e-News #97: Zero Net Energy Case Studies

October 26, 2015
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California is a global a leader in energy efficiency where many technical and policy innovations are developed. In the past four decades, per capita energy consumption has risen nationally while California has leveled its energy use.

Case Study No. 1: Packard Foundation Headquarters Building
Case Study No. 1: Packard Foundation Headquarters 
Building Type: Two-story office
Location: Palo Alto, CA
Gross Square Footage: 49,000 sf
On-Site Renewable Energy System Installed:285 kW
Measured EUI (site) 14.1 kBtu/sf-yr
Key Features: very sophisticated HVAC system that includes two large underground chilled water storage tanks charged at night (off peak) using cooling towers as opposed to mechanical cooling. Coupled with a low temp low pressure drop design, the system achieves free cooling for much of the year.

In 2010, the California Public Utilities Commission authorized California’s investor-owned utilities to launch a series of innovative programs to address the statewide energy efficiency goals outlined in the 2008 California Energy Efficiency Strategic Plan. Pacific Gas and Electric (PG&E) established a Zero Net Energy (ZNE) Pilot Program as a particular way to meet these goals. PG&E’s effort to disseminate the findings gained from examining the case studies from early ZNE projects is essential as California implements AB 32 (the Global Warming Solutions Act) requirements. PG&E published a monograph, Zero Net Energy Case Study Buildings, written by Edward Dean in September 2014. This report outlines lessons learned from six early PG&E ZNE nonresidential projects. Here are the highlights.

The fundamental aspect of a zero net energy building is that it generates as much energy as it consumes over a one year period.This represents a paradigm shift in how building owners and developers work with design professionals to build efficient buildings.  The report delineates three methods of defining net zero:  Site ZNE, Source ZNE and “Time-Dependent Valuation” (TDV) ZNE. 

A Site ZNE building has an energy demand equal to the amount of energy generated on site.  This is the one ZNE metric that can be directly metered and measured on site.  Source ZNE takes into account the transmission losses from the power plant to the building site.   TDV energy, as defined by the California Energy Commission (CEC), puts a higher value on energy produced or used during the peak demand periods. 

Peak is when the demand is the highest and it’s those periods of time when it is the most costly for the grid operators to produce, procure and deliver electricity. 

Case Study No. 2: Stevens Library at Sacred Heart Schools
Case Study No. 2: Stevens Library at Sacred Heart School
Building Type: K-12 School
Location: Atherton, CA
Gross Square Footage: 6,300 sf
On-Site Renewable Energy System Installed: 40 kW
Measured EUI (site) 19.0 kBtu/sf-yr
Key Features: Low cost high performance design, displacement ventilation, gray water recycling.
Image: Bruce Damonte

Regardless of the three different ZNE definitions above, the essential design process focuses first on achieving the maximum level of load reduction and energy efficiency in the building through integrated design practices. Once minimum loads are determined, equipment and  on-site renewable energy systems are selected and sized to balance out the remaining energy requirement.  It is very important to note the strong emphasis on energy efficiency within the definition of ZNE.  As the saying goes, the least expensive kilowatt-hour is the one that you don’t have to generate!

In all of the case studies in the ZNE report, the design teams and owners followed a four-step process that included the following:

  • Set the energy performance target
  • Design to this target
  • Build to this design
  • Monitor, diagnose, and correct actual performance

These steps must be adopted early in the design process.  The architect and engineers must work together, using an integrated design process.  These four steps in the project process are further described below:

Set the energy performance target

Case Study No. 3: IDeAs Office Building
Case Study No. 3: IDeAs Office Building
Building Type: Office
Location: San Jose, CA
Gross Square Footage: 6,557 sf
On-Site Renewable Energy System Installed: 30 kW
Measured EUI (site) 18.7 kBtu/sf-yr
Key Features: Deep retrofit, including Building Integrated Photovoltaics (BIPV) in the façade overhangs, radiant floor for heating and cooling, detailed plug load sensors and controls.
Image: EHDD

In each of the six case studies, it was critical to define the goals of the project.  For the Packard Foundation, Net Zero and a low EUI were well established from the beginning.  ZNE was not initially the goal for the Stevens Library at Sacred Heart Schools, but it was adopted after the design team advocated the concepts to the owner.  The IDeAs Office Building was a “deep retrofit” with ZNE and low energy design concepts set early in the process.  The IDeAs building is owner-occupied by an engineering firm, so they wanted to be able to showcase the ZNE features.   The Watsonville project was not originally ZNE.  The owner wanted to demonstrate “environmental stewardship” however.  The two projects at UC Davis were designed for low energy impact and the on-site renewable system will be built out in the future.

Design to this target

Once the goals are established, it is essential that the design team sticks to the plan.  In the case of the Packard Foundation, the iterative design process took many years to complete.  The result is a very sophisticated set of integrated systems.  Several of these projects had tight budget constraints including Sacred Heart.  There the design team had to be careful to use “value engineering” in the selection of the systems. 

Case Study No. 4: Watsonville Water Resources Center
Case Study No. 4: Watsonville Water Resources Center
Building Type: Office/Laboratory
Location: Watsonville, CA
Gross Square Footage: 16,000 sf
On-Site Renewable Energy System Installed: 96 kW (roof system) 254 kW (ground system)
Measured EUI (site) 51.4 kBtu/sf-yr
Key Features: Low cost, 'state-of-the-shelf' system selection. Design-build team.
Image: Bruce Damonte

The Watsonville team had a mandate to select “state-of-the-shelf” systems to maintain the low cost goals.  Each project included the integration of the building envelope with the lighting and ventilation systems to utilize daylighting and natural ventilation when possible.

Build to this design

For many of the projects, a construction team was selected early and was involved throughout the process.  It’s essential for the contractor to be aware of the ZNE goals early on to ensure the building can meet those goals.  The design team must also be prepared to offer construction guidance during the build out process.  In the case of the Watsonville project, the team applied a design-build method of construction. 

Monitor, diagnose, and correct actual performance

In every one of the ZNE case studies, the post construction phase required a significant amount of attention.  In some cases, some systems required adjustments and even replacement.  The Commissioning process can sometimes take more than one year to complete.  ZNE buildings usually have more monitoring systems for the Building Management System (BMS) to handle. 

Case Study No. 5: UC Merced Science & Engineering Building I
Case Study No. 5: UC Merced Science & Engineering Building I
Building Type: Laboratory
Location: Merced, CA
Gross Square Footage: 180,339 sf
On-Site Renewable Energy System Installed: N/A
Measured EUI (site) 188 kBtu/sf-yr
Key Features: Central plant, on site renewables will be installed in the near future

These complex sensor and controls require additional effort to calibrate.  In several cases, these measurement and control systems did not function as expected.   Operational issues are even ongoing at some of these buildings. 


As discussed in the zero net energy case study report, there are several commonalties in each of the six buildings included in this study.  The trends that were noted are as follows:

  • Environmental goals, as defined during the initial stages of development are critical in determining the performance objectives.
  • Modeling the building’s energy performance is critical to forecast how the facility will operate.   It takes both art and science to accurately predict energy loads.  Comparing the modeling results to the actual measured performance will help industry stakeholders learn from the lessons contained in these early projects.
  • Metering issues can be expected in these often complicated buildings.  ZNE facilities usually have features and systems that require adjustments during the post occupancy phase.  These efforts go beyond the normal commissioning process.
  • Monitoring the systems and using the measured data to diagnose and correct improper functioning of the building systems can be expected after all of the systems have been commissioned.
  • ZNE certification program standards are needed within our industry.

Case Study No. 6: UC Merced Classroom and Office Building
Case Study No. 6: UC Merced Classroom and Office Building
Building Type: Laboratory
Location: Merced, CA
Gross Square Footage: 103,000 sf
On-Site Renewable Energy System Installed: N/A
Measured EUI (site) 36 kBtu/sf-yr
Key Features: Central plant, on site renewables will be installed in the near future.
Image: Kacey Jurgens

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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.

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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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