Book - School of Science and Technology

Solar heat gains 7740.0Temperature (Deg. C) July30.020.010.0Inside temp.Outside temp.0.00.00 4.00 8.00 12.00 16.00 20.00 24.00Time (hours)Figure 3.6 Computer plot of temperature without air-conditioningstructure, furniture, carpets and other contents. The subject has been explored exhaustivelyby Loudon*at the Building Research Establishment in evolving a method wherebythe excessive heat experienced in buildings without air-conditioning could be predicted.As a result of this work and later refinements, a number of computer programs havebeen produced which, as exemplified by Figure 3.6, enable the relationship between insideand outside temperatures to be seen. The more sophisticated of such programs enablemany parallel effects to be examined in detail and pet-mit evaluation of structural formsand the dynamic response to these by engineering systems. The method involves theconcept of environmental temperature, which has been referred to previously, thus takingaccount of both the mean radiant temperature and the air temperature within a space.Advantage may be taken of the same basic approach, including use of the admittancevalue of the various components of the building structure, to evaluate the extent to whichthose components are able to smooth out diurnal temperature swings resultant uponintermittent solar heat gains.Glass has certain unique characteristics as far as heat transfer is concerned in that ithas different transmissivities at different wavelengths: it is, moreover, virtually opaqueto radiation from any source having a surface temperature less than about 250 C.When a glazed surface is insolated, some part of the total incident radiation will bereflected away, some will be transmitted through the material, and some will beabsorbed by it. This simple pattern of heat transfer is confused by re-radiation outwardsand by other emissions in both directions, with the result that the final pictureis quite complex.It is of course much more effective to prevent solar radiation reaching glazed areas, inwhole or in part, by making use of blinds, etc., fitted externally, than to permit exposure,*Loudon, A. G., `Summertime temperatures in buildings without air-conditioning', JIHVE, 1970, 37, 280.