Mind the trap: avoid these common carbon modelling mistakes

Craig Jones, director, Circular Ecology says you have to get the data right.

While the process of modelling carbon is not rocket science, the plethora of data available can be overwhelming. Choosing the right data, in the right combinations, is fundamental to a model’s integrity, and potential pitfalls exist that could undermine this. With care and attention, they can be avoided.

"An asset with high capital carbon will look carbon-heavy at the beginning of its life, but if its design keeps operational carbon at a minimum throughout its life, this could be preferable to alternative low capital/high operational carbon solutions."

The need to obtain reliable data should go without saying, and in the UK there are many reputable sources. The Department for Environment, Food & Rural Affairs (Defra) offers regularly updated data on fuel and electricity, and the carbon footprint of various freight transport modes. Mott MacDonald’s CapIT carbon modelling tool contains data from the Institution of Civil Engineers Inventory of Carbon and Energy and the Hutchins UK Building Blackbook.

Capital carbon must be modelled alongside operational carbon to build a true picture. The carbon within the walls of a building may be less obvious than the electricity running through its lighting and ventilation systems, but they both count toward the total.

Ensure carbon data is being read correctly. For example, when evaluating the carbon content of different material choices, do not fall into the trap of simply comparing the same weight of each. 1kg of timber and 1kg of steel will offer different volumes of material with different structural properties and lifespans. Only by adjusting your data to account for this will you gain accurate insight.

Similarly, ensure you have all the necessary data. The carbon footprint of freight transport is measured using tonne-kilometres (tkm), which refer to 1t of material being moved 1km. However, assigning a single carbon ‘rate’ to 1tkm is not accurate enough; transport mode options need to be differentiated to reflect the carbon difference between, for example, road, rail and sea freight.

Consider also the carbon footprint of your asset over its whole life, as an initial snapshot could give a false picture. For example, an asset with high capital carbon will look carbon-heavy at the beginning of its life, but if its design keeps operational carbon at a minimum throughout its life, this could be preferable to alternative low capital/high operational carbon solutions.

Carbon models don’t have to be 100% accurate – that would not be practicable. But taking the time to ensure you have appropriate data in the appropriate configuration will result in a more credible model, and more valuable insights.

Seven strategies for effective carbon modelling

1. Specify activity boundaries early on

Activity boundaries are the limits defining which parts of a project will be measured for carbon, and which will be omitted. The model could include quantities of energy consumed by activities such as materials extraction, manufacturing, transport, installation, removal and recycling, as well as aspects of operational carbon. Specifying boundaries before modelling begins is essential to ensure consistency across all data sets.

2.  Define desired business outcomes

Knowing at an early stage is how the model will be used to drive business outcomes is critical to ensuring the model will serve those requirements. Measuring and modelling alone won’t reduce carbon; it’s what the business does with that data that matters. Likewise, an individual calculation will have limited value – real advantages come from repeated modelling and optioneering as the project evolves.

 3. Model carbon before it’s too late

The carbon reduction curve shows that the potential to reduce carbon is inversely proportional with the progression of a project. The greatest opportunity lies at the investment appraisal and early design stages, because the optimal way to minimise capital carbon is to minimise new construction. Modelling carbon at an early stage empowers you to capitalise on these early opportunities to make the greatest carbon and cost savings.

4.  Source reliable, appropriate data

Obtain data for carbon emissions factors from reputable sources such as DEFRA, the Institution of Civil Engineers and industry tools such as Mott MacDonald’s CapIT. This data is often available at no cost. Suppliers can also provide emissions data on their own products, but this data must be transparent to ensure it aligns with the rest of the model.

5.  Find the optimal degree of accuracy

It may feel counter-intuitive to build models with data that is not precisely accurate: A carbon model must be a basis for confident business decisions, so it should be as accurate as is reasonably practicable. But the pursuit of perfection can paralyse. It is better to use a less-than-100% accurate model – which can still provide valuable insight and reliable guidance – than to wait years before taking any decisions.

6.   Look at data in context

Beware false conclusions. The outcome of comparisons between the carbon footprint of different materials, products or design options can be changed in light of different quantities needed, waste produced (any excess materials ordered for wastage also contribute to a project’s carbon footprint, even if they are not used), expected design life, and operation carbon emissions.

7.  Use shortcuts for indicative data

Indicative calculations can facilitate more informed decisions at an early stage, as long as the approach to measurement is consistent. Anglian Water developed a Carbon Modeller tool containing raw carbon data on suppliers’ materials, typical site activities, construction methods and transport options. Although not perfectly accurate, this tool allows engineers to quickly and effectively consider carbon when optioneering at the design stage.

For more on crunching carbon, cutting cost talk to Davide Stronati, Mott MacDonald Group sustainability manager.