Flows represent products and materials that move throughout a life cycle, interconnected within the process network, and take form of inputs, outputs, energy, or emissions. Flows can be substances, products, materials, energy carriers, emissions, or other types of inputs or outputs. A flow is characterized by its name, flow type, and reference flow property (unit category in which the flow is expressed). Examples of flows include electricity, water, CO2 emissions, aluminium, and so on.

In general, openLCA distinguishes three flow types:

• Elementary flows: These flows represent material or energy entering the system that has been drawn from the environment without previous human transformation, or material or energy leaving the system and released into the environment without further human transformation. For example, crude oil extracted from the ground, or emissions released into the air.

• Product flows: These are all the flows that are not elementary or waste flows, and represent the materials or energy exchanged between processes within the product system.

• Waste flows: Waste flows are any substances or objects that the holder needs to dispose of, like by-products with no market value or those requiring more resources to recycle than their economic return.

Each flow created in openLCA must be associated with a reference flow property, such as mass, volume, area, and so on. Though, it is also possible to have multiple flow properties for the same flow (e.g. uranium can be measured using both mass and radioactivity units, gasses can be measured using both mass and volume units, etc.)

Note: Certain waste flows can also be modelled as product flows. In databases this is usually stated in the name. Waste paper is a great example. As it can be used in the production of paper, waste paper isn’t necessarily modelled as a waste flows but instead as a product flow.