Biomedical Engineering Reference
In-Depth Information
as reference flow. This small example shows that it is essential to include the
duration or number of uses in the definition of the functional unit.
The second crucial step of the LCA goal definition is the determination
of the system and
system boundaries
, which are also based on the product
function. A system consists of a set of processes that combine to perform
one or several functions. Each process in the system can have inputs from
the environment (consumption of resources, energy, or land area) and out-
puts to the environment (emissions to air, water, or soil). Theoretically, the
system boundary should include all economic processes required to achieve
the system function, from cradle to grave (creation to disposal), which gener-
ally involves the following phases: extraction of energy and raw materials,
manufacturing (infrastructure production, input products, product manu-
facturing), transportation (e.g., to the consumer), use phase (including main-
tenance), and the short- and long-term emissions and extractions associated
with waste treatment. In practice, the system boundaries cannot include all
necessary processes. For example, to model the function of 100 days of use of
a T-shirt, the T-shirt manufacturing process should be included, but should
the manufacturing of the T-shirt-manufacturing machine also be included?
And the machine that manufactured this machine? System boundaries
should cover the same functional reality in all scenarios, with a cutoff per-
centage that only includes processes that contribute more than this percent-
age of the total mass of extractions or emissions for each scenario. System
boundaries are particularly important in the context of nanoproducts to
ensure that alternative and innovative materials are evaluated within the
same framework even if their production chains are extremely different.
11.1.2.2 Inventory of Extractions and Emissions
Once the system boundary has been determined, all processes are detailed
in a Process LCA. For each process, the inventory of resource extractions and
environmental emissions (e.g., the carbon dioxide emissions at the T-shirt
production plant in India) must be recorded, as well as the intermediary
flows that link the different unit processes together (e.g., the quantity of cot-
ton used in T-shirt manufacturing). To also estimate the inventories associ-
ated with indirect processes in the supply chain (e.g., electricity production
in India), LCA experts have created large inventory databases, such as the
EcoInvent database,
4
that grandly facilitate the realization of the inventory
and enable the practitioner to mostly focus on the core manufacturing pro-
cesses, using existing data for the supply chain in a first screening.
For applications such as services, it is difficult to assess what is actually
important. One way to ensure that all relevant processes are included is to
use a method known as Input-Output LCA (IO LCA). Rather than using
physical fluxes, IO LCA inventories the monetary fluxes between different
economic sectors involved in the production chain for a given product, pro-
cess, or activity. Each sector is studied to determine the resource extraction
