Integration: Net-zero energy design
- Become familiar with commonly used definitions of net-zero energy.
- Know which metric is used to evaluate the energy performance of buildings and the practical maximum target for most zero energy building applications.
- Learn about building design strategies that contribute to achieving net-zero energy goals.
In January 2008, ASHRAE published a report titled ASHRAE Vision 2020: Providing tools by 2020 that enable the building community to produce market-viable NZEBs (Net-Zero Energy Buildings) by 2030. Several other organizations in the design and building sectors (see a white paper published by Building, Design + Construction: 2011 Zero and Net-Zero Energy Buildings and Homes) have put forward similar plans to transform the built environment from one of the largest consumers of energy in the United States to energy self-sufficient using clean renewable energy. In the five years since ASHRAE Vision 2020 was published, significant progress toward net-zero energy buildings has been made.
As net-zero energy and low-energy design projects become more prevalent, engineers must be prepared to collaborate with all members of a project team including architects, energy specialists, lighting designers, builders, and owners in order to accomplish net-zero energy goals with little to no cost premium. Is this possible today or will it take another 10 or more years to get there?
There are many examples of completed projects demonstrating that not only is this possible, but it has been done in all regions of the country using readily available building products and common construction methods. So what’s the secret? It’s all about the design.
Net-zero energy defined
The term “net-zero energy” is abundantly used, but a single universally accepted definition does not exist. In general terms, a net-zero energy building (NZEB) has greatly reduced energy needs achieved through design and energy efficiency, with the balance of energy supplied by renewable energy. In an effort to clarify the issue, the National Renewable Energy Laboratory (NREL) published a paper in June 2006 titled “Zero Energy Buildings: A Critical Look at the Definition,” in which it defined the following four types of NZEBs:
- Net Zero Site Energy: A site NZEB produces at least as much renewable energy as it uses in a year, when accounted for at the site.
- Net Zero Source Energy: A source NZEB produces (or purchases) at least as much renewable energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to extract, process, 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 based on the utility’s source energy type.
- Net Zero Energy Costs: In a cost NZEB, the amount of money the utility pays the building owner for the renewable 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 (or purchases) enough emissions-free renewable energy to offset emissions from all energy used in the building annually. Carbon, nitrogen oxides, and sulfur oxides are common emissions that zero-energy buildings offset. To calculate a building’s total emissions, imported and exported energy is multiplied by the appropriate emission multipliers based on the utility’s emissions and on-site generation emissions (if there are any).
A subsequent paper was published by NREL in June 2010 titled “Net-Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options,” where four classifications of NZEBs were defined:
- NZEB:A: Building generates and uses energy through a combination of energy efficiency and renewable energy (RE) collected within the building footprint.
- NZEB:B: Building generates and uses energy through a combination of energy efficiency, RE generated within the footprint, and RE generated within the site.
- NZEB:C: Building generates and uses energy through a combination of energy efficiency, RE generated within the footprint, RE generated within the site, and off-site renewable resources that are brought on site to produce energy.
- NZEB:D: Building uses the energy strategies described for NZEB:A, NZEB:B, and/or NZEB:C buildings, and also purchases certified off-site RE such as Renewable Energy Certificates (RECs) from certified sources.
In the ASHRAE Vision 2020 report, net-zero site energy is the building type chosen through an agreement of understanding between ASHRAE, the American Institute of Architects (AIA), the U.S. Green Building Council (USGBC), and the Illuminating Engineering Society (IES). When working on a net-zero energy project, engineers must have a clear understanding of the type of net-zero energy building being pursued as this greatly influences project goals and design decisions. While Net Zero Site Energy is often used for projects, one of the other types may be selected for any particular project depending on the objectives of the owner.
Integrated building design
Integrated building design is a process that promotes holistic collaboration of a project team during all phases of the project delivery and discourages the traditional sequential philosophy. According to ASHRAE, the purpose of the integrated design process is to use a collaborative team effort to prepare design and construction documents that result in an optimized project system solution that is responsive to the objectives defined for the project. NZEB must be designed collaboratively using a “whole systems” approach recognizing that the building and its systems are interdependent. As such, the integrated building design process has proven to be effective for net-zero energy projects.
The project vision and goals are clearly defined at the onset of design. This includes specifying the type of NZEB (site, source, costs, or emissions) and targets for energy reductions compared to a baseline building. The design team comprises all project stakeholders including the building owners (administration, facility manager, users, staff), design professionals (architects, engineers, designers), consultants (energy specialist, USGBC LEED project administrator, commissioning authority), construction professionals (general contractor, subcontractors, construction management), and code enforcement (zoning, building, environmental). All parties are involved throughout the design process, with decisions made collectively rather than in isolation. More time and energy is invested early in the design process. Systems are considered in relation to others, allowing for full optimization with an emphasis on lifecycle costs and benefits rather than up-front costs. Success is driven by having a team that is aware of and buys into the vision and goals and that communicates effectively.
Commissioning is an important part of every project, and for NZEB projects the commissioning authority should be a member of the design team and involved throughout the design process. Best practices for HVAC commissioning can be found in the ACG Commissioning Guideline, and for lighting commissioning in the IES Guide for The Commissioning Process Applied to Lighting and Control Systems.
Energy reduction is the most important design criteria for a NZEB. The architectural design of the building (orientation, insulation, building envelope, passive solar strategies) will account for much of the energy savings. Engineers must design and specify building systems that use the least amount of energy possible while providing the required amount of space conditioning and power for the building operations and occupants.
An energy reduction target should be set, and this will drive design decisions. Keep in mind that all the energy needed by the building must be supplied with renewable energy. Conventional payback analysis for efficiency measures must be avoided. Rather, efficiency opportunities must be compared to the cost of generating the equivalent amount of renewable energy. It is almost always easier and less expensive to save energy than to produce it, so efficiency measures that may not be financially viable on a typical building may prove to be effective on a NZEB.
Building energy modeling is a key part of the design process. It is used to assess building energy use and the interactive effects of energy efficiency measures. A baseline building design should be established early and modeled. As the building and system designs are refined, the energy model should be updated so the design team can determine whether the energy reduction goals are on track. Some building design applications contain a built-in energy modeling component. Often specialized energy modeling software is used, such as eQUEST (commercial buildings), BEOpt (residential buildings), or one of the many building energy software tools listed on the U.S. Dept. of Energy’s Energy Efficiency & Renewable Energy website.
Energy use intensity (EUI, measured in kBtu/sq ft/yr) is the metric used to evaluate the energy performance of buildings. The U.S. Energy Information Administration publishes the Commercial Buildings Energy Consumption Survey (CBECS), which is a national sample survey that collects information on the stock of U.S. commercial buildings, their energy-related building characteristics, and their energy consumption and expenditures. The most recently published survey contains data from 2003, and work is underway to compile data for 2012.
The 2003 national average EUI of all U.S. commercial buildings is 93 kBtu/sq ft/yr. In the New Buildings Institute research report “Getting to Zero 2012 Status Update: A First Look at the Costs and Features of Zero Energy Commercial Buildings,” the EUI of the zero-energy buildings considered in the study was 9 to 35 kBtu/sq ft/yr. The National Renewable Energy Laboratory (NREL) considers “highly energy efficient” buildings as those using 60% to 70% less than CBECS and refers to a target in the 25 to 30 kBtu/sq ft/yr range as a practical maximum for most zero-energy building applications. Achieving energy reductions of this magnitude is a big challenge, but as demonstrated by example projects across the country, it is achievable.
ASHRAE Vision 2020 established a baseline for buildings referencing ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings representing the “turn of the millennium.” Energy reduction targets from the baseline are phased in over time until 2030 when all buildings should be designed for net-zero energy. ASHRAE Standard 189.1: Standard for the Design of High-Performance, Green Buildings Except Low-Rise Residential Buildings, will be updated over time as the energy reduction targets increase.
Some of the most useful design resources are the Advanced Energy Design Guides (AEDG) published by ASHRAE and developed in collaboration with the AIA, the IES, the USGBC, and the U.S. Department of Energy (DOE). The original series of guides have an energy savings target of 30% over ASHRAE Standard 90.1-1999, while the most recent series of guides have an energy savings target of 50% over ASHRAE Standard 90.1-2004. Each guide addresses a specific building type.
The guides contain prescriptive energy-saving recommendations for each of the eight U.S. climate zones. They also contain a chapter on how to implement recommendations. By following the recommendations, advanced levels of energy savings can be achieved.
It is beneficial to know which areas to target for energy use reduction. The areas of highest energy use hold the greatest potential for reduction in terms of the overall total. For example, if water heating accounts for 2% of overall energy use, a 50% reduction is not nearly as significant as a 20% reduction in lighting, which accounts for 25% of overall energy use. Figure 1 shows the energy use intensity (EUI) breakdown for typical commercial office buildings. Lighting, heating, and cooling account for the majority of energy use, and these are the areas typically targeted for efficiency savings. Figure 2 shows the EUI for an example NZEB, NREL’s Research Support Facility in Golden, Colo. As lighting, heating, and cooling are optimized for efficiency, other categories constitute a more significant portion of the total energy use. In this case, the data center is the most significant energy use. For office building types and perhaps others, computing should be an area of focus for engineers.
Building energy savings
The following are energy-saving strategies for engineers to consider for some of the more significant end-use categories.
Lighting—With an abundance of efficient lighting products available, significant energy savings can be achieved through lighting design. The best strategy is to eliminate the need for artificial lighting. This is best accomplished though the building design allowing sufficient natural daylight to enter interior spaces. This must be done with caution because heat gain comes with daylight. Additional heat may or may not be desirable (depending on geographic region, time of year, and/or building use), so daylighting strategies must be carefully evaluated for the site.
Both fluorescent and LED fixtures offer outstanding efficacy (W/sq ft). Illumination levels for spaces should comply with IES guidelines. While LED still carries a price premium, the benefits often make it the right choice for a NZEB. The inherent dim-ability of most LED lamps and fixtures makes them well-suited for daylighting applications. In areas not in continuous use (offices, restrooms, conference rooms, etc.) occupancy/vacancy sensors should be used so that lights are automatically turned off when they are not needed.
A reduction in lighting loads may result in an ancillary benefit to the HVAC system. All of the electrical energy supplied for lighting eventually becomes heat. In spaces that are cooled, the reduction in heat load from efficient lighting results in a lower peak cooling load and less energy required to cool the space.
HVAC—Geothermal heat pump, or ground source heat pump, systems are a popular choice for NZEB. With coefficients of performance much greater than one, the ability to both heat and cool, and the fact that they operate on electricity that can be produced with on-site renewable energy systems (rather than fossil fuel), they are a natural fit. While geothermal is considered a form of renewable energy, it is categorized as a demand-side technology. Similar to passive solar space heating and solar ventilation air preheaters, geothermal reduces the need for energy production. This is in contrast to supply-side renewable energy technologies such as solar photovoltaics and wind turbines that contribute toward the balance of energy in order to achieve net-zero energy status.
In environments with simultaneous heating and cooling loads, variable refrigerant flow (VRF) systems are an energy-efficient option. Another efficiency trend is the shift from air to water as the medium for energy. Radiant heating and cooling systems have been successfully used in several NZEBs.
Ventilation control is an area that can result in energy savings. A dedicated outdoor air system (DOAS) combined with a parallel mechanical system has the potential to use less energy than a conventional variable air volume (VAV) system by eliminating the need for excess airflow and by reducing the energy needed for terminal reheating. Heat or energy recovery should be employed wherever possible (for example, in the DOAS). In regions where natural ventilation is viable, it should be used instead of mechanical cooling whenever conditions are feasible.
Temperature setpoints should be evaluated and lowered (winter) and raised (summer) as appropriate. One advantage of radiant heating and cooling systems is that more aggressive setpoints can be used while still maintaining a satisfactory level of comfort for occupants. Aggressive setbacks and reduced ventilation should be employed during unoccupied periods.
After the building energy needs have been reduced as much as possible, the remaining energy must be generated with renewable energy. Ideally, all generation is located within the building footprint as shown in Figure 3. For many buildings this is not possible due to space constraints, in which case the second-best scenario is for all generation to be within the site. In addition to rooftop solar, this may require a ground-mounted solar array, a solar-carport structure in the parking lot, a wind turbine on the property, or all three as shown in Figure 4.
Many NZEB initiatives allow for a limited amount of renewable energy to be produced or purchased off-site. For example, ASHRAE Vision 2020 states that renewable energy credits (RECs) should not be permitted to offset building nonrenewable energy use or carbon emissions for more than 50% of the building’s net energy consumption. If this was not limited, a building could simply buy its way to net-zero energy status without reducing energy needs or deploying renewable energy on-site. Some programs are more stringent; for example, Architecture 2030 allows a maximum of 20% off-site renewable energy generation for the required reduction targets.
The design of renewable energy systems is a key part of every net-zero energy project. It is advisable to include on the project team a firm or consultant with specialized knowledge of and experience with renewable energy technologies. System modeling and energy production analysis must be completed to ensure that sufficient energy can be generated to achieve a net-zero energy designation. Commissioning renewable energy systems is also important and will require a commissioning authority with renewable energy experience.
Net-zero energy buildings are the future of the building industry, and for some the future is today. While it may take some time to get the entire building community there, an increasing number of architects, engineers, and building professionals are well on the way to fulfilling the vision of making net-zero energy buildings the norm by 2030.
Scotte Elliott is an electrical engineer and energy specialist at Metro CD Engineering in Powell, Ohio. He is a Certified Energy Manager, a NABCEP Certified PV Installation Professional, and past division chair of the American Solar Energy Society.
See NZEB success stories and how to save energy on data centers below.