3.8 and 0.034 mm³/kWh for the decisional mixes.

The criterion “relative economic size” helps to better decide on the appropriate LCI model to be applied in specific LCA case studies supporting any kind of decisions. Being quantitative, the “relative economic size” criterion is comparable to the criteria used to delimitate the product system (cut-off criteria mass, energy and environmental impact). The delimitation values proposed are still preliminary and still show a certain degree of ambiguity. Nevertheless, it proves to be both a practical and potentially relevant criterion. The case study of the French and European electricity mixes shows that a distinction of different decision contexts is required and feasible. Using official statistical information and published forecasts issued by the relevant industrial associations or governmental bodies significantly reduces the potential bias related to the determination of possible change-oriented electricity mixes.

The relative economic size of the object of investigation is a quantified criterion to decide on the most appropriate modelling approach in life cycle inventory. It is recommended to apply the criterion in the goal and scope phase of any LCA and to apply it on the production volume on the one hand and on purchase volumes on the other. Production and purchase volumes can be expressed in either economic or physical quantities and be related to the totals of economic sectors or political entities such as nations or international organisations. The current paper deals with the appropriate modelling approach illustrated with the electricity mix. The electricity sector is only one of many sectors where the choice of the modelling approach may reveal important differences in the overall environmental impacts. Thus, it is worthwhile to extend the concept presented in this paper to other economic sectors such as agriculture, mining or paper and pulp. This would help to better substantiate or to adjust the delimitation values and to gain more experience with the threshold criteria proposed.