| Neglected
aspects of solar input through the building envelope |
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| Thanos N.
Stasinopoulos Laskou 30, GR-156 69 Papagou, Greece delaxo@central.ntua.gr |
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TNS
web home |
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| Abstract | |||||
| Solar control is usually regarded as a geometric issue, where the typical aim is to protect the transparent portions of the building envelope from the direct solar rays –especially on the equatorial side- according to seasonal thermal requirements and solar angles. | |||||
| However, the direct radiation received by an equatorial vertical plane during the warm season is quite lower than the sum of diffuse & ground reflected energy. At the same time, incident energy on the east/west sides of a building can be substantially higher than on the equatorial one. | |||||
| Using radiation data from London, Athens, and Riyadh as examples, the present study elaborates on those observations, highlighting their consequences for shading design. | |||||
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Additionally, solar input through opaque elements (roof & walls) is assessed, showing that its effects on heat transfer through the building fabric deserve more attention in practice. |
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Conference
topic : Solar design |
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| solar control in common practice | |||||
| Solar control is frequently considered as mainly a geometric issue, in which the typical objective is to protect the transparent portions of the building envelope from the direct solar rays. This is generally applied on the equatorial elevation, according to seasonal thermal requirements and solar angles. | |||||
| The opaque portions of the envelope, the diffuse & reflected solar components, as well as directions other than the equator frequently draw less attention on the drawing board. | |||||
| Such approach disregards certain significant aspects of solar heat transfer into the building and results in only partial solar protection, without eliminating the risk of overheating. | |||||
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The aim of this study is to highlight a number of such aspects, in order to enhance designers’ awareness on the broad nature of solar control. |
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| shading | |||||
| The direct component of solar radiation received by a vertical plane can be substantially lower than the sum of the diffuse & ground reflected energy, depending on season & orientation. The graphs in Figure 1 illustrate this fact, showing the monthly ratio of direct / global irradiation on vertical planes at various orientations in three locations separated by about 15º of latitude (average sky radiation data from [1] calculated as in [2]). | |||||
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| This study focuses on the 5 warmest months of the year in each location, that is from May to September hereafter referred as ‘summer’ (see Table 1; the term ‘summer’ is not quite suitable for London, but it is used here for uniformity). Figure 2 depicts the proportion between the summer sums of direct & global radiation incident on vertical planes directed from south to north (0-180º). | |||||
| equatorial orientation |
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| In all three locations, it is clear that direct irradiation on south facing vertical planes accounts for less than 50% of the global energy during the summer period. | |||||
| That means than, even if we provide so-called “100%” summer protection from direct beam on an equatorial window, we will affect less than half of the total incident radiation; therefore further measures are required for the remaining substantial amount of diffuse & reflected energy. | |||||
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| Shading devices do block diffuse energy from a portion of the sky (D’ in Figure 3), but at the same time they emit reflected radiation (R’) through their ‘inner’ side, that is the one facing the protected opening. If the reflectivity of the ground and/or the inner side is high, then the decrease of diffuse energy might be fully offset by the additional reflection. | |||||
| Unfortunately, the only way to completely block diffuse & reflected radiation is to cover the entire opening, but that would also hinder daylight & view. | |||||
| Following these observations, we conclude that for the efficient solar protection of an equatorial window it is essential to couple direct shading with lessening the reflectivity of the nearby surfaces, including the shading device itself. | |||||
| Alternatively, one could reduce global radiation as a whole -without separate consideration for the direct component- by ‘filtering’ the solar rays through special glass (reflective or absorbing) or perforated metal sheets that cover the entire opening. These elements are independent of the seasonal solar geometry, therefore they will decrease solar gains in winter too, unless they are (re)movable. | |||||
| east/west orientation |
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| In Athens & Riyadh, global radiation on vertical planes facing south is lower than on planes facing up to 110º (Athens) or 130º (Riyadh) off south, as shown in Figure 4 (in London, the difference is negligible). This is caused by the similar pattern of the direct component (Figure 5). | |||||
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| Therefore summer overheating is more likely due to solar input through the east/west side and less through the south one (assuming identical size & thermal properties). | |||||
| Consequently, a shading device obstructing the solar rays on the eastern/western elevations is more beneficial than on the south one, because it applies to a larger portion of the global radiation. | |||||
| opaque elements | |||||
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Typically solar gains through opaque elements (roof & walls) are seldom considered as a reason for solar protection other than surface colour. |
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| But most part of the building envelope is usually opaque, therefore even a low intensity energy flow through it can yield a substantial total heat load to the interior –especially in buildings of high Surface-to-Volume ratio (F/V). | |||||
| sol-air temperature |
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| Figure 6 illustrates the effect of solar radiation on a cube located in Athens facing the cardinal points: Using hourly irradiance data from [1], the outer surface temperature increase due to incident energy ( ‘sol-air temperature’) has been calculated as described in [3], assuming a concrete structure with absorptivity a=0.7, emissivity e=0.9, and external surface resistance Rso=0.05. | |||||
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| The temperature increase in this simple example reaches up to +25ºC for the top, +15ºC for the south side, +20ºC for east/west and +5ºC for north. Adding that increase to the ambient temperature, the out-indoors temperature difference Dt can cause a substantial heat flow to the interior through the opaque envelope. Given the increase of Dt, summer heat input can be even higher than the winter heat losses which normally determine thermal insulation requirements. | |||||
| avoiding overheating |
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| A common assumption in practice is that the time lag of the opaque elements will delay the heat flow until the discharge period after sunset, when the air temperature drops thus curbing the effects of heat released by the walls. | |||||
| This may be true for heavyweight structures, but lightweight elements -like metal panels or plasterboard partitions- raise the risk of overheating during daytime, so they require extra attention. | |||||
| A sensible precaution is to reduce absorbed radiation using light colours and reflecting materials, especially on the roof which faces the sun more directly and much longer than the other surfaces of the building. | |||||
| Similarly, one could possibly decrease incident radiation by shading the opaque surfaces like openings, without the restrictions of view or daylight. | |||||
| additional issues | |||||
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| thermal radiation | |||||
| High emissivity building elements acting as shading devices can often undermine their protective function due to the thermal radiation they emit when heated by the solar rays, thus adding to the heat load on the opening. Other nearby elements can generate similar effects, like a concrete floor in front of the opening (Figure 7a). The glazing of the opening -being almost opaque to long wave radiation- can keep most of the emitted heat out, but still the warmed-up air volume next to the opening might increase heat transfer to the interior through conduction. | |||||
| A way to avoid that is by employing materials of low emissivity (e.g. aluminium or grass, selected paints, wood, etc.), balanced with the need for reducing diffuse & reflected short wave radiation as mentioned earlier. | |||||
| warm air pockets |
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| A horizontal shading device can be heated up not only by solar radiation but also by warm air trapped underneath, thus increasing thermal emittance onto the protected surface and the adjacent air volume (Figure 7b). | |||||
| Such risk can easily be prevented by avoiding cross sections which restrict air movement or by providing ventilation gaps. | |||||
| conclusions | |||||
| Solar control is not just a geometric issue related to the equatorial elevation of a building. For optimum results, the designer should bear in mind a number of –frequently neglected- key points related to the reduction of solar input through the entire building envelope: | |||||
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| These issues should of course be integrated into the broader thermal function of the building, including the cold season. | |||||
