Solar Energy
Introduction
One of the most outstanding characteristics of solar control window film is its ability to regulate the amount of solar energy that passes through glass. Safety and security protection is another tremendous benefit offered by various types of window film.
Solar Energy and Heat Transfer
Energy always flows from higher to lower temperatures.
Solar control window films are designed to reflect infrared heat waves and to filter out most ultraviolet radiation.
3 % ultraviolet
44 % visible light
53 % infrared (invisible light)
Heat Transfer Methods
Conduction
Describes the transfer of energy (heat), through a solid such as window glass, by physical contact.
Convection
Occurs between a solid and a liquid surface that are brought into contact. It is the movement of heat being re-radiated (transfer by wind motion).
Measurement of Heat:
BTU: British thermal unit amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit.
Shading Coefficient: measurement often used in the window film and glazing industries, which measures the performance of glass window with film versus clear glass. The lower the coefficient number for a particular film or coating, the better it is able to reduce solar heat.
What happens when sunlight strikes glass
When sunlight strikes glass three things happen:
Energy can pass through the glass (transmittance) - 89%
Energy can be reflected away from the glass - 6%
Energy can be absorbed by the glass - 5%
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Visible Light Transmittance (VLT): percentage of daylight that passes through a glazing system. All solar control films reduce visible light transmittance in order to provide solar heat control.
Total Solar Energy Rejection (TSER): is an important measurement of a window film's ability to reject solar heat in the form of visible light and invisible infrared radiation.
Visible Light Reflectance (VLR): percentage of visible light that is being reflected by the window film.
Ultraviolet (UV) Absorption: measures the percentage of ultraviolet (UV) rays, which are being absorbed by the window film.
U-Value measurement of heat transfer due to outdoor/indoor temperature differences, the lower the U-value, the less the heat that transfers.
R-Value denotes a material's ability to act as insulator, the higher the R-value, the less heat transference.
Solar Energy Technical Definitions
Visible Light Transmission (VLT) the percent of total visible light that passes trough a glazing system.
Visible Light Reflectance (VLR) the percent of total visible light that is reflected by a glazing system.
Total Solar Absorption (TSA) the percent of incident solar radiation that is absorbed by a glazing system.
Total Solar Transmittance (TST) the percent of incident solar radiation that directly passes through a glazing system.
Total Solar Reflectance (TSR) the percent of incident radiation that is reflected by a glazing system.Ultraviolet (UV) Radiation Rejection percent of total ultraviolet light that is prevented from passing through a glazing system
Total Solar Energy Rejection (TSER) the percent of total energy (heat) rejected by a glazing system
Shading Coefficient (SC) ratio of solar heat gain passing through a glazing system to the solar heat gain that occurs under the same conditions if the window was made with clear glass
U-Value amount of conductive heat energy (BTU's) transferred through one square foot of a specific glazing system for each 1 degree F temperature difference between the indoor and outdoor air
Luminous Efficacy (LE) determined by dividing the visible light transmission by the shading coefficient
Emissivity is a measure of surface's ability to absorb or reflect far-infrared radiation (sensed by the body as heat)
Seasonal Differences in Sunlight have an important bearing on heat gain; this creates a seasonal difference in the directions from which sunlight is brightest trough windows
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