Net-zero energy buildings (NZEB) have been the subject of research initiatives at the National Renewable Energy Laboratory and in the Department of Energy in recent years. In 2006, we and our NREL colleague Michael Deru and our DOE colleague Drury Crawley published “Zero Energy Buildings: A Critical Look at the Definition,” an early attempt to reach a common definition, or even a common understanding, of what the term “zero energy building” means.

With the passage of the Energy Independence and Security Act of 2007, the pace of activity surrounding net-zero energy buildings quickened. EISA 2007 authorized the Department of Energy to host industry-led Com-mercial Building Energy Alliances and to establish the Net-Zero Energy Commercial Building Initiative, whose mandate is to support the goal of net-zero energy for all new commercial buildings by 2030.

EISA 2007 further specifies a net-zero energy target of 50% of all U.S. commercial buildings by 2040 and a net-zero standard for all commercial buildings, new and existing, by 2050. Toward this end, the Department of Energy has set a goal of creating the technology and knowledge base for cost-effective net-zero energy commercial buildings (NZEBs) by 2025.

In response to this aggressive agenda, in 2009 we, along with Dru Crawley, took the next step in our discussion of net-zero energy buildings with the publication, in ASHRAE Journal, of “Getting to Net Zero.”  Last year, we added another dimension to the definitions based on a hierarchy of possible renewable energy supply options for NZEBs, in “Net-Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options.”

This chapter summarizes the key points in our effort to create a workable set of definitions for NZEBs, based on these three documents. The formulation of the definitions was guided by two basic principles: 1) energy efficiency and demand-side technologies need to be optimized first, before renewable energy supply is considered; it is almost always easier to save energy than to produce it; and 2) the fewer the number of energy transfers, the better. Readers of this White Paper who wish to follow our discussion more closely are invited to access the original articles online.

SEEKING A WORKABLE CONSENSUS

The quest for ever greater precision in measuring energy performance has uncovered the need for greater precision in the definition of  “net-zero energy performance.” What do design and construction professionals, building owners, energy experts, government officials, and others involved in the built environment mean by this term?

In concept, an NZEB is a building with greatly reduced operational energy needs. In such a building, sufficient efficiency gains will have been made such that the remaining portion of the building’s energy needs could be offset by renewable technologies. An NZEB should have no adverse energy or environmental impacts associated with its operation. In other words, an NZEB should be highly energy efficient and capable of producing at least as much energy over the course of a year as it draws from the utility grid.

To arrive at a consensus definition, Building Teams involved in an NZEB project must evaluate two interrelated concerns:
• How will the team account for energy use? Some projects may target net-zero energy at the site. Others might allow purchased renewable energy to supplement on-site renewables, with that energy accounted for at the source. Still others might put primary emphasis on energy cost, with the goal being to offset any purchased energy with the sale of revenues from on-site renewable energy. Lastly, some might target net-zero emissions of greenhouse gases.
• What are the physical boundaries for choosing among renewable energy options? If a project targets net-zero energy use at the site, that limits the choice of renewables to sources and technologies available within the building footprint or at the site. Other projects might use renewable energy sources from beyond the site (e.g., biomass) to produce power at the site, while others might incorporate purchased renewables, such as renewable energy certificates.

Agreeing on energy-use accounting and the choice of renewables is pivotal to determining the design goals and strategies of NZEBs.

These factors guided us in formulating the following definitions for various types of net-zero energy buildings (note: NZEBs are assumed to be grid-connected):
Net Zero Site Energy: A site NZEB produces at least as much 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 sources. 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 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 NZEBs offset. To calculate a building’s total emissions, imported and exported energy is multiplied by the appropriate emissions multiplier, based on the utility’s emission and on-site generation emissions (if any).

CLASSIFICATION SYSTEM BASED ON RENEWABLES

More recently, we have added to our definitions by developing a classification system based on the renewable energy sources used in the four types of NZEBs. This classification system starts with the premise that all NZEBs must first reduce site energy use through energy efficiency and demand-side renewable building technologies, including such strategies as daylighting, insulation, passive solar heating, high-efficiency HVAC equipment, natural ventilation, evaporative cooling, and ground-source heat pumps.

The classification system breaks down NZEBs into two groups, one that uses on-site supply options, another that uses off-site renewables. At the highest level of the classification system is NZEB:A, a building that offsets all its energy use from renewable sources within its footprint. Next in rank is NZEB:B, which obtains some or all of its renewable energy from the project site—for example, photovoltaics that are mounted on the ground.

NZEB:C buildings use renewables from off the site, such as biomass or wood pellets. At the lowest end is NZEB:D, which uses a combination of on-site renewables and off-site purchases of renewable energy credits.

There is no “best” definition of net-zero energy buildings, nor is there a “best” method for accounting for energy use. Each has its merits and drawback, and Building Teams should select the appropriate approach for each project to align with the client’s goals. 

However, across all NZEB definitions and classifications, one design rule remains constant: reduce energy demand to the lowest possible level first, then address energy supply. NZEB teams should use all possible cost-effective energy-efficiency strategies first before incorporating renewables. Preference should be given to sources available within the footprint, such as solar hot water. Using on-site renewables minimizes the NZEB’s overall environmental impact by reducing losses incurred from transportation, transmission, and conversion losses of off-site renewable energy sources.

OFF-GRID NET-ZERO ENERGY BUILDINGS

Achieving an NZEB without the grid is very difficult, largely because the current generation of energy storage technologies is limited. Most off-grid buildings rely on outside energy sources such as propane for space heating, water heating, and backup generators. Off-grid buildings cannot feed their excess energy production back onto the grid to offset other energy uses. As a result, the energy production from renewable resources must be oversized. In many cases (especially during the summer), excess generated energy cannot be used.

It is possible, though, to have a grid-independent NZEB. To do this, any backup energy needs would have to be supplied from renewable resources such as wood pellets or biodiesel. An off-grid building that uses no fossil fuels could be considered a pure NZEB, as no fossil fuels or net annual energy balances would be needed or used.

NET-ZERO ENERGY BEYOND SINGLE BUILDINGS

As NZEBs become technically and economically feasible, extending their boundaries to groups of buildings—net-zero energy campuses, communities, towns, bases, and cities—may become more and more realistic. Extending the net-zero energy boundary beyond a single building addresses the emergence of communities, neighborhoods, and campuses that would generate renewable energy for a certain group of buildings; however, the energy would not necessarily connect directly to a specific building’s utility meter. This would be considered a community-based renewable energy system that would be connected to the grid or to a district heating or cooling system.

For a large organization or neighborhood, it is often more cost-effective and efficient to generate renewable energy in a central location on campus or in the community, rather than on (or in addition to) individual buildings. Community-scale systems allow for a single point for all maintenance and offer economies of scale—larger, central systems can be better optimized and cost less per kilowatt of generation capacity.
Community-based renewable energy systems, however, have some transmission and distribution losses when providing energy directly to a building. Inefficiencies and costs such as distribution piping and wiring, pumping losses, distribution transformers, and thermal losses are often associated with district distribution systems, whereas this is generally not the case with a building-based renewable energy generation systems.

The energy use accounting methods and renewable energy supply hierarchy concepts we have developed for standalone NZEBs still apply to net-zero energy communities. A parallel definition system further defines net-zero energy communities and extends the single-building net-zero concepts to multiple buildings with districtwide renewable energy systems.

ENCOURAGING BUILDING TEAMS TO ACT

This classification system begins ranking energy supply options in the NZEB context. As Building Teams and property owners look to design NZEBs, they must begin a discussion of which classification to seek in order to set workable goals for their projects. Since the publication of the initial NZEB definition paper we have applied these definitions to multiple real-world NZEB examples with various renewable energy options. Some of the buildings used to evaluate these definitions can be found in the Zero Energy Buildings Database, which was developed by the U.S. Department of Energy.

In addition to refining the definitions, we felt that it would be beneficial to classify buildings based on how well they achieve NZEB status by considering which renewable energy supply options they use. We have developed a simple flow chart that illustrates how to navigate the prerequisites and classification requirements to classify NZEBs.

This classification system is meant to encourage, when possible, energy-efficiency strategies, followed by the use of footprint and on-site renewable energy to power buildings. The long-term benefits of these options are numerous:
1.   Optimized usability of power-generation capacity in the NZEB context 
2.   Less reliance on the grid (and therefore less need for investment in the grid)
3.   Less energy required because energy losses through conversion, transmission, and distribution would be minimized
4. Fewer peak demand problems with utilities 

Ultimately, it is our hope that Building Teams will be encouraged to create more energy-efficient, high-performance structures if the buildings must generate their own energy. BD+C

Source: 

Author: Paul Torcellini, PhD, PE, and Shanti Pless, LEED A

Date: March 2011