solar gain (also known as solar thermal gain, solar heat gain or passive solar gain) refers to the increase in the thermal energy of a space, object or structure after being exposed to incident solar radiation.[1] The solar gain experienced by a space depends on the incident solar irradiance and the ability of the materials to transmit or resist the radiation.
Objects struck by sunlight absorb their visible and short-wave infrared components, increasing their temperature, and then re-radiate that heat at longer infrared wavelengths. Although transparent building materials, such as glass, allow visible light to pass through them almost unimpeded, once the light is transformed into long-wave infrared radiation by the materials inside, it cannot escape through the window, as glass is opaque to these longer wavelengths. The trapped heat causes solar gain through a phenomenon known as the greenhouse effect. In buildings, excessive solar gain can cause overheating within the space, but can also be used as a passive heating strategy when heat is desired.
Solar gain properties in windows
Contenido
La ganancia solar se aborda con mayor frecuencia en el diseño y selección de ventanas, puertas y otros acristalamientos. Por ello, se recogen de forma común las propiedades térmicas de los acristalamientos para poder cuantificar la ganancia solar. En Estados Unidos, la American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE)[2] y el National Fenestration Rating Council (NRFC)[3] mantienen estándares para el cálculo y la medición de estos valores.
Shading coefficient
Shading Coefficient (SC) is a measure of the radiative thermal behavior of a glazed unit (panel or window) in a building. It is defined as the ratio of solar radiation at a given wavelength and angle of incidence that passes through a glazing unit to the radiation that would pass through a 3 millimeter (0.1 in) reference window .[3] Since the quantities compared are functions of both wavelength and angle of incidence, the shading coefficient for a glazing is usually given for a single typical wavelength of incoming solar radiation normal to the plane of incidence. glass. This quantity includes both the energy that is transmitted directly through the glass and the energy that is absorbed by the glass and the frame and re-radiated into space, and is given by the following equation:[4].
Solar control glazing
Introduction
solar gain (also known as solar thermal gain, solar heat gain or passive solar gain) refers to the increase in the thermal energy of a space, object or structure after being exposed to incident solar radiation.[1] The solar gain experienced by a space depends on the incident solar irradiance and the ability of the materials to transmit or resist the radiation.
Objects struck by sunlight absorb their visible and short-wave infrared components, increasing their temperature, and then re-radiate that heat at longer infrared wavelengths. Although transparent building materials, such as glass, allow visible light to pass through them almost unimpeded, once the light is transformed into long-wave infrared radiation by the materials inside, it cannot escape through the window, as glass is opaque to these longer wavelengths. The trapped heat causes solar gain through a phenomenon known as the greenhouse effect. In buildings, excessive solar gain can cause overheating within the space, but can also be used as a passive heating strategy when heat is desired.
Solar gain properties in windows
Contenido
La ganancia solar se aborda con mayor frecuencia en el diseño y selección de ventanas, puertas y otros acristalamientos. Por ello, se recogen de forma común las propiedades térmicas de los acristalamientos para poder cuantificar la ganancia solar. En Estados Unidos, la American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE)[2] y el National Fenestration Rating Council (NRFC)[3] mantienen estándares para el cálculo y la medición de estos valores.
Shading coefficient
Shading Coefficient (SC) is a measure of the radiative thermal behavior of a glazed unit (panel or window) in a building. It is defined as the ratio of solar radiation at a given wavelength and angle of incidence that passes through a glazing unit to the radiation that would pass through a 3 millimeter (0.1 in) reference window .[3] Since the quantities compared are functions of both wavelength and angle of incidence, the shading coefficient for a glazing is usually given for a single typical wavelength of incoming solar radiation normal to the plane of incidence. glass. This quantity includes both the energy that is transmitted directly through the glass and the energy that is absorbed by the glass and the frame and re-radiated into space, and is given by the following equation:[4].
Clear Float Glass
where λ is the wavelength of the 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 reemitted into the space. The global shading coefficient is given by the relationship:.
The shading coefficient depends on the radiative properties of the glazing. These properties are transmissivity "T", absorptivity "A", emissivity (which is equal to the absorptivity for any given wavelength) and reflectivity. These are all dimensionless quantities that add up to 1.[4] Factors such as color, tint, and reflective coatings affect these properties, which prompted the development of the shading coefficient as a correction factor to take this into account. The ASHRAE table of solar heat gain factors[2] provides the expected solar heat gain for 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 varies from 0 to 1. The lower its value, the less solar heat is transmitted through the glass and the greater its shading capacity.
Window design methods have moved away from the shading coefficient toward the Solar Heat Gain Coefficient (SHGC), which is defined as the fraction of incident solar radiation that actually enters a building through the entire glazed envelope as heat gain (not just the glass portion). Although the shading coefficient is still mentioned in manufacturers' product literature and in some industry software,[5] it is no longer mentioned as an option in industry-specific texts[2] or in building codes.[6] In addition to its inherent inaccuracies, another flaw of SC is its counterintuitive name, which suggests that high values equal high shading, when in fact just the opposite is true. The industry recognized the limitations of SC and moved toward SHGC in the United States (and analogous g value in Europe) before the early 1990s.[7].
The conversion from SC to SHGC is not necessarily straightforward, as each takes into account different heat transfer mechanisms and paths (glazed assembly enclosure vs. all-glass). To perform an approximate conversion from SC to SHGC, multiply the value of SC by 0.87.[3].
Solar g factor or total transmittance of solar energy
The solar g factor (sometimes also called total solar energy transmittance) is the coefficient commonly used in Europe to measure the solar energy transmittance of windows. Despite there being small differences in the modeling standards compared to the SHGC, both values are equivalent. A g value of 1 represents the total transmission of all solar radiation, while a g value of 0 represents a window with no solar energy transmission. However, in practice, most g values will range between 0.2 and 0.7, with a g value less than 0.5 for solar control glazing.[8].
Solar thermal gain coefficient (SHGC)
The solar thermal gain coefficient (SHGC) is the successor to the shading coefficient used in the United States and is defined as the ratio between the transmitted solar radiation and the incident solar radiation for a glazed enclosure. Its value ranges from 0 to 1 and refers to the solar energy transmission of a window or door as a whole, taking into account the glass, frame materials (if present), divider bars (if present) and screens (if present).[3] The transmittance of each component is calculated in a similar way to the shading coefficient. However, unlike the shading coefficient, the directly transmitted fraction of the solar heat gain coefficient is given by:[4].
where is the spectral transmittance at a given wavelength in nanometers and is the incident solar spectral irradiance. When integrated over the wavelengths of shortwave solar radiation, the total fraction of solar energy transmitted at all solar wavelengths is obtained. The product is, therefore, the fraction of energy absorbed and re-emitted in all the components of the glazed enclosure. 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 in most cases a good estimate for a wide range of angles, up to 30 degrees from the normal.[3].
The SHGC can be estimated through simulation models or measured by recording the total heat flux through a window with a calorimetric camera. In both cases, the NFRC standards describe the calculation, testing and validation procedure of the SHGC.[9].
Although the SHGC is more realistic than the SC, both coefficients are overly simplified approximations when complex elements such as shading devices are included, which offer more precise control over the treatments of the glazing itself when the glazed enclosures receive shadows that limit solar gain.[10].
References
[1] ↑ Daniel D. Chiras, (2010), The Solar House: Passive Heating and Cooling, Nueva York, pág. 216.
[2] ↑ a b c ASHRAE (2013). «Chapter 15. Fenestration». ASHRAE Handbook: Fundamentals. Atlanta, GA: ASHRAE.: http://www.ashrae.org
[3] ↑ a b c d e ANSI/NFRC 200-2017: Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence., NFRC, 2017, consultado el 9 de noviembre de 2017 .: http://www.nfrc.org
[8] ↑ «British Fenestration Rating Council». BFRC. Consultado el 9 de noviembre de 2017.: https://www.bfrc.org/
[9] ↑ 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 |fechaacceso= requiere |url= (ayuda).
[10] ↑ Lechner, Norbert (2009). Heating, Cooling, Lighting: Sustainable Design Methods for Architects (3ª edición). John Wiley & Sons. pp. 250-252. ISBN 9780470048092.
Clear Float Glass
where λ is the wavelength of the 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 reemitted into the space. The global shading coefficient is given by the relationship:.
The shading coefficient depends on the radiative properties of the glazing. These properties are transmissivity "T", absorptivity "A", emissivity (which is equal to the absorptivity for any given wavelength) and reflectivity. These are all dimensionless quantities that add up to 1.[4] Factors such as color, tint, and reflective coatings affect these properties, which prompted the development of the shading coefficient as a correction factor to take this into account. The ASHRAE table of solar heat gain factors[2] provides the expected solar heat gain for 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 varies from 0 to 1. The lower its value, the less solar heat is transmitted through the glass and the greater its shading capacity.
Window design methods have moved away from the shading coefficient toward the Solar Heat Gain Coefficient (SHGC), which is defined as the fraction of incident solar radiation that actually enters a building through the entire glazed envelope as heat gain (not just the glass portion). Although the shading coefficient is still mentioned in manufacturers' product literature and in some industry software,[5] it is no longer mentioned as an option in industry-specific texts[2] or in building codes.[6] In addition to its inherent inaccuracies, another flaw of SC is its counterintuitive name, which suggests that high values equal high shading, when in fact just the opposite is true. The industry recognized the limitations of SC and moved toward SHGC in the United States (and analogous g value in Europe) before the early 1990s.[7].
The conversion from SC to SHGC is not necessarily straightforward, as each takes into account different heat transfer mechanisms and paths (glazed assembly enclosure vs. all-glass). To perform an approximate conversion from SC to SHGC, multiply the value of SC by 0.87.[3].
Solar g factor or total transmittance of solar energy
The solar g factor (sometimes also called total solar energy transmittance) is the coefficient commonly used in Europe to measure the solar energy transmittance of windows. Despite there being small differences in the modeling standards compared to the SHGC, both values are equivalent. A g value of 1 represents the total transmission of all solar radiation, while a g value of 0 represents a window with no solar energy transmission. However, in practice, most g values will range between 0.2 and 0.7, with a g value less than 0.5 for solar control glazing.[8].
Solar thermal gain coefficient (SHGC)
The solar thermal gain coefficient (SHGC) is the successor to the shading coefficient used in the United States and is defined as the ratio between the transmitted solar radiation and the incident solar radiation for a glazed enclosure. Its value ranges from 0 to 1 and refers to the solar energy transmission of a window or door as a whole, taking into account the glass, frame materials (if present), divider bars (if present) and screens (if present).[3] The transmittance of each component is calculated in a similar way to the shading coefficient. However, unlike the shading coefficient, the directly transmitted fraction of the solar heat gain coefficient is given by:[4].
where is the spectral transmittance at a given wavelength in nanometers and is the incident solar spectral irradiance. When integrated over the wavelengths of shortwave solar radiation, the total fraction of solar energy transmitted at all solar wavelengths is obtained. The product is, therefore, the fraction of energy absorbed and re-emitted in all the components of the glazed enclosure. 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 in most cases a good estimate for a wide range of angles, up to 30 degrees from the normal.[3].
The SHGC can be estimated through simulation models or measured by recording the total heat flux through a window with a calorimetric camera. In both cases, the NFRC standards describe the calculation, testing and validation procedure of the SHGC.[9].
Although the SHGC is more realistic than the SC, both coefficients are overly simplified approximations when complex elements such as shading devices are included, which offer more precise control over the treatments of the glazing itself when the glazed enclosures receive shadows that limit solar gain.[10].
References
[1] ↑ Daniel D. Chiras, (2010), The Solar House: Passive Heating and Cooling, Nueva York, pág. 216.
[2] ↑ a b c ASHRAE (2013). «Chapter 15. Fenestration». ASHRAE Handbook: Fundamentals. Atlanta, GA: ASHRAE.: http://www.ashrae.org
[3] ↑ a b c d e ANSI/NFRC 200-2017: Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence., NFRC, 2017, consultado el 9 de noviembre de 2017 .: http://www.nfrc.org
[8] ↑ «British Fenestration Rating Council». BFRC. Consultado el 9 de noviembre de 2017.: https://www.bfrc.org/
[9] ↑ 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 |fechaacceso= requiere |url= (ayuda).
[10] ↑ Lechner, Norbert (2009). Heating, Cooling, Lighting: Sustainable Design Methods for Architects (3ª edición). John Wiley & Sons. pp. 250-252. ISBN 9780470048092.