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| Fig. 1: USGBC LEED Gold Certification Logo. (Source: Wikimedia Commons) |
As sustainability initiatives become ubiquitous throughout infrastructure, novel criteria for evaluation of sustainability have become widely adopted throughout the fields of architecture and engineering. Perhaps the most commonly adopted green building certification program is known as LEED (Leadership in Energy and Environmental Design). The LEED certification is granted by the U.S. Green Building Council. The certification logo for a "LEED Gold" certified building is shown in Figure 1. The rubric used to assess buildings for the LEED criteria contains multiple categories to holistically evaluate a building for environmental sustainability. [1] The LEED requirement guidelines incorporate multiple categories, awarding points based on specified qualitative and quantitative criteria. [1]
This analysis will evaluate the effectiveness of the "Minimum Energy Performance" prerequisite of the LEED Building Design and Construction requirements. Within the LEED v4 Reference Guide for Building Design and Construction, this particular prerequisite states that the proposed building must demonstrate a 5% improvement (for new construction) in building performance rating compared with the baseline building. [1] The guide states that the baseline building performance is calculated using the ANSI/ASHRAE/IESNA Standard 90.1-2010, Appendix G. [1] This is a cost-based metric derived from an energy simulation model. [2] This metric is unfortunately dependent on prescribed utility rates and fuel prices that are inherently unstable.
For the sake of a physics-based analysis, it is more appropriate to utilize a stable, empirical metric to quantify the magnitude of energy reduction. The Energy Use Intensity (EUI) is a widely-adopted metric of the energy efficiency for a given building. It is calculated by dividing the total annual energy consumption by the total floor area. Applying the 5% reduction in terms of EUI will provide a reasonable estimate of the emissions impact as a result of the prerequisite.
In order to do this analysis, consider a mid-size office building of roughly 100,000 square feet. The Energy Use Intensity (EUI) will be compared between a baseline building and an analogous proposed building with a 5% improvement in EUI as outlined by the LEED requirements.
The national median site EUI for office buildings can be estimated to be approximately 67.3 kBtu per square foot per year. [3] For the sake of calculation in SI units, Joules will be used as the unit for energy. To get the total energy consumption per year for the baseline site, the EUI of the baseline site is multiplied by the total square footage of the building to get
| Total Yearly Energy Consumption | = | 67.3 kBTU feet-2 year-1 × 1000 BTU kBTU-1 |
| × 1055 Joules BTU-1 × 1.0 × 105 feet2 | ||
| = | 7.10 × 1012 Joules year-1 |
Thus, a 5% reduction of this value for a LEED certified office reduces the total energy consumption by
| Reduction in Yearly Energy Consumption | = | 0.05 × 7.10 × 1012 Joules year-1 |
| = | 3.55 × 1011 Joules year-1 |
In order to calculate the mass of natural gas required to generate this amount of energy we will assume a power plant efficiency of 40%. The result is
| Yearly Reduction in Natural Gas Consumption | = | 3.55 × 1011 Joules
year-1 0.4 × 5.5 × 1010 Joules tonne-1 |
| = | 16.14 tonnes year-1 |
This calculation demonstrates that a 5% reduction in EUI results in a reduction of 16.14 tonnes per year of natural gas consumption. To convert this to the total yearly CO2 emissions reduction, this value will be multiplied by a factor of 44/16. This factor corresponds to the molar mass of CO2 divided by the molar mass of methane. The resulting multiplication results in a total yearly CO2 emissions reduction of 44.375 metric tonnes of CO2 per year due to the 5% reduction in building EUI within a 100,000 square foot office building.
Upon performing the mathematical analysis, it becomes clear that the reduction in CO2 emissions as a result of the "Minimum Energy Performance" prerequisite in the LEED guidelines is rather small. To provide context to the value of 44 metric tonnes of CO2 per year, it is helpful to compare with the value of annual per capita CO2 emissions. According to a scientific paper published by Lucas Chancel, the global average per-capita CO2 emissions was estimated to be 6 tonnes per year. [4] Thus, the reduction of 44 metric tonnes of CO2 per year can be equated to removing the CO2 emissions from just 7 people per year.
Thus, in order to truly make an impactful reduction of carbon emissions and support LEED's stated goal of improving sustainability, the "Minimum Energy Performance" prerequisite must be adjusted beyond the marginal 5% improvement in building performance rating. Furthermore, the implementation of concrete, nonvolatile metrics for energy usage, such as Energy Use Intensity (EUI), in lieu of the existing cost-based building performance metrics would allow for improved thermodynamically-based calculation of a building's energy efficiency and carbon emissions.
© Ethan Yang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "LEED v41: Building Design and Construction." U.S. Green Building Council, July 2023.
[2] "ANSI/ASHRAE/IES Standard 90.1-2010 Performance Rating Method Reference Manual," Pacific Northwest National Laboratory, PNNL-25130 May 2016.
[3] L. Chancel, "Global Carbon Inequality over 1990 - 2019," Nat. Sustain. 5, 931 (2022).
[4] C. Yang and J.-H. Choi, "Energy Use Intensity Estimation Method Based on Façade Features," Procedia Eng. 118, 842 (2015).
