Net-Zero Energy: A Directional Goal for Commercial Buildings

By Jeff Harris, Alliance to Save Energy

One question raised repeatedly in almost all of the CBC Working Group calls was “What do we really mean by a ‘net-zero energy (NZE) building’?”  And equally important, what does DOE mean by net-zero, and what did Congress have in mind when authorizing a “zero net energy commercial buildings initiative” in section 422 of the 2007 Energy Independence and Security Act (EISA)? 

The closer we look, the more complicated this question becomes – and there may not be a single “correct” answer.  Fortunately, though, it may not be a question that we have to answer precisely, right now, in order for the Consortium to make meaningful progress on our current tasks for DOE in technology roadmapping and market and policy assessments.  This article is based in part on a recent paper by Kate Marks et al. to be presented at the ACEEE Summer Study this month.  We provide some background on different approaches to defining net-zero, references to other sources of information, and a brief overview of issues that may be worth more discussion among members of the Consortium. 

Agreeing on a common definition of net-zero energy is not a simple task. In the EISA-2007 legislation Congress defined “zero-net-energy commercial building’’ as “…a high-performance commercial building that is designed, constructed, and operated—

• “to require a greatly reduced quantity of energy to operate;
• “to meet the balance of energy needs from sources of energy that do not produce greenhouse gases;
• “in a manner that will result in no net emissions of greenhouse gases; and
• “to be economically viable.”

A paper by Paul Torcellini of the National Renewable Energy Laboratory (NREL) offered four practical definitions:

• Net Zero Site Energy: A building that produces at least as much energy as it uses in a year, when accounted for in site energy (not including electricity system losses).
• Net Zero Source Energy: A building that produces at least as much energy as it uses in a year, when accounted for in source (primary) energy, including electricity system losses.
• Net Zero Energy Costs: A building that receives at least as much annual revenue from the utility for on-site energy exported to the grid as the amount paid to the utility (or utilities) for energy used over the year.
• Net Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources. (Torcellini et al. 2006).

These alternative definitions raise several important questions. There may be no single definition that works in all cases, yet it is important to establish some common ground for purposes of policy and program direction, setting research goals, or specifying performance expectations for a new building design or retrofit.

In general, a net-zero energy building will start with an integrated whole-building design process; maximize energy-efficient envelope, equipment, and design features (including daylighting and passive heating/cooling/ventilation where possible); and carefully monitor and control all installed mechanical and electrical systems (including plug loads) to assure that they only operate when needed. With these strategies reducing annual energy demand by about 80% compared with a typical building today, the remaining loads are at a scale where they can be met with on-site power generation (most commonly solar PV). 

Analyses by NREL show that the feasibility and cost of reaching NZE performance varies significantly by building type; the potential also varies by climate zone and site-specific opportunities or constraints.  Thus, we should think of the NZE goal as an average to be achieved across all new commercial construction – and eventually for the entire stock. But the need to average together performance of many buildings brings us back to the question of exactly what metric(s) to use to aggregate (or to compare) building energy performance. While “zero is zero” using any measurement scale, this does not eliminate the metrics question, since: a) a building’s needs may be met by a combination of electricity and fuel (natural gas), and b) to verify the net-zero annual goal we need to aggregate a building’s performance at different times of the year, to see if the energy surpluses at one time offset energy deficits at other times. Here are some of the considerations to keep in mind:

-          Site vs source energy – Energy performance metrics for commercial buildings such as ENERGY STAR benchmarking/“Portfolio Manager” are leaning toward source (primary) energy accounting since it considers off-site energy losses in energy production and distribution, especially for electricity.  Source energy also tracks more closely with energy costs and with carbon, compared with site (delivered) energy, and avoids inadvertently signaling an efficiency gain when an existing building changes from on-site fuel consumption to electricity (off-site fuel consumption) for a particular end-use.

-          Economics – For widespread adoption, NZE criteria must also consider economic feasibility; the total cost of ownership (including payments to amortize first-costs and the annual energy bill plus and other operating costs) needs to be in the same range for an NZE building as for a conventional building – at least after externality costs are considered or perhaps compensated for.

-          Grid connectivity vs grid-independence – Zero-energy does not necessarily imply grid-independent, and in fact most zero-energy buildings will likely continue to be grid-connected (except for facilities at remote sites). The concept of a net zero building implies grid-connectivity, although such buildings are more likely to be resilient and able to function in the event of a grid failure or brownout.

-          Peak electrical demand, load shape, and grid impact – A building whose peak electricity demand coincides with the electricity grid peak is a liability rather than an asset to all other customers on the grid. Nor is excess power from a net-zero building as valuable if it is produced during off-peak hours.  In other words, the timing of both electrical and thermal loads in an NZE building may be as important as the magnitude of those loads, and the ability to control or shift the timing of loads, or to store energy on site, can have added value well beyond the “annual net” energy use.  This value also applies to short-term control for purposed of demand-response, based on a utility price signal or control signal.

-          Renewable energy credits (and urban density) – In some cases, use of off-site, purchased renewable electricity or “renewable energy credits” may be included in the NZE equation. It could be argued that off-site renewable energy purchases should count toward reducing a building’s annual net energy consumption, especially where the physical or economic feasibility of producing renewable power may be greater not on the building’s rooftop but at another location – whether close by in a campus setting, in the same community, or elsewhere in the region.   

-          Net-zero building or “community” – The flip side of this argument is that requiring each building to produce the needed renewable energy within its own footprint is also an argument for low-rise, and thus low-density, sprawling development (Carlisle et al 2009, Malin 2010).  We could modify the NZE criterion to suggest that a building’s energy use be reduced to the level that could be met with rooftop PV (for example) if the building were only 2 or at most 3 stories. And then for higher-rise buildings that are part of a denser urban form one could aim at the same energy consumption target but allow the renewable energy to come from another site. As NZE as a concept continues to gather momentum, we must be mindful that it does not dilute density or propel development towards low-rise sprawl which may technically satisfy NZE for an individual building. 

-          Indirect and induced energy use (and “Scope 3” GHG emissions) – An NZE building located at a great distance from other development may have a much larger total energy or carbon footprint. So the performance metric(s) need to somehow reflect the energy used to transport building occupants and other users (as well as operating supplies and waste materials) to and from the building; as the LEED rating system demonstrates this can affect not only building location decisions but also community-scale decisions about development density; access to transit, bicycle, or pedestrian paths; and the mix of land-uses in the building’s vicinity.

-          Embodied energy (and carbon) – Embodied energy in building materials, equipment fabrication, and the construction process itself currently represents about 15% of the energy used directly during the building’s occupied lifetime. The relative importance of this embodied energy will grow as in-use energy is reduced; for a NZE building at ~20% of today’s average, embodied energy might approach the same magnitude as use-energy, so it merits attention as we progress toward NZE.

-          Beyond energy:  overall building performance – Building owners, today’s leading designers, and green building rating systems such as LEED, Green Globes, and the Living Building Challenge fully recognize that a low-energy building will not be acceptable to occupants or commercially viable unless it also provides the expected level – and preferably superior levels – of building services and amenities, and in particular thermal comfort, lighting quality and visual comfort, and  indoor air quality.

-          Lifetime performance – With most new commercial buildings expected to remain in service for many decades, we clearly care about not only initial performance but sustained energy performance (and indoor environmental quality). A related concept is that a new or renovated building might be configured not only to maintain performance but to improve it in the future – in other words to be “future-proofed” or “NZE-ready.” For example, while some costly features (such as rooftop PV, solar hot water panels, or a ground-loop for heating and cooling) are not installed initially, roof space, mechanical room access, and wiring or plumbing can be made available to more easily add them later.

-          Water Consumption – Water use is increasingly important in many regions, and municipally-supplied potable water also embodies significant amounts of energy in supply and post-use treatment (as well as water heating for some uses).

Although the CBC has not adopted a formal definition of a NZE building, we accept the need to consider all the elements mentioned above, and perhaps others.  What we do know is that a high-performance commercial NZE building must not only maximize energy efficiency to achieve very low energy use at an annual level that could be met with on-site renewable energy, but must also take into account:  GHG emissions; economic feasibility; load-shape impacts on the utility grid and other customers; indoor environmental quality and occupant comfort and amenity; energy embodied in construction; transportation energy indirectly required for occupant and user access; and efficient use of water and other non-energy resources.  At present, the CBC will focus on how to achieve major reductions in energy while keeping these other factors in mind, and recognizing that a more precise definition will be more important as we get closer to zero. 

SELECTED REFERENCES:

Carlisle, N. et al. 2009. “Definition of a Zero Net Energy Community.” NREL technical report NREL/TP-7A2-46065.  http://www.nrel.gov/docs/fy10osti/46065.pdf

Madsen, Jana J. 2007. “Zero-Energy Buildings Defined.” http://www.buildings.com/ArticleDetails/tabid/3321/ArticleID/4987/Default.aspx.Cedar Rapids, IA.: Buildings.com.

Malin, N. 2010.  “The Problem with Net-Zero Buildings (and the Case for Net-Zero Neighborhoods).”  Environmental Building News, 19:8. August. http://www.buildinggreen.com/auth/article.cfm/2010/7/30/The-Problem-with-Net-Zero-Buildings-and-the-Case-for-Net-Zero-Neighborhoods/

Torcellini, P., et al. 2006. “Zero Energy Buildings: A Critical Look at the Definition.” Paper presented at the ACEEE Summer Study, Pacific Grove, Calif., August 14-18. http://www.nrel.gov/docs/fy06osti/39833.pdf

“Zero-Energy Buildings.”  Wikipedia article.  http://en.wikipedia.org/wiki/Zero-energy_building

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