Book - School of Science and Technology

74 The building in summertime of peak gain may not be released from the inner surfaces until after rooms therehave ceased to be occupied.As will be appreciated therefore, the temperature difference between outside air (t ao )and inside air (t ai ), does not represent the situation properly since the outside surfacetemperature of the material will have been raised by the heat absorbed, as noted above. Aconvenient but approximate method of dealing with this situation is to make use of theconcept of sol±air temperature (t so ), a hypothetical scale which takes into account not onlythe outside air temperature but also increments to it which represent the increase due tothe effects of solar radiation. Tables of sol±air temperature, hour by hour during thecritical months of insolation, are given in the Guide Section A2 for a number of orientationsand with respect to both `light' and `dark' coloured building surfaces.The subject of dynamic response, including the influence of time lag () and decrementfactors ( f ) was discussed briefly at the end of the previous chapter. In summer, when heatgain is a penalty rather than a bonus, some attempt must be made in calculationsperformed manually to take these factors into consideration. The application of sol±airtemperature tables to the problem takes account of the average of such temperatures over24 hours as well as that relevant to the duration of the time lag, making use of thefollowing equation to determine heat gain at the time taken for peak loading, hours:Q/AU ˆ (t som t ai ) ‡ f (t so t som )which may be transposed to:Q/AU ˆ (t s t ai )whereQ/AU ˆ heat flow into the space at the time, , which has been taken for peakloading (W/m 2 ):t som ˆ mean sol air temperature over 24 hours ( C):t so ˆ sol air temperature at time, , i:e: prior to the appropriate time lag, hours ( C):t s ˆ t som (1 f ) ‡ f (t so )( C):Selected values of this notional temperature (t s ) for a medium coloured building surface(i.e. one between `light' and `dark' and which absorbs 70% of total radiation) and for anumber of combinations of time lag and decrement factors are listed in Table 3.1. Thesevalues apply strictly to south-east England in the month of July, but may be used for suchlatitudes over the months of May to September without too much loss of accuracy.Further, the temperatures listed may be applied, within the relevant margin of error, inconjunction with most published figures for time lag and decrement factor, includingthose given in Table 3.2 which represent these characteristics for a range of typicalconstruction elements. Routine calculations are simple, as illustrated by the followingexample:A wall facing south-east is constructed of 220 mm solid brick, with lightweight plasterinternally. The internal air temperature is to be maintained at 21 C, the peak cooling loadfor the space concerned being taken at 16.00 hours BST in July.