Stevens Library at Sacred Heart Schools: Atherton, Calif.
Taking Conservation to Heart
For schools in drought-stricken areas, net zero energy and water strategies help future-proof against utility rate hikes. But, the price tag for net zero can be too high for school budgets. Fortunately, a library project at Sacred Heart School in northern California illustrates that it is possible to deliver a net zero energy building within a conventional budget while teaching kids about the value of conserving resources.
The Stevens Library is the learning hub for the K–8 campus and was built as part of a campus modernization project. The library makes resource conservation part of the everyday experience for students through energy use displays and info graphics on the rainwater harvesting system.
Saving Energy, Costs
The design team pursued a simple design, maximizing daylighting and natural ventilation. With a good building envelope and flat-roof space that allowed for the installation of solar photovoltaic panels, the team laid the foundation to reach net zero energy.
The design team reconsidered original plans, identifying opportunities for cost and energy savings, including:
- Using metal framing in place of wood framing;
- Pursuing a flexible design, which reduced the total square footage;
- Using evaporative cooling and an air-to-air heat pump over the preferred radiant flooring, which has a higher first cost;
- Placing ample operable windows throughout the space to maximize daylight and natural air circulation; and
- Installing high-efficiency linear fluorescent direct/indirect light fixtures that can be continuously dimmed and can be overridden by staff.
Envelope. The switch from wood to metal framing caused a 17% loss in envelope efficiency, dictating the use of rigid insulation. The resulting R-15 walls and R-38 white reflective roof create a well-insulated envelope.
Operable windows take advantage of local breezes and maximize cross ventilation. The high performing low-e glazing is color neutral.
Daylight autonomy drove much of the design. On the southerly side, a wall of windows is deeply recessed, providing ample daylight penetration while minimizing glare and excess heat gain. Interior daylighting is provided by rooftop solar tubes, which are positioned to avoid casting shadows on the PV panels.
HVAC Systems. The construction budget and the warm but dry climate influenced choices for the HVAC systems. The team selected a packaged unit with an efficient air-to-air heat pump that has indirect and direct evaporative cooling sections. It also includes a compressor for mechanical cooling on rare extreme temperature days.
To reduce energy use, air is distributed via displacement ventilation in the main room. Rather than moving air through the use of large, energy-consuming fans, the air-to-air indirect direct system works differently. It uses small fans and injects air via low-velocity air nozzles at the perimeter windows. Natural pressure exhausts the air via openings high above the library stacks. (Figure 1)
Figure 1 Ventilation and Daylighting. Displacement ventilation uses small, efficient fans to pull air from the windows. Daylighting from solar tubes and the ample windows reduce lighting demand.
Additionally, oversized ductwork reduces pressure losses, and the return air side uses no ducts. Although this system is usually reserved for larger footprints, the application was advantageous for this scale due to the reduced system noise and avoidance of air drafts. Since air can be injected at a higher temperature (65°F rather than 55°F), “free cooling” can be used for more hours during the year.
Flexible Design. The school wanted an open, flexible learning environment that would accommodate traditional library functions while supporting cross-disciplinary and project-based learning. The school also wanted to create an outdoor learning space where students could experience a hands-on connection to nature.
As a result, 90% of the library’s spaces are flexible. Its adaptable floor plan with modular furniture can be easily reconfigured into different learning areas. These flexible features accommodate a multitude of educational, administrative and community needs. The multipurpose design is an 8% reduction in total square footage and a corresponding cost decrease.
Solar panels on the Stevens Library’s roof produce twice the electricity that the building uses on an annual basis. While Sacred Heart Schools did not originally seek a net zero energy building, the design team proposed this goal without increasing the project budget.
Renewable Energy Source
The building’s design led to ample flat roof for an appropriately-sized solar photovoltaic (PV) system. The roof space is filled by the high-efficiency system (250 W per panel).
Since the PV panels are densely packed on the roof, they were placed horizontally to avoid one panel shading another. This arrangement conformed to a local ordinance that restricts visible solar panel arrays on buildings.
Regular maintenance is critical to keep the PV panels performing at optimal levels, since debris and water stains can easily collect on the horizontal surfaces. Despite this challenge, the PV system‘s annual electricity production exceeds the library’s demand.
During the performance period from January 2014 to December 2014 (Figure 2), the building produced much more energy than it consumed. The building used 24,394 kWh and generated 56,811 kWh, delivering 32,417 kWh back to the grid.
Figure 2 Monthly Energy Use Breakdown. (Credit: Edward Dean FAIA, © PG&E)
The model showed a predicted energy use intensity (EUI) of 27 kBtu/ft2·yr, not including the water recycling pumps, which serve the whole campus. Due to the actual use patterns of the occupants, the building is performing at an exceptionally low EUI of 13.2 kBtu/ft2·yr.
A major factor for the difference between modeled and actual is that the building management system (BMS) was not programmed to produce all meaningful data during the measurement period. To resolve this problem, a second set of power meters (separate from the BMS) was installed with additional measurement points. This additional data helped uncover a flaw with the BMS recording and provide a more detailed picture of how the building was using energy.
Figure 3 Low-Energy Design Strategies. The library's design emphasizes indoor air quality. Daylight is used as light source, operable windows let fresh air in, and fans circulate air.
In addition, limitations of modeling software can contribute to discrepancies in modeled versus actual data. The energy modeling software used for the library has difficulty modeling the effect of daylighting in interior zones, resulting in a predicted use of interior lighting despite daylighting from the roof.
The school’s emphasis on resource conservation also apparently led to lower use of electric lights than predicted by the model. The library staff simply turn off the lights when the space is under-occupied, contrary to the model that assumed continuous automatic dimming.
Actual plug load use was half that predicted by the model. This energy use data indicates a vigilant use of the "off" switch by the occupants.
Lighting energy use was also lower than modeled by 33%, which is attributed to ease of control (override switches), daylighting and an educational message that promotes conservation. Actual HVAC energy use was 10% to 30% better than modeled each month.
The differences in modeled versus actual energy point out that occupant use intensity, scheduling and the impact of technologies such as occupancy sensors impact measured results versus modeled.
1. The lighting energy use was much lower than predicted: less than 1 kBtu/ft2 annually compared to the modeled total of 10 kBtu/ft2. A variety of factors account for this reduction: the complexity of the interactions among the lighting system, the available daylight and the daylight occupancy controls, and occupant behavior. We attribute this energy-saving behavior to the awareness of students and staff to the environmental impacts of energy use.
2. Complaints that the building was too cold in the morning caused the setback temperature of the heat pump to be set at 65°F versus the calculated 55°F, resulting in increased measured heating energy consumption compared to the model.
3. More mechanized shading and better comfort modeling will likely assist in the perceived glare from the front windows during a few hours of the winter.
While Sacred Heart Schools never planned to pursue a net zero energy building, this project illustrates that such a design can be possible without increasing costs or compromising program goals. The design team’s thorough communication helped reassure the school regarding perceived risks of net zero. And the lower-than-predicted building energy use shows that staff and students are taking the building’s message of conservation to heart.
About the Author
Pauline Souza, AIA, LEED Fellow, is a partner and sustainability director at WRNS Studio in San Francisco. She is also a USGBC National Green Schools Advocate.