Control of Solar Heat Gain in Buildings


Introduction
In hot climates room temperatures can increase to uncomfortable levels due to the heating effect of the sun.  A good explanation of this is given in Wikipedia:-

“Objects struck by sunlight absorb the short-wave radiation from the light and reradiate the heat at longer infrared wavelengths. Certain materials and substances, such as glass, are more transparent to the shorter wavelengths than the longer; when the sun shines through such materials, the net result is an increase in temperature — solar gain.”

If we control Solar heat gain then we can reduce the use of costly power for air conditioning.

Glass and its Manufactured Properties
One way to minimise solar heat gain in buildings is to minimize the size and or number of windows, conversely we can control in the manufacturing process the Solar Heat Gain Coefficient (SHGC) of the glass.
SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. It is expressed as a value between 0 and 1. A material with a small coefficient transmits more energy than a High coefficient. In choice of windows we have to look at a number of factors it is not only The SHGC that is important.
The U value of the glass pane is very important. It is the overall heat transfer coefficient or thermal conductivity of the material and allows us to calculate how much heat power (in Watts) is lost (or gained) in an environment with an particular external to internal heat differential due to a material and its surface area.
 
Φ = A × U × (T1 - T2)
Where Φ is the heat power lost or gained
A is the surface area of the material
U is our U value or heat transfer coefficient
T1 is our the outside temperature
T2 is our the inside temperature
 
Any specification of glass must take into account SHGC, U value, and the frame material. But glass is not good unless we can see through it so we must also look at the Visible Light transmittance or VLT of the glass.
Good glass panes will have:-
  • Low U Value (Does not transfer heat from one side to the other very well)
  • Low SHGC (Low Re-radiation of incident light waves as infrared)
  • High VLT (good transmittance of the visible light spectrum)
Good target values for these properties are
  • U   0.28 – 0.33
  • SHGC 0.27 - 0.35
  • VLT 65 - 70%
The frame of the glass pane can also have a significant effect and it is generally recommended that non -metal frames are better for stopping heat transfer (metal is a good conductor of heat).
Solar heat gain can have positive effects as well as negative effects. In winter it provides free heating (look at the effects green houses and poly-tunnels). In summer it can raise the internal room temperature in a building to uncomfortable levels making expensive to run air conditioning systems vital. Glass panes can be manufactured in numerous ways and it is accepted that Pyrolytic glass coatings are better in warm climates whilst Silvered coatings are better in cooler climates.
Active Solutions for Solar heat Gain
Control of Glass
Home and Building Designers have a number of options when it comes to control of solar heat gain. The correct choice of glass is one of these. If we wanted to apply a more active approach to the control of solar heat gain with some home/building automation techniques we may look to control the properties of the glass. Smart Glass has been marketed as a means by which the householder can control solar heat gain and hence reduce air conditioning running costs. There is very little evidence for this and major manufacturers are not making such bold claims. Where they claim it has an effect they state that the effect is minimal and does not form a basis for its installation based on economic factors. i.e.
Cost of Glass (film) + cost of installation + cost of ownership 
>> Any Potential Savings
Where such glass has its uses is mainly where privacy is required. The ability to use it as occasional projection screens is also well known.
Typical Glass Solar behaviours
INTERIOR SOLUTIONS
Interior solutions may include:-
  • Blinds
  • Curtains
  • Internal Shutters
A discussion of the individual merits of each is not in the scope of this document. But suffice it to say that the following Solar heat gain reductions are possible with suitable choices and suitable installation:-
  • Blinds 45% reduction
  • Curtains/Drapery 33%
  • Internal Shutters 50%
There are possibilities for the automation of all of these.
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​EXTERIOR SOLUTIONS
Exterior solutions may include:-
  • Awnings
  • Shutters including
    • Roller Shutters
    • Hinged Shutters
    • Slide over shutters
  • Mesh Window Screens
  • Shades
External measures are generally more effective than internal ones and figures in excess of 70% reduction in solar heat gain are achievable with automation being easily applied.
​AUTOMATION AND SMART BUILDING TECHNOLOGY
The control of solar heat gain cries out for some level of automation. It is very easy to apply what is effectively remote control of motors on shutters, blinds, curtains etc. This is what a lot of manufacturers and smart home/ building system integrators call smart, especially if it is controlled by a smartphone or tablet. In a truly smart system the building (or rather sensors in the building) will instruct blinds, shutters and shades to open/close or tilt the slats to a different angle dependent upon conditions in the building.
With a few strategically placed sensors measuring temperature/light levels and computer algorithms that are not excessively complicated we can effectively tell when a particular environment or group of environments are likely to suffer from solar heat gain and hence when we should begin the processes of control such as unfolding awnings, adjusting slats on shades and venetians or just simply closing blackout curtains and roller blinds.
Such operations may be integrated alongside other typical smart home/ audio visual systems and control can initiates from a number of wall mount or wireless control panels around the building. Often lighting control may be included in such a system. This is very useful as we can control lighting in working environments automatically to compensate for any reduced lighting levels. This calls for energy efficient lighting such as LED lighting and the ability to dim /un-dim luminaires as and when necessary.
Such functions also lend themselves to Building Management (BMS) and SCADA (Supervisory control and data acquisition) systems for large buildings, campuses and towers. Large savings can be made if we reduce the power required by air conditioning by 70% to say 200 offices on the west/south west faces of a 25 floor tower. Savings for this cooling could be over $500 per day for all the offices dependent upon the efficiency of the AC systems installed.
NJS Smart Technical Services LLC Technology Division specialise in the Design, Consultancy and Installation  of electronic infrastructure systems for buildings be they homes, towers, offices of factories in the Gulf Region. Solar heat gain control is one of their main activities. They work with Somfy (www.somfy.com) for control of Blinds Curtains and Shutters, Austrian company RESI (www.resi.cc) for LED dimming systems and their SCADA.3d  building and Security Management systems and SATER (http://www.sateruae.com) for all shutter systems.
Smart Homes and Buildings are one of the many areas NJS can provide services in. 
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