The New ZEB: Revising our Definition of "Net Zero"
The time has arrived for ZEB to be redefined
All images are courtesy of author.
When The 2030 Challenge was issued by Edward Mazria, FAIA, 20 years ago, it was predicated on the notion that our buildings are a major contributor to global greenhouse (GHG) emissions. According to the United Nations data at the time, buildings were responsible for about 40% of total GHG emissions. The 2030 Challenge set thresholds for fossil fuel use reductions.
The playbook for architects and engineers at the time was simple: make our buildings as energy efficient as possible and then use renewable energy resources to account for the remaining requirements. The building design and construction industry galvanized efforts around this notion and the concept of "net zero energy" emerged - which generally indicated that a building would produce at least as much energy as it required over the course of a year.
And born was the acronym ZEB to represent "zero energy buildings."
Realizing the need to better define and distinguish the various approaches to "net zero" buildings, the National Renewable Energy Laboratory (NREL) soon thereafter published four definitions of ZEB in a June 2006 white paper:
Net Zero Site Energy: A site ZEB produces at least as much energy as it uses in a year, when accounted for at the site.
Net Zero Source Energy: A source ZEB produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site. To calculate a building’s total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers.
Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.
Net Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.
From Zero Energy Buildings to Zero Emission Buildings
In the years since the NREL terms have served as the de facto definitions of "net zero" and the primary focus of ZEBs has been on energy efficiency. An emphasis on reducing energy consumption makes sense as this is an architect or engineer's primary lever for GHG emissions reductions.
However, energy efficiency alone is a limited and incomplete approach to a building solution that seeks to completely eliminate its GHG emissions.
The time has arrived for ZEB to be redefined. Rather than Zero Energy Buildings, we should pursue Zero Emissions Buildings. In addition to energy efficiency, we need buildings that eliminate the need for on-site fossil fuel use and only use zero emissions electricity.
The National Definition of a Zero Emission Building
Building upon this notion of zero emissions, the White House, in partnership with the Department of Energy, released the National Definition of a Zero Emissions Building in June 2024 to aid in the effort of reducing GHG emissions from the building sector.
It states that at a minimum, a zero (operational) emissions building (ZEB) must meet the following criteria:
1. Energy efficient: The building is among the most efficient.
2. Free of on-site emissions from energy use: The building’s direct greenhouse gas (GHG) emissions from energy use equals zero.
3. Powered solely from clean energy: All the energy used by the building, both on-site and off-site, is from clean energy sources.
Figure: The National Definition of a Zero Emissions Building. Figure by Daniel Overbey.More Specifically...
The National Definition of a Zero Emission Building goes on to unpack the criteria and clarifies:
- The Definition applies to both new construction and existing buildings with respective criteria.
- Electric vehicle supply equipment (EVSE) is not considered part of the building load.
- Though direct GHG emissions from energy use must equal zero, there is an exception for use of emergency backup generators when grid power is unavailable.
- All energy used by the building must be clean energy, obtained through any combination of on- and off-site sources, as long as the GHG emissions from that clean energy equals zero.
- The clean energy must also meet at least one of four listed requirements.
- If the building obtains heating or cooling from a district energy system, the district energy must be generated from clean sources.
AEC Alignment Around the New Definition of ZEB
The purpose of the National Definition of a Zero Emission Building is to set forth criteria that public and private entities can use to assess whether a building is a zero emissions building. It sets a consistent definition for ZEB that the building design and construction industry can coalesce around. AIA, ASHRAE, USGBC, and others have applauded the Definition and are moving to align climate action programs and information resources alike. For instance, the latest requirements for LEED Zero Carbon and LEED Platinum certifications now incorporate the three principles of the Definition.
