With respect to the question ‘What technology has to be assessed?’, two major definitions have to be considered. First, there is the definition of the functional unit. If one aims, for example, at comparing a fossil resource-based technology with a renewable resource-based technology, one needs to define a functional unit allowing comparison. In the past, products have been considered frequently as the functional unit. However, the sustainability idea, which, according to the UN definition of it in 1987, must be ‘fulfilling the needs’, makes a shift from product to service: products are only a vehicle to deliver the service one uses to fulfil the needs of the population. This fits with the one-liner ‘doing more with less’ and the dematerialization concept.
Next to an appropriate definition of the functional unit, sustainability metrics also need to be based on a good definition of system boundaries of the technology to be assessed. System boundaries chosen in a too restrictive way may lead to erroneous conclusions. This is nicely illustrated in Figure 3.2 with hydrogen as energy resource. Hydrogen is nowadays perceived as a clean energy source, resulting only in water emissions when it is combusted. However, hydrogen as such is neither a renewable nor a non-renewable resource. It is only an energy carrier that can be manufactured both from renewable and non-renewable resources. This means that assessment of hydrogen-based technogies is well served by a cradle to grave approach. Only in this way, does it become obvious what type of resources (renewable or not) are involved in the overall production chain.