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DCD Magazine

High-Performance Glass for High-Performance Schools
By Glenn Miner, Director, Construction PPG Industries

Daylighting strategies can have a significant impact on energy consumption in educational facilities, particularly those that incorporate advanced solar control, low-e glasses such as double-silver-coated Solarban 60 glass and triple-silver-coated Solarban 70XL glass by PPG, both of which were specified for CY Middle School (featured on page 36 of this issue).

To quantify the energy-saving capability of these products, which are designed to maximize light transmittance while blocking the sun’s radiant heat energy, PPG commissioned an independent energy and environmental analysis firm to conduct energy modeling studies on a prototypical 200,000 square-foot middle school.

Study Parameters
To ensure accurate and objective results, the testing corporation used the U.S. Department of Energy’s (DOE) 2.2 Building Analysis Tool, which is regarded as the most accurate and well- documented energy modeling software in the U.S.

The DOE 2.2 tool calculates hour-by-hour energy consumption at prototype buildings over an entire year. Input includes hourly climate data for the building’s location as well as local utility costs, heating and air conditioning systems and controls, interior and exterior building mass, shading and fenestration, hourly scheduling of occupants, lighting equipment, thermostat settings and numerous other variables.

For comparative purposes, the testing firm modeled three glazing types:

  • Solarban 70XL glass
  • Solarban 60 glass
  • Dual-pane, spectrally selective, tinted glass (light-green)

Relevant performance data for each glazing type is included in the following table:

Building Prototype
The three architectural glasses were glazed on the prototype school: one with punched windows on each façade and another with complete window walls on each exposure. It was equipped with a packaged VAV air-handling system, DX coils for cooling, an economizer, hot-water boilers for the heating plant, and a gas water heater.

Operating hours were from 7:00 a.m. to 9:00 p.m. on weekdays from September to June, and from 10:00 a.m. to 3:00 p.m. on weekends during July and August. The heating and cooling temperatures were 72oF and 76oF, respectively.

Internal peak load assumptions were 125 square-feet per-occupant, and 1.1 watts of lighting and .45 watts of equipment per-square-foot.

Climate Considerations
To provide a representative sample, the prototype school was tested with climate data from 10 major U.S. cities. In addition to representing a range of weather conditions, the cities had widely fluctuating prices for natural gas and electricity, which were obtained and factored into the models.

In the end, 120 energy-modeling simulations generated precise calculations for building load, cooling equipment size, energy costs and HVAC cooling costs.

The Results
The following chart compares the performance of Solarban 60 and Solarban 70XL glasses to more commonly specified spectrally selective, dual-pane tinted glass in the prototype, window-walled middle school.

Single-story middle school, four window walls, Total Floor Area: 200,000 ft2; Total Glass Area: 45,027 ft2

Annual Energy Cost Savings
The study showed that schools constructed with Solarban 60 glass instead of dual-pane tinted glass can expect annual energy savings that range from 5 percent ($7,204) in St. Louis to 8 percent ($13,510) in Seattle and Chicago ($19,395). Average annual energy savings with Solarban 60 glass across the 10 U.S. cities was 7 percent ($18,992).

While the energy savings from Solarban 60 glass were impressive, they were significantly less those realized with Solarban 70XL glass.

When substituted for dual-pane tinted glass in the same middle school building, Solarban 70XL glass generated energy savings of 8 percent ($23,137) in Philadelphia to 12 percent in Phoenix ($26,967) and Houston ($42,727). The 10-city average for energy cost reductions was more than 10 percent ($27,726).

HVAC Equipment Cost Savings
Because of their ability to block heat and transmit light, schools with Solarban 60 and Solarban 70XL glasses require less cooling capacity than those glazed with less-advanced products, which enables architects and school administrators to spend less on HVAC equipment for their buildings.

With Solarban 60 glass in place of dual-pane tinted glass, initial HVAC equipment cost savings for the prototype middle school averaged 9 percent ($111,947), including 10 percent reductions in such climactically diverse cities as Phoenix ($134,171), St. Louis ($125,090) and Boston ($122,169).

In schools modeled with Solarban 70XL glass, HVAC equipment costs were slashed by 14 percent in Chicago ($182,603) and Philadelphia ($174,383), and 19 percent ($229,919) in Los Angeles. The average equipment cost savings for all 10 cities was nearly 17 percent ($207,613).

Lower Operational Costs. More Educational Resources.
Energy modeling demonstrates that schools equipped with advanced architectural glass can reap the rewards of an investment that pays for itself many times over.

Over the 40-year lifetime of a typical middle school, annual energy savings of $25,000 can total $1 million, enough to pay for 2,000 computers or 50,000 textbooks. What’s more, the value of that investment continues to grow with the escalation of energy prices, a trend that will accelerate well into the future.

Even more important, however, is the investment in our country’s schoolchildren. It has never been clearer that schools with abundant daylight, fresh air and a strong visual connection to the outdoors help provide the best possible environments for learning and growth. By investing in the latest solar control, low-e glasses, architects and school administrators can get energy savings they need while promoting the academic performance they demand. In the end, that makes them a winner for students, parents and taxpayers.

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