Finding Common Ground: Defining Zero Energy Buildings
Buildings that produce more energy than they consume have moved from concept to increasingly common reality in recent years. But until a few months ago, no general, industry-wide agreement existed as to what exactly defined such a building. The U.S. Department of Energy (DOE) adopted “A Common Definition for Zero Energy Buildings,”1 a report prepared for the DOE by the National Institute of Building Sciences in September 2015, with hopes that it will spur the addition of such buildings, grow the expertise required to design and operate them, and, ultimately, make them more affordable.
The resulting common definition for a zero energy building (ZEB) also applies to a net zero energy or zero net energy building.
Zero Energy Building: 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.
Before arriving at the common definition, the Department of Energy evaluated current definitions for zero energy buildings and solicited industry input. This project expanded initial zero energy building definition efforts from the DOE and National Renewable Energy Laboratory (NREL).2
The resulting definition and accompanying nomenclature (See “Nomenclature,”) uses commonly available measurements and national conversion factors to define a zero energy building on a source energy basis, as well as definitions for zero energy portfolios, campuses, and communities.
A commonly accepted ZEB definition benefits the building industry by:
Providing guidance to building owners wanting to or who are required to meet ZEB requirements; and
Allowing public entities and utilities to recognize or incentivize ZEBs in a consistent manner.
Paradigm Shift
The dropping cost of renewable energy during the past 10 years and advancement of energy-efficient technologies have resulted in a strong interest in the concept of ZEBs—a paradigm shift from buildings being consumers to being producers of energy. This new breed of buildings has a tremendous potential to shift building energy use from nonrenewable sources to a more sustainable future.
Overhead active chilled beams bring comfort and ventilation to one of many collaboration spaces in the David & Lucile Packard Foundation Headquarters. The energy-efficient active chilled beam system is a key strategy used to meet the project’s zero energy design goals.
Definition Development: Project Goals
A commonly accepted definition and methods of measurement for ZEBs are designed to bring the market together around the concept of buildings producing as much energy as they consume. Reducing the ambiguity of any term helps set boundaries, which can be used for common goal setting, providing market direction.
The project team used the following guiding principles in developing a ZEB definition for commercial, industrial, and institutional buildings. The team sought a definition that would:
• Create a standardized basis for identification of ZEBs for use by industry;
• Be capable of being measured and verified, and be rigorous and transparent;
• Influence the design and operation of buildings to substantially reduce building operational energy consumption;
• Be clear and easy to understand by industry and policy makers; and
• Set a long-term goal and be durable for some time into the future.
Engaging Industry Expertise
Early in 2014, the National Institute of Building Sciences, with funding and support from the DOE BuildingTechnologies Office, began working to establish a common national ZEB definition. Creating a broadly agreed upon and supported definition of ZEB required participation from many organizations that have a stake in the outcome.
The project team surveyed existing publications in North America and Europe, and interviewed subject-matter experts working on ZEBs from across the building industry to identify issues to address and to draft a set of definitions and metrics. The project team presented these findings to ZEB industry stakeholders, who were invited to provide their input. After posting the revised document in the Federal Register,3 the project team further evaluated and considered public comments received, and refined the final report.
Understanding Boundaries
The new document specifies methodology for establishing boundary conditions, conducting energy measurements and accounting, calculating source energy, and using renewable energy certificates (RECs). The RECs must be owned by the building or, at minimum, have been retired—but not have been resold to others in order to meet their renewable energy requirements. Being resold would be double-dipping the renewable energy attributes of the power and is the subject of several rulings by the Federal Trade Commission.,4
Figure 1 illustrates the site boundary of energy transfer for zero energy accounting. The site boundary for a ZEB could be around the building footprint if the on-site renewable energy source is located within the building footprint. Or, it could be around the building site if some of the renewable energy sources are on site.
Delivered energy and exported energy are measured at the site boundary. Only on-site renewable energy (not delivered renewable fuels) may be used to offset energy delivered through the site boundary.
The key is to define the boundary for the building and closely related functions. Typically, this boundary is at the point of the utility meters or the point of delivery of fuel (such as oil and propane).
ZEB energy accounting for delivered energy through the boundary includes energy used for heating, cooling, ventilation, domestic hot water (DHW), indoor and outdoor lighting, plug loads, process energy and transportation within the building. Vehicle charging energy from on-site renewable sources for transportation outside of the building is included in the exported energy.
The term grid-independent building typically refers to a building that is detached from the electrical utility. Often these buildings purchase nonrenewable fuels, such as oil or propane.
This energy must be accounted as delivered energy when making the determination for a zero energy building. These energy flows must be metered where they cross the boundary, which is typically a utility meter or the point of delivery for fuels.
Renewable energy used on-site need not be metered, as it doesn’t cross the site boundary. Electricity from nonrenewable cogeneration systems must be metered, as this nonrenewable energy cannot be exported from the site.
In addition to establishing a definition for ZEB, definitions were needed for collections of buildings where renewable energy resources are shared. In some cases, buildings and renewable energy sources may be colocated on campuses or portfolios of buildings, and renewable energy resources may be owned in different geographic areas.
Not all buildings can achieve the lofty state of being a zero energy building; however, all buildings can reduce their energy consumption and produce some of their energy and receive the rest from another building or a common renewable energy resource. An early study on the potential of zero energy buildings showed that with technology 10 years ago, the entire commercial portfolio could approach zero energy with just rooftop photovoltaics.5
Three variations on the ZEB definition were provided to expand the boundary around multiple buildings.
Zero Energy Campus. An energy-efficient campus 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 Portfolio. An energy-efficient portfolio 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 Community. An energy-efficient community where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy.
The front of the DPR Construction Phoenix Regional Office is characterized by a 79 kW photovoltaic array and shower towers, which extend above the roof. The shower towers help facilitate natural venti-lation by increasing airflow and cooling in the office, reducing energy demands. The project was featured in the Spring 2014 issue of High Performing Buildings.
Energy Metric
Site energy consumption measures the performance of the building and the building systems, but it does not tell the whole story of impacts from resource consumption and emissions associated with the energy use. In addition, site energy is not a good comparison metric for buildings that have different mixes of energy types, buildings with on-site energy generation such as photovoltaics, or buildings with cogeneration.
To assess the relative efficiencies of buildings with varying fuel types, it is necessary to convert these types of energy into equivalent units of raw fuel consumed in generating one unit of energy consumed on site. To achieve this equivalency, the convention of source energy is used.
When energy is consumed on site, the conversion to source energy must account for the energy consumed in the extraction, processing, and transport of primary fuels such as coal, oil and natural gas; energy losses in thermal combustion in power generation plants; and energy losses in transmission and distribution to the building site.
The ZEB definition uses national average source-site ratios to accomplish the conversion to source energy because doing so ensures that no specific building will be credited (or penalized) for the relative efficiency of its energy provider(s).
Source energy is calculated from delivered energy and exported energy for each energy type using source energy conversion factors. The con-version factors are applied to convert energy delivered and exported on-site into the total equivalent source energy. These factors are specified in Table J2-A of ANSI/ASHRAE Standard 105-2014, Standard Methods of Determining, Expressing, and Comparing Building Energy Performance and Greenhouse Gas Emissions. While on-site renewable energy is a carbon-free, zero-energy-loss resource, when it is exported to the grid as electricity, it displaces electricity that would be required from the grid.
Table 1 summarizes the national average source energy conversion factors for various energy types. Exported nonrenewable energy has no value in the calculation method.
Example Energy Accounting
All Electric Building. A building has an actual annual delivered energy of 300,000 kBtu of electricity. The on-site renewable exported energy is 320,000 kBtu electricity from photovoltaics. (Notice that the equation is using energy transferred across the site boundary and does not include on-site renewable energy consumed by the building.)
Using the source energy formula, the annual source energy balance would be:
Esource = (300,000 kBtu × 3.15)
– (320,000 kBtu × 3.15)
= 945,000 kBtu
– 1,008,000 kBtu
= –63,000 kBtu
Combined Heat and Power (CHP) Building. A building with CHP has the following actual annual delivered energy types: 120,000 kBtu of electricity and 260,000 kBtu of natural gas. The on-site renewable exported energy is 210,000 kBtu of electricity from photovoltaics. On-site nonrenewable energy exported would not be included in the accounting.
Using the formula above, the annual source energy balance would be:
Esource = [(120,000 kBtu × 3.15)
+ (260,000 kBtu × 1.09)]
– (210,000 kBtu × 3.15)
= 661,400 kBtu
– 661,500 kBtu
= –100 kBtu
Since Esource ≤ 0, the building would be a zero energy building.
Using the Designations
The designation zero energy building (ZEB) and net zero energy building (NZEB) should be used only for buildings that have demonstrated through actual annual measurements that the delivered energy is less than or equal to the on-site renewable exported energy.
Owners of buildings designed to be zero energy, but that have not had a full year of operation demonstrating that they meet the requirements, are encouraged to identify their intent to be or return to being a zero energy building.
Conclusion
Zero energy buildings provide the inspiration to change how we think about energy in buildings. The com-mon definition specifies a goal that can be measured—the first step in achieving any mass-market shift. It is hoped that the definition will unite building owners, utilities and government agencies, allowing them to march in the same direction to provide clean energy for the future.
This common definition establishes metrics and boundaries to assist in verification and consistency of messaging. Many people were involved in helping to put this definition together. Inevitably, questions will arise as the definition is implemented, and we encourage this dialogue and will track the questions to help provide clarity for implementation of the definition. •
Note: Two of the primary authors of the Zero Energy Building definitions, nomenclature and guidelines in the “A Common Definition for Zero Energy Buildings” report are Kent Peterson and Paul Torcellini. Also, a portion of this article will be included in a Journal of the National Institute of Building Sciences article also authored by Peterson and Torcellini (2016).
References
1. DOE. 2015. “A Common Definition for Zero Energy Buildings.” U.S. Department of Energy prepared by National Institute of Building Sciences. http://tinyurl.com/zeroenergybldgs.
2. Torcellini, P; S Pless; M Deru; D Crawley. 2006. “Zero Energy Buildings: A Critical Look at the Definition.” Proceedings (CD-ROM) of the ACEEE Summer Study on Energy Efficiency in Buildings. http://tinyurl.com/6pvtua7
3. DOE. 2015. “Request for Information (RFI) for Definition for Zero Energy Buildings.” Federal Register 80(3): 499–500. http://tinyurl.com/fedreg-zeroenergy.
4. Federal Trade Commission. 2012. “Guides for the Use of Environmental Marketing Claims, Final Rule, Sect. 260.15(d).” Federal Register 77(197). http://tinyurl.com/phatujn.
5. Griffith, B; P Torcellini; N Long; D Crawley; J Ryan. 2006. “Assessment of the Technical Potential for Achieving Zero-Energy Commercial Buildings.” NREL/CP-550-42144. National Renewable Energy Laboratory and U.S. Department of Energy. Proceedings of the 2006 ACEEE Summer Study on Energy Efficiency in Buildings. 4:100-111. http://tinyurl.com/qz7yug3.
About the Authors
Kent W. Peterson, P.E., Presidential Member/Fellow ASHRAE, is vice president and chief engineer at P2S Engineering in Long Beach, Calif.
Paul Torcellini, Ph.D., P.E., Member ASHRAE, is a principal engineer at the National Renewable Energy Laboratory in Golden, Colo. and on the faculty at Eastern Connecticut State University.
