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Solar gain (also known as solar heat gain or passive solar gain) is the increase in thermal energy of a space, object or structure as it absorbs incident
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
. The amount of solar gain a space experiences is a function of the total incident solar
irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
and of the ability of any intervening material to
transmit
Transmit is a file transfer client program for macOS. Developed by Panic, Transmit is shareware. After a seven-day trial period, the product can only be used for seven-minute sessions until it has been purchased. Originally built as an FTP client ...
or resist the radiation.
Objects struck by sunlight absorb its visible and short-wave infrared components, increase in temperature, and then re-radiate that heat at longer
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
wavelengths
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
. Though transparent building materials such as glass allow visible light to pass through almost unimpeded, once that light is converted to long-wave infrared radiation by materials indoors, it is unable to escape back through the window since glass is opaque to those longer wavelengths. The trapped heat thus causes solar gain via a phenomenon known as the
greenhouse effect
The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
. In buildings, excessive solar gain can lead to overheating within a space, but it can also be used as a passive heating strategy when heat is desired.
Window solar gain properties
Solar gain is most frequently addressed in the design and selection of windows and doors. Because of this, the most common metrics for quantifying solar gain are used as a standard way of reporting the thermal properties of window assemblies. In the United States, The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (
ASHRAE
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE ) is an American professional association seeking to advance heating, ventilation, air conditioning and refrigeration (HVAC&R) systems design and constructio ...
)
and The
National Fenestration Rating Council (NFRC)
maintain standards for the calculation and measurement of these values.
Shading coefficient
The
shading coefficient Shading coefficient (SC) is a measure of thermal performance of a glass unit (panel or window) in a building.
It is the ratio of solar gain (due to direct sunlight) passing through a glass unit to the solar energy which passes through 3mm Clear Fl ...
(SC) is a measure of the radiative thermal performance of a
glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
unit (panel or window) in a
building
A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fun ...
. It is defined as the ratio of solar radiation at a given wavelength and angle of incidence passing through a glass unit to the radiation that would pass through a reference window of frameless Clear Float Glass.
Since the quantities compared are functions of both wavelength and angle of incidence, the shading coefficient for a window assembly is typically reported for a single wavelength typical of solar radiation entering normal to the plane of glass. This quantity includes both energy that is transmitted directly through the glass as well as energy that is absorbed by the glass and frame and re-radiated into the space, and is given by the following equation:
Here, λ is the wavelength of radiation and θ is the angle of incidence. "T" is the transmissivity of the glass, "A" is its absorptivity, and "N" is the fraction of absorbed energy that is re-emitted into the space. The overall shading coefficient is thus given by the ratio:
The shading coefficient depends on the
radiation properties of the window assembly. These properties are the
transmissivity "T" ,
absorptivity "A",
emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
(which is equal to the absorptivity for any given wavelength), and
reflectivity
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
all of which are dimensionless quantities that together sum to 1.
Factors such as
color
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
, tint, and reflective coatings affect these properties, which is what prompted the development of the shading coefficient as a correction factor to account for this. ASHRAE’s table of solar heat gain factors
provides the expected solar heat gain for ⅛” clear float glass at different latitudes, orientations, and times, which can be multiplied by the shading coefficient to correct for differences in radiation properties.
The value of the shading coefficient ranges from 0 to 1. The lower the rating, the less solar heat is transmitted through the glass, and the greater its shading ability.
In addition to glass properties, shading devices integrated into the window assembly are also included in the SC calculation. Such devices can reduce the shading coefficient by blocking portions of the glazing with opaque or translucent material, thus reducing the overall transmissivity.
Window design methods have moved away from the Shading Coefficient and towards the
Solar Heat Gain Coefficient (SHGC), which is defined as the fraction of incident solar radiation that actually enters a building through the entire window assembly as heat gain (not just the glass portion). The standard method for calculating the SHGC also uses a more realistic wavelength-by-wavelength method, rather than just providing a coefficient for a single wavelength like the shading coefficient does.
Though the shading coefficient is still mentioned in manufacturer product literature and some industry computer software,
it is no longer mentioned as an option in industry-specific texts
or model building codes. Aside from its inherent inaccuracies, another shortcoming of the SC is its counter-intuitive name, which suggests that high values equal high shading when in reality the opposite is true. Industry technical experts recognized the limitations of SC and pushed towards SHGC in the United States (and the analogous g-value in Europe) before the early 1990s.
A conversion from SC to SHGC is not necessarily straightforward, as they each take into account different heat transfer mechanisms and paths (window assembly vs. glass-only). To perform an approximate conversion from SC to SHGC, multiply the SC value by 0.87.
g-value
The g-value (sometimes also called a Solar Factor or Total Solar Energy Transmittance) is the coefficient commonly used in Europe to measure the solar energy transmittance of windows. Despite having minor differences in modeling standards compared to the SHGC, the two values are effectively the same. A g-value of 1.0 represents full transmittance of all solar radiation while 0.0 represents a window with no solar energy transmittance. In practice though, most g-values will range between 0.2 and 0.7, with solar control glazing having a g-value of less than 0.5.
Solar heat gain coefficient (SHGC)
SHGC is the successor to the shading coefficient used in the United States and it is the ratio of transmitted solar radiation to incident solar radiation of an entire window assembly. It ranges from 0 to 1 and refers to the solar energy transmittance of a window or door as a whole, factoring in the glass, frame material, sash (if present), divided lite bars (if present) and screens (if present).
The transmittance of each component is calculated in a similar manner to the shading coefficient. However, in contrast to the shading coefficient, the total solar gain is calculated on a wavelength-by-wavelength basis where the directly
transmitted portion of the solar heat gain coefficient is given by:
Here
is the spectral transmittance at a given wavelength in nanometers and
is the incident solar spectral irradiance. When integrated over the wavelengths of solar short-wave radiation, it yields the total fraction of transmitted solar energy across all solar wavelengths. The product
is thus the portion of absorbed and re-emitted energy across all assembly components beyond just the glass. It is important to note that the standard SHGC is calculated only for an angle of incidence normal to the window. However this tends to provide a good estimate over a wide range of angles, up to 30 degrees from normal in most cases.
SHGC can either be estimated through simulation models or measured by recording the total heat flow through a window with a calorimeter chamber. In both cases, NFRC standards outline the procedure for the test procedure and calculation of the SHGC. For dynamic fenestration or operable shading, each possible state can be described by a different SHGC.
Though the SHGC is more realistic than the SC, both are only rough approximations when they include complex elements such as shading devices, which offer more precise control over when fenestration is shaded from solar gain than glass treatments.
Solar gain in opaque building components
Apart from windows, walls and roofs also serve as pathways for solar gain. In these components heat transfer is entirely due to absorptance, conduction, and re-radiation since all transmittance is blocked in opaque materials. The primary metric in opaque components is the Solar Reflectance Index which accounts for both solar reflectance (albedo) and emittance of a surface.
Materials with high SRI will reflect and emit a majority of heat energy, keeping them cooler than other exterior finishes. This is quite significant in the design of roofs since dark roofing materials can often be as much as 50 C° hotter than the surrounding air temperature, leading to large thermal stresses as well as heat transfer to interior space.
Solar gain and building design
Solar gain can have both positive or negative effects depending on the climate. In the context of passive solar building design, the aim of the designer is normally to maximize solar gain within the building in the winter (to reduce
space heating
Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...
demand), and to control it in summer (to minimize cooling requirements).
Thermal mass
In building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...
may be used to even out the fluctuations during the day, and to some extent between days.
Control of solar gain
Uncontrolled solar gain is undesirable in hot climates due to its potential for overheating a space. To minimize this and reduce cooling loads, several technologies exist for solar gain reduction. SHGC is influenced by the color or tint of glass and its degree of
reflectivity
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
. Reflectivity can be modified through the application of reflective metal oxides to the surface of the glass.
Low-emissivity
Low emissivity (low ''e'' or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect, and emit radiant energy according to Planck's law but here, the primary co ...
coating is another more recently developed option that offers greater specificity in the wavelengths reflected and re-emitted. This allows glass to block mainly short-wave infrared radiation without greatly reducing
visible transmittance
Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is th ...
.
In climate-responsive design for
cold and mixed climates, windows are typically sized and positioned in order to provide solar heat gains during the heating season. To that end, glazing with a relatively high solar heat gain coefficient is often used so as not to block solar heat gains, especially in the sunny side of the house. SHGC also decreases with the number of glass panes used in a window. For example, in
triple glazed windows, SHGC tends to be in the range of 0.33 - 0.47. For
double glazed windows SHGC is more often in the range of 0.42 - 0.55.
Different types of glass can be used to increase or to decrease solar heat gain through fenestration, but can also be more finely tuned by the proper orientation of windows and by the addition of shading devices such as
overhangs,
louver
A louver (American English) or louvre (British English
British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". Mor ...
s, fins,
porch
A porch (from Old French ''porche'', from Latin ''porticus'' "colonnade", from ''porta'' "passage") is a room or gallery located in front of an entrance of a building. A porch is placed in front of the facade of a building it commands, and form ...
es, and other architectural shading elements.
Passive solar heating
Passive solar heating is a design strategy that attempts to maximize the amount of solar gain in a building when additional heating is desired. It differs from active solar heating which uses exterior water tanks with pumps to absorb solar energy because passive solar systems do not require energy for pumping and store heat directly in structures and finishes of occupied space.
In direct solar gain systems, the composition and coating of the building glazing can also be manipulated to increase the greenhouse effect by optimizing their radiation properties, while their size, position, and shading can be used to optimize solar gain. Solar gain can also be transferred to the building by indirect or isolated solar gain systems.
Passive solar designs typically employ large south facing windows with a high SHGC and overhangs that block sunlight in summer months and permit it to enter the window in the winter. When placed in the path of admitted sunlight, high thermal mass features such as concrete slabs or
trombe wall
A Trombe wall is a massive equator-facing wall that is painted a dark color in order to absorb thermal energy from incident sunlight and covered with a glass on the outside with an insulating air-gap between the wall and the glaze. A Trombe wall is ...
s store large amounts of solar radiation during the day and release it slowly into the space throughout the night. When designed properly, this can modulate temperature fluctuations. Some of the current research into this subject area is addressing the tradeoff between opaque thermal mass for storage and transparent glazing for collection through the use of transparent phase change materials that both admit light and store energy without the need for excessive weight.
See also
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Double-skin facade
The double-skin façade is a system of building consisting of two skins, or façades, placed in such a way that air flows in the intermediate cavity. The ventilation of the cavity can be natural, fan supported or mechanical. Apart from the type o ...
*
Heating degree day
Heating degree day (HDD) is a measurement designed to quantify the demand for energy needed to heat a building. HDD is derived from measurements of outside air temperature. The heating requirements for a given Architectural structure, building at ...
*
Insulated glazing
Insulating glass (IG) consists of two or more glass window panes separated by a space to reduce heat transfer across a part of the building envelope. A window with insulating glass is commonly known as double glazing or a double-paned window, ...
*
Low-emissivity coatings
References
{{DEFAULTSORT:Solar Gain
Atmospheric radiation
Meteorological phenomena
Low-energy building
Glass engineering and science
Glass architecture
Glass physics
Shading