Solar Gain Through Building
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4 Solar Gain Through Building

 

4.1 Fenestration

Fenestration refers to any glared aperture in the building envelope, including

(a) glazing material, either glass of plastic,

(b) framing, mullions, munitions and dividers,

(c) external shading devices,

(d) internal shading devices, and

(e) integral (between-glass) shading systems.

Purposes of fenestration are:

(a) to satisfy human needs of visual communication with the outside world,

(b) to admit solar radiation to provide supplement daylight and heat, and

(c) to enhance the exterior and interior appearance of a building.

Fenestration affects building on thermal heat transfer, solar heat gains ,air leakage and daylighting. The net effect depends on:

(a) the characteristics and orientation of the fenestration,

(b) the weather and solar radiation conditions, and

(c) the operation of the building thermal system.

 

4.2 Type of Window Glasses

 

In order to provide satisfactory visual effects, better solar heat control and thermal insulation, various types of window glass have been produced. The window glass which are used widely in buildings nowadays is classified as follows:

 

4.2.1 Clear Plate or Sheet Glasses

These are the types of glass which provide fine visual qualities and also a greater transmittance of solar radiation.

 

4.2.2 Tinted Heat Absorbing Glasses

These types of glass are manufactured to have bronze, grey and blue-green colours. Tinted heat absorbing glasses absorb a greater amount of infra-red with some reduction of visible light.

 

4.2.3 Reflective Coated Glasses

These types of glasses have a microscopically thin metallic layer of ceramic layer coated on one of the surfaces of the float glass. The reflecting coating provides reflecting characteristic in the infra-red region with comparatively less reduction of visible light transmission.

 

4.2.4 Insulating Glasses

These are made of two or three pieces of glasses separated by metal or rubber spacer around the edge and sealed in a stainless steel structure. The dehydrated airspace between the glass panes is usually at a thickness of 6 to 12 mm which enhances the thermal insulation of the unit.

 

4.3 Greenhouse Effect

Window glass allows short-wave solar radiation get into an interior space. This radiation is absorbed by the interior of the building. The interior then radiates long-wave, thermal radiation. Glass is opaque (not transparent) to this long wave radiation. Thus energy is trapped in the building and the indoor air temperature rises. This is known as the green effect.

Figure 6 Distribution of Solar Radiation Falling on 3mm Clear Plate or Float Glass

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4.4 Solar Gain Through Fenestration

When solar radiation strikes on an unshaded window, as shown in Figure 6,

(a) part of the radiant energy is reflected back outdoors,

(b) part of the radiant energy is absorbed within the glass,

(c) the remainder is transmitted directly indoor, and

(d) the absorbed portion comes out again and flows either outward and inward.

 

Therefore, heat admission or loss through these fenestration areas is affected by:

(a) solar radiation intensity and incident angle (see Figures 7 and 8),

(b) outdoor-indoor temperature difference,

(c) velocity and direction of air flow across the exterior and interior fenestration surfaces,

(d) low temperature radiation between the surfaces of the fenestration and the surroundings, and

(e) exterior and/or interior shading.

 

Figure 7 Solar Transmission through a 6mm Float Glazing Glass (Green)

 

Figure 8 Solar Transmission through a 6mm Reflective Glass (Gold)

 

4.4.1 Solar Heat Gain

The solar heat gain is the sum of the transmitted radiation and the portion of the absorbed radiation that flows inward.

 

4.4.2 Shading Coefficient

The shading coefficient (SC) is the ratio of solar heat gain through a glazing system under a specific conditions to solar gain through a single pane of the reference glass (double -strength sheet glass with 0.86 transmittance, 0.08 reflectance, and 0.06 absorptance at normal incidence) under the same conditions.

(11)

 

4.4.3 Solar Heat Gain Factor

Solar heat gain factor SHGF (W/m2) is the solar heat gain through a sunlit double strength sheet glass at any specified orientation and any day-light hour.

Since SHGF at a fixed window orientation varies through the year because of the continuous change of incident sunlight direction., maximum solar heat gain factor (max. SHGF) is introduced. Max. SGHF is the maximum value of SHGF on the 21st day of each month for a specific latitude. Table 2 lists the maximum SHGFs of the 22 degree north latitude. These max. SGHFs are values on average cloudiness days. At high elevations and very clear days, the actual max. SGHF may be 15% higher, and in very dusty industrial areas, they may be 20 to 30% lower.

 

Table 2 Maximum Solar Heat Gain Factor for Sunlit Glass on Average Cloudiness Days

 

Month

Maximum solar heat gain factor for 22 degree north latitude, W/m2

 

North

North-east /

north-west

East / west

South-east /

south-west-

South

Horizontal

January.

February.

March.

April

May

June

July

August

September

October

November

December

88

97

107

119

142

180

147

123

112

100

88

84

140

265

404

513

572

589

565

502

388

262

142

101

617

704

743

719

687

666

671

694

705

676

606

579

789

759

663

516

404

355

391

496

639

735

786

790

696

578

398

210

139

134

140

223

392

563

686

730

704

808

882

899

892

880

877

879

854

792

699

657

 

4.4.4 Solar Heat Gain through Fenestration Area

Solar heat gain (Qes) through fenestration is calculated as:

(12)

where max. SHGF = maximum solar heat gain factor for window glass

max. SHGFsh = maximum solar heat gain factor for the shaded area on window glass (see Table 3).

 

Table 3 Maximum Solar Heat Gain Factor of Shaded Area

Month Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
SHGFsh, W/m2 98 107 114 126 137 142 142 133 117 107 101 95

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