Solar Mirrors
Outdoor mirrors can be used to reflect skylight and often sunlight into a room to lighten the room and reduce electric lighting use.
This article discusses mirrors mounted almost horizontally outside windows at cill level. The mirrors slope down at around 5 degrees to avoid glare, and shed water and dirt. These are a visually unintrusive option (in small sizes).
A small 6" deep window sill mirror is unobtrusive, but adds noticeably to the interior light level. A 15" deep mirror can double interior light level. Although twice as much light is not entering the room, the upward reflected light is used more effectively, meeting a white ceiling rather than the darker floor.
Large mirrors are not popular due to poor external appearance, but can be a practical option for off-grid homes, sheds & workshops. For outbuilding use, the mirror can be hinged, thus acting as a fold-up shutter when not in use, and the horizontal mirror is only visible when the building is used.
These mirrors also harvest a small amount of heat in winter, and are often enjoyed as cat sunbathing balconies.
Skylight or Sunlight
The usual aim of these is to refect in skylight, not sunlight. Skylight is present all day long, on all sides of the house, and gives a diffuse white light on the ceiling. Skylight also doesn't bring in significant heat in summer, whereas reflecting sunlight in does.
Reflector Material
Aluminium foil is an unsatisfactory reflector.
- It doesn't lie flat properly
- It causes quite a lot of glare.
- It isn't durable outdoors
Silvered mylar on greased sheet material would behave better.
Silvered rigid plastic is prone to damage and deterioration over time.
Polished stainless steel is unbreakable, but causes glare.
Conventional glass mirrors give the best result visually, as being perfectly flat they don't add glare, and the smooth finish looks good. Glass mirrors should be painted on the rear to prevent eventual deterioration.
Ali foil on cardboard can be held in place as a test to see how much extra light you get, but it does create a lot of glare which doesn't happen with a good quality reflector.
Mirror Angle
Sloping at 5 degrees gives no significant glare with very good light yield. The mirror is sloped so that rain falls away from the wall.
Tilting the mirror up further would give more light, but also glare.
Mirror Size
A small 6" deep mirror is unobtrusive, but adds noticeably to the interior light level. The extra light is immediately noticeable on the ceiling above the window, but the mirrors usually bring some light in to the full depth of the room.
A 15" deep mirror can roughly double interior light level.
Even larger mirrors can return more, but each additional inch of depth returns less extra light, as its further away from the window.
Payback
The mirrors give a small amount of winter heating payback, reducing primary fuel use slightly. ROI depends primarily on the cost of the mirror: the lower the cost, the better the payback.
- Use of second hand mirrors at minimal cost can ensure payback in under a year.
- New mirrors give low ROI.
Summer
Where light is wanted, these mirrors are more useful on the north side of the house, where interior light levels are lower, and hot direct sun is not reflected in in summer.
Where primarily heat is wanted, the south facing side will reflect direct sunlight in. However on the south facing side, heat is also harvested in summer, which is a real problem. There are a few posssible ways to address this:
- Hinged mirrors folded away in summer
- Place a plant on the shelf in summer to cast shade on some of the mirror
- Plant shade from a deciduous climber or tree
Rot
Mirrors should not be mounted onto the top of softwood exterior cills. This tends to trap water and dirt & discourage painting, and can result in rot.
Where wooden window frames are in use, the mirror should be mounted 2-3" away from the frame to let sunlight dry the windowframe, and should be mounted a little above the cill to prevent muck buildup by the frame.
Fixing
The mirror should be secured well enough to survive storms. Hinged mirrors can be locked shut when a storm approaches.
Used mirrors often come in wooden frames, but should not be secured by these. Such frames are neither strong enough nor durable enough.
2mm galvanised iron wire overlapping the mirror corners is a simple way to make a fixing framework tough enough to survive storms.
A possible alternative is to design the mirror light and flexible enough that it can become detached in severe weather, eg thin rigid plastic sheet with metallised mylar.
These mirrors should not be used on high rise buildings. Winds are more severe at higher floors, and a normally trivial detachment could have grave consequences.
Alternatives
Limestone chippings on the ground by the window will reflect skylight up through a ground floor window. Light yield is less, but still significant. This maintains a conventional appearance.
Its also possible to use more determined solar collectors, such as tracking heliostats, but these are much harder to set up and have significant issues, so are rarely used. Where extreme energy efficiency is required, these can be an effective option. Heliostats should always avoid having a point of focus, as point focus mirrors are a fire risk. Slight dish distortion of a parabolic dish can produce a line of focus instead.
Stainless steel decking has been used as a giant solar mirror before. Its rarely chosen on cost grounds, but can be an effective solution to long dark interiors, where all windows are at one end. By bouncing light onto the ceiling it carries further into the room. Beware of huge heat gains from sunlight though.
Finally small stainless steel topped paving stones (made of concrete) reflect light without reflecting an optical image, since the angle of each stone varies. Cutting up scrap stainless steel and casting the concrete is laborious, but if it transforms a dark unpleasant living space it can be worth it. A quick method is to place many pieces of chopped stainless steel in a shallow mould and pour concrete in to make a large slab. Steel pieces are best kept small, otherwise they can be slippery to walk on. The concrete between them provides grip for walking.