29-04-2014, 03:17 PM
Zero Energy Buildings: A Critical Look at the Definition
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ABSTRACT
A net zero-energy building (ZEB) is a residential or commercial building with greatly
reduced energy needs through efficiency gains such that the balance of energy needs can be
supplied with renewable technologies. Despite the excitement over the phrase “zero energy,” we
lack a common definition, or even a common understanding, of what it means. In this paper, we
use a sample of current generation low-energy buildings to explore the concept of zero energy:
what it means, why a clear and measurable definition is needed, and how we have progressed
toward the ZEB goal.
The way the zero energy goal is defined affects the choices designers make to achieve
this goal and whether they can claim success. The ZEB definition can emphasize demand-side
or supply strategies and whether fuel switching and conversion accounting are appropriate to
meet a ZEB goal. Four well-documented definitions—net-zero site energy, net-zero source
energy, net-zero energy costs, and net-zero energy emissions—are studied; pluses and minuses
of each are discussed. These definitions are applied to a set of low-energy buildings for which
extensive energy data are available. This study shows the design impacts of the definition used
for ZEB and the large difference between definitions. It also looks at sample utility rate
structures and their impact on the zero energy scenarios.
Introduction
Buildings have a significant impact on energy use and the environment. Commercial and
residential buildings use almost 40% of the primary energy and approximately 70% of the
electricity in the United States (EIA 2005). The energy used by the building sector continues to
increase, primarily because new buildings are constructed faster than old ones are retired.
Electricity consumption in the commercial building sector doubled between 1980 and 2000, and
is expected to increase another 50% by 2025 (EIA 2005). Energy consumption in the
commercial building sector will continue to increase until buildings can be designed to produce
enough energy to offset the growing energy demand of these buildings. Toward this end, the
U.S. Department of Energy (DOE) has established an aggressive goal to create the technology
and knowledge base for cost-effective zero-energy commercial buildings (ZEBs) by 2025.
Zero-Energy Buildings: Boundary Definitions and Energy Flows
At the heart of the ZEB concept is the idea that buildings can meet all their energy
requirements from low-cost, locally available, nonpolluting, renewable sources. At the strictest
level, a ZEB generates enough renewable energy on site to equal or exceed its annual energy use.
The following concepts and assumptions have been established to help guide definitions for
ZEBs.
Grid Connection Is Allowed and Necessary for Energy Balances
A ZEB typically uses traditional energy sources such as the electric and natural gas
utilities when on-site generation does not meet the loads. When the on-site generation is greater
than the building’s loads, excess electricity is exported to the utility grid. By using the grid to
account for the energy balance, excess production can offset later energy use. Achieving a ZEB
without the grid would be very difficult, as the current generation of storage technologies is
limited. Despite the electric energy independence of off-grid buildings, they usually rely on
outside energy sources such as propane (and other fuels) for cooking, 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.
Zero-Energy Buildings: Definitions
A zero energy building can be defined in several ways, depending on the boundary and
the metric. Different definitions may be appropriate, depending on the project goals and the
values of the design team and building owner. For example, building owners typically care
about energy costs. Organizations such as DOE are concerned with national energy numbers,
and are typically interested in primary or source energy. A building designer may be interested
in site energy use for energy code requirements. Finally, those who are concerned about
pollution from power plants and the burning of fossil fuels may be interested in reducing
emissions. Four commonly used definitions are: net zero site energy, net zero source energy,
net zero energy costs, and net zero energy emissions.
Each definition uses the grid for net use accounting and has different applicable
renewable energy sources. The definitions do apply for grid independent structures. For all
definitions, supply-side option 2 can be used if this resource will be available for the life of the
building. Off-site ZEBs can be achieved by purchasing renewable energy from off-site sources,
or in the case of an off-site zero emissions building, purchasing emissions credits. In support of
DOE’s ZEB research needs, the following definitions refer to ZEBs that use supply-side options
available on site. For ZEBs that have a portion of the renewable generation supplied by off-site
sources, these buildings are referred to as “off-site ZEBs.”
Net Zero Energy Cost Building
A cost ZEB receives as much financial credit for exported energy as it is charged on the
utility bills. The credit received for exported electricity (often referred to net energy generation)
will have to offset energy, distribution, peak demand, taxes, and metering charges for electricity
and gas use. A cost ZEB provides a relatively even comparison of fuel types used at the site as
well as a surrogate for infrastructure. Therefore, the energy availability specific to the site and
the competing fuel costs would determine the optimal solutions. However, as utility rates can
vary widely, a building with consistent energy performance could meet the cost ZEB goal one
year and not the next.
In wide-scale implementation scenarios, this definition may be ineffective because utility
rates will change dramatically. As energy-efficient building technologies and renewable energy
installations increase, the effects of large numbers of energy-efficient buildings must be
considered in a given utility’s service area. In addition to purchasing fuel to generate electricity,
electric utilities must provide dependable service, maintain capacity to meet potential loads, meet
obligations for maintaining and expanding infrastructure, and provide profitability for
shareholders. The fixed costs associated with these activities result in rate structures that provide
only limited incentive for consumers to create cost ZEBs. Trends in other utility sectors, such as
water districts, indicate that as buildings become more efficient, and consequently have lower
consumptive charges, the costs associated with infrastructure are increased. If significant
numbers of buildings achieved a zero energy cost, financial resources would not be available to
maintain the infrastructure, and the utility companies would have to raise the fixed and demand
charges.