With the ecoinvent 3.1 database, released with SimaPro 8.0.4, a new system model is introduced: Allocation, recycled content. This new system model is available in addition to the two system models already present in SimaPro 8.0.0 to 8.0.3: Allocation, default and Consequential.
That raises the question: which system model should you use? To make your decision easier, we briefly explain the three system models below.
The Allocation, default System Model
The Allocation, default system model is based on two methodological decisions:
- It uses the average supply (unconstrained supply) of products, as described in market activity datasets.
- It uses partitioning (allocation) to convert multi-product datasets to single-product datasets.
Average supply means that the model doesn’t use constrains due to markets and technology. In such a model, the product, material or service can be always provided, even if it is scarce. This differs from the Consequential model, which uses a constrained supply of products.
The model allocates reference products and by-products economically, i.e. according to the market value of the products. The Allocation, default system model’s approach to attribution was also available in ecoinvent v2.
The Allocation, recycled content System Model
The Allocation, recycled content system model (called Allocation, cut-off by classification by the ecoinvent Centre) uses the same two decisions concerning average supply and allocation as the Allocation, default system model. The difference with the Allocation, default system model is that Allocation, recycled content does not take into account any benefit related to the recycling of a material. In this model, recyclable materials are available burden-free to recycling processes, which means that secondary (recycled) materials bear only the impacts of the recycling processes. The model does not give any credit to producers of wastes for the recycling or re-use of products from any waste treatment. This approach was also available in ecoinvent v1 and v2.
The Allocation, default system model allocates waste (and by-products) at the point of substitution. Here, the benefit from recycling materials is attributed to the market processes that provide the secondary materials or by-products (such as heat or electricity from the incineration of waste). Check the ecoinvent website for more information on the allocation of waste and by-products.
The Consequential System Model
The Consequential system model handles the two methodological decisions used in both Allocation system models differently:
- It uses a constrained supply of products, based on market activity data and on information about technology level.
- It uses substitution (system expansion) to convert multi-product datasets into single-product datasets.
In a constrained market, a change in demand does not result in a corresponding change in supply but instead in a change in consumption elsewhere. For example, by-product markets are constrained because their production volumes depend on the production volumes of the reference products, and it’s the demand for the reference product that drives the production. This system model is intended to reflect the consequences of small-scale, long-term decisions by taking into account the constraints that apply at this scale and time horizon.
Because of the constrained supply of products, the allocation of by-products is avoided by using substitution, and analyses a product which replaces or substitutes the by-product of the reference product. The emissions of that replacing product are then subtracted from the reference product emissions.
The possibility of using consequential allocation gives practitioners an alternative to the attributional approach.
Choosing A System Model
As you can see, each model has specific advantages, depending on the goal and context of you LCA. The availability of three models allows you to select the most appropriate model for your study. For instance, should you wish to align to ecoinvent v2 modelling, the closest match is the Allocation, recycled content system model. More information on the background of the various modelling choices is available on the ecoinvent website.