Chapter 5Energy and Light

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 1161

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5.1 Energy and Thermal Properties of Glass

5.1.1 General

The energy and thermal properties of glass is defined by light transmission, internal and external reflection, solar heat gain coefficient and U-value. The performance requirement for glazing shall be suitably selected. Minimum performance requirement is mentioned in the Annex A.

Specific methods of calculating light and energy factors for glass in buildings are covered here and their characteristic data can serve as a basis for light, heating and ventilation calculations of rooms and can permit comparison between different types of glazing. It is applicable to all flat glass material.

5.2Light Reflectance

5.2.1 External Light Reflectance of Glazing

External light reflectance corresponds to the amount of sunlight reflected by the external glass surface. The

amount of light reflection depends on the type of base glass (substrate tint) and the position of coating on the glass surface.

The external light reflectance of glazing $\rho_{\mathrm{v,o}}$ shall be calculated using the following formula:

\rho_ {\mathrm{v,o}} = \frac {\sum_ {\lambda = 3 8 0 \mathrm{nm}} ^ {7 8 0 \mathrm{nm}} \rho_ {\mathrm{o}} (\lambda) D _ {\lambda} V (\lambda) \Delta \lambda}{\sum_ {\lambda = 3 8 0 \mathrm{nm}} ^ {7 8 0 \mathrm{nm}} \mathrm{D} _ {\lambda} V (\lambda) \Delta \lambda}

where

$\rho_{0}(\lambda) =$ spectral external reflectance of glazing, and $D_{\lambda}, V(\lambda), \Delta \lambda$ and the integration procedure are defined in 5.1.11.4.

For multiple glazing, the calculation of the spectral external reflectance $\rho_{\mathrm{o}}(\lambda)$ shall be performed using the same methods as given in 5.1.11.4 for the calculation of the spectral transmittance $\tau (\lambda)$ .

For the spectral external reflectance $\rho_{\mathrm{o}}(\lambda)$ as a function of the spectral characteristics of the individual components of the unit, the following formulae are applied:

a) For double glazing;

\rho_ {\mathrm{o}} (\lambda) = \rho_ {1} (\lambda) + \frac {\tau_ {1} ^ {2} (\lambda) \rho_ {2} (\lambda)}{1 - \rho_ {1} ^ {\prime} (\lambda) \rho_ {2} (\lambda)}

b) For triple glazing:

\rho_ {\mathrm{o}} (\lambda) = \rho_ {1} (\lambda) + \frac {\tau_ {1} ^ {2} (\lambda) \rho_ {2} (\lambda) \left[ 1 - \rho_ {2} ^ {\prime} (\lambda) \rho_ {3} (\lambda) \right] +}{\left[ 1 - \rho_ {1} ^ {\prime} (\lambda) \rho_ {2} (\lambda) \right] \cdot \left[ 1 - \rho_ {2} ^ {\prime} (\lambda) \rho_ {3} (\lambda) \right]} - \tau_ {2} ^ {2} (\lambda) \rho_ {1} ^ {\prime} (\lambda) \rho_ {3} (\lambda)

For multiple glazing with more than three components, relationships similar to above equations are found to calculate $\rho_{\mathrm{o}}(\lambda)$ of such glazing from the spectral characteristics of the individual components.

As an example for calculating $\rho_{\mathrm{o}}(\lambda)$ , a glazing composed of five components may be treated in the same way as described in 5.1.11.4.

5.2.2 Internal Light Reflectance of Glazing

Internal light reflectance corresponds to the amount of light reflected by the inner glass surface. The amount of light reflection depends on the type of base glass (substrate tint) and the position of coating on the glass surface.

The internal light reflectance of glazing $\rho_{\mathrm{v,i}}$ shall be calculated using the following formula:

ρ_{v, i} = \frac{\sum_{λ = 380 n m}^{780 n m} ρ_{i} (λ) D_{λ} V (λ) Δ λ}{\sum_{λ = 380 n m}^{780 n m} D_{λ}}

5.3.2Division of Incident Solar Radiation Flux

The incident solar radiant flux per unit area $\varphi_{\mathrm{e}}$ is divided into the following three parts (see Fig. 7):

a) Transmitted part, $\tau_{\mathrm{c}}\varrho_{\mathrm{e}}$ :

b) Reflected part, $\rho_{\mathrm{e}}\varphi_{\mathrm{c}}$ ; and

c) Absorbed part, $\alpha_{\mathrm{e}}\varphi_{\mathrm{e}}$

where

$\tau_{\mathrm{e}} =$ solar direct transmittance,

$\rho_{\mathrm{e}} =$ solar direct reflectance, and

$\alpha_{\mathrm{e}} =$ solar direct absorptances.

The relationship between the three characteristics is:

\tau_ {\mathrm{e}} + \rho_ {\mathrm{e}} + \alpha_ {\mathrm{e}} = 1

SUN

FIG. 6 SOLAR HEAT GAIN COEFFICIENT

KEY

1 OUTER PANE

2 SECOND INNER PANE

3 \text {UNIT INCIDENT RADIANT FLUX}

\rho_ {\mathbf {e}} = 0. 3 8; q _ {\mathbf {e}} = 0. 1 7;

\tau_ {e} = 0. 4 1; q _ {i} = 0. 0 4; \text {therefore} g = 0. 4 5

FIG. 7 COMPONENTS OF INCIDENT SOLAR RADIANT FLUX

PART 6 STRUCTURAL DESIGN — SECTION 8 GLASS AND GLAZING

The absorbed part $\alpha_{\mathrm{e}}\varphi_{\mathrm{e}}$ is subsequently divided into two parts $q_{\mathrm{i}}\varphi_{\mathrm{e}}$ and , which are energy transferred to the inside and outside, respectively:

\alpha_ {\mathrm{e}} = q _ {\mathrm{i}} + q _ {\mathrm{e}}

where

$q_{\mathrm{i}} =$ secondary heat transfer factor of the glazing towards the inside; and

$q_{e}$ = secondary heat transfer factor of the glazing towards the outside.

(

λ

)

Δ

λ

∑λ=380nm780nmρo(λ)DλV(λ)Δλ

1−ρ1′(λ)ρ2(λ)τ12(λ)ρ2(λ)

o

(

λ

)

=

ρ1(λ)+

[1−ρ1′(λ)ρ2(λ)]⋅[1−ρ2′(λ)ρ3(λ)]τ12(λ)ρ2(λ)[1−ρ2′(λ)ρ−

τ22(λ)ρ1′(λ)ρ3(λ)

ρ_{v, i} = \frac{\sum _{λ = 380 nm}^{780 nm} ρ _{i} ( λ ) D _{λ} V ( λ ) Δ λ}{\sum _{λ = 380 nm}^{780 nm} D _{λ} V ( λ ) Δ λ}

where

$\rho_{i}(\lambda) = \text{spectral internal reflectance of glazing, and } D\lambda, V(\lambda), \Delta\lambda \text{ and the integration procedure are as defined in light transmittance.}$

For multiple glazing, the calculation of the spectral internal reflectance $\rho_{\mathrm{i}}(\lambda)$ shall be performed using the same methods as given in 5.1.11.4 for the calculation of the spectral transmittance $\tau (\lambda)$ .

For the spectral internal reflectance $\rho_{\mathrm{i}}(\lambda)$ as a function of the spectral characteristics of the individual components of the unit, the following formulae are applied.

a) For double glazing:

\rho_ {\mathrm{i}} (\lambda) = \rho_ {2} ^ {\prime} (\lambda) + \frac {\tau_ {2} ^ {2} (\lambda) \rho_ {1} ^ {\prime} (\lambda)}{1 - \rho_ {1} ^ {\prime} (\lambda) \rho_ {2} (\lambda)}

b) For triple glazing:

\rho_ {i} (\lambda) = \rho_ {3} ^ {\prime} (\lambda) + \frac {\tau_ {3} ^ {2} (\lambda) \rho_ {2} ^ {\prime} (\lambda) \left[ 1 - \rho_ {2} (\lambda) \rho_ {1} ^ {\prime} (\lambda) \right] +}{\left[ 1 - \rho_ {3} (\lambda) \rho_ {2} ^ {\prime} (\lambda) \right] \cdot \left[ 1 - \rho_ {2} (\lambda) \rho_ {1} ^ {\prime} (\lambda) \right]} - \tau_ {2} ^ {2} (\lambda) \rho_ {3} (\lambda) \rho_ {1} ^ {\prime} (\lambda)

For multiple glazing with more than three components, relationships similar to above expressions are found to calculate $\rho_{\mathrm{i}}(\lambda)$ of such glazing from the spectral characteristics of the individual components.

1

e

q_{\mathrm{e}}\varphi_{\mathrm{e}}

3

(

λ

)

]

+

(

λ

)

ρ1′

(

λ

)

(

λ

)

ρ1′

(

λ

)

]

τ32(λ)ρ2′(λ)[1−ρ2(λ)ρ1′(λ)]+

−