Carbon Footprint: Concept, Methodology and Calculation
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four different categories of functional units: use of feedstock (e.g. 1 ha, 1 ton of
biomass), single product (e.g. 1 kg of product, 1 MJ of product), function of single
product (e.g. 1 MJ of electricity, 1 person*km), and multifunctional (e.g. 1 biorefinery). They suggested that 1 biorefinery could be the most suitable functional unit
for LCA models of biorefineries with multiple functions. Sills et al. [75] analyzed the
influence of three different functional units (1 MJ fuel, 1 kg animal feed, and 1 ha of
production area) on the LCA results of an algal biorefinery. An area based functional
unit was indicated as the best choice, even if it does not significantly influence the
CF results.
In the building materials sector, the effect of functional unit on the LCA and
CF results was evaluated for the concrete production. In particular, Panesar et al.
[61] compared the LCA results obtained using six functional units with different
complexity. They concluded that the global warming impact category is largely
influenced by the functional unit and it should capture the concrete’s functional
performance metrics specific to its application.
4.2 Consequential Versus Attributional Approach
LCA and CF studies can be carried out following two different modelling approaches:
consequential (CLCA) and attributional (ALCA). ALCA and CLCA approaches are
defined in the UNEP/SETAC guidance on LCA (UNEP/SETAC, 2011) as “system
modelling approach in which inputs and outputs are attributed to the functional unit
of a product system by linking and/or partitioning the unit processes of the system
according to a normative rule” and “system modelling approach in which activities
in a product system, are linked so that activities are included in the product system to
the extent that they are expected to change as a consequence of a change in demand
for the functional unit”, respectively. Thus, ALCA allows to calculate the CF of the
processes used to produce, use and dispose of a given product while CLCA evaluates
indirect effects arising from changes in the level of output of the product, defining
a cause-effect relationship between a change in demand and the related changes in
supply. However, CF standards, such as ISO 14067, do not define which approach is
considered or if it is recommended to include the system-wide change in emissions
due to a variation in demand for the product. In literature, several studies were carried
out to evaluate how the different modelling approaches affect the GWP of a product.
Kua and Kamath [39] analyzed the GWP impact of replacing concrete with bricks
in Singapore. They found that while, using ALCA approach, the GHG emissions
increase due to the highly energy intensive process of brick manufacturing, with
the CLCA such substitution might result in small reductions in GWP because the
domestic changes in demands for concrete and bricks produce a change in the imports
of these products.
Also in the building materials sector, Kua and Lu [40] evaluated the impact of
the modelling approach on the CF results of replacing tempered glass with polycarbonate at different percentages in the Singapore building industry. This study
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four different categories of functional units: use of feedstock (e.g. 1 ha, 1 ton of
biomass), single product (e.g. 1 kg of product, 1 MJ of product), function of single
product (e.g. 1 MJ of electricity, 1 person*km), and multifunctional (e.g. 1 biorefinery). They suggested that 1 biorefinery could be the most suitable functional unit
for LCA models of biorefineries with multiple functions. Sills et al. [75] analyzed the
influence of three different functional units (1 MJ fuel, 1 kg animal feed, and 1 ha of
production area) on the LCA results of an algal biorefinery. An area based functional
unit was indicated as the best choice, even if it does not significantly influence the
CF results.
In the building materials sector, the effect of functional unit on the LCA and
CF results was evaluated for the concrete production. In particular, Panesar et al.
[61] compared the LCA results obtained using six functional units with different
complexity. They concluded that the global warming impact category is largely
influenced by the functional unit and it should capture the concrete’s functional
performance metrics specific to its application.
4.2 Consequential Versus Attributional Approach
LCA and CF studies can be carried out following two different modelling approaches:
consequential (CLCA) and attributional (ALCA). ALCA and CLCA approaches are
defined in the UNEP/SETAC guidance on LCA (UNEP/SETAC, 2011) as “system
modelling approach in which inputs and outputs are attributed to the functional unit
of a product system by linking and/or partitioning the unit processes of the system
according to a normative rule” and “system modelling approach in which activities
in a product system, are linked so that activities are included in the product system to
the extent that they are expected to change as a consequence of a change in demand
for the functional unit”, respectively. Thus, ALCA allows to calculate the CF of the
processes used to produce, use and dispose of a given product while CLCA evaluates
indirect effects arising from changes in the level of output of the product, defining
a cause-effect relationship between a change in demand and the related changes in
supply. However, CF standards, such as ISO 14067, do not define which approach is
considered or if it is recommended to include the system-wide change in emissions
due to a variation in demand for the product. In literature, several studies were carried
out to evaluate how the different modelling approaches affect the GWP of a product.
Kua and Kamath [39] analyzed the GWP impact of replacing concrete with bricks
in Singapore. They found that while, using ALCA approach, the GHG emissions
increase due to the highly energy intensive process of brick manufacturing, with
the CLCA such substitution might result in small reductions in GWP because the
domestic changes in demands for concrete and bricks produce a change in the imports
of these products.
Also in the building materials sector, Kua and Lu [40] evaluated the impact of
the modelling approach on the CF results of replacing tempered glass with polycarbonate at different percentages in the Singapore building industry. This study