The Challenge of Defining "Net-Zero"

Welcome to 2020, the year of reckoning. For many governments, organizations, trade associations, and industrial sectors, 2020 is a target year for fulfilling certain environmental goals. For example, Germany and Sweden committed to reducing primary energy consumption by 20% as compared to 2008, according to the 2015 Energy Efficiency Market Report by the International Energy Agency. The 2007 Paris Climate Action Plan called for 25% reduced energy consumption, 25% reduced emissions, and 25% of energy from renewable sources by 2020. The U.S. aimed to improve lighting efficiency by 70% and mandate net-zero energy in all new federal buildings exceeding 5,000 square feet. In California, all new residences must now be constructed to a zero-net energy standard. According to the World Wildlife Fund, 2020 must be “the year for coordinated, comprehensive climate action.” In many cases, this action involves the attainment of net-zero energy.

Yet, what exactly does net-zero energy mean? What seems to be a self-explanatory term is, in fact, not so clear-cut. According to Ming Hu, an assistant architecture professor at the University of Maryland, more than 70 different definitions for reduced or net-zero energy are currently in use. Her book Net Zero Energy Building: Predicted and Unintended Consequences (Routledge, 2019) reveals the inconsistencies between the different standards, and the unanticipated ramifications of these discrepancies. Hu argues that we must establish a holistic framework for the consistent calibration of net-zero energy goals moving forward.

In her book, Hu outlines the four primary established definitions, which pertain to matters of site, source, cost, and emissions. A net-zero site energy building consumes no more energy than it produces “when accounted for at the site.” Although such a structure maintains the energy balance sheet, this definition does not account for differences in potential fuel types. A net-zero source energy building consumes no more energy than it produces when measured at the source. This definition addresses the inefficiencies of centralized energy production.

For example, in one National Renewable Energy Laboratory calculation, a unit of electricity requires 3.37 units of natural gas at the source. A net-zero energy cost building is one that produces enough energy (or receives enough subsidies) to “zero-out” its power meter from a cost perspective, thus requiring no utility payment. This definition, however, fails to address price differences between different utilities, and building owners must prepare for inevitable price fluctuations in electricity costs.

A net-zero emissions building consumes no more energy from emissions-generating sources than it produces without emissions. Like the net-zero source definition, this approach most closely approximates the fundamental goal of net-zero consumption—although it too can necessitate complicated energy conversion calculations.

Not only do these definitions reveal accounting inconsistencies, but they are also inherently incomplete. Because these measures focus on operating energy, they fail to address other forms of energy and resources used throughout the life cycle of a building. The first unintended consequence is the missing ecological impact related to the embodied energy of materials and assemblies. This energy is estimated to comprise the bulk of a building’s energy footprint for the first 15 to 20 years of its life.

The second unintended consequence is the emphasis on individual buildings and not their contexts. Such an approach ignores the adverse environmental ramifications of low-density, suburban-style development with its inherent infrastructural inefficiencies and reliance on the automobile. The third unintended consequence is the increased—and unaccounted for—ecological impact of some operational energy-optimizing assemblies, such as energy-intensive solar systems or hyper-insulated building enclosures.

Hu addresses both the inconsistency and incompleteness of the current standards with a new proposed methodology she calls Zero Impact Building (ZIB). This framework includes calculations of embodied energy, occupant transport energy, induced energy (“fringe social and economic impacts from the interaction between individual buildings and their surrounding urban context”), and other factors. The ZIB goals include zero impact on land use, the environment, water, and human health. Organized according to the four main categories of energy, water, environment, and health, Hu’s framework has a clear aspiration: “A zero impact building is a building that has no negative environmental or human health impacts.”

Measuring ZIB involves the application of four representative methods: life cycle energy assessment, life cycle water assessment, life cycle environmental impact assessment, and life cycle health assessment. These calculations are converted to the triple bottom line impact categories of environmental, economic, and social. Once determined, these figures are then applied to three different scenarios—conventional, conservative, and progressive—to establish a range of potential outcomes for a proposed building and its site.

Sound simple? Unfortunately, no. Such a comprehensive analysis requires a concomitant amount of effort, and the methodologies used in certain phases—such as scenario analysis—are not well understood. Readers seeking a correspondingly thorough explanation of the required steps to calculate ZIB will be disappointed, as Net Zero Energy Building merely introduces the framework concept. As a result, we are left to ponder questions such as how to measure each component, how to apply appropriate weights to each factor (for example, is water LCA equal to energy LCA?), how to normalize the various units, how to conduct error-checks and address interoperability issues, and so on. Most significantly, the holistic nature of ZIB environmental impacts suggests that achieving zero impact may be virtually impossible. It is difficult to imagine that any building could, at least within its first several decades of use, produce enough energy to offset all of its embodied and operational impacts.

However, if Net Zero Energy Building merely scratches the surface of a “unified theory” of buildings’ environmental impact, the book does reveal the known unknowns. Hu convincingly demonstrates the extent to which our current understanding of net-zero energy is wholly inadequate and misleading. Furthermore, her ZIB framework—despite its lack of development and testing—seems to be a compelling and sound approach to estimating the true ecological effects of the built environment. As we take stock in 2020 and beyond, this approach is a welcomed and much-needed contribution.