How to calculate solar heat gain through windows?
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The heat gain components through glass consists of solar radiation and conduction. Solar radiation is considered in two parts - direct and diffuse (or scatter). Diffuse radiation is the solar radiation that is absorbed, stored and scattered in the atmosphere. The glass can be in the sun (direct and diffuse radiation) or in the shade (diffuse or scatter radiation). Conduction heat gain occurs due to the difference in temperature on either side of the glass. Conduction heat gain is positive if the outdoor air temperature is greater than indoor air temperature and it is negative (heat loss from the space) if the indoor air temperature is greater. Solar radiation is always positive.
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Direct solar radiation is the vector component of the absolute (total) solar radiation that is perpendicular to the glass surface. The Solar Cooling Load (SCL) Factor for a window is based on this value. So for any given hour, the SCL values for windows with different azimuth and tilt angles will have different SCLs although the absolute solar radiation is the same for all windows.
Q-solar = A * SC * SCL
Q-cond = A * U * CLTD
A = Glass Area , SC = Glass Shading Coefficient, U = Glass heat transfer coefficient.
CLTD = Cooling Load Temperature Difference. CLTD for glass depends mainly on the difference between indoor and outdoor temperatures but as with walls and roofs it is affected by the mass and properties of the glass material.
When solar radiation strikes a glass surface, some of it is transmitted, some of it is absorbed and some of it is reflected. The absorbed component increases the temperature of the glass and the heat is slowly conducted (released) to the outside and inside depending on the difference in temperature. Unlike walls that are thick and have high densities, the absorbed portion of the solar radiation is relatively small compared to transmitted and reflected components.
For example, a particular tinted glass has a transmissivity of 0.6 (60% of radiation is transmitted), reflectivity of 0.3 (30% of radiation is reflected) and absorbtivity of 0.1 (10% of radiation is absorbed). The direct solar radiation value is the component of the absolute solar radiation that is perpendicular to the glass surface.
Shading Coefficient (SC) is the ratio of the solar heat through a given glass type under specific conditions to the solar heat gain through a standard reference unshaded glass that was used to determine Solar Cooling Load (SCL) factors. The reference glass is one-eighth thick inch clear double strength single glass and it has an SC = 1 under the specific conditions.
Figure ?
Direct solar radiation is the vector component of the absolute (total) solar radiation that is perpendicular to the glass surface. The Solar Cooling Load (SCL) Factor for a window is based on this value. So for any given hour, the SCL values for windows with different azimuth and tilt angles will have different SCLs although the absolute solar radiation is the same for all windows.
Q-solar = A * SC * SCL
Q-cond = A * U * CLTD
A = Glass Area , SC = Glass Shading Coefficient, U = Glass heat transfer coefficient.
CLTD = Cooling Load Temperature Difference. CLTD for glass depends mainly on the difference between indoor and outdoor temperatures but as with walls and roofs it is affected by the mass and properties of the glass material.
When solar radiation strikes a glass surface, some of it is transmitted, some of it is absorbed and some of it is reflected. The absorbed component increases the temperature of the glass and the heat is slowly conducted (released) to the outside and inside depending on the difference in temperature. Unlike walls that are thick and have high densities, the absorbed portion of the solar radiation is relatively small compared to transmitted and reflected components.
For example, a particular tinted glass has a transmissivity of 0.6 (60% of radiation is transmitted), reflectivity of 0.3 (30% of radiation is reflected) and absorbtivity of 0.1 (10% of radiation is absorbed). The direct solar radiation value is the component of the absolute solar radiation that is perpendicular to the glass surface.
Shading Coefficient (SC) is the ratio of the solar heat through a given glass type under specific conditions to the solar heat gain through a standard reference unshaded glass that was used to determine Solar Cooling Load (SCL) factors. The reference glass is one-eighth thick inch clear double strength single glass and it has an SC = 1 under the specific conditions.
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