Life-Cycle Assessment

A life-cycle assessment (LCA) measures the resource use, emissions and potential environmental impacts associated with a fuel throughout its entire value chain, rather than only during combustion. It compiles inputs and outputs across defined stages and expresses impacts relative to a functional unit, such as one megajoule of energy or one vehicle-kilometre.

Stages covered

For fuels, the system boundary is commonly described as well-to-wheel, comprising upstream production and downstream use. The precise boundary must be stated because excluding a stage can change the result.

• Well-to-tank covers feedstock extraction or cultivation, processing or refining, transport, storage and distribution.
• Tank-to-wheel covers fuel use in the vehicle, including combustion emissions and delivered energy.
• A broader study may also include infrastructure, vehicle manufacture and end-of-life treatment, depending on its stated scope.

Environmental burdens measured

LCA first records material and energy flows, then converts them into selected categories of potential environmental impact. It can therefore reveal trade-offs that a comparison based only on tailpipe emissions would miss.

• Climate impact is generally expressed as greenhouse-gas emissions in carbon-dioxide equivalent over the life cycle.
• Other indicators may include fossil energy demand, water consumption, land occupation, resource depletion and emissions contributing to air pollution, acidification or eutrophication.
• For biofuels such as ethanol, assessment may include cultivation inputs, irrigation, fertilisers, processing energy, transport, land-use change and the treatment of co-products.

Method and interpretation

Under ISO 14040 and ISO 14044, an LCA comprises goal and scope definition, inventory analysis, impact assessment and interpretation. Valid comparison requires the same functional unit, system boundary and consistent assumptions.

• Results are sensitive to crop yield, irrigation source, energy mix, transport distance and methods used to allocate impacts among co-products.
• LCA estimates potential environmental burdens; it does not by itself provide a complete judgement of economic, social or distributional sustainability.

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