Solar gain

Last updated February 07, 2024

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. The amount of solar gain a space experiences is a function of the total incident solar irradiance and of the ability of any intervening material to transmit 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 wavelengths. 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. 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.^[1]

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),^[2] and The National Fenestration Rating Council (NFRC)^[3] maintain standards for the calculation and measurement of these values.

Shading coefficient

The shading coefficient (SC) is a measure of the radiative thermal performance of a glass unit (panel or window) in a building. 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 3 millimetres (0.12 in) Clear Float Glass.^[3] 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:^[4]

$F(\lambda ,\theta )=T(\lambda ,\theta )+N*A(\lambda ,\theta )$

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:

$S.C.=F(\lambda ,\theta )_{1}/F(\lambda ,\theta )_{o}$

The shading coefficient depends on the radiation properties of the window assembly. These properties are the transmissivity "T" , absorptivity "A", emissivity (which is equal to the absorptivity for any given wavelength), and reflectivity all of which are dimensionless quantities that together sum to 1.^[4] Factors such as color, 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^[2] 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.^[5]

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.^[4] Though the shading coefficient is still mentioned in manufacturer product literature and some industry computer software,^[6] it is no longer mentioned as an option in industry-specific texts^[2] or model building codes.^[7] 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.^[8]

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.^[3]

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.^[9]

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).^[3] 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:^[4]

$T=\int \limits _{350\ nm}^{3500\ nm}T(\lambda )E(\lambda )d\lambda$

Here $T(\lambda )$ is the spectral transmittance at a given wavelength in nanometers and $E(\lambda )$ 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 $N*A(\lambda ,\theta )$ 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.^[3]

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.^[10] 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.^[5]

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.^[11] 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.^[5]

Solar gain and building design

Solar gain can have either 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 demand), and to control it in summer (to minimize cooling requirements). Thermal mass 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. Reflectivity can be modified through the application of reflective metal oxides to the surface of the glass. Low-emissivity 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.^[3]

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, louvers, fins, porches, 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.^[12]

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 equator 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 walls store large amounts of solar radiation during the day and release it slowly into the space throughout the night.^[13] 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.^[14]

Related Research Articles

A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent material, a sash set in a frame in the opening; the sash and frame are also referred to as a window. Many glazed windows may be opened, to allow ventilation, or closed, to exclude inclement weather. Windows may have a latch or similar mechanism to lock the window shut or to hold it open by various amounts.

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 a passive solar building design strategy that adopts the concept of indirect-gain, where sunlight first strikes a solar energy collection surface in contact with a thermal mass of air. The sunlight absorbed by the mass is converted to thermal energy (heat) and then transferred into the living space.

In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.

Daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that direct or indirect sunlight can provide effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved from the reduced use of artificial (electric) lighting or from passive solar heating. Artificial lighting energy use can be reduced by simply installing fewer electric lights where daylight is present or by automatically dimming or switching off electric lights in response to the presence of daylight – a process known as daylight harvesting.

Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material is converted to electromagnetic radiation. All matter with a temperature greater than absolute zero emits thermal radiation. At room temperature, most of the emission is in the infrared (IR) spectrum. Particle motion results in charge-acceleration or dipole oscillation which produces electromagnetic radiation.

In physics, Planck's law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature $T$ , when there is no net flow of matter or energy between the body and its environment.

In heat transfer, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium. It is a special case of Onsager reciprocal relations as a consequence of the time reversibility of microscopic dynamics, also known as microscopic reversibility.

Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body. It has a specific, continuous spectrum of wavelengths, inversely related to intensity, that depend only on the body's temperature, which is assumed, for the sake of calculations and theory, to be uniform and constant.

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 not visible to human eyes. A portion of the thermal radiation from very hot objects is easily visible to the eye.

Low 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 concern is a special wavelength interval of radiant energy, namely thermal radiation of materials. In common use, especially building applications, the temperature range of approximately -40 to +80 degrees Celsius is the focus, but in aerospace and industrial process engineering, much broader ranges are of practical concern.

Building insulation is material used in a building to reduce the flow of thermal energy. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation. Often an insulation material will be chosen for its ability to perform several of these functions at once.

The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient value that is large represents a beam becoming 'attenuated' as it passes through a given medium, while a small value represents that the medium had little effect on loss. The (derived) SI unit of attenuation coefficient is the reciprocal metre (m⁻¹). Extinction coefficient is another term for this quantity, often used in meteorology and climatology. Most commonly, the quantity measures the exponential decay of intensity, that is, the value of downward e-folding distance of the original intensity as the energy of the intensity passes through a unit thickness of material, so that an attenuation coefficient of 1 m⁻¹ means that after passing through 1 metre, the radiation will be reduced by a factor of e, and for material with a coefficient of 2 m⁻¹, it will be reduced twice by e, or e². Other measures may use a different factor than e, such as the decadic attenuation coefficient below. The broad-beam attenuation coefficient counts forward-scattered radiation as transmitted rather than attenuated, and is more applicable to radiation shielding. The mass attenuation coefficient is the attenuation coefficient normalized by the density of the material.

The National Fenestration Rating Council (NFRC) is a United States 501(c)3 non-profit organization which sponsors an energy efficiency certification and labeling program for windows, doors, and skylights.

Window insulation reduces heat transfer from one side of a window to the other. The U-value is used to refer to the amount of heat that can pass through a window, called thermal transmittance, with a lower score being better. The U-factor of a window can often be found on the rating label of the window.

Shading coefficient (SC) is a measure of thermal performance of a glass unit (panel or window) in a building.

The cooling load temperature difference (CLTD)calculation method, also called the cooling load factor(CLF) or solar cooling load factor(SCL) method, is a method of estimating the cooling load or heating load of a building. It was introduced in the 1979 ASHRAE handbook.

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, triple glazing or a triple-paned window, or quadruple glazing or a quadruple-paned window, depending upon how many panes of glass are used in its construction.

A skylight is a light-permitting structure or window, usually made of transparent or translucent glass, that forms all or part of the roof space of a building for daylighting and ventilation purposes.

<span class="mw-page-title-main">Quadruple glazing</span> Type of insulated glazing

Quadruple glazing is a type of insulated glazing comprising four glass panes, commonly equipped with low emissivity coating and insulating gases in the cavities between the glass panes. Quadruple glazing is a subset of multipane (multilayer) glazing systems. Multipane glazing with up to six panes is commercially available.

In the study of heat transfer, Schwarzschild's equation is used to calculate radiative transfer through a medium in local thermodynamic equilibrium that both absorbs and emits radiation.

References

↑ Frota, Anesia Barros (1999). Manual de conforto termico. Sueli Ramos Schiffer (3rd ed.). Sao Paulo: Studio Nobel. ISBN 85-85445-39-4. OCLC 46753712.
1 2 3 ASHRAE (2013). "Chapter 15. Fenestration". ASHRAE Handbook: Fundamentals. Atlanta, GA: ASHRAE.
1 2 3 4 5 6 ANSI/NFRC 200-2017: Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence., NFRC, 2017, retrieved 9 November 2017
1 2 3 4 McCluney, Ross (1996), Fenestration Solar Gain Analysis, Florida Solar Energy Center/University of Central Florida, CiteSeerX 10.1.1.30.2472
1 2 3 Lechner, Norbert (2009). Heating, Cooling, Lighting: Sustainable Design Methods for Architects (3rd ed.). John Wiley & Sons. pp. 250–252. ISBN 9780470048092.
↑ "WINDOW Documentation" . Retrieved 7 October 2017.
↑ ICC (2009). 2009 International Energy Conservation Code.
↑ McCluney, Ross (1991). "The Death of the Shading Coefficient?". ASHRAE Journal (March): 36–45. Retrieved 7 October 2017.
↑ "British Fenestration Rating Council". BFRC. Retrieved 9 November 2017.
↑ ANSI/NFRC 201-2017: Procedure for Interim Standard Test Method for Measuring the Solar Heat Gain Coefficient of Fenestration Systems Using Calorimetry Hot Box Methods, NFRC, p. 19
↑ "Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces". West Conshohocken, PA: ASTM International. 2001. doi:10.1520/E1980-11.
↑ Balcomb, J.D; Hedstrom, J.C; McFarland, R.D (1977). "Simulation analysis of passive solar heated buildings—Preliminary results". Solar Energy. 19 (3): 2–12. Bibcode:1977SoEn...19..277B. doi:10.1016/0038-092X(77)90071-8.
↑ Balcomb, J.Douglas, editor. Passive Solar Buildings. MIT Press, 1992.^{[ page needed ]}
↑ Manz, H; Egolf, P.W; Suter, P; Goetzberger, A (1997). "TIM–PCM external wall system for solar space heating and daylighting". Solar Energy. 61 (6): 369. Bibcode:1997SoEn...61..369M. doi:10.1016/S0038-092X(97)00086-8.

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[1] Frota, Anesia Barros (1999). Manual de conforto termico. Sueli Ramos Schiffer (3rd ed.). Sao Paulo: Studio Nobel. ISBN 85-85445-39-4. OCLC 46753712.

[ASHRAE-2] 1 2 3 ASHRAE (2013). "Chapter 15. Fenestration". ASHRAE Handbook: Fundamentals. Atlanta, GA: ASHRAE.

[NFRC200-3] 1 2 3 4 5 6 ANSI/NFRC 200-2017: Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence., NFRC, 2017, retrieved 9 November 2017

[FloridaSolar-4] 1 2 3 4 McCluney, Ross (1996), Fenestration Solar Gain Analysis, Florida Solar Energy Center/University of Central Florida, CiteSeerX 10.1.1.30.2472

[HCL-5] 1 2 3 Lechner, Norbert (2009). Heating, Cooling, Lighting: Sustainable Design Methods for Architects (3rd ed.). John Wiley & Sons. pp. 250–252. ISBN 9780470048092.

[WINDOW-6] "WINDOW Documentation" . Retrieved 7 October 2017.

[7] ICC (2009). 2009 International Energy Conservation Code.

[DeathOfSC-8] McCluney, Ross (1991). "The Death of the Shading Coefficient?". ASHRAE Journal (March): 36–45. Retrieved 7 October 2017.

[9] "British Fenestration Rating Council". BFRC. Retrieved 9 November 2017.

[10] ANSI/NFRC 201-2017: Procedure for Interim Standard Test Method for Measuring the Solar Heat Gain Coefficient of Fenestration Systems Using Calorimetry Hot Box Methods, NFRC, p. 19

[ASTM1980-11] "Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces". West Conshohocken, PA: ASTM International. 2001. doi:10.1520/E1980-11.

[12] Balcomb, J.D; Hedstrom, J.C; McFarland, R.D (1977). "Simulation analysis of passive solar heated buildings—Preliminary results". Solar Energy. 19 (3): 2–12. Bibcode:1977SoEn...19..277B. doi:10.1016/0038-092X(77)90071-8.

[13] Balcomb, J.Douglas, editor. Passive Solar Buildings. MIT Press, 1992.^{[ page needed ]}

[14] Manz, H; Egolf, P.W; Suter, P; Goetzberger, A (1997). "TIM–PCM external wall system for solar space heating and daylighting". Solar Energy. 61 (6): 369. Bibcode:1997SoEn...61..369M. doi:10.1016/S0038-092X(97)00086-8.

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