©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no.5, May 2019. The complete article can be purchased from the ASHRAE bookstore and is free to members through the ASHRAE Technology Portal.
About the Author
Michael P. Sherren, P.E., is a mechanical principal/vice president at James Posey Associates, located in Baltimore.
The 106,221 ft2 (9868 m2) Wilde Lake Middle School in Columbia, Maryland exemplifies the Howard County Public School Systems (HCPSS) commitment to sustainability and 21st century learning through environmental stewardship. The school was designed as a net zero energy (NZE) building, representing a new era in educational design innovation. The resulting project’s measured energy performance exceeded the goals established by the owner, design team and other key project stakeholders.
NZE performance was achieved by gaining an in-depth understanding of the anticipated occupant behavior and building operations, exploring interactions and relationships between building system components, maximizing the overall efficiency of these systems, and offsetting building energy usage with renewable energy. Reliability, simplicity of operation, and long-term maintainability were critical criteria for the school’s design.
Energy Efficiency
While the school’s energy performance was compared to the LEED baseline building requirements set forth in Appendix G of ASHRAE/IESNA Standard 90.1-2007, the predicted energy use intensity (EUI) served as the critical metric for evaluating energy performance during design.
Energy performance data contained within the U.S. Energy Information Administration’s 2012 Commercial Buildings Energy Consumption Survey (CBECS) indicated an average site EUI of 54.9 kBtu/ft2·yr (623.5 MJ/m2·yr) for K – 12 elementary and middle school buildings. A 25.0 kBtu/ft2·yr (283.9 MJ/m2·yr) site EUI goal was established for this project to economically justify the initial cost of the photovoltaic (PV) system required to support the school’s energy production demands.
Achieving Wilde Lake Middle School’s NZE goal required optimizing the building envelope, lighting, and kitchen components prior to focusing on the mechanical systems. Through ongoing design team collaboration and multiple energy studies performed using simulation software, the school’s envelope was life-cycle cost optimized with the use of R-28.5 masonry cavity wall and R-25 insulated metal wall panel construction. The solar heat gain coefficient of the eastern, western, and southern glazing was reduced to 0.27, with exterior shading devices added as appropriate.
Lighting power density was reduced to 0.39 W/ft2 (4.2 W/m2). Both occupancy-based lighting control and building receptacle control were implemented. Kitchen equipment was designed to allow for the use of a Type II kitchen exhaust hood, reducing the amount of makeup airflow required.
This optimization process was important to achieve NZE, as it provided a reduction in the required capacity and initial cost of the school’s mechanical and PV systems.
Geothermal technology was used to meet the school’s ultralow energy consumption requirements and included one-hundred twelve 400 ft (121.9 m) deep vertical boreholes that use the earth as a heat source/heat sink for the school’s HVAC equipment.
Energy modeling simulations were performed to assist with balancing the building’s annual heating/cooling energy, maximizing ground temperature stability while minimizing ground temperature extremes. This energy balance allowed for an all-water geothermal heat pump loop fluid, promoting improved heat transfer and lower piping pressure drop compared to traditional glycol-based fluid solutions.