Building a MFA Model
A typical MFA model looks like this:
It consists of the following components:
- System Boundary
You have to define in space and time which data you want to consider. The spatial system boundary could be e.g. the logistical border of a company or the spatial border of a region, the system boundary in time e.g. one balance sheet year.
For a model only those processes are interesting that are located within the system boundary. They will be balances using the law of mass conservation. Of course there also exist processes outside of the system boundary (displayed light grey in the diagram above) but they are not taken into account. Processes might contain stocks or subsystems that can be described by an extra model.
We distinguish between internal flows (they connect processes within the system) and flows that cross the system boundary. They are called import or export flows respectively.
Not allowed are:
- Processes that are situated on the system boundary. They have to be either inside or outside of the system boundary.
- Flows that split. If you want to model such behavior you have to use an additional splitting process.
The result of a MFA is a diagram, where every flow and stock should have a value.
The following kind of display is preferred:
- The flow ovals display mass flows.
- A stocks within a process is symbolized by small rectangle. It contains data about the amount of stock at the beginning of the considered period of time +/- the change in stock during the period.
- A legend shows the units of the displayed data.
- The system boundary is labeled with the name of the system and the considered period. Additional information about the sum of imports, exports, stocks and changes in stocks are displayed.