Glossary:Glossary-Plus

Glossary: WUFI-Plus

Solar Heat Gain Coefficient

• The Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from sunlight. The SHGC is the fraction of the heat from the sun that enters through a window, both directly transmitted, and absorbed and subsequently released inward.
• The SHGC replaced the shading coefficient an the standard indicator of a window‘s shading ability
• SHGC is expressed as a number between 0 and 1.
• SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.
• The lower a window's SHGC, the less solar heat it transmits.
• For incident radiation that is diffuse, the hemispherical SHGC must be used.

[[1]] [[2]] ASHRAE Handbook 2005 Fundamentals

g-Value

The g-value is a value for the pass of energy through a transparent component. It is composed of direct solar radiation and heat emission by secondary absorptance to the inner space.

$g={\tau }_e+q_i[1]$

The radiation power ${\varphi }_e$ through the component can be separated as following:

transmitted part ${\tau }_e{\varphi }_e$ direct transmittance ${\tau }_e$

reflected part ${\rho }_e{\varphi }_e$ direct reflectance ${\rho }_e$

absorbed part ${\alpha }_e{\varphi }_e$ direct absorptance ${\alpha }_e$ [1]

Where

${\tau }_e+{\rho }_e+{\alpha }_e=1$ [1]

For the calculation of the g-value are the direct transmittance ${\tau }_e$ and the direct absorption ${\alpha }_e$ needed. From the ${\alpha }_e$ only the inward flowing fraction of the energy, the secondary absorptance $q_i$ is relevant.

Direct transmittance ${\tau }_e$

${\tau }_e=\frac{\sum S_{\lambda }\tau (\lambda )\mathrm{\Delta }\lambda }{\sum S_{\lambda }\mathrm{\Delta }\lambda }$ [1]

Where

$S_{\lambda }$ spectral distribution of solar radiation

$\tau (\lambda )$ spectral transmittance of the glazing

$\mathrm{\Delta }\lambda$ range of wavelength

Inward flowing fraction of energy $q_i$:

single glazing fenestration

$q_i=\frac{{\alpha }_e{h}_i}{h_e+h_i}$ [1]

Where

$h_e$ 23 W/(m²K)

$h_i$ 8 W/(m²K) – for borosilicate- und soda-lime-glass

${\alpha }_e$ solar absorptance

($h_e$ and $h_i$ are mean values with arranged conditions)

g-values are dimensionless and between 0 and 1

[1] DIN EN 410:1998 Glas im Bauwesen, Bestimmung der lichttechnischen und strahlungsphysikalischen Kenngrößen von Verglasungen