What is a compartment?

A compartment is a concept from the field of Life Cycle Assessment (LCA). The basic idea is that the impact of an emission from a factory, say, depends on where the emission ocurrs. If CO2 is disolved in a lake it will have a different impact from being disolved in a river which will also be different from releasing it into the atmosphere. Compartments are used to classify emissions into different categories to help estimate what impact they have.

The Earthster Core Ontology (ECO) is a core domain ontology for LCA. The first release of ECO modeled compartments as a subspace of the environment defined by properties such as a medium (e.g. air or water), a longevity (either long term or not) and a population density (urban or not). These were represented in OWL as individuals rather than as classes. However, as I wrote the code to turn the list of EcoInvent and ILCD compartments into RDF, I decided to think again about the nature of compartments and how the they might be modelled in OWL.

One of the key inputs I received when first working on ECO was a concern from domain experts that the idea of compartments was rather limiting. The suggestion was that there is a need to be able classify specific flows in arbitrary ways based on arbitrary properties of a flow

The way the term compartment is used in LCA can be a little confusing. The LCA literature talks about emissions to compartments. ISO 14044, EcoInvent and ILCD name compartments with terms like emission to air. So if you accept these terms, the literature is really saying emission to emission to air, which is a bit odd. This is done because compartments are used not just for emissions but also resource consumption. So air is not enough to identify a compartment because resources from air and emissions to air are two different compartments. The first version of ECO represented the direction of a flow as a property of a flow rather than a property of a compartment. Overall, that works well, but it does make it difficult to give URIs to the compartments as defined in EcoSpold and ILCD. Hence the need to reconsider.

I tried to track down a definition of what a compartment is, which proved to be quite elusive until I found the ILCD nomenclature and other conventions document.

Compartments are used to classify elementary flows. However, as they are referred to in the LCA literature, they are not classes or types, but individuals. The literature does not refer to elementary flows of type emission to air, but of an emission to the compartment emission to air. So compartments are individuals used for classification, rather than classes themselves. This may not be an ideal conceptual model, but it is the one we have for now and so ECO should be able to represent it. It may evolve in future.

Properties compartments, as currently conceived, have include:

• whether it is a resource consumption compartment or an emissions compartment
• the medium to or from which a flow occurs, e.g. air or sea water
• the time frame of the flow, e.g. long term or not
• population density - whether urban or not
  

A goal of ECO is to be extensible so that new concepts, such as new compartments, can be easily added in the future. Another goal is to make effective use of OWL's capabilities so that emissions to sea water can be automatically recognised as a subcategory of emissions to water. ECO now defines classes corresponding to the values of this extensible list of properties. There are disjoint subclasses of Compartment that are resource consumption compartments and emissions compartments. There are subclasses that correspond to the various compartment media such as air, water and sea water. There are respectively disjoint subclasses corresponding to long term and not long term effects and to compartments with urban and rural population densities. Any current EcoInvent or ILCD compartment can be defined as an instance of one or more of these classes.

It ispossible to extend this set of classes and to use OWL inference to determine subclass relationships amongst them.

As reported above, one of the constraints on the first version of ECO was to support a more general classification mechanism than compartments. The present design offers a choice for future extensibility. The concept of compartment can be extended and refined and that can be the primary mechanism for classifying flows. Alternatively, flows may be classified more directly in terms of their immediate properties. A combination of both is also possible.

Thus the present design reflects the concept of compartments as they are currently conceived, enables of the concept of a compartment to be extended and refined and also supports other classification schemes.