Net Zero Energy - Building Operation Decarbonization
The US Department of Energy defines a zero-energy building as “an energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.” Zero energy buildings typically combine energy efficiency and renewable energy in a building to result in net zero energy consumption over the course of a year.
The 2021 IECC includes Zero Code appendices for both residential and commercial buildings. The residential Zero Code appendix is based on the Energy Rating Index (ERI) path of the code. It requires more efficiency than is required in the base code and requires enough onsite or offsite renewable energy production to achieve an ERI score of zero. Renewable energy compliance may happen through a combination of onsite power production, energy generated through community renewable energy facilities, and renewable energy purchase contracts or leases.
The commercial Zero Code appendix is based on the Architecture 2030 ZERO Code, which requires a building to meet the minimum code requirements plus enough on-site or off-site renewable energy to compensate for all the energy anticipated to be consumed by the building.
These appendices focus on the energy use of the building, not the carbon emissions from its energy use. A zero-energy building may still have significant carbon emissions if it utilizes fossil fuels (including natural gas) for space heating, water heating, cooking, or clothes drying.
Operational Energy Use
When energy codes mandate higher-performing systems, these quickly evolve from being perceived as risky and unfamiliar special-order systems to being business as usual, at which point the costs moderate. This happens because suppliers, design teams and builders start competing for market share under the new rules, and they develop smarter, more economical means to comply.
To minimize overall energy, use long-term, the code development priority “stacking order” should be based on key strategies with the greatest direct impact on energy consumption:
- Optimize Envelope
- System Efficiency
- Renewable and Recovered Energy
The additional construction cost of high-performance buildings is typically repaid many times over during the life of the building through reduced utility bills. A balance point should be defined to determine the best investment balance point between more efficient systems and more renewables, recognizing that the cost of each continues to fall over time. A second and perhaps more elusive balance point should be selected between on-site and off-site renewable energy generation.
Renewable energy, defined in the energy code as the “energy derived from solar radiation, wind, waves, tides, landfill gas, biogas, biomass or the internal heat of the earth,” emits little or no carbon. Achieving a zero energy or a zero-carbon building requires a combination of energy efficiency measures to reduce the building’s load, with remaining end uses met using on-site or off-site renewable energy. The most common type of on-site renewable energy are solar photovoltaic panels. Off-site options can include community renewable energy facilities, power purchase agreements, or other mechanisms enabled by state or local laws and utility policies. The electricity grid, itself, is increasingly powered with renewable energy. However, the specific grid mix varies widely by location. Jurisdictions will need to determine the amount and type of renewable energy appropriate for buildings in their climate zone and grid mix. California, for example, includes a requirement in Title 24, Part 6 that requires solar photovoltaic systems on most new homes.
Renewable Energy Certificates (RECs) represent the “environmental attributes” of energy production from solar or wind, but by themselves are not the equivalent of building and owning a rooftop solar array. Ideally, one would be able to own shares of some large-scale solar array or wind farm and have the energy produced there be treated just as one’s own rooftop solar. Such an arrangement does exist in “community solar” installations, but these are not widely available, especially at large scale.
Low-carbon buildings, especially those which are fully electrified, must be able to shift loads through demand responsiveness, to ensure that functions that draw a relatively large load like storage water heating, building preheating or precooling, or electric vehicle charging, can be done during times when demand is lower on the grid, or when the grid is powered by lower-carbon energy.
Advanced energy codes and regulations, often known as “stretch codes” or “reach codes,” provide a means to advance energy efficiency in a jurisdiction. They are a tool to allow more progressive cities and counties with ambitious climate goals to move forward, without having to drag the more conservative parts of the state along. As these stretch code cities demonstrate the constructability and affordability of higher-performance buildings, statewide adoption of such rules is facilitated, generating a virtuous cycle. Notable examples include:
The NYStretch Energy Code is developed by the New York State Research and Development Authority (NYSERDA). Some municipalities, such as Beacon and Hastings on Hudson in New York State, have gone one step further by adopting the advanced model energy code that is more stringent than the State-adopted Energy Code.
The Massachusetts Stretch Energy Code has now been adopted by most of the jurisdictions in that state, providing a much higher level of energy efficiency than is provided by the base code. As most construction now meets the stretch code provisions, further upgrades to the base code may become less problematic for designer teams and contractors.
Washington state has one of the more progressive codes in the country, and the City of Seattle maintains a code that is higher performing still. Seattle’s provisions are regularly adopted by the state in subsequent code cycles.
Boulder, Colorado, is determined to reach a zero-energy code by 2031 and requires buildings over 500,000 square feet to use energy modeling and achieve progressively more stringent efficiency targets. https://bouldercolorado.gov/plan-develop/energy-conservation-codes
California has a well-developed “reach code” program, with dozens of cities and counties participating. This includes not only energy efficiency, but separate paths for advanced code requirements in electrification, renewables, process loads, water use, and electric readiness.
Advanced Codes with a Path Forward
Both Toronto and Vancouver have developed code structures that are strategically moving towards net zero. These are performance-based standards, meaning that they are based purely on energy modeling results rather than imposing specific requirements for equipment or envelope assemblies.
Whereas most North American performance-based energy codes are focused only on overall energy use, Toronto (and Vancouver) codes require compliance with three metrics: TEUI, TEDI and GHGI.
- TEUI – Total Energy Use Intensity – overall annual energy use, in kWh per unit of floor area
- TEDI – Thermal Energy Demand Intensity – annual space heating energy, in kWh per unit of floor area.
- GHGI – GreenHouse Gas Intensity – annual carbon emissions per unit of floor area.
Version 3 of the Toronto Green Standard (TGS) is Toronto’s 2019 step towards a 2030 zero-energy code. Developers of the most common building types can choose either the standard Tier 1 code or else a Tier 2 that’s a 20 – 30% improvement and provides a hefty partial refund of development fees. (15% of projects in the last code version chose the Tier 2 path.) The current Tier 2 will then become Tier 1 for the next code cycle, and in fact Tiers 3 and 4 have also been defined to flesh out the remaining steps to Toronto’s 2030 zero energy standard
In addition, the TGS includes several prescriptive requirements which are important for ensuring long-term real-world energy performance but can’t be evaluated by using energy modeling. These include
- Solar readiness
- District energy connection readiness
- Air tightness testing
- Building commissioning
- Building labeling and disclosure
While the Toronto system is among the most comprehensive and well-vetted of energy code pathways, it is based entirely on energy modeling, which is notoriously over-optimistic when predicting the energy use of highly efficient buildings. Also, it only applies to a handful of specific building types: office, multifamily, retail and education, since high process load buildings (lab, hospital, restaurant, etc.) don’t lend themselves to hard energy use targets. On the other hand, such performance codes reward efficient design choices for building form, orientation and other aspects that would be difficult to pin down in a prescriptive code.