A functional unit is the reference measurement that an LCA study is scaled against. ISO 14040 and ISO 14044 require this to be defined clearly during the Goal and Scope definition, but the standards don’t tell you how to choose the right one.
That choice comes down to understanding WHY you’re doing the study. Get this wrong and your entire assessment answers the wrong question.
We’ll use microalgae biodiesel, a sector where we have conducted substantial LCA work, to show the correct decision making process.
Comparing Microalgae Biodiesel to Fossil Diesel
So, say we have the production of microalgae derived biodiesel, and that of fossil derived diesel. How shall we compare them?
Perhaps, logically, we could compare the production and use of 1 kg of each fuel?
No, that will not work, because the energy densities differ; microalgae biodiesel delivers roughly 38 MJ/kg whilst fossil diesel provides about 42 MJ/kg. You’ve compared unequal energy.
In that case, shall we compare per MJ then? Again, no. These fuels behave differently in engines – different polyaromatic hydrocarbons, different particulate matter, different tailpipe emissions. Equal energy doesn’t mean equal performance.
Therefore, we need to take a step back and think, WHY are we making these fuels at all? To drive vehicles.
Therefore the functional unit is 1 km driven in a vehicle which could take either fuel. This captures what the fuels are actually produced for, to deliver transportation.
Declared Units vs Functional Units
There is often confusion, especially around EPDs, of how declared units and functional units compare. Declared units are much simpler. They’re physical quantities – mass, volume, energy. All easy to measure quantities. Whereas, as previously explained, functional units describe purpose. What does this thing achieve?
ISO 14040 distinguishes these clearly: functional units enable comparison between systems delivering the same function. Declared units just report inventory data for a physical quantity.
