Solar Load

Solar Load

Fenestration solar heat gain has two components:

First is the directly transmitted solar radiation. The quantity of radiation entering the fenestration directly is governed by the solar transmittance of the glazing system. Multiplying the incident irradiance by the glazing area and its solar transmittance yields the solar heat entering the fenestration directly.

The second component is the inward-flowing portion of the absorbed solar radiation, radiation that is absorbed in the glazing and framing materials of the window and is subsequently conducted, convicted, and radiated to the interior of the building.

The total solar gain (heat flow per unit area) q_b that enters the space due to the incident solar radiation is    q_b = E_D(T+NA)  W/m2   = E_D( SHGC)

ED is the direct solar irradiance incident upon a single pane of glass, T is its solar transmittance, A is the solar absorptance, and N be the inward flowing fraction of the absorbed radiation. In this case,

Direct beam solar heat gain qb:  qb = A ED  SHGC(θ) IAC   (13)

Diffuse solar heat gain qd:   qd = A (Ed + Er) ⟨SHGC⟩D  IAC (14)

Conductive heat gain qc:   qc = UA(Tout – Tin)      (15)

Total fenestration heat gain Q:   Q = qb + qd + qc (16)

where

A = window area, m2

ED, Ed, and Er = direct, diffuse, and ground-reflected irradiance, calculated using the equations in Table 14

SHGC(θ) = direct solar heat gain coefficient as a function of incident angle θ; may be interpolated between values in Table 13 of Chapter 30

⟨SHGC⟩D = diffuse solar heat gain coefficient (also referred to as hemispherical SHGC); from Table 13 of Chapter 30

Tin = inside temperature, °C        &  Tout = outside temperature, °C

U = overall U-factor, including frame and mounting  orientation from Table 4 of Chapter 30, W/(m2·K)

IAC = inside shading attenuation coefficient, = 1.0 if no inside shading device

Cooling load associated with nondoor fenestration is calculated

q_fen = A x CF_fen                (24)

CF_fen = U(∆t - 0.46DR) + PXI x SHGC x IAC x FFs    (25)   where

q_fen = fenestration cooling load, W

CF_fen = surface cooling factor, W/m2

A = fenestration area (including frame), m2

U = fenestration NFRC heating U-factor, W/(m2'K)

∆t = cooling design temperature difference, K  & DR= daily range

PXI = peak exterior irradiance, including shading modifications, W/m2

SHGC = fenestration rated or estimated NFRC solar heat gain coefficient

IAC = interior shading attenuation coefficient, Equation (29)

FFs = fenestration solar load factor, Table 13

Peak Exterior Irradiance (PXI). PXI = T_xE_t (for unshaded fenestration) & T_x=1

From

 Cooling Load Estimation for a Multi-story office building Thesis

3.2.1.2 Heat Gain through Glass

Heat is transmitted through glass due to solar radiation. The heat gain through glass areas

constitutes a major portion of the load on the cooling apparatus. This could be direct in the form

of sunrays or diffused radiation due to reflection from other objects outside. Heat transmitted

through a glass depends on the wavelength of radiation and physical and chemical characteristics

of glass. Part of the radiation is absorbed, part is reflected and the rest is transmitted. The heat

transfer of glass takes place in the two ways, transmission heat gain and solar heat gain. The

following equations are used to calculate heat gain from glass areas.

Fig 3.4 heat gain through glass

Transmission heat gain through glass:

Q =UA(CLTD) corr …………... (3.4)

By solar radiation:

Q = A´SHGF ´SC´CLF max ………………….(3.5)

SHGF max = maximum solar heat gain factor (W/m2)

SC = shading coefficient depends on type of shading

CLF = cooling load factor