Energy Conservation Efforts Get Boost from New Solar Spectral Standard
Architects and engineers now have another tool they can rely on as they work to improve buildings’ energy demands. A new standard published by ASTM International in October promises to provide more accurate data on solar spectral distribution that can be used to calculate how various wavelengths affect buildings in terms of energy consumption, heat load and air conditioning and heating requirements.
“For a long time, fenestration people and architects have utilized solar energy distribution as a function of wavelength to evaluate the performance of windows in building structures based on what are now rather dated models and computations and assumptions about atmospheric constituents,” says Daryl Myers, senior scientist II in the Resource Integration Group of the Electricity, Resources and Buildings Integration Center at the National Renewable Energy Laboratory (NREL), and a member of the Subcommittee G03.09 on Radiometry that developed the standard.
“The problem was that a lot of the data that those computations were based on were, with respect to the solar spectrum, developed in the early 1980s,” he goes on to say. “And of course they’ve come a long way in terms of knowledge of atmosphere and the knowledge of the mechanics that occur in modifying the solar spectral distribution, the power distribution, as a function of wavelength as the solar rays propagate from the top of the atmosphere to the bottom of the atmosphere.”
The new ASTM standard, ASTM G197, Table for Reference Solar Spectral Distributions: Direct and Diffuse on 20° Tilted and Vertical Surfaces, is based on a solar radiation model called SMARTS. Developed by a former scientist at the Florida Solar Energy Center, Dr. Christian Gueymard, SMARTS is now available as a free download through the NREL website.
By incorporating the SMARTS model into G197, the committee updated the spectra that would be used in solar spectral distribution computations. The standard also lets users employ the model to understand the effects of the sun on buildings throughout the day.
Myers explains that “as the sun moves through the sky, the atmosphere modifies it in different ways in different parts of the day because there is more atmosphere to go through in the morning and afternoon then there is at noon, and therefore that solar power spectral distribution changes throughout the day. So now the architects can use the same model that is used to produce the reference spectrum to actually produce realistic spectra through different parts of the day and see how that impacts, say, the performance of their windows.”
The new standard also provides engineers and architects with more appropriate data for their applications.
“The standards that the community was using before were actually oriented toward the photovoltaic community, and they addressed only a 37° tilt, and direct normal radiation,” says Myers. “The new configuration addresses tilts that are more likely to be included in a building in terms of vertical surfaces and rooftop skylights.”
By having more accurate information about the effects of sunlight throughout the day—and having that information described using more relevant geometries for buildings—the standard will make it easier to design more energy-efficient buildings. These may include buildings that rely on daylighting or passive solar heating and cooling as a way of reducing energy consumption.
G197 is the third standard to be released by the ASTM G03 Committee on Weathering and Durability based on SMARTS. The other two are ASTM G173, Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface, and ASTM G177, Tables for Reference Solar Ultraviolet Spectral Distributions: Hemispherical on 37° Tilted Surface.