05.04 Solar gain

Categories: Thermal Properties

Light and heat
The visual and thermal conditions within a building are two of the most important factors affecting a person’s comfort, and it is the building envelope which plays the most important part in achieving an acceptable indoor environment.  The building envelope acts as a barrier to the harsh conditions which exist in the natural environment, and may be used to completely isolate the internal environment from that outside or to act as a filter which allows some heat and light energies to pass between the two.

Although a massive and well-insulated building envelope would provide excellent thermal isolation from an external environment, it would be considered unacceptable by most people if it did not also provide the opportunities to enjoy the warmth of the sun and delight in a view through a window.  However, the advantages of a window are not provided without cost.  The massive well-insulated envelope has been punctured and no longer will it provide the same degree of thermal isolation; heat will now be required to make up for that which is lost through the window in winter, and cooling may be needed to reduce the overheating from sunlight in a hot summer.

Adding a window does not inevitably result in additional energy costs however; light through the window will reduce the energy needed for electric lighting and sunlight on a cold winter day will help keep the building warm.  The complex balance between energy gains and losses through a window considerably increases the complexity of designing a facade and it can be quite difficult to accurately predict the nett balance between the energy gains and energy losses.

The window is rarely a static building element and its characteristics are modified to make the most of particular features of the glazing.  Because ordinary window glass transmits solar heat as effectively as it does light, an external shutter may be used to limit the sunlight entering a building in midsummer and the poor insulation of such glass may be compensated for by using the shutters to keep in the heat on a cold winter night.  Curtains may be drawn at night to retain heat,  and this also softens the interior decor to make a room more comfortable than it would appear with black windows harshly reflecting the electric lights.  Net curtains may be used to alter the appearance of the window and obscure the view into a building, and thus provide more privacy than possible with clear transparent glazing on its own.

These techniques are all examples of modifying the overall behaviour of the window so that favourable aspects of the glazing are exploited whilst the consequences of some of the less attractive features of glazing are ameliorated.  Advanced glazings are being developed so that some of the selectivity possible in the overall window design can be incorporated within the glazing itself.
 

The physics of light and heat, as it relates to buildings, can be divided into two clear subject areas - solar radiation and heat transfer.
 

• Solar radiation is energy in the form of the electromagnetic spectrum.  The parts of the spectrum that are of interest to the building engineer are the ultraviolet (wavelengths from approximately 300x10^-9 m (300 nm) to about 380x10^-9 m (380 nm)), visible light (wavelengths in the region of 380x10^-9 m (380 nm) to 780x10^-9 m (780 nm)) and near infra-red (wavelengths from 780x10^-9 m (300 nm) to approximately 3000x10^-9 m (3000 nm)).

•  Heat transfer comprises three basic processes by which energy is transferred from one point to another - conduction in solid and stationary fluids, convection in moving fluids and radiation heat transfer in the far infra-red.

Near- and far-infrared
It is essential to make the distinction between shortwave (solar) and intermediate/longwave (‘room’) infrared

• clear float glass is relatively transparent to solar infrared

clear float glass is relatively opaque to room infrared

The distinction between near- and far-infrared is important.  All surfaces at a temperature above absolute zero emit electromagnetic radiation.  For a surface which behaves as a black body the wavelengths at which radiation is emitted is a function of the surface temperature, with a peak energy emitted at some wavelength which is directly related to the surface temperature by Wien’s law

l_max  T  =  2.896 x 10^-3 mK

l_max    is the wavelength of maximum radiation intensity, in m
T         is the surface temperature, in K

and a distribution across wavelengths which is given by Planck’s law

E_l   is the energy flux density emitted in the wavelength range l to l+dl, in W/m²
l     is the wavelength of radiation, in m
T     is the surface temperature, in K
c₁ and c₂ are constants (c₁=3.74x10^-16 Jm²/s, c₂=1.4388x10^-2 mK)
 

The total energy flux emitted per unit area of a ‘black’ surface is given by Stefan’s law

q  = s T⁴  W/m²

q    is the energy emitted per unit area across all wavelengths, in W/m²
s    is the Stefan-Boltzmann constant (s = 5.67x10^-8 W/m²K⁴)

A surface at a temperature of 6000 K (the surface temperature of the sun) will emit radiation in a distribution around a peak wavelength of 483 nm, which is in the visible light region.  A surface at a temperature of 1000 K will emit radiation at a wavelength of 2896 nm, which is at the edge of the near-infrared.  A warm building surface, at a temperature of 313 K (40^oC) emits radiation around a peak wavelength of 9252 nm, which is deep into the far-infrared.

The distribution of radiation is shown in this image for solar radiation at high altitude, and for a black body at 6000 K and 313 K.  Note that the vertical axis of the 303 K line has been greatly magnified - a black body at 313 K emits 544 W/m² whilst a black body at 6000 K emits 73.5 MW/m², some 135,000 times more energy.  Heat transfer, which is concerned with energy transfers at low temperatures, is clearly concerned with the far-infrared, whereas solar radiation is concerned with the near-infrared.

It should be noted that dust, water and other chemicals in the atmosphere absorb some of the solar radiation before it reaches the ground.  The distribution of energy at sea level is therefore slightly lower than the distribution shown in the image.
 

Emissivity
For a real surface the emission of energy does not follow the black-body curve - less energy is radiated but over the same range of wavelengths.  The ratio of the actual emitted energy to the energy that would be emitted by a black body at the same temperature is termed the emissivity of the surface

e  =  q  /  s T⁴

e    is the emissivity
q    is the actual radiation emitted per unit area of the surface, in W/m²

Emissivity is a function of the wavelength of the radiation, and is also a function of the direction from which the surface is viewed.  If it is assumed that a surface has the same emissivity at all wavelengths then the surface is termed ‘grey’, and this assumption is usually made for building surfaces.  The effect of viewing direction (i.e. the direction in which the radiation is emitted - radiation is emitted in all directions from a surface) is then allowed for either by obtaining a true emissivity spectrum, by considering only a normal viewing direction, or by taking an average over a solid hemisphere.

Some typical values of emissivity for surfaces at room temperature are given in the table below (note that emissivity is not strongly affected by the colour of a surface - colour is only relevant to visible light, whereas these emissivity values are for surfaces at room temperature, where radiation is emitted in the far-infrared):
 
 

¹ from CIBSE Guide Part C3 [1986]       ² hemispherical emissivity from CALEX [1980s]
 

 The emissivity of a surface is usually either high (0.7 to 1.0) or low (0.02 to 0.4).  As a rule of thumb a surface which conducts electricity has a low emissivity (e.g. polished metals, some metal oxides), and a surface which resists electricity has a high emissivity (glass, concrete, some heavily oxidised metals).

This image shows typical distributions of emissivity at a single wavelength with viewing angle for a low emissivity surface and a high emissivity surface.
 

Solar-radiated energy
All solar radiation is energy.  When solar radiation strikes a surface a proportion of the radiation, whether it is ultraviolet, visible light or near-infrared, is absorbed, thus increasing the energy content of the surface.  The absorbed energy causes the surface temperature to increase, and this then increases the emission of energy by the surface in the far-infrared.

All solar radiation therefore increases the energy content of a building.  This can give rise to overheating in summer, but it is not possible to exclude all of the solar energy from a building because visible light is also energy.  If 100% of the light is admitted, but none of the ultraviolet or infrared, then 54% of the total solar energy would still be admitted (Button and Pye [1993] pp166).  This represents the best performance in terms of admitting all of the light but rejecting unwanted heat.  Further gains in performance require that some of the light is also excluded, but this can then lead to an increase in the requirement for artificial lighting.

There are of course important issues beyond energy performance.  The admission of ultraviolet and visible light at the blue end of the spectrum may lead to the fading of some colour finishes (it is not sufficient just to exclude the ultraviolet; the blue component of visible light also influences fading (Button and Pye [1993] pp101-103).  Ultraviolet can also lead to problems with degradation of materials, particularly some polymers.  The admission of natural light is essential to human comfort, but too much strong direct light can lead to problems with glare.  The admission of infrared may lead to overheating in summer, but can be very beneficial in winter.
 

Reflection, absorption and transmission
When any form of radiation strikes a surface three things can happen - the radiation is either reflected, absorbed or transmitted, and usually a mix of all three.  Furthermore, reflection, absorption and transmission can be wavelength- and angle-dependent.  Additionally, any radiation that is absorbed will increase the temperature of the surface and result in greater emission of radiation in the far-infrared.  A surface can also have a diffusing effect, so that specular radiation becomes diffusely reflected and transmitted.

The selective nature of reflection, absorption and transmission can cause problems, but can also be highly desirable.  Natural light appears white because it contains an even mix of colours.  Tinted glasses filter out some of the light, changing the spectral composition, and this is what gives them their colour.  However, changing the colour of transmitted light can have detrimental effects on the colour rendering of surfaces.

The spectrum of light from the sky can vary a great deal and our eyes have developed to adapt to a range of correlated colour imperatives.  Therefore problems should only occur where there is a gross distortion or sudden change in transmission; ideally a glazing material should change the spectrum of the visible part of the solar spectrum gradually, so that the light passing through the glazing gives a true rendition of colours, although brightness may be limited.  However, reducing brightness too much can lead to a feeling of isolation, because the human eye and brain are used to high levels of natural light (there is evidence from physiological studies that gross distortion of the light spectrum can affect biological development, but there is no evidence that the modifications to light passing through current glazing types adversely affects human biology).

The fact that reflection, absorption and transmission depend upon the angle of incidence of the solar radiation can be used to improve the performance of the glazing.  Button and Pye [1993, pp161] give the example of a south-facing glazing at 54^o latitude, with the sun at 60^o elevation, for which the direct solar energy transmission is 73% with the glazing vertical, but only 51% with the glazing tilted outwards by 15^o.  In this way the peak solar gain is reduced, but when the sun is at low altitudes in the winter the transmission is better.
 

The analysis of solar radiation
Consider a simple surface on which the sun may shine at some moment in time.  The surface receives ultraviolet, light and infrared, in a proportion that depends upon the angle of the surface, its geographical position (latitude, longitude and altitude), the time of day, the time of year and the condition of the sky and any shading elements between the sun and the surface.

The total radiation reaching an exposed surface may comprise a direct component from the sun (sunlight), a scattered component from the clear sky (skylight), a reflected and diffused component from clouds (daylight) or radiation reflected from the ground and other buildings (ground light), as shown in this image.  Within a building these various components may directly irradiate a surface or reach it after reflection from internal surfaces in the room.

The radiation I that falls on any surface must be either reflected, absorbed or transmitted, and usually some combination of the three.  The fraction that is reflected is

 r I      whilst

aI     is absorbed and

 t I      is transmitted, where

a + r + t = 1

 However, the parameters a, r and t are dependent upon the angle of incidence q and the wavelength of the radiation.  Thus light and infra-red will behave differently when striking a surface, and the behaviour of the surface at near-normal incidence (incidence perpendicular to the surface, q = 0) can be very different to near-parallel incidence (q = 90°).  Furthermore, the path that the sun makes through the sky is not simple, and the solar altitude and solar azimuth are complex functions of other parameters.
 
 

Solar radiation on a layered structure
The structure shown in this image is a simple layered component, such as a multiple glazing unit with a suspended film.  Each layer has a known reflection, absorption and transmission characteristic, together with a known characteristic for the emission of far-infrared.  Of the incident radiation I a total part rI is reflected and a total part tI is transmitted.  The remainder aI is absorbed, but each layer may absorb a different amount, depending upon its particular properties.  Note also that the part that is reflected consists of direct reflection from each of the layers, together with the transmitted part of each repeated reflection between layers - i.e. each pair of adjacent layers may reflect light and infra-red between them, losing a proportion of the energy by absorption and transmission each time a surface is encountered.

To analyse the multiple reflections between layers, and the absorption and transmission at each surface, is difficult.  However, it can generally be assumed that only solid parts of the structure absorb any energy directly (the gaseous parts of a structure absorb energy by heat transfer) and that the absorption, reflection and transmission at each surface is independent of wavelength and direction.  This at least allows an estimate to be made of the total energy reflected, transmitted and absorbed.
 

Service temperature ranges
The following examples are of annual service temperature ranges for materials used in normal circumstances in the United Kingdom:

External surface temperatures:
 

Internal temperatures:
 

These figures are taken from BRE Digest 228.  The examples are for normal buildings and conditions only.  More extreme temperatures can be expected in exceptional circumstances such as dark surfaces under glazing, materials in cold rooms or stores, and materials adjacent to heating or cooling systems.   For sun-facing sloping surfaces the designer shall increase the upper temperature limits by an appropriate amount.

Components such as glazing gaskets and joint sealants shall be able to withstand these temperature ranges, and thermal movements shall be estimated for these ranges, unless calculation or measurement shows that some other range of temperatures is more appropriate.

The service temperature range is affected by air temperature, solar radiation (which may depend on the orientation of the surface and any shading devices, including adjacent buildings), and by effects such as clear night sky radiative cooling.  Generally the range will be from a winter night-time minimum to a summer daytime maximum, representing actual extremes.  It is also possible to calculate summer daytime maximum temperature using the method outlined in the CIBSE Guide Part A8.

Glazing thermal stress calculations will be based on maximum daily variation.  Glazing thermal stress is generated by the temperature difference between the exposed centre-of-glazing, and the shaded edge-of-glazing.  Conduction heat transfer within the glass will limit the centre-to-edge temperature difference, but the rate at which the centre-of-glazing is heated is significant.  Tests for thermal stress shall be performed on realistically mounted samples, and at sensible rates of heating and cooling.  The results will be system-specific.

Guidance on reductions in environmental temperature due to clear night sky radiative cooling is given in the CIBSE Guide Part A2, and this may also be referred to as a night-time sol-air temperature.