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

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Month	Exposure	Solar air-conditioning 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 metal-framed windows and the area of the glass for wooden-framed 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. Air-to-air 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 Type of glazing and shading Correction factor Single glass + internal Venetian blinds Ordinary or plate glass 1.00 Heat-absorbing (bronze) 0.96 Heat-reflecting (gold) 0.49 Double glass + internal Venetian blinds Ordinary 1.08 Plate 1.00 Heat-absorbing (bronze)* 0.81 Double glass + Venetian blinds between the sheets Ordinary 0.55 Plate 0.49 Heat-absorbing (bronze)* 0.47

*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 Orientation Dec Jan/Nov Feb/Oct Mar/Sep Aug/Apr Jul/May Jun North 45 55 75 100 120 135 135 NE/NW 45 55 110 240 405 505 530 E/W 205 255 410 545 660 695 695 SE/SW 425 490 625 695 685 645 625 Horizontal 175 235 385 560 725 820 850

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 m-2. 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 m-2 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 heavy-weight 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