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.
