©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 5, May 2013.
By Hormoz Janssens, P.E., Member ASHRAE
About the Author
Hormoz Janssens, P.E., is a principal at Interface Engineering in San Francisco. He is a member of the Golden Gate chapter.
Using advanced engineering tools, the design team for City College of San Francisco explored several iterations of the building envelope, floor plan and roof skylight layout to create one of the largest naturally ventilated and passively cooled buildings in the United States. The challenges to the team included using the natural environment as much as possible instead of mechanical cooling, minimizing mechanical equipment room space (all equipment had to be in a mechanical room due to corrosion issues with the site), maximizing net useable floor area, minimizing energy use, and providing a comfortable atmosphere year-round. In addition to the design challenges, the project had to be delivered by the construction manager at risk with no cost premium.
The 105,000 ft2 (9755 m2) building, which houses classrooms, administrative offices, specialized laboratories, computer lab, study spaces, childcare/family training center, meeting rooms, café, and other spaces for student development, has been operational since August 2010 and is outperforming client and design team expectations.
Mechanical strategies include engineered natural ventilation, radiant heating and cooling in the building slab, a geothermal central plant (by others), a dual-purpose atrium smoke removal and mixed mode ventilation system, and building-integrated photovoltaics.
On Target to Save Energy and Water
The team’s energy model predicted the building would use 40%+ less energy than a building designed to code and perform 70% better than most existing California community college facilities of similar scope. In addition, it was designed to reduce water use by 30%.
The engineering firm is tracking the building’s energy consumption to compare the energy model with actual performance. Based on 12 months of utility bills, the building’s energy use is performing as designed, modeled, and predicted. Despite record high temperatures in August 2010 and record cold temperatures in December 2010, the building’s energy use has performed better than expected. The building’s energy model estimated an energy use intensity of 26.4 kBtu/ft2•yr (297 553 kJ/[m2•yr]). The actual performance is slightly higher, with an energy use intensity of 28.1 kBtu/ft2•yr (319 131 kJ/[m2•yr]).
Tools have been provided to the owner to facilitate sustained energy and water savings. These tools include plug load and device level energy consumption monitoring and controls, feedback visualization tools for tracking energy use, flow meters for water consumption, and a Btu/h (kW) and hydronic system water meters for HVAC energy use.
Furthermore, outdoor wind speed, air temperature, and dew-point temperature monitoring helped the owner to further optimize the operation of the HVAC system. Lighting controls and daylighting provide additional tools for saving energy.
Building That Breathes
This “breathing building” features a central atrium that functions as the building’s “lungs” to organize circulation and facilitation of natural ventilation to serve all occupied spaces. The administrative spaces on the south end of the building on the first, mezzanine, second and third floors are naturally ventilated. Each of these spaces has operable windows.
The classrooms are automatically naturally ventilated with demand-controlled ventilation using CO2 sensors. The building, at 105,000 ft2 (9755 m2) does not require an air handler and has no mechanical rooms for air distribution and no roof-mounted air handlers.
A louvered wind-driven central atrium and skylight system takes advantage of natural ventilation by allowing airflow from the perimeter classrooms to flow into the central atrium and is then exhausted through the glazed skylights at the roof level, which uses both stack effect and wind-driven forces and maximizes air circulation and passive cooling. The building takes advantage of the cool temperatures and on-site wind pressure of its natural surroundings and only uses mechanical radiant cooling during peak seasonal extremes.
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