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 kg-1 K-1, 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, so-called, ‘pull-down’ 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 pull-down load. Figure 7.19 illustrates the possible effect of such a surplus load. The importance of pull-down load is open to question: outside dry-bulb 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 pull-down 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 pull-down load on the air conditioning system. |
When the window has internal blinds these absorb part of the radiation and convect and re-radiate 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 re-radiation 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 heat-absorbing 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 m-2. 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 m-2. 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 double-glazed window, shaded by internal Venetian blinds, facing south-west, at 15.00 h sun-time, 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 single-glazed 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 m-2 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 m-2. 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, heat-reflecting (bronze) glass, facing SW, at 15.00 h sun-time in June in London, for a floor slab density of 500 kg m-2. Use the storage load factors in Table 7.12 and the maximum total solar intensity from Table 7.11.
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