e-News #86: Zero Net Energy Buildings

October 10, 2012
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Zero Net Energy:
The zero net energy future starts today

"In five years, anyone would be crazy to design a building that isn't green.  But I'll bet you that ... in five years [it] won't just be about green buildings.  It will be about zero net energy buildings, and about technologies to increase the amount of excess energy building owners can sell on the grid."

-Former President Bill Clinton, Greenbuild Keynote Address, November 2007

The ZNE Goal

Five years ago, zero net energy (ZNE) may have seemed lofty, but in 2012 it is, indeed, all about ZNE buildings. As part of the statewide "Big and Bold Energy Efficiency Strategies" goals, California will require all new residential construction to be zero net energy by 2020 and all new commercial construction to be zero net energy by 2030.  This is in alignment with other nationwide voluntary programs such as the 2030 Challenge that calls for incremental reduction in fossil fuel required to provide energy to buildings with an end goal of carbon neutral by 2030.  In order to achieve these goals, we must understand how to define and quantify ZNE, approach ZNE by project type, and move forward with ZNE.

Defining ZNE

To achieve ZNE goals, it is first important to understand the definition of ZNE. Some similar concepts to ZNE include carbon neutral, climate neutral, and carbon advantage. However, these definitions do not necessarily key in on energy as the defining factor.  There is general agreement that a ZNE building will produce as much energy as it uses.  This is made possible first with reduced energy needs achieved through energy efficiency measures and second with the balance of energy needs supplied by renewable technologies.  Further, ZNE energy balance is generally defined on an annual basis, meaning a building might consume more energy than is generated at some points in the year, but then compensate by generating more energy than consumed at other points.  This broad definition still leaves room for interpretation and more detailed definitions of ZNE have been and are still being developed. 

Currently, four Zero Net Energy definitions developed by the National Renewable Energy Laboratory (NREL) are the most robust and widely accepted.  These definitions are based on boundary and metric associated with different scales of a project.  The four definitions of Zero Net Energy defined by NREL as Net-Zero Site Energy, Source Energy, Energy Cost, and Energy Emission. These definitions are outlined below:

  • Net Zero Site Energy: A site ZNE building produces at least as much energy as it uses in a year, when accounted for at the site.
  • Net Zero Source Energy: A source ZNE building produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site.
  • Net Zero Energy Cost: In a cost ZNE building, the amount of money the utility pays the building owner for the 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 Emission: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.

More information about ZNE and the NREL definitions can be found here.

Quantifying ZNE

With the variation of definition of ZNE, it is important to understand how to quantify ZNE depending on the definition used for ZNE.  Since the goal is to reduce energy, it is important to recognize energy is delivered to buildings as electricity and/or gas.  Electricity is typically measured using kilowatt-hours or kWh. Gas is typically measured using therms; however, for larger customers the measurement is sometimes in thousands of cubic feet of gas or Mcf.  Mcf must be multiplied by the thermal content of the gas to determine therms.  Depending on the ZNE definition, understanding these two energy sources at several levels is important.

The most common approach to ZNE is through site energy.  Electricity and gas are converted to a common energy unit of British Thermal Units or Btus.  kWh and therms convert simply to Btus giving a common unit allowing analysis to compare reductions in electricity or gas consumption.  This approach encourages reduction in all energy consuming devices from electricity-consuming appliances to gas-consuming hot water heating.

Figure 1Figure 1: Typical Commercial Energy End Use

Figure 2Figure 2: Typical Residential Energy End Use

Source: DOE Energy Trends Commercial and Residential

Analysis for source energy is more complex and requires an understanding of primary energy and how it is generated, stored, transported and delivered to the ZNE site.  Under this approach, electricity and gas will carry different weight, referred to as source-site ratios, depending on the losses incurred in generation, storage, transportation and delivery of each energy type. To calculate a building's total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers. Electricity typically has a site-to-source conversion of approximately 3.3, while natural gas has a conversion of about 1.1. This varies locally because it takes into account specific variables, thus every site will carry a unique source-site ratio.

Energy cost inherently uses the dollar as the common denominator.  Analysis by energy cost then seems to be the easiest way to analyze ZNE. However, this poses complications due to the variability of cost of different fuels by availability.  This fuel availability changes by time, location, and demand.  Energy by costs only provides a snapshot in time based on resource cost and availability at the time of analysis.

Energy emission is the most complex definition of ZNE requiring an in-depth analysis of the emissions associated with energy generation and transportation.  Emissions commonly evaluated in this analysis include Carbon, NOx, and SOx.  In calculating impact, imported and exported energy is multiplied by the appropriate emission multipliers based on the utility's emissions and on-site generation emissions. Energy emission, then, inherently takes into consideration of source energy, but takes this a step further to analyze the correlated emission impact.

ZNE Buildings and Communities

Different building types and communities must focus on different energy issues.  Commercial and residential buildings have different load profiles and different occupant behavior, so they must address different energy efficiency measures to approach ZNE.  Commercial buildings typically operate with regular scheduled hours of operation, are controlled by a building automation system, and have predictable process loads, so energy efficiency strategies can be more predictably incorporated to reduce demand.  Residential buildings have less predictable loads and are more dependent on appliances and occupant behavior.  Also, heating and cooling is a larger fraction of the total energy consumption in residential buildings than it is in commercial buildings.  Research has found that the ability to achieve ZNE in any particular dwelling unit is highly correlated with occupant operational behavior.  For both commercial and residential ZNE buildings, the remaining energy must be supplied by on-site renewable energy.

Figure 3Figure 3: Milestones for ZNE Communities

Source: Definition of a "Zero Net Energy" Community, Pless et al, Nov.2009, Figure 2.

A ZNE community offers additional solutions to producing on-site energy such as community energy storage and residential energy storage.  Energy storage in a community will allow electric and thermal energy, including locally generated renewable energy, to be stored and used locally, reducing the load on the grid infrastructure.  A ZNE community considers energy uses including energy for vehicles, thermal, and electric energy within the community.  The thermal and electric energy within a community are to be divided by building and industry use.  As with ZNE buildings, a ZNE community reduces energy needs through energy efficiency measures and also addresses the remainder of vehicle, thermal, and electrical energy needs within the community with renewable energy.

Infrastructure for ZNE

ZNE buildings will incorporate renewable energy technologies that produce electricity; therefore, the infrastructure for electricity will be affected by the change to ZNE buildings.  California's energy policy recognizes an electricity "loading order" for meeting electricity demands. The loading order is (1) energy efficiency and demand response, (2) renewable resources, and (3) clean and efficient natural gas-fired power plants.  Further, AB 32 requires a 33% reduction in greenhouse gas emissions by 2020.  ZNE buildings will help to address these goals. However, their effect on the electricity grid is uncertain. ZNE buildings will export and import electricity to and from the grid, which will change the relationship between the utility customer and the utility.

As noted, ZNE is assessed on an annual basis. A ZNE building will affect the grid on a weekly, daily, hourly, and even near-instantaneous basis.  The daily and weekly grid profile shown (below) exemplifies the change in demand.  A grid -connected customer will demand less electricity from the grid; however, that customer will still be using grid services to export and import electricity. Most current rate structures are based on use by volume of electricity, yet these rate structures do not account for infrastructure use.  With an increase in electricity-producing buildings and communities, it is likely the rate structure will change based on new factors that better account for the infrastructure required to import and export decentralized electricity.  This may take into account fluctuations in the demand profile such as timing and magnitude.

Moving Forward Toward ZNE

Figure 4Figure 4: A grid-connected customer with distributed energy resources demands less total electricity from the grid, but uses grid services to export and import electricity while the demand profile (timing and magnitude) could change dramatically.

Source: Sustainable Zero Net Energy - Identifying the Essentials for Solutions, Lacy et al., ACEEE 2012

Five years ago, ZNE buildings were still considered nearly experimental with a few progressive private and public sector buildings attempting ZNE at various scales.  These forerunners worked with new technologies, strategies, and even project management structures  -- such as integrated design -- to create an example for the next generation of ZNE buildings and communities. Developing to a ZNE standard is currently voluntary. However, standards are being updated across the nation with goals like California's "Big and Bold Energy Efficiency Strategies" requiring ZNE for all new residential construction by 2020 and all new commercial construction by 2030.  Mandatory and voluntary requirements are now in place to help the building industry phase into ZNE including CalGreen requirements and local Reach Codes.  Title 24 will continue to move toward these ZNE goals as well, allowing the industry to learn how to reach ZNE over the next 8 to 18 years.  ZNE pilot programs, educational programs, and design competitions also are encouraging the building community to become more educated and incentivized to start moving forward toward ZNE now.

Training Highlights

California utilities offer outstanding educational opportunities that focus on the design, construction and operation of energy-efficient buildings. Listed here are a few of the many upcoming classes and events; for complete schedules, visit each utility's website.

Zero Net Energy Homes "People and Plug Loads" - Part III
People and Plug Loads addresses the role of occupant choices and behaviors in energy use, and outlines strategies that can be designed and built into homes to support ZNE goals. Topics to discuss include the relationship between water and energy use, water conservation strategies, plug loads and how to curb them, home automation, energy monitoring, education, and home energy management.

October 26 (Friday, 8:30 am to 3:30 pm)
Irwindale - Energy Education Center
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DesignShift: Integrated Design for ZNE

DesignShift is a process and set of tools to advance integrated design to achieve Zero-Net Energy (ZNE) buildings. This full day workshop will introduce a repeatable integrated design process, integrated design project management structures and early design tools that provide quick and quantifiable results to inform and advance the design process. Participants will be provided with a set of tools to immediately implement on projects seeking high energy goals and ZNE. Please bring your laptop. We will email you with further information on access to tools we will use during the

November 1 (Thursday, 8:30 am to 4:30 pm)
Irwindale - SCE EEC
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e-News is published by Energy Design Resources (www.energydesignresources.com), an online resource center for information on energy efficiency design practices in California.

Savings By Design (www.savingsbydesign.com) offers design assistance and incentives to design teams and building owners in California to encourage high-performance nonresidential building design and construction.

Energy Design Resources and Savings By Design are funded by California utility customers and administered by Pacific Gas and Electric Company, Sacramento Municipal Utility District, San Diego Gas and Electric, Southern California Edison and Southern California Gas Company, under the auspices of the California Public Utilities Commission.


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