Book - School of Science and Technology

Solar heat gains 73800South600Intensity W/m 2400EastWest200North(b)0Diffuse0 2 4 6 8 10 12 14 16 18 20 22 24Time of day G.M.T.S. and N. faces 24 hour mean = 241 W/m 2E. and W. faces 24 hour mean = 236 W/m 2Figure 3.5 Solar heat on a vertical surface: (a) 21 June; (b) 22 March and 22 Septemberautumn equinox: the intensity on the south face at a noon altitude of 40 now exceeds thepeaks on the east and west faces. A measure of the relative intensities of radiation is givenby the 24 hour mean values as stated below the two parts of the diagram.Solar gain through opaque surfacesWhen direct and diffuse radiation fall upon the opaque surfaces of a building, a proportionis reflected back into space but the greater part will warm the surface of the material.Of this latter component, some is lost by re-radiation and some by convection to thesurrounding air but the remainder is absorbed into the material to a degree which dependsupon the nature and colour of the surface. For example, a black non-metallic surface mayretain as much as 90% of that remainder, whereas a highly polished metal surface willtake in as little as 10%. The majority of building materials absorb some proportion withinthe range 50±80% and, of this, the greater part will be transmitted through the material byconduction at a rate which will depend upon the U value (W/m 2 K).In instances where the building element has a negligible thermal capacity, as wouldbe the case with thin unlined cement sheeting, then heat will be transferred at once butwhere there is mass of any real significance, then a time lag will occur. When thematerial has both sufficient thickness and substantial mass, this time delay will beprolonged to the extent that solar radiation which falls upon it during the early part ofthe day may not penetrate to the inside face until the outer surface is in shadow. Thecurious situation may then occur where the heat stored in the structure will be flowingin both directions, to outside and into the building. In some cases, even on east tosouth facades which receive solar exposure earlier in the day, the heat absorbed at the