Air conditioning load due to solar gain through glass
The solar radiation which passes through a sheet of window glazing does not constitute an immediate load on the air conditioning system. This is because
(a) air is transparent to radiation of this kind, and
(,b) a change of load on the air conditioning system is indicated by an alteration to the air temperature within the room.
For the temperature of the air in the room to rise, solar radiation entering through the window must first warm up the solid surfaces of the furniture, floor slab and walls, within the room. These surfaces are then in a position to liberate some of the heat to the air by convection. Not all the heat will be liberated immediately, because some of the energy is stored within the depth of the solid materials. The situation is analogous to that considered in section 7.17 for heat gain through walls. There is, thus, a decrement factor to be applied to the value of the instantaneous solar transmission through glass, and there is also a time lag to be considered.
Figure 7.18 illustrates that, in the long run, all the energy received is returned to the room, but, because of the diminution of the peak values, the maximum load on the air conditioning system is reduced.
Modern buildings have most of their mass concentrated in the floor slab, which will, therefore, have a big effect on the values of the decrement factor and the time lag. Since the specific heat of most structural materials is about 0.84 kJ kg1 K1, the precise composition of the slab does not matter very much. Although most of the solar radiation entering through a window does strike the floor slab and get absorbed, the presence of furniture and floor coverings, particularly carpeting, reduces the influence of the slab. Wooden furniture has a smaller mass, hence any radiation received by it and absorbed will be subjected to only a small time lag and will be convected back to the room quite soon. The insulating effect of carpets means that the floor behaves as if it were thinner, resulting in a larger decrement factor. There is, thus, a tendency for a furnished carpeted room to impose a larger load on the air conditioning system, and to do so sooner than will an empty room.
Another factor of some importance is the time for which the plant operates. Figure 7.18 shows what happens if an installation runs continuously. Under these circumstances there is no, socalled, ‘pulldown’ load. If the plant operates for only, say, 12 hours each day, then the heat stored in the fabric of the building is released to the inside air during the night and, on start up next morning, the initial load may be greater than expected. This surplus is termed the pulldown load. Figure 7.19 illustrates the possible effect of such a surplus load. The importance of pulldown load is open to question: outside drybulb temperatures fall at night and, in the presence of clear skies, the building is then likely to lose a good deal of the stored heat by radiation. There is an initial load when the sun rises, but the major increase in load is unlikely to occur, in an office block for example, until people enter at
09.0 h and lights are switched on. This may swamp the effect of pulldown load and render its presence less obvious.
100% 
Time lag for externally shaded windows 
Air conditioning plant runs continuously 
16 Time in hours 
Fig. 7.18 Instantaneous solar heat gain through glass and the load on the air conditioning system. 
Load on a. c. plant for externally shaded windows Load on a. c. plant for internally shaded windows 
Instantaneous heat gain
Fig. 7.19 The possible effect of pulldown load on the air conditioning system. 
When the window has internal blinds these absorb part of the radiation and convect and reradiate it back to the room. The remaining part is considered as direct transmission and so is susceptible to storage effects. The load imposed by convection and reradiation is virtually instantaneous because the mass of the blinds is small and air is not entirely transparent to the long wavelength emission from the relatively low temperature blinds. The same argument holds for heatabsorbing glass.
For cases where the windows have internal Venetian blinds fitted, the air conditioning cooling load may be calculated directly by means of Tables 7.9 and 7.10.
Table 7.9 refers to what are commonly called lightweight buildings. The term lightweight is described in the CIBSE Guide A5 (1999) as referring to buildings having demountable partitions and suspended ceilings, with supported, uncarpeted floors, or solid floors with a carpet. The thermal response factor, defined by CIBSE Guide A9 (1986) as the ratio CL(AY) + nV/3)/’Z(AU) + nV/3), should only be used with caution to describe the weight of a building structure when determining the load due to solar heat gain through glazing, because it may lead to the wrong conclusion. For the purposes of Table 7.9, a lightweight building is defined as one having a surface density of 150 kg m2. This is typical of most modern office blocks. Surface density is determined by calculating the mass of the room enclosure surfaces, using half the known thicknesses of the walls, floor, ceiling etc., applied to the relevant areas and densities. The mass of the glass is ignored. The sum of the calculation, in kg, is divided by the floor area of the room to yield its surface density in kg m2. When the floor slab is covered with a carpet, or provided with a supported false floor, its density is halved for purposes of the calculation.
EXAMPLE 7.15
Calculate the load arising from the solar heat gain through a doubleglazed window, shaded by internal Venetian blinds, facing southwest, at 15.00 h suntime, in June, at latitude 51.7°N, by means of Tables 7.9 and 7.10.
Answer
Reference to Table 7.9 shows that the load is 224 W m~2 for singleglazed windows shaded internally by Venetian blinds of a light colour. Reference to Table 7.10 gives a factor of 1.08 to be applied to the value of 224 W m2 when the window is double glazed with ordinary glass. The load on the air conditioning system is, therefore, 1.08 x 224 = 242 W m~2.
Note that if the blinds had been fitted between the sheets of glass, the factor would have been 0.55, and the load would then have been 0.55 x 224 = 123 W m2. Compare the simplicity of this with example 7.11.
For windows not fitted with internal Venetian blinds, where the direct use of Tables 7.9 and 7.10 is not appropriate, the air conditioning load may be determined by taking the maximum total solar intensity normal to a surface (Table 7.11) and multiplying this by factors for haze, dew point, altitude, hemisphere, storage (Table 7.12) and shading (Table 7.6).
EXAMPLE 7.16
Calculate the air conditioning load arising from solar gain through a window fitted with unshaded, single, heatreflecting (bronze) glass, facing SW, at 15.00 h suntime in June in London, for a floor slab density of 500 kg m2. Use the storage load factors in Table 7.12 and the maximum total solar intensity from Table 7.11.
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Month 
Exposure 
Solar airconditioning loads (W m 2) 

Sun time (hours) 06.00 07.00 
08.00 
09.00 
10.00 
11.00 
12.00 
13.00 
14.00 
15.00 
16.00 
17.00 
18.00 

W 
9 
9 
16 
19 
22 
22 
22 
50 
114 
174 
218 
228 
199 

NW 
3 
6 
6 
9 
9 
9 
13 
13 
19 
44 
69 
88 
88 

October 23 
N 
0 
3 
6 
6 
9 
9 
9 
9 
9 
9 
9 
9 
9 
And 
NE 
32 
44 
41 
32 
19 
13 
9 
9 
9 
6 
6 
6 
3 
February 20 
E 
95 
142 
164 
161 
130 
85 
50 
38 
32 
28 
22 
19 
16 
SE 
0 
91 
174 
228 
256 
246 
212 
152 
91 
60 
50 
41 
27 

S 
32 
69 
139 
205 
249 
278 
284 
265 
180 
161 
79 
50 
35 

SW 
9 
13 
19 
22 
28 
69 
142 
199 
246 
262 
240 
183 
79 

W 
6 
9 
13 
16 
16 
16 
16 
38 
85 
133 
164 
174 
152 

NW 
3 
3 
3 
3 
6 
6 
6 
6 
9 
22 
35 
44 
44 

November 21 
N 
0 
3 
6 
6 
6 
6 
6 
6 
6 
6 
6 
6 
6 
And 
NE 
9 
13 
13 
9 
6 
3 
3 
3 
3 
3 
3 
3 
3 
January 21 
E 
57 
85 
98 
98 
79 
54 
32 
22 
19 
16 
13 
13 
9 
SE 
0 
73 
139 
183 
205 
199 
171 
123 
73 
50 
41 
32 
22 

S 
28 
63 
127 
186 
228 
256 
262 
243 
167 
148 
73 
47 
32 

SW 
6 
9 
15 
16 
22 
57 
117 
164 
199 
212 
196 
148 
63 

W 
3 
6 
6 
9 
9 
9 
9 
22 
50 
79 
101 
104 
91 

NW 
0 
0 
0 
0 
0 
0 
3 
3 
3 
6 
9 
13 
13 

December 22 
N 
0 
0 
3 
6 
6 
6 
6 
6 
6 
6 
6 
6 
6 
NE 
6 
9 
9 
6 
6 
3 
3 
3 
3 
3 
3 
0 
0 

E 
41 
63 
73 
73 
57 
38 
22 
16 
16 
13 
9 
9 
6 

SE 
0 
66 
127 
167 
189 
183 
155 
114 
66 
44 
38 
28 
19 

S 
28 
57 
117 
171 
212 
234 
240 
224 
152 
136 
66 
44 
32 

SW 
6 
9 
13 
16 
19 
50 
104 
152 
183 
193 
177 
136 
60 

W 
3 
3 
6 
6 
6 
6 
6 
16 
38 
60 
73 
79 
66 

NW 
0 
0 
0 
0 
0 
0 
0 
0 
3 
6 
9 
9 
9 
Values are for single plate or float glass and, where correction is necessary for other types of glazing, should be multiplied by the factors given in Table 7.10. The area to be used is the opening in the wall for metalframed windows and the area of the glass for woodenframed windows. A haze factor of 0.9 has been allowed. It is assumed that shades are not provided on the windows facing north but that all other exposures have blinds which will be raised when the windows are not in direct sunlight. Scattered radiation is included and the storage effect of the building mass taken into account. Airtoair transmission is excluded. (Reproduced by kind permission of Haden Young Ltd.) 
188 Heat gains from solar and other sources 
Table 7.10 Correction factors for Table 7.9

*Inner leaf ordinary or plate glass.
The above factors are obtained from the ratios of the shading coefficients to that of ordinary plate glass from Table 7.6.
(Reproduced by kind permission of Haden Young Ltd.)
Table 7.11 Maximum total solar intensities normal to surfaces for latitude 51.7°N in W m 2

Note that the above figures should be increased by 7 per cent for every 5° by which the dew point is less than 15° for an application other than in London.
Answer
The maximum total solar intensity normal to a SW surface in June is 625 W m2. Assume a haze factor of 0.95 for London. The shading coefficient (Table 7.6) is 0.27 and the storage factor (Table 7.12) is 0.62. Then
Qac = 625 x 0.95 x 0.27 x 0.62 = 99 W m2 of glass surface
The data for cooling loads from heat gains through glazing for London, given in C1BSE Guide A5 (1999), are based on measurements of solar irradiances that were not exceeded on more than 2.5 per cent of occasions at Bracknell (latitude 51 °33’N), in the period 1976— 95. The same source provides a computer disc giving cooling loads, based on theoretical predictions, for latitudes 0° to 60°N and 0° to 60°S. When using the data for the southern latitudes the tabulated values must be increased because the sun is nearer to the earth in their summer. See Table 7.4, section 7.16 and example 7.20.
Tables 7.13 and 7.14 give details of the CIBSE cooling loads through glazing for latitude 51°33’N and the related correction factors for that latitude.
Tables 7.13 and 7.14 assume that the system maintains a constant dry resultant temperature in the conditioned space and a correction is given in the CIBSE method to determine the load when the air temperature is held at a constant value, as is the invariable practical case. The answers are tabulated in the usual way for internally shaded glass and corrections are given to cover the case of heavyweight buildings and various shading/glass combinations. The corrections to give loads in terms of a constant room temperature yield results that are less than those for a constant resultant temperature.
The calculation is then simple and is typified by the following equation:
(7.28)
Where Qs = cooling load due to solar gain through glass in W Aw = area of the glass or of the opening in the wall in m2 Fb = shading factor Fc = air point control factor
Qs = specific cooling load due to solar gain through glass in W trf2
The method used in the ASHRAE Guide calculates solar gain by the product of the glass area, a shading coefficient, the maximum solar heat gain and a cooling load factor generated by the use of transfer functions according to Mitelas and Stephenson (1967), Stephenson and Mitelas (1967), and Mitelas (1972). The method is slightly similar in appearance to that using storage load factors, as in Table 7.12 but is based on a sounder theoretical foundation.
The three methods mentioned in the foregoing for estimating the cooling load that occurs by solar gain through windows yield answers that are in approximate agreement, although sometimes with a difference of phase as well as amplitude. It is not possible to say that any one method is correct and the others wrong. However, the CIBSE Guide A5 (1999) method yields solar cooling loads for glass that are somewhat larger than the other methods. See Example 7.18. Due to the large number of variables and imponderables concerned, particularly with regard to the thermal inertia of the building, it is probable that the sensible heat gain through the window of a real room could never be measured with enough accuracy to verify one method. For all its occasional inadequacy in coping with the moving shadows across a building face, it is worth noting that the Carrier method (using Tables 7.11 and 7.12) has been tested extensively throughout the world over many years and, in spite of its apparently inadequate theory, it seems to give answers that work. This is a worthy recommendation. It is also to be noted that air conditioning systems do not maintain constant dry resultant temperatures and calculations based on the assumption that they do are not realistic.
Posted in Engineering Fifth Edition