©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 5, May 2014.
By Brent Eubanks, P.E., Member ASHRAE; and Glenn Friedman, P.E, Member ASHRAE
About the Authors
Brent Eubanks, P.E., is a mechanical engineer and certified permaculture designer and Glenn Friedman, P.E., is a principal at Taylor Engineering in Alameda, Calif. Both are members of the Golden Gate ASHRAE Chapter.
The Oakland Unified School District (OUSD) is one of the largest in California with more than 80 schools and 36,000 students, primarily from low income communities. Funded by a bond, the La Escuelita Education Center (LEEC) project replaces an outdated campus—consisting primarily of decrepit portable classrooms—with a modern facility. OUSD-LEEC Phase 1 includes a kindergarten and elementary school, as well as district support facilities.
The new LEEC campus meets the requirements of the California Collaborative for High Performance Schools (CA-CHPS), including compliance with ASHRAE Standards 55 and 62.1 (2010 versions), while maintaining the long-standing district policy of avoiding compressor-based cooling for classrooms and administrative offices.
These goals were achieved by a combination of building orientation, careful lighting and daylighting design, and an HVAC strategy that capitalizes on Oakland’s Mediterranean climate with its reliable breezes and significant diurnal temperature swings.
While the data center, clinic, and TV studio use a high-efficiency, single-duct VAV HVAC system, the classrooms do not use compressor-based cooling. Maintaining comfort required a multi-pronged conditioning strategy that relies on local climate, thermal mass and sophisticated controls.
The classrooms and offices have operable windows (with switches that disable HVAC), allowing for occupant control and passive natural ventilation. When windows are closed these spaces are cooled by unconditioned 100% outdoor air from central AHUs. Displacement diffusers deliver air to the classrooms, which improves ventilation effectiveness and IAQ, and also rejects occupant heat from the space rather than mixing it into the rooms’ air mass. Air supply is controlled by demand and outdoor air temperature, delivering large volumes when the weather is cool but reducing to ventilation minimum when weather is warm.
Additional cooling is provided by thermal mass: a 4 in. (25 mm) concrete floor slab and a 2 in. (25 mm) thick cement plaster layer on interior walls. Thermal mass is charged (cooled) at night by a high volume purge cycle, leveraging a cooling season diurnal temperature swing of 20°F (11°C) or more. This cycle is controlled based on outdoor air temperature, room temperature, and thermal mass temperature from sensors embedded in the floor and walls to minimize fan energy and avoid over-cooling. This provides a thermal flywheel effect to maintain comfort even on warm days.
A final element of the classroom cooling strategy is automatically controlled high volume, low speed (HVLS) ceiling fans. The fans are off during the first stage of cooling, when the supply air is cool, so that displacement ventilation provides beneficial stratification in the occupied zone. As the outdoor temperature (and, thus SAT) rises, the ceiling fans activate to provide up to 4°F (2°C) additional effective cooling.
The ceiling fans also assist in heating, which is by parallel fan-powered VAV (FP-VAV) boxes with hot water coils in each room. Heated air is delivered via the same displacement diffusers used for cooling, while the ceiling fan operates at low speed to destratify the space and ensure uniform heating.
These cooling and heating strategies are also applied in the great room, which has a dedicated single-zone AHU with economizer to provide heating, ventilation and 100% outdoor air cooling. However, the great room is periodically subject to high occupant densities, up to 10 ft2 (0.9 m2) per person. This exceeds what outdoor air cooling and thermal mass alone can support, so a pair of passive evaporative downdraft towers (“cool towers”) is employed to address these loads.
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