What does net zero carbon mean in buildings?

Now that building efficiency has become codified, engineers and designers have a process where building professionals across the industry refer to ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings based local building codes, sustainability ratings systems and grant programs with the goal of delivering efficient, comfortable buildings. ASHRAE Standard 90.1 provides the underpinnings of numerous state energy codes, municipal energy codes, energy metrics for sustainability rating systems and is the basis for the vast majority of the energy modeling efforts in the commercial space.

Standard 90.1 is the compliance standard for state energy codes for 38 states, all versions of the U.S. Green Building Council LEED rating system, International Green Construction Code, Federal Energy Management Program projects and ASHRAE Standard 189. Utility incentive programs and building financing/loan programs such as Commercial Property Assessed Clean Energy also rely on Standard 90.1 to standardize building energy design and allow for the program to create a level playing field for all participants.

At the core of Standard 90.1’s individual building component requirements is the determination of what is the most efficient, yet still cost-effective and viable level of efficiency for a given building component. This is the strength that has kept Standard 90.1 relevant, bringing industry stakeholders, building design and construction teams and code officials on board. While ASHRAE 90.1 originated in 1975 only needing to deliver cost-effective energy savings, increasing calls from the scientific community for carbon emission reductions have prompted building code officials to ask, “What can the standard do to advance carbon and climate goals?”

ZNC code drivers

What is driving the need for a zero net carbon energy code? Architecture 2030 defines the ZNC building as, “A highly energy-efficient building that produces on-site, or procures, enough carbon-free renewable energy to meet building operations energy consumption annually.”

Buildings that generate as much energy as they use annually are often called “zero energy buildings,” “zero net energy” or “net-zero energy buildings,” depending on the program, agency or scientific paper you are looking at. The important features of this definition are that the building generates, most commonly with solar photovoltaics, as much energy as it uses in a year.

Because the science tells us that greenhouse gas emissions are the culprit when it comes to climate change, we really need to think beyond energy use to what emissions are generated as a result of building operation. Electricity creates emissions at the power plant, so offsetting a grid supplied power with solar PV produced energy will result in an elimination of emissions related to building activity.

Many buildings, such as 50-story skyscrapers or buildings heavily shaded by terrain will likely never be able to generate enough on-site energy to be ZNC. To bring these buildings to ZNC, building managers can buy into a community solar project to offset electricity use. This may not meet the strictest definition of ZNC, but in some cases, this is the only path forward.

Because natural gas burned for space heating, domestic hot water or process loads emit greenhouse gases on-site, a building would need to sequester this carbon on-site or buy into a community carbon sequestration project to offset these emissions and be considered a ZNC building. Because these systems are not currently technologically viable, building electrification, where no fossil fuels are burned on-site, is the only path to ZNC buildings currently available.

Electrified, ZNC buildings are technologically feasible and currently exist in nearly all climate zones. Figure 3 shows a municipal office building in climate zone 5B renovated to electrified ZNC building status using on-site solar PV. Note that this office looks like any other non-ZNC building.

According to the Energy Information Administration, operation of buildings accounts for 28% of energy-related GHG emissions, while transportation, industrial processes and transmission losses make up the rest in roughly equal amounts. When the electric transmission system required to deliver electricity to buildings is added to buildings’ tab, building-related GHG emissions jumps to 40% of energy-related emissions.

At the 2019 U.N. Climate Action summit, the need for all new buildings to be ZNC by 2030 was established as the necessary to limit global average temperature rise to 1.5°C. In turn, the 1.5°C limit is what the U.N.’s Paris Agreement is structured around and represents a target that could help avoid the worst possible impacts of global climate change. Because energy code is the minimum backstop for energy design, either net zero emissions targets in new construction will have to become standard practice or building codes will need to mandate it.

Standard 90.1 and International Energy Conservation Code have succeeded in driving reductions in energy use where they are adopted. The Department of Energy estimates that energy codes will save 12.82 quads (quadrillion Btus) of energy in the period of 2010 to 2040. This equals the energy used by six million cars over the same 30-year period and $128 billion in energy bill savings. That is a very significant achievement, but considering that the annual energy usage of buildings in the U.S. was 21 quads in 2019, the work that must be done to increase the impact of energy code is clear.

Scientific consensus and policy initiatives at all levels of government are beginning to ask when can we expect all buildings to be ZNC? These goals demand new thinking and design techniques. The question becomes, how does the code that has gone so far in advancing energy savings match up to ZNC code goals?

ZNC hurdles

When asking, “What is preventing a building designed to any level of energy code from being a ZNC building?” the answer is, “For most building projects, nothing.” Any electrified building with enough renewable energy generation can zero out its annual electric consumption, and any code that mandates this is a ZNC code.

Figure 3: A municipal office building was renovated to be electrified and have an energy use index low enough that energy use could be offset by on-site renewable generation. The project was intended to demonstrate that older buildings can become net zero energy buildings. Courtesy: PCD Engineering[/caption]

For example, prescriptive U-values for mass, metal building and steel framed exterior walls can vary by 30% or more, depending on the climate zone where the building is located. The same is true for a typical packaged rooftop system versus a ground source heat pump solution — both will comply with prescriptive code requirements, but will result in very different energy outcomes.

Jurisdictions with energy codes that are moving most quickly toward net zero typically solve this problem by requiring designs to hit stricter energy use targets via performance-based compliance. It is difficult to find data on the proportion of buildings using the prescriptive compliance path of Standard 90.1 versus the performance path, but anecdotally the majority of buildings choose the former.

It should be noted that not all energy design improvements have positive life cycle cost savings compared to code minimum alternatives. However, building code contains many other requirements that code officials and society at large have determined to be socially beneficial enough that they are required without the need for financial payback as justification.

For example, fire safety systems and building accessibility under the Americans With Disabilities Act could be viewed as falling into this category. As with these building code requirements, it is up to the acumen of design professionals to assist building developers in navigating energy code requirements and minimizing the budgetary impact of compliance.

Unregulated loads such as kitchen equipment, computers, screens or monitors and plug-in refrigerators are problematic within a ZNC code because two buildings with identical systems can have very different energy outcomes when unregulated loads differ significantly. In 2010, the DOE estimated that 33% of commercial building electricity use (equal to 26% of total energy use) is made up of plug and process loads that are unregulated in Standard 90.1.

As successive versions of Standard 90.1 drive down the energy use from service hot water, lighting and heating, ventilation and air conditioning systems, this portion is projected to grow to 49% by 2030. ASHRAE 90.1-2016 has begun to address this issue by adding commercial refrigeration systems and elevators to baseline requirements. This motivates design and construction teams to increase efficiency of these components by allowing credit to be taken for exceeding the code minimum values. Because many unregulated loads are consume electricity, these are loads that could still be offset by renewables, but addressing them within a ZNC code would make the code more able to be implemented by reducing the sizing and demand for renewable generation.

Renewable energy generation is the final piece to make an energy code a ZNC code. To codify the requirement to fully offset energy with renewable generation, a ZNC code needs to provide a procedure for estimating the energy use of a building design, then mandate that this energy be offset. Standard 90.1 and IECC both contain the beginnings of this process via performance-based modeling compliance, but lack offset mandates. This is where authorities having jurisdiction can and are stepping in to develop this requirement.

Despite the hurdles mentioned, some code authors are moving in the direction of a ZNC code. ZERO Code by Architecture 2030 was published in 2018 and combines ASHRAE 90.1-2016 compliance with a renewable energy or carbon offset mandate. This code represents a giant leap toward a ZNC code by adding the requirement for design teams to carbon offset, which is unique among codes.

However, because the code allows for ASHRAE Standard 90.1 prescriptive compliance, it has to rely on tables containing energy use estimates for nine building types. The likelihood that this code will produce a ZNC building depends on the degree to which the actual, post-construction energy use is less than the energy use from the tables in the code. The hurdles of requiring electrification and the possibility that the renewable system installed will not meet building requirements remain for future code improvement.

Standard 90.1 has and continues to make impressive strides toward ZNC code by driving reduced energy use through incremental, consensus-driven building design improvements. To meet the goals of creating a code that mandates net zero buildings by 2030, significant hurdles of code adoption, cost-effective electrification, process load regulation and renewable mandates must be overcome. This will require legislative will on the part of authorities having jurisdiction and engineering diligence on the part of building design teams and construction, but the goal is achievable.

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