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.
A small 6" deep 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.
The mirrors are usually 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 mirrors also harvest a small amount of heat in winter, and are often enjoyed as cat sunbathing balconies.
Skylight & Sunlight
The main 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.
Reflector Material
I found aluminium foil an unsatisfactory reflector. It does not lie flat properly, and causes quite a lot of glare. Silvered mylar on greased sheet material would behave much better, and conventional glass mirrors, which I used, give the best result visually, as being perfectly flat they don't add glare.
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.
Glass mirrors should be painted on the rear to prevent eventual deterioration.
Mirror Angle
Sloping at 5 degrees gives no significant glare with very good light yield.
Tilting the mirror up further gives more light, but also glare. Tilting the mirror down gives less light.
Mirror Size
A small 6" deep mirror is unobtrusive, but adds noticeably to the interior light level. The light is seen most on the ceiling above the window.
A 15" deep mirror can roughly double interior light level.
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 can ensure payback in under a year.
Summer
These mirrors are more useful on the north side of the house, where interior light levels are lower. If used on the south side, they will reflect heat in in summer. Much of this heat can be blocked by placing a plant on the shelf in summer, so that it casts a lot of shade on the mirror.
Details to Watch
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.
The mirror should be secured well enough to survive storms. This is relatively easy with hinged mirrors, which can be locked shut when a storm approaches.
Mirrors should not be secured by their frames where fitted, 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.
An 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, as winds are more severe at higher floors, and a normally trivial detachment could have grave consequences.
Alternatives
Solar mirrors are usually ruled out on the grounds of appearance. Where this is an issue, there 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 is easy and maintains a familiar appearance.
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.
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 very laborious, but if it transforms a dark unpleasant usable living space it might be worth it. The quickest method is to place many pieces of steel in a shallow mould and pour concrete on the whole lot to make a large slab. A dollop of mix on each steel piece helps weight them down during filling. Steel pieces are best kept small, otherwise they can be slippery to walk on. The concrete between them provides grip for walking.
See Also
Heliostats