09.10 Alternative technologies

Categories: Advanced Glazings

Introduction
Advanced glazings are used to:

• Reduce solar gain
• Reduce heat loss
• Reduce the need for artificial lighting
• Reduce glare

Alternative strategies may be adopted.  For instance the area and orientation of the glass may be varied.  Other technical solutions are also possible.  These include modifications to windows such as blinds and shutters and separate solutions such as light pipes.  A further alternative is to improve the efficiency of the lighting by using low energy light fittings or better control systems.  The main alternative technologies are described below.
 

Light-pipes and light-shelves
Light-pipes are used to channel natural light from the outside of a building to the inside, through a duct with suitable reflectors.  A light-pipe may be used to channel light from the roof of a building into a basement area, but may also be used to channel light into the back of a deep room or office.  This could have an impact on the use of refractive glazings, and the designer must consider the relative value of using the glazing to divert light to the back of the room against the use of a duct system which will possibly use up valuable commercial space.  Issues of cleaning and maintenance will also need to be considered.

A light-shelf is a somewhat simpler device to install, comprising a reflective shelf mounted across the glazing and used to reflect light up towards the ceiling whilst also shading the area below the light-shelf.  The principal issue with light shelves is one of cleanliness - dust will gather on the shelf and will need to be cleaned off regularly, whilst a shelf that is fixed will quickly become a home for potted plants.  A good light-shelf therefore needs to be hinged so that it is not used as storage, but this may create conflicts over control - the light-shelf could be angled in such a way that it is used primarily for shading and so reflects the useful light straight back out of the window.  It is suggested that a light-shelf is more efficient when placed on the outside of the window, has reflective surfaces that diffuse the light and is combined with a room ceiling that has a high reflectance (UCD-OPET [1994]).  However, an external light-shelf, as with any external shading device, may limit access to the window for cleaning.  A light-shelf is also of little use for dealing with low winter sun.  Better control can be achieved with adjustable blinds which have a reflective surface - this also gives a degree of control to the room occupant, allowing cloudy days to be dealt with.

Light-pipes and light-shelves are discussed in some detail by Littlefair [1996].
 

Shutters, blinds and curtains
A traditional approach to modify the performance of a glazing system is to use some device to block solar radiation (and reduce heat transfer by creating an additional trapped air-space).  In the UK this takes the form of curtains, which are pulled across the inside of the window at night to reduce heat loss, and may remain in place during summer days to prevent excessive solar heat gain (to the detriment of pot plants!).  In other European countries shutters may be used to cover the outside of the window and perform the same function.  Alternatively blinds can be used, which may be installed on the room-side of the glazing or between the panes of a sealed glazing unit.

The use of shutters, curtains and blinds depends on the application.  In domestic housing cost is often the governing factor - it costs less to fits shutters, blinds or curtains than to buy better glazing - although privacy issues may be of equal importance.  For commercial buildings there are issues of control, which may mean that although blinds would reduce direct light and solar gain they can become the subject of a control struggle with those seated nearest the blind insisting that they remain closed, whilst those seated away from the glazing wanting the blinds to be left open.  Blinds in commercial and public buildings are also more likely to be damaged by careless use.

An important point is that the best place for a shading device is on the outside of a glazing system, to prevent the solar radiation from even reaching the glazing.  However, this requires consideration of possible shading lines across the glazing, and may also prevent access to the glazing for cleaning (of course, if designed sensibly a shading device may also provide a safe working platform for cleaning - if the shading device stops rain from reaching the glazing surface then this may also influence the rate at which the glazing becomes dirty).  Shading devices fitted between the panes of a double glazing unit introduce issues of maintenance, although the control may be taken out of the hands of the room occupier.  Shading devices on the room-side suffer from damage, accumulate dirt and lead to control struggles.

In the UK curtains or blinds are only used on the room-side of the window, creating an additional insulation layer on the warm-side of the glazing (in winter and at night).  In winter although this reduces heat loss is also has the effect of lowering the temperature of the room-side glass, increasing the risk of condensation.  This is particularly undesirable where the fabric of the curtain or blind comes into contact with the wet glass and becomes stained or otherwise damaged as a result.  In summertime the use of blinds or curtains on the room-side to reduce solar gain leads to a build up of heat adjacent to the glazing, and this might cause glass to fail due to higher levels of thermal expansion.  In other European countries shutters and blinds are used on the outside of the building, preventing solar radiation from reaching the glazing and resulting in higher room-side glass surface temperatures during winter night-time conditions!

The use of blinds between the panes of a glazing unit, or between coupled lights in a casement window, offers an intermediate level of performance, and introduces control issues - facility must be provided to adjust the blinds, and this may require additional operations on the part of the window fabricator and installer.  However, sealed units containing blinds are already widely available.
 

Low-energy artificial lighting
A simple alternative to advanced glazings is to combine some form of external shading device with energy efficient lighting to compensate for reductions in natural light levels.  A shading device can be arranged to block out summer sun, which is at a high elevation in the sky, whilst allowing the sun into the building during winter, when it is lower in the sky.  In this case some artificial lighting may be needed to compensate for low natural light levels on cloudy summer days, and low energy lighting will be preferred.

A reasonable introduction to energy efficient lighting, including a number of cost-based case studies, is given in BRECSU-OPET [1992].
 

Design
As well as using other low-energy technologies it is also sensible to consider design issues - i.e. how can we design to minimise problems:

Glare arises when the light entering a room has a strong direct component.  Reflections of direct light from adjacent buildings and from surfaces within the room will compound matters.  However, glare is a problem of contrast - strong light is more of a problem if it contrasts with dull adjacent surfaces, or is the only light entering an otherwise dark room.

Glare from a window can be minimised by reducing the contrast.  Button and Pye [1993, pp100-101] suggest using deeper windows or windows in more than one wall to increase general light levels, and using light-coloured matt finishes around windows, with splayed reveals to decrease the contrast between window and wall.  Using glazings with diffusing glasses may not be helpful, because the incident light is scattered and the whole of the glazing may become illuminated.  This can cause a significant glare problem that can then be seen from anywhere within the room, because the glazing is diffusing the light.
 

Daylight and passive solar gain when used properly  can allow the designer to provide an energy efficient building without the expense of advanced glazing systems.  The balance between natural and artificial lighting is not well understood, and should be researched further.