The Thermal Effects of Solar Gain

Introduction

Article Nine of the European Directive on the Energy Performance of Buildings (Ref.1) requires the “regular inspection of air conditioning systems of an effective rated output of more than 12kW” with a view to determining if required cooling loads cannot be met or at least reduced by alternative solutions such as solar shading and/or more effective solar-control glazing systems. This means that the designers of new buildings and the inspectors of existing ones will need to make quite complex and far-reaching assessments as to the potential effectiveness of different shading and glazing systems.

To do this, they will turn to the manufacturers of such systems and to the wide range of energy and thermal analysis tools currently available. However, the data that the manufacturers will provide and that the analysis tools will require are a simple set of solar transmittance values for the glass/shading combination. At the high end these will be given for a range of different solar incidence angles or, more typically, as a single average value.

However, as many commercial buildings in much of the world rely solely on blinds and other internal shading devices to protect from direct sunlight and glare, the relationship between solar gains and overheating can be a very complex one - dependant on and often governed by a whole range of configuration/operational and radiative/convective factors not even considered in such analysis.

As a result, neither the designer nor the inspector/auditor can expect to rely solely on a comparison of solar transmittance values or the results of simple simulations when considering the true effects of solar gains in buildings. Instead, common sense and a sound knowledge of basic building physics can often achieve much more in a design than the most sophisticated set of analysis results.

Solar Gains & Energy Use in Buildings

The rest of this article looks at the different factors that govern the potential effects of solar gains on energy use in buildings. This includes a consideration of the obvious factors such as transmission and opacity as well as the not-quite-so obvious factors such as the use of vents and pelmets or how the building is zoned.

Window Size and Glazing Type

In most simplified energy analysis algorithms, window size and glazing type are almost the same thing. A large area of glass with a low solar transmittance is effectively the same as a lesser area of glass with a proportionately higher transmittance. Thus the effective area of glazing is usually given by multiplying its physical area by its average solar transmittance.

In these cases, solar transmittance is given as a Solar Heat Gain Coefficient (SHGC). This refers to the ratio of solar heat gain entering a space through a window compared to the total incident solar radiation falling on the outside surface of that window. This includes both directly transmitted solar heat and the solar radiation actually absorbed by the glass, which is then re-radiated, conducted or convected into the space.

This is an important point as some older tinted glasses used to claim a very low transmittance. However they were usually created by suspending metal particles within the glass, which would heat the glass up quite considerably under direct sun. Whilst the direct sunlight travelling through was relatively low, a very large portion of the solar gain was still conducted to the inside surface of the glass and then into the space behind.

This means that you must be conscious of how a particular solar control glass is achieving its reduced transmittance. The fundamental laws of conservation of energy means that the heat must go somewhere. If due to a reflective coating then the heat is likely reflected away, potentially to a nearby building or into the eyes of passers-by. If it is absorbed in the pigment of the glass, then this must be located as the outer pane of a double-glazing system or coated internally with a low-e film to reduce radiant emissions into the building.

To a large extent, the regulatory requirement for manufacturers to specify the SHGC of glazing systems can assist here as it represents the fraction of heat from the Sun that actually entered a test space through a window fitted with that type of glass. Thus, unless the transmittance of a glass is given as a SHGC, you should always be a bit skeptical of claims of high performance.

External Shading vs Internal Shading

Obviously if there is an opaque obstruction between the Sun and a window, direct solar gain will be blocked. Diffuse radiation from the sky and reflected radiation from the ground and other external surfaces may still get through, but this is a relatively small load when compared to direct solar gains.

External shading devices usually work best because the gains are prevented from entering the space in the first place. To reach internal blinds, louvres or curtains, the solar radiation must have already passed through the window. Whilst it is blocked by the internal obstruction, it is still incident on that obstruction so its energy acts to incease surface temperatures. You will often find that, under direct sun, temperatures in the air gap between the blinds and a window may be up to 20°C higher than average internal air temperatures.

In many buildings a combination of external and internal shading systems are used, with the internal blinds primarily there to combat glare at times of low sun angle. However, badly designed external shading devices may result in people actually closing the internal blinds to protect against direct sun penetration. When this happens, they typically turn on the electric lights to make up for a reduction in daylight levels. This means a double load: more gains to the space through solar radiation on the blinds and more electricity use (and therefore heat emission) from the extra lights.

Paradoxically then, at higher latitudes where the sun is generally lower in the sky, good solar shading is not just about solar protection, but also preventing external glare and at the same time maximising internal daylight.

Internal Shading and Blinds

Once solar gains have passed through a window and hit an internal blind, they are already inside the space. Only if the blind surface is highly reflective and the solar rays redirected straight back out the window will this not result in some heat build-up. Thus, whilst the blinds may effectively block glare and daylight, conduction, convection and radiation will usually convey a large portion of the heat to the internal space.

If the space between the blind and the window is open at top and bottom, a convection current will likely result. This means that air warmed by the Sun will rise out of the top, causing a negative pressure in the gap between the blind and the window which will act to draw in cool air from near the floor in through the bottom. Such a system is a very efficient heat transfer mechanism which can be used to good effect in winter, but is rarely desirable in summer.

Open-Top Blind/Curtain System

Figure 4 shows a calculated example of such a phenomenon. In this case a simple room was used with a window fitted with a closed fabric blind (or curtain) approximately the same height as the window but offset from the wall the distance of an average curtain rail. This window was then subjected to direct solar radiation of 660W/m² for a period of 30 minutes and computational fluid dynamics (CFD) used to calculate the spatial distribution of air temperature within the space.

This example shows that the thermo-syphoning effect of such a window/blind configuration can very quickly lead to significant temperature stratification within a small room. The exact nature of this effect depends on the solar absorptance of the blind fabric, the U-value of both the window and the blind itself (conducting the built-up heat either outside through the window or inside directly through the blind itself) as well as the size of the air-gaps at top and bottom of the blind.

You can also see that, at around 30 minutes, the warmed air is sufficiently low within the room to start being drawn back through the gap at the bottom. This will lead to a whole new feedback loop that will further accelerate the heating of the room.

Horizontal Louvres

Figure 5 shows the same time sequence, but this time with a set of horizontal louvres fitted to the window, spaced a similar distance away and also open at the top and bottom. In this case the blinds were angled at 45 degrees to allow some diffuse light in but to completely block direct sunlight.

This shows a very similar effect, but note that because the blinds are partially open, the feedback loop where warm air is drawn back into the system starts much earlier. This means that air at the top of the room is warmed much quicker, but still takes a similar time to stratify temperatures as the open-top blind system.

Obviously the same factors affect the nature of this effect, except that the heat is free to enter the room through the open louvres so the U-value of the louvre material becomes less important with the increased angle of the open louves.

Sealed Pelmet

Figure 6 shows the effect of adding a sealed pelmet to the top of the blind/curtain system used in Figure 4 above. This completely closes the gap between the blind and the wall above the window - thus preventing the warmed air escaping out the top.

In this configuration the air is trapped inside the gap and may become very hot indeed as it absorbs more and more solar radiation. In this case, the effectiveness of the seal at the top and the U-value of the fabric is very important as both serve to contain the heat within the gap. Whilst not shown here, there will come a point where some of the warmed air will begin to spill out the bottom of the blind and into the room.

Sealed Pelmet and External Exhaust Vent

Figure 7 shows the same system as in Figure 6, but with the addition of an external vent at the top of the window. This vent allows the warm air building up in the gap between the blind and the window to exhaust to the outside - to be replaced by room air. Assuming the room is connected to others within the building, this may be used to draw air across the building from the non-Sun side and thus assist with natural ventilation.

The true thermal effects of solar gains and internal shading systems therefore depend greatly on the detailed nature of the air flow that is induced. However, a typical thermal/energy analysis will only consider solar transmittance and the additional insulating characteristics the shading may impart to the window. To properly consider convection effects requires a very detailed 3D model of the window-blind configuration and a complex computational fluid dynamics (CFD) solution.

Neither designers nor inspectors/auditors are likely to apply such solutions, and manufacturers are unlikely to provide such detailed data on all potential installations and configurations of their window/blind systems. Thus, all you are left with is your own common sense and ability to think through the likely heat flow paths, convective loops and operational regimes within each space. However, this level of basic thinking can prevent all sorts of heat build-up issues and facilitate huge potential savings.

Exposed Internal Mass

Glass is relatively transparent to those frequencies of the solar spectrum from short-wave infra-red through visible light, ultra-violet and above. However, it is relatively opaque to long-wave infra-red radiation. Thus, when solar radiation passes through a window, a significant amount of its energy is allowed through, though not all.

When solar radiation falls directly on a material’s surface, the incident energy acts to increase the surface temperature of the material. As it does this, the temperature differential between the material’s surface and the material immediately underneath it also increases. This results in a flow of heat from the surface deeper into the material itself. The rate of this heat flow depends upon the conductivity of the material.

For most high thermal mass materials with high conductivity, this occurs faster than the surface heat can be lost by radiation or convection into the air layer immediately above it. This means that surface temperature rises remain relatively low with the heat energy quickly dispersed over a much greater mass of material. Inside a space, this means that both air and mean radiant temperatures remain relatively unaffected. However it also means the heat is stored within the fabric of the space for release later when internal temperatures fall.

For low thermal mass or low conductivity (i.e.: highly insulating) materials, very little of the surface heat is conducted away internally. This means that the rise in temperature at the surface is much higher, leading to the majority of heat being lost immediately by radiation and convection. This has an almost immediate impact on both air and mean radiant temperatures within a space.

Thus, with an appropriate choice of materials, the designer has enormous control over the response of each space to internal and solar gains. This means the placement of carpets, tiling, wall-coverings and furniture all play an important thermal role that must be accounted for.

Zones and Zoning

Almost all thermal analysis algorithms calculate a single temperature for each zone - this being the average over the whole space. This is calculated by summing all the gains and losses across the zone. If one part of a zone is subject to high solar or radiation gains, it is likely to have a higher local temperature. However, this will only be indicated by a slightly increased zone average, the specific local temperature will not be calculated.

If a large space were subdivided into a number of smaller “virtual” zones, it would be possible to quantify these localised variations, however each calculated temperature will still be the spatial average for each “virtual” zone. Thus, if you create very small “virtual” zones near a window, the thermal analysis may not properly account for air movement across the space - meaning that this will likely overemphasise the thermal effect of the solar gains. If you use large “virtual” zones, the same averaging effects will tend to underemphasise the solar gains.

Unfortunately there is no simple answer for the optimum zone size. As a result, it is important to know the extent to which different relative “virtual” zone sizes affect solar gains in office buildings, using different thermal analysis engines and a different methods of characterising the inter-zonal interface.

A follow-up article in this series will look in detail at the numerical effects of different thermal zoning strategies and room sizes.

References