Book - School of Science and Technology

Solar heat gains 79allow penetration into the building, and then to search for means to curb the nuisance.Table 3.3 lists a number of combinations of glazing and shading devices and ranks thesein order of efficacy. It will be noted that substitution of double for single glazing offers amuch less dramatic improvement in this context than it does in reduction of U value. Thecomputer based plots of Figure 3.7 provide parallel information and illustrate the effectupon peak air temperature within a south-west facing room which results from variationin types of protection and rates of air change.Special solar control glasses reduce the total rate of heat transfer by increasing eitherthe absorbed or the reflected component: a variety of different types is available:Absorbing glasses. There are two main types here: body-tinted and metallic ion coated.The former type (BTG) is coloured throughout its thickness and, in consequence, thecolour density and solar control properties increase with glass thickness: colouringsavailable are commonly grey, bronze and green. The alternative variety has a layer ofions, which absorb incident radiation, injected into the surface of clear plate glass. Sincethe layer is below the surface and therefore protected, the glass may be used as a singlepane.Reflective glasses. These have a metallic oxide or similar coating on one surface and arethus normally used in sealed double glazing units. They are manufactured in a standardcolour range of silver, grey, bronze or blue but other colours may be obtained.Solar control films. Solar control films are available in a wide range of colours, and areapplied to the internal side of the glass. They produce an effect similar to that ofreflective glass and can provide an economic solution as a retrofit measure to existingsingle glazed windows. A recent addition to the materials available is a film which actslike a photochromic glass, darkening as the incident sunlight increases its intensity.Smart windows. These so-called materials are in the course of development for commercialuse. Typically, the glass has four coatings, a transparent conductor, an electrochromiclayer, an ion storage layer and a second transparent conductor. By the application of avoltage to the glass, the transmissivity property of the coating is changed and may beadjusted to control the amount of solar heat entering the space.Low emissivity coatings. These are, strictly speaking, more appropriate to retention ofsolar heat within a space than to the control of its entry.In order to take account of the surfaces and other features of a space within a buildingstructure which absorb, to a greater or lesser degree, the solar heat admitted throughglazing, the Guide Section A5 includes a group of tables which list cooling loads, as distinctfrom heat gains. These apply to lightweight buildings, defined as having demountablepartitions, suspended ceilings, and floors which are suspended or, if solid, have either acarpet or a wood block finish: a heavyweight building, conversely, has an overall solidconstruction with no soft finishes. Each such table applies to a single latitude and toa plant operational period of ten hours: correction factors are provided to cover someother circumstances. These data are represented here for four representative months byTable 3.4, which relates to unprotected clear glass, and by the parallel Table 3.6 whichrelates to a combination of glass with an internal white venetian blind. Correction factorsfor other glass and building combinations are provided by Tables 3.5 and 3.7 respectively.In the case of the principal Tables 3.4 and 3.6, all the values listed assume thatcalculations for internal design conditions have been based upon dry resultant temperature.If air temperature were to be used, the correction factors at the ends of Tables 3.5and 3.7 must be applied. Diffuse radiation from earth and sky is included in the dataprovided but, where the ground floor of a building abutts to a wide city pavement subject