Glossary:Glossary-Plus

Glossary: WUFI-Plus

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 the direct transmittance ${\tau }_e$ and the direct absorption ${\alpha }_e$ are 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

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

Solar heat gain coefficient is the ration of solar heat gain entering the space through the fenestration product to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and that portion of the absorbed solar radiation which is the reradiated, conducted or convected into the space. [1]

The different solar heat gain coefficients:

SHGC_f - all frame and sash members

SHGC_d - divider area

SHGC_e – Edge-of-glazing area

SHGC_de – Edge-of-divider area

SHGC_c – Center-of-glazing area

SHGC_t – Total fenestration product

$SHGC={\tau }_s+N_i\cdot {\alpha }_s$ [2]

Where

${\tau }_s$ = solar transmittance

$N_i$ = inward-flowing fraction of absorbed radiation

${\alpha }_s$ = solar absorptance

The total product SHGC

$SHG{C}_t=\frac{(SHG{C}_f\cdot A_f)+(SHG{C}_d\cdot A_d)+(SHG{C}_e\cdot A_e)+(SHG{C}_{de}\cdot A_{de})+(SHG{C}_c\cdot A_c)}{A_{pf}}$ [1]

Where

$A$ relative area m² (edge-of-areas are 63.5 mm)

$A_{pf}$ projected fenestration product area

SHGC are calculated separately

for frame and divider the SHGC is 0.3 or 0.5 by default

SHGC are dimensionless and between 0 and 1

[1] National Fenestration Rating Council Incorporated, NFRC 200-2004, Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence

[2] National Fenestration Rating Council Incorporated, NFRC 201-2001, Interim Standard Test Method for Measuring the Solar Heat Gain Coefficient of Fenestration Systems Using Calorimetry Hot Box Method