What does it mean to be a zero energy building? That question is now being asked by the U.S. Department of Energy (DOE), which recently sent out a request for public comment on their official definition of zero energy buildings (ZEBs). What seems like a simple matter of semantics is actually an incredible opportunity for members of the public to influence energy policy for the coming decades.
As you may know, many definitions of ZEBs already exist. However, no single definition has been formally adopted by the U.S. Department of Energy. This common definition is important because it will set a precedent that governments, utilities, and other organizations will adopt when developing policy or incentive programs.
This definition is also important due to an evolution in energy regulation. Current energy codes require designers to predict building energy use or prescriptively follow performance requirements to achieve efficiency. However this approach does not affect “unregulated” energy uses such as elevators or plug loads, nor does it consider the operational behaviors that can dramatically affect real energy use. Only recently are local or state governments proposing to adopt outcome-based codes that establish performance on actual measured energy consumption, inclusive of all loads. Thus, as the nation shifts to this new model, a consistent definition of ZEBs will set the precedent for the scope of building energy measured by regulators.
Complexities of a Common Definition
So with that context in mind, let’s take a look at the definition provided by the DOE for comment:
“An energy-efficient building where the actual annual source energy consumption is balanced by on-site renewable energy.”
Seems simple enough, right? In theory yes, though there are many nuances to this definition that are under debate by the building community. We can summarize these into the following four topics:
Efficiency Requirement. Exemplary energy-efficiency is commonly referenced in definitions for ZEBs. In the DOE’s proposed definition, the energy efficiency target is set by the renewable generation capacity of the building footprint or site. Essentially, this means there is no limit on energy use as long as renewable capacity is available. While this seems logical, it has the side effect of incentivizing buildings with lower density or larger sites over those with more compact massing (and less capacity to generate renewable energy).
Other organizations have more specific energy efficiency expectations for ZEBs, setting either relative comparisons or absolute targets. For example, the New Buildings Institute notes in their 2014 Getting to Zero Update that ZEBs typically use 25% of the energy consumed by an average building (relative), and most commercial ZEBs achieve an energy use intensity of 30 kBtu per square foot or less (absolute). The DOE has also published a series of Advanced Energy Design Guides, noting that achievement of a predicted energy demand of 50% below an ASHRAE 90.1-2004 baseline (commonly used energy code) is the first step to achieving a ZEB.
So this shows that ZEBs must be substantially more energy efficient than other buildings or code to achieve zero energy. But which of these is the right target? Or is a target based on renewable energy site capacity enough?
Energy Boundary. The DOE makes one clear distinction in the definition regarding energy consumption: that the balance is based on source energy. This is important because many ZEB definitions use site energy, which is a simple calculation of all energy used within the site boundary. By contrast, source energy includes the energy consumed on site AND the losses incurred in storing, transferring, and delivering energy to the site. The positive side of this approach is that it is a more realistic measure of energy burdens to the grid. However, it unfortunately incentivizes the use of fossil fuel-based energy such as natural gas, oil or propane, which is delivered to the site with fewer losses. Additionally, the source energy ratios used represent national averages, so electricity grids that are less efficient or more carbon-based are treated the same as cleaner, more efficient grids.
Renewable Energy Location. The DOE definition clearly defines a limit to on-site renewable energy production. While it is suggested that renewables should be limited to the building footprint, it does give an allowance for use of the area to the edge of the property boundary. While this approach allows more flexible application of renewable energy systems based on solar – where placement is critical to performance – it also incentivizes larger sites with less compact development.
To address this issue, further discussion is given to whether ZEBs should be permitted to locate (or purchase) renewable energy systems off-site, such as with the purchase of a renewable energy credit (REC). While this dramatically extends the potential land allowance for one building, it also permits taller, denser buildings in urban areas to achieve some form of ZEB status.
Combustion Allowance. Although the proposed DOE definition specifies what is considered a renewable energy source, it does not specifically address carbon or other greenhouse gas emissions on the consumption side. This discussion may be more relevant to a zero carbon building, but some question the role of combustible fuels in ZEBs if the ultimate intent of a zero energy future is to eliminate resource constraints and the environmental impacts of greenhouse gas emissions.
Where Do We Stand on the Issues?
While you absorb this information and formulate your own opinion on the ZEB definition, here’s how I’d like to see the DOE’s definition evolve:
- ZEBs should be given a minimum energy efficiency target based on building type rather than site renewable capacity. Not only do I think this helps projects meet the intent of ZEBs, from my experience as a consultant I know it also helps owners focus investments in energy efficiency rather than renewable energy systems.
- If source energy is used it should reflect variation in utility performance at the sub-region level rather than a national average. This will incentivize utility companies to balance their grids as well as lower the energy burden to project teams that can demonstrate a lower localized energy factor.
- ZEBs should be permitted to claim renewable energy production from the footprint of the building and any adjacent structures, but not from what is generated on open land. To accommodate urban sites with limited area, renewable energy produced from off-site locations or via RECs should be classified under a separate ZEB definition.
- Fuels with lower greenhouse gas potential – such as biogas or natural gas – offer an important interim step to a carbon-free future, but no combustion should be permitted under the ZEB definition.
Now It’s Your Turn
How do you think the definition should be modified to best define zero energy?
Leave your thoughts in our comments section or prepare your own statement for the DOE, due Friday, February 20th. To submit your own definition, or comment on the draft statement prepared by the DOE, please visit the Federal Register page.