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©2018 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 60, no. 5, May 2018.
By Ray Beaufait, P.E., Member ASHRAE
About the Author: Ray Beaufait, P.E., is a principal and mechanical engineer with CMTA Inc., in Louisville, Ky.
Discovery Elementary School in Arlington, Va., is the nation’s largest zero energy elementary school and has become a model for other local school districts seeking to reduce their energy consumption. The two-story, 97,600 ft2 (9067 m2) school was built in 2016 for a total occupancy of 715 teachers and students. Its name honors astronaut John Glenn who traveled in the Discovery space shuttle 36 years after his historic Mercury flight.
Arlington Public Schools did not start with aspirations to reach zero energy. The Request for A/E Services Proposal was similar to the district’s previous requests. The budget matched the last school constructed, and the project had a sustainability goal of LEED Silver.
The evolution of Discovery into the largest zero energy school in the U.S. demonstrates that exceptional outcomes can occur when the design team and owner share a common vision to design a school that provides a better environment for student education and a building that can be a part of the education process.
Energy Efficiency
This project was designed with the goal of having the lowest energy consumption (energy use index [EUI]), integrating zero energy within the budgetary cost constraints, positively impacting the region’s watershed and creating an immersive energy dashboard linking the building and education curriculum.
The design started with energy modeling to identify building massing. The architect provided the engineer with multiple schematic sketches to determine a massing model that best met the energy goal, site constraints and desired student learning experience. This early collaboration set expectations for the entire design process.
The energy model indicated a school EUI of 21.2 kBtu/ft2·yr (240.8 MJ/m2·yr based on a building envelope cooling load of less than 50 tons (176 kW). The ventilation load was calculated at another 32 tons (113 kW), resulting in a building block load and final geothermal ground heat exchanger designed for 133 tons (468 kW) or 731 ft2/ton (19.3 m2/kW).
The five major systems that consume energy in a school are HVAC, lighting, kitchen, IT, and plug loads, so strategies were developed to reduce energy consumption of each.
HVAC
A ground source heat pump system (GSHP) was provided with variable speed heat pump units and a single dedicated outdoor air system (DOAS). Emphasis was placed on “right sizing” the heat pump units during the design phase. One heat pump unit serves two classrooms to maximize efficiency and reduce maintenance and construction cost. The water pumping system was also distributed with an individual water pump at each heat pump. All heat pump units were installed in mechanical rooms or closets to allow easier maintenance access and proper sound attenuation. A single DOAS unit was installed to serve the entire school, taking full advantage of building occupant diversity. This resulted in reduced first cost (by providing smaller equipment and system tonnage). The DOAS unit airflow is varied based individual room CO2 measurements.
The building has 38% glazing, but the solar heat gain was controlled through building orientation, large canopies and external solar shading devices. The exterior wall system is insulated concrete forms (ICF), and Discovery is the first school in the district to use an ICF system. The building was air pressure tested to avoid undesired air infiltration.
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