CASE STUDIES

North-facing roof monitors and white roof. Photo: Steven Spencer

Project: Florida Solar Energy Center, Cocoa, FL

Owner: University of Central Florida

Architect: Architects Design Group

Mechanical/Electric Engineer: Brian Cummings & Associates

CASE STUDY FEATURES:
Orientation
Daylighting
Glare
Top Lighting

 

Typical Glazing Properties:
Insulating low-solar-gain low-E glass
U-factor=0.31
SHGC=0.28
VT=0.56

Florida Solar Energy Center

The Energy Center design starts with the premise that in most office buildings, electric lighting is the largest energy load, and much of it is unnecessary. In Florida's warm and humid climate, excess electric lighting only adds to air-conditioning loads. If a building is to keep occupants cool and dry without consuming much energy, it must effectively address daylighting in its basic form and development.

With this concept as a starting point, the Energy Center office and lab building is designed as a long, thin rectangle oriented on an east-west axis. This basic design gesture accomplishes three energy-efficiency objectives:

  • Orientation exposes the smallest east- and west-facing building surfaces to hot morning and afternoon sun. By design, daylight principally enters through the south and north facades.
  • Form maximizes usable space on the building perimeter, to capitalize on sidelighting. With a building depth of 60 feet, toplighting must light the deep interior, non-perimeter zones. Roof monitors can provide daylight within the building core.
  • The design facilitates the development of independent north and south HVAC zones, so interior air temperatures can be controlled based on solar exposure. South-facing zones can thus be cooled without wasting energy cooling the north side.

Overall view of Florida Solar Energy Center. The Visitor Center is in foreground, with office beyond. Photo: Steven Spencer

Design development, such as glazing specifications, reinforces these form decisions. For instance, in the Visitor Center, the southeast-facing window wall receives significant solar exposure, yet little solar gain. The insulated glass unit—and the thin, almost-transparent metallic, spectrally selective film sandwiched inside it—provides a relatively high 56% visible transmittance, with a solar heat gain coefficient of 0.28. The glazing admits sunlight but minimizes solar heat. This high-performance window system costs more than conventional ones, but energy savings—from reducing air-conditioning costs and using daylight rather than electric light for illumination—paid back that additional expense in about two years.

View of the Visitor Center window wall, facing southeast. Photo: Steven Spencer

Linked with the window system are the shading devices, which were monitored with regard to their ability to reduce electric lighting loads. The architects designed exterior light shelves as an integral facade element, but FSEC staff added interior light shelves too, after significant testing. Interior light shelves, traditional blinds and a control window without blinds or light shelves were all tested for half a year to gauge relative performance. The data indicated that the light shelves increase daylight penetration, thereby reducing electric light loads and giving glare reduction. In contrast, more traditional interior blinds had a negative impact on daylighting. Based on the monitoring, all south-facing offices were subsequently fitted with interior light shelves.

Monitoring also revealed the importance of the window frames. The window unit's overall U-factor of 0.31 Btu/hr-sf-°F provides effective control of conductive gains under peak load conditions but the highly conductive metal window frames seriously degrade the assumed U-factor. Monitoring reveals that the actual overall assembly U-factor cannot be less than about 0.7 Btu/hr-sf-°F. In short, the window assembly's assumed U-factor—used in DOE-2 runs and operational energy assumptions—was erroneous, as learned in documenting the building's actual energy use. As the glazing system in each office—approximately 56 square feet—is about 44 square feet of glass and 12 square feet of frame, the high-performance glazing could be improved with better, less conductive frames. A key lesson is that window specification must address frame as well as glazing, unless a serious compromise in performance can be accepted.

Moving from the perimeter offices and exterior spaces, in the building core, roof monitors (also referred to as lightscoops) replace windows to provide daylight. Florida's solar-intense climate make skylights problematic—they cause hot spots from direct-beam sunlight, fade interior furnishings, and create too much internal heat in general. Roof monitors, projecting 10 feet above the flat roof, provide light without the drawbacks of skylights. Each monitor's north-facing surface is a highly efficient glazing system that lets in cool light from the northern sky but no hot, direct-beam sunlight. The light is directed downward into the building's core, reducing electrical lighting loads.

Looking up to the roof monitor. Splayed surfaces around the monitor increase the toplighting efficiency of glazing and provide a more gradual visual transition to the brighter view of the sky. Photo: Steven Spencer

If the overall design strategy is to turn the building itself into a daylight fixture that captures light, there is certainly a need to reject light too. With cooling loads dominating the building in this hot-humid climate, the facility envelope cannot be a heat sink for solar gains. To keep the Energy Center's air-conditioning load down, the facility's white roof reflects almost 80% of the sun's energy. Even though the building's walls are blue, rather than white, they still block heat gain through the use of a radiant barrier. This aluminum foil surface in the air space behind the exterior finish blocks the transfer of solar-generated heat.

Within the Center's climate, a typical single-story office building has 30% of its annual cooling load attributed to heat produced by electric lighting, 20% to solar gain through windows, 15% to roof heat gain, and 13% to heat from internal equipment. The lighting system is not only the largest cooling load component, but second only to the HVAC in electric consumption. By directly tackling lighting improvements—through glazing, daylight and occupancy sensors, and good design strategies—as well as other energy saving measures, the facility uses half the energy of a similar building.

Source:
Carmody, J. S. Selkowitz, E. Lee, D. Arasteh, T. Willmert. Window Systems for High-performance Buildings. Norton, 2004.

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