Varying the Daylight Transmission of Facades


All year around we should be trying to maximise the daylight contribution of the external sky to the lighting of the internal spaces.  The primary reason for this is to save energy by reducing the electric lighting load but reductions in the buildings cooling load can also be gained if the facade system is designed correctly.  This is because natural daylight has a high luminous efficiency compared to artificial light, which means it provides more light for the same heat output.  By maximising the daylight, whilst rejecting a lot of the solar heat, we can often turn off the electric lights and thus reduce the cooling load by approximately 10W/m2.

Blinds and shading devices primarily provide protection against intrusion, glare and direct sunlight, and if external or mid-pane, they also provide reductions in solar heat gains.  The use of  blinds can lead to considerable energy savings, if controlled correctly and adjusted several times a day. For example, the blind could be left open, to utilise natural daylight, when the summer sun is not shining directly onto a window. Then during hours of maximum solar heat gain, the blind could be progressively closed to reflect incident radiation back out of the window. This pattern of operation can result in reductions to the summer cooling load as well as the electric lighting load, but such savings rely on the occupants manually adjusting the blinds every hour to suit the overall energy strategy.

Unfortunately, field studies show that occupants very rarely adjust blinds, especially the slat angle. The research identified overheating and glare as principal parameters that stimulate occupants to manually operate window blinds; after having made such an adjustment, occupants very rarely adjust the blinds for the rest of the day. This behaviour leads to the unnecessary use of artificial lighting in parts of the day when sufficient natural light is available.

Such factors have resulted in the increasing use of automated photoelectric control to ensure that blinds are not left down when the sky is overcast and the risk of glare and overheating has passed, thus providing potential for energy saving. However, the maximisation of daylight alone is not a guarantee of low energy consumption; appropriate lighting control is also required to dim lights gradually when daylighting levels increase.  In addition the use of zoned occupant sensors in a lighting control system can yield significant energy savings.