Book - School of Science and Technology

72 The building in summerIncidence of solar radiationThe total of solar radiation to reach the surface of the earth has two components: directand diffuse. Of the former, some 1% is ultraviolet, 40% is visible light and the remainderis infra-red. Diffuse, sometimes known as sky or scattered radiation, results from absorptionby, and reflectance from, vapours and dusts, etc., in the atmosphere. It is at amaximum with cloud cover and a minimum with a clear sky.For a latitude of 50 in the British Isles, the accepted value for the intensity of directsolar radiation on a horizontal surface at sea level (or up to about 300 m above) will beabout 800 W/m 2 at noon in June under a clear sky. Diffuse radiation received on the samehorizontal surface, again at noon in June, will be about 100 W/m 2 and 300 W/m 2 for clearand cloudy skies respectively. Intensities on surfaces other than the horizontal will beproportional to the angle of incidence, which is the angle between the direction of the sun'srays and the perpendicular to the surface.As far as vertical surfaces (walls and windows) are concerned, some part of the diffuseradiation is received from the sky and the remainder by reflection from adjacent groundsurfaces. The intensity of radiation received on the ground will be that for any horizontalsurface, as already noted, but the amount reflected will depend upon the nature andcolour of the surface. In the past, correction factors related to grass, tarmac, water andsnow were used but since these were too precise, the choice has been reduced in number torepresent light and dark surfaces only. For the case of a city pavement, a convenient(albeit arbitrary) way to take account of ground reflection is to add a margin of 10% tothose solar heat gains attributed to glazed areas.The manner in which solar radiation varies throughout the day, for east, south and westfaces at the summer solstice, is illustrated in Figure 3.5(a). It will be noted that since thesun altitude is high at midday (64 ), the peak solar intensity on the south face is lowerthan that on the east and west faces which occur at times when the altitude is less. Figure3.5(b) is a similar plot but shows the situation prevailing at either the spring or the800600EastSouthWestIntensity W/m 2400200NorthNorthDiffuse00 2 4 6 8 10 12 14 16 18 20 22 24(a)Time of day G.M.T.S. and N. faces 24 hour mean = 245 W/m 2E. and W. faces 24 hour mean = 390 W/m 2