Solar Thermal & Optical Performance
The selection of the appropriate architectural glass products for a project demands finding a balance between the aesthetic, performance, cost (initial and life cycle) and, most importantly, compliance with existing building and energy codes.
When governing the performance parameters of the glass, however, there are several aspects that need to be considered individually, and then in a holistic view. These aspects exist and are a natural outcome of the fact that the sun radiates solar energy or sunlight by electromagnetic waves over a range of wavelengths known as the Solar Spectrum, 280 – 2500 nanometres, that is divided into three bands:
While the energy allotted to each band of the solar spectrum is approximately:
It is important to note that the shorter the wavelength (i.e. the lower the nanometres), the higher is the energy associated with the radiation. This is highlighted by the fact that it is the shorter wavelength; the high energy UV light, that causes humans to sunburn, fabrics to fade and plastics to deteriorate. On the other hand, the longer-wavelength and lower-energy radiations; produced by the visible light and infra-red bands; are less damaging.
As visible light and infra-red account for 98% of solar energy, they are extremely important considerations when deciding the choice of glass. Solar control glass types are either body tinted and / or coated to absorb or reflect the sun’s energy and reduce the solar heat gain transmitted through the glass.
Solar, Thermal & Optical Performance
Visible Light Transmittance: Expressed as the percentage of visible light transmitted through glass. It is defined by VTL, LT and is directly proportional to the amount of light transmitted.
Visible Light Reflectance (external): The percentage of visible light reflected externally from the glass surface(s). It is defined by LRout.
Visible Light Reflectance (internal): The percentage of visible light reflected internally from the glass surface(s). It is defined by LRin.
Solar Energy Transmittance: The percentage of ultra-violet, visible and infra-red energy directly transmitted through glass.
Solar Energy Reflectance: The percentage of solar energy reflected from the glass surface(s).
Solar Heat Gain Coefficient (SHGC) or Total Solar Energy Transmittance: The proportion of directly transmitted and absorbed solar energy that enters into the building’s interior. The lower the number is, the better the glass is able to exclude solar radiation.
Shading Coefficient (SCo.): The ratio of total solar radiation through a particular glass type, relative to the total solar radiation through 3mm clear float glass. The lower the number, the better the glass performs in reducing heat gain.
U-value (expressed in W/m2K): The measure of air-to-air heat transfer (either loss or gain) due to thermal conductance and the difference between indoor and outdoor temperatures. The lower the number is, the better the insulating qualities of the glass.
Relative Heat Gain: This is the total amount of heat gain through a glass taking into consideration short and long wave heat. RHG is calculated by using the shading coefficient and the U-value. (It is expressed in W/m²
RHG = (637 x SCo.) + (U-value x 7.8)
Light to Solar Gain (LSG): The ratio of visible light transmittance to the Solar Heat Gain Coefficient. This ratio is helpful when both high light transmittance and low Solar Gains are important. LSG is particularly important in structures and building designed around daylighting conditions.