Glass is used in a building to harvest natural light inside it and there by reducing the internal artificial lighting requirements, in turn saving energy. The two major sources for heat entering the building is solar heat and non-solar heat; solar heat is nothing but the direct solar heat entering the building through visible light, non solar heat is caused by various factors like conduction, convection and radiation. Out of the heat sources, solar heat is the major one and requires more attention, even though the effects of non-solar heat could not be ignored as well, as it plays a major role in certain structures.
When we consider the solar spectrum, major part of Sun’s energy is made up of invisible infra-red and ultra violet rays, and the remaining is visible light carries rest of the energy; 50% heat is carried by the visible light and 50% is carried by infra-red and ultra-violet. Ultra-violet rays are the major cause for fading colors. Then what does it mean when we say solar control? Solar control process is nothing but letting in maximum visible light and blocking the heat entering the building. Since 50% of heat entering the building comes with visible light, if we are asking for 100% of visual light, we must expect 50 % of the heat from the sun also to enter the building.
Visible light, when it hits on glass, could be either transmitted through, reflected or absorbed (depending on the glass type). As mentioned above, since energy is carried along by visual light, transmission, reflection and absorption also happens with energy (Direct Energy Transmission). Solar control works on the principle of increasing reflectance and absorption of energy and increasing the light transmission, by choosing the right kind of glass.
There are a few important parameters which should be understood while choosing the right glass type for a building, out of which the most important ones are Solar Factor or Solar Heat Gain Coefficient (SHGC), Shading Coefficient (SC), Light Transmittance, Light Reflectance, DET, Energy Reflectance and Energy Absorption. Solar Factor (SF) is the fraction of solar radiation let in through glass, either transmitted directly, or absorbed and transmitted inwards later. A low SF signifies that the glass lets in less solar heat, and SF could also be expressed as for the entire window assembly and not just for the glass. 3 mm clear glass is considered to have the maximum SF, i.e. it lets in maximum solar heat. The SF of a particular glass used in a building relative to the SF of 3 mm clear glass is referred to as the Shading Coefficient (SC) of that particular glass. Light transmittance (LT) is nothing but the percentage of visible light let in by the glass, similarly, Light Reflectance (LR) is the percentage of visible light reflected by the glass (both internal and external LR). Direct energy transmittance (DET) is the percentage of total solar energy transmitted directly through the glass. Energy Reflectance (ER) and Energy Absorption are the percentage of heat reflected and absorbed by the glass respectively.
Since the primary objective of using glass is to let in maximum visual light, it becomes important to do maximum solar control as well. Selectivity value of a particular glass is used to compare two glasses in terms of visual light transmittance (VLT) and SF. For instance, if LT and SF of a particular glass is 32% and 52% respectively , and for another glass it is 26% and 36% respectively, then the selectivity for both glasses are 0.61 and 0.72 respectively (Selectivity = LT/SF). In this case, since the selectivity value is greater for the second glass, it becomes a better choice than the second glass.
Non-solar heat control to be discussed in the next post.