Q8: LCA Beyond Energy and CO2

What are LCA metrics besides energy use and carbon dioxide emissions, and how important are they?

Author: John Anderson; Contributors: Kate Simonen, Martha VanGeem

The comparison of alternative structural systems requires a common metric. Typical LCA metrics are energy use and carbon dioxide (or abbreviated simply as carbon) emissions. For countries with fossil fuel dependent energy systems (e.g., the U.S.), energy use and carbon dioxide emissions are typically codependent – more energy use results in more carbon dioxide emissions. While energy use and carbon emissions are ubiquitous metrics, there are numerous other environmental variables of interest for structural engineers.

Environmental metrics can be either inventory data or impact categories. Carbon dioxide is an example of inventory data, whereas climate change (i.e., the combination of numerous greenhouse gases – carbon dioxide, methane, nitrous oxide, hydro-fluorocarbon, and sulfur hexafluoride) is a midpoint impact category. Using life cycle inventory (LCI) data can allow for faster environmental comparisons and give good approximate results when comparing specific items such as a particular compound being emitted to air. Impact categories on the other hand are useful as the final results are presented in terms of the impacts of concern (e.g., acidification, stratospheric ozone depletion). Impact categories often require weighting of inventory data, which can add a level of uncertainly to the final comparison. The figure and table below illustrate the relationship between inventory data and impact categories.

Figure 8.1: Illustration of the relationship between inventory data and impact categories for coal fired electricity. The numbers represent the different weight of the emission on the impact category (Anderson and Thornback 2012).

Table 8.1: Summary and description of impact categories (Curran 2006)

Common impact categories include land use, water use, resource use, climate change, acidification, eutrophication, fossil fuel depletion, habitat alteration, smog, eco-toxicity, ozone depletion, and human health. For inventory data the US Environmental Protection Agency Toxic Release Inventory Program lists 682 chemicals and chemical categories that are of interest (e.g., aluminum, chromium, lithium carbonate, phosphine). The EPA publishes the TRACI characterization factors that are commonly used to report impacts in the U.S. (Bare n.d.). LCA research often combines inventory data and midpoint impact categories (e.g., equivalent SO₂, CO, NO₂, volatile organic compounds, particulate matter < 10 micrometers (PM-10), global warming potential, hazardous waste generated, toxic air emissions) to give a detailed description of the environmental performance of a building (Hendrickson and A Horvath 2000).

The benefit of using multiple metrics for structural engineers is that it reveals trade-offs in environmental impacts that would not be seen when only evaluating in terms of energy and carbon emissions.  This is illustrated by the dominance of PM-10 emission from structural materials (Junnila et al. 2006). Simply using energy use and carbon dioxide emissions might undervalue the importance of structural materials in the overall environmental performance of the building. At the same time the functional unit (e.g., emissions per square-meter of the building) must also be evaluated to ensure a fair comparison of structural alternatives. For example while concrete structures may have a higher mass per square-meter of the building, the high embodied energy of steel can result in a slightly higher embodied energy per square-meter of the structure (Hsu 2009).

It is important to note that life cycle assessment only captures a certain set of environmental impacts, and does not typically report impacts such as habitat loss, species diversity, land use change.  Structural engineers interested in reducing these other impacts should consider additional environmental certification programs and evaluation methods.

The numerous LCI data and impact categories illustrate that there is not a single metric for LCA. A good source for further detailed information on LCA in general and specifically for impact categories can be found in the report “A guide to understanding the embodied impacts of construction products” published by the Construction Products Association (Anderson and Thornback 2012). Each project needs to be reviewed by the engineer, client, and owner to determine the environmental goals with the understanding that there may be trade-offs between different impacts.   

References

Anderson, J., and Thornback, J. (2012). A guide to understanding the embodied impacts of construction products. London, 48.

Bare, J. (n.d.). Tool for the reduction and assessment of chemical and other environmental impacts (TRACI).
Curran, M. A. (2006). Life cycle assessment: principles and practice. Cincinnati, Ohio, 80.

Hendrickson, C., and Horvath, A. (2000). “Resource use and environmental emissions of US construction sectors.” Journal of Construction Engineering and Management, American Society of Civil Engineers, 126(1), 38–44.

Hsu, S. L. (2009). “Life cycle assessment of materials and construction in commercial structures: variability and limitations.” Massachusetts Institute of Technology.

Junnila, S., Horvath, Arpad, and Guggemos, A. A. (2006). “Life-Cycle Assessment of Office Buildings in Europe and the United States.” Journal of Infrastructure Systems, 12(1), 10–17.

Return to Top 10 Home