Integration: Net-zero energy design
ASHRAE has a goal: net-zero energy for all new buildings by 2030. What do engineers need to know to achieve this goal on their projects?
- 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.