Figuring Solar Heat Gain: Calculation Procedure, Definition of Shading Coefficients, Solar Heat Gain Factor, Solar Heat Coefficient, & Solar Transmittance
Define solar input and shading coefficients in solar heating design
Definition of solar heat gain factor & solar heat gain coefficient
Definition of solar transmittance
Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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How to calculate solar gain for a building: this article discusses the method for calculating solar heat gain through windows. We include definitions of solar input, shading coefficient, solar heat gain faactor, solar heat gain coefficient, and solar transmittance. Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Also see PASSIVE SOLAR DESIGN KEY ELEMENTS and see PASSIVE SOLAR ENERGY MONITORING. Readers should also see SOLAR ENERGY SYSTEMS.
The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Figuring Solar Heat Gain: Calculation Procedure, Definition of Shading Coefficients, Solar Heat Gain Factor, Solar Heat Coefficient, & Solar Transmittance
Question: What is Solar Heat Gain and How is it Calculated?
I am having a hard time determining which specification to use when calculating solar heat gain through windows.
Does one use visible light transmission or some other figure? 3M's Sungain Film brochure mentions "Solar Input," which represents the shading coefficient times 0.87.
What exactly does that give you? How do visible light transmittance, and shading coefficient relate to one another? - Jeff Pendl, Enervision, Cincinnati OH
Answer: the basics of calculating solar heat gain.
Solar heat gain is calculated with the solar shading coefficient and the solar heat gain factor (not to be confused with the solar heat gain coefficient - see below).
Definition & Uses of Solar Transmittance
The solar transmittance of a window glazing or window is the percentage of visible and near-infrared light that passes directly through a window. Solar transmittance is used to compare different types of glass. But solar transmittance does not include the solar heat that the glass absorbs and re-radiates into the building, so alone it is not accurate enough for calculating total solar gains through windows.
Definition of Solar Input - Solar Heat Gain Coefficient & Calculating the Shading Coefficient
The "solar input" in the 3M Sungain film brochure, also known as the solar heat gain coefficient, is the solar transmittance modified to account for re-radiated heat.
If you divide by 0.87 (the coefficient for 1/8-inch clear glass), you get the shading coefficient you need to calculate solar heat gain.
Solar Heat Gain Coefficient (SHGC) measures how well a
product blocks heat from the sun. SHGC is expressed as a
number between 0 and 1. The lower the SHGC, the better
a product is at blocking unwanted heat gain. Blocking solar
heat gain is particularly important during the summer
cooling season.
This is a different figure from visible transmittance (VT) of light, also defined by the NFRC as follows:
Visible Transmittance (VT) measures how much light
comes through a product. VT is expressed as a number
between 0 and 1. The higher the VT, the higher the
potential for daylighting.
Watch out: other very important factors need to be considered in arriving at an accurate picture of solar heat gain through windows. Here are added factors defined by the NFRC as of 2005:
Air Leakage (AL) measures how much outside air comes
into a home or building through a product. AL rates
typically fall in a range between 0.1 and 0.3. The lower the
AL, the better a product is at keeping air out. AL is an
optional rating, and manufacturers can choose not to
include it on their labels. This label displays AL in U.S. units.
Labels on products sold in markets outside the United
States may display AL in metric units.
See ASHRAE Fundamentals, Chapter 27,(2005 ASHRAE Handbook : Fundamentals at Amazon) for lists of solar heat gain factors. Calculating solar gains by hand is a laborious, imperfect process. Many computer programs make the job easier by calculating solar heat gain for you.
The higher its solar transmittance or solar heat gain coefficient, the better a glazing is for solar heating use.
The lower those numbers are, the more suitable a glazing is for cooling.
Solar-Heat-Gain Coefficient Rating for Windows: SHGC
A window with an SHGC of .70 captures about 70% of the
available solar energy falling on the window. Clear double
glazing has an SHGC of about .75 versus .60 to .70 for
standard low-E and about .40 for spectrally selective low-
E. Which type of glazing is optimal for a given project depends
on the climate, summer and winter fuels costs, and
how glass is used in the house design.
Low SHGC. Low-solar-gain glass blocks unwanted
solar gain and provides significant savings in both
peak and annual cooling loads in hot climates. For
example, switching from clear double glass to low-
SHGC glazing can reduce air-conditioning bills by
15 to 20% in a typical home in Phoenix or Miami (See Figure 3-10 above). Spectrally selective glass , introduced in
the 1990s, combines very low solar gains with high
visible light transmittance and high R-values (up to
R-4 with gas fill). While this yields the greatest savings
in hot climates, it is also a good choice in any
climate with significant cooling loads or large
amounts of un shaded west-facing glass. (See Spectrally Selective Window Glazing).
Moderate SHGC. In northern cities like Boston or
Chicago or mixed climates with more-or-less equal
heating and cooling costs, moderate-gain glass is a reasonable
choice, balancing moderate solar gains in winter
with moderate blocking of solar gains in summer.
High SHGC. High solar-heat-gain glass is a good
choice in cold climate homes with enough south glass
to take advantage of passive solar gain, called “sun
tempering.” Savings on winter heating bills will be
partially offset by the increased cooling load in
summer, however. To avoid overheating, south-facing
glass should range from 4 to 7% of the total floor area
(avoid sloped glass, which tends to overheat in summer
and fall).With south glass in excess of 7% of floor
area, thermal mass may be needed to prevent daytime
overheating and to store heat for nighttime use. A designer
with expertise in passive solar can help determine
the right amounts of glass and thermal mass.
The question-and-answer article above, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.
Q&A on Figuring Solar Heat Gain- PDF version, use your browser's back button to return to this page. Original article, Solar Age Magazine, November, 1985, adapted and updated for InspectAPedia.com December 2010.
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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
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Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: info@buildingadvisor.com
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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ASHRAE Fundamentals Handbooks: Where to buy: The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
2005 ASHRAE Handbook : Fundamentals : Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
"2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
"The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 AShrae Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
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Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
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