Solar Lighting

Daylighting

Is the practice of placing windows, or other transparent media, and reflective surfaces so that, during the day, natural light provides effective internal illumination.

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Within the overall architectural design of a building, particular attention is given to daylighting when the aim is to maximize visual comfort, productivity, or to reduce energy use. Energy savings from daylighting are achieved in two ways–either from the reduced use of electric lighting, or from passive solar heating or cooling.

Electric lighting energy savings can accrue because occupants choose not to switch their lights on, or because an automatic lighting control system (photocontrol system) switches the lights off or dims them to a lower level.

In passive solar technique, buildings are designed such as to account for local climate, in particular the luminance of the sky. For instance, in cooler parts of northern countries with largely overcast sky, a house will be designed with minimal windows on the north side but more and larger windows on the south side. This is because in the Northern Hemisphere, above the Tropic of Cancer, there is no direct sunlight on the north wall of a house from the autumnal equinox to the spring equinox, north-side windows are ineffective at daylighting. South-side windows receive at least some direct sunlight on any sunny day of the year, so they are effective at daylighting areas of the house adjacent to the windows. One disadvantage of relying on conventional window space for daylighting is that, especially during mid-winter, it tends to be highly directional light that casts deep shadows.

Windows

Windows are the most common way to admit daylight into a space. Their vertical orientation means that they selectively admit sunlight and diffuse daylight at different times of the day and year. Therefore windows on multiple orientations must usually be combined to produce the right mix of light for the building, depending on the climate and latitude.

Light reflectors

Once in extensive use in office buildings, the adjustable light reflector is seldom seen, having been supplanted by a combination of other methods in concert with artificial illumination. The reflector found favor where the choices of artificial light provided poor illumination compared to modern electric lighting.

Light shelfs

An effective way to enhance the lighting from windows on the equator-facing side of a structure is to replace a white or reflective metal light shelf outside the window. Usually the window will be protected from direct summer season sun by a projecting eave. The light shelf projects beyond the shadow created by the eave and reflects sunlight upward to illuminate the ceiling. This reflected light can contain little heat content and the reflective illumination from the ceiling will typically reduce deep shadows, reducing the need for general illumination.

Skylights

Skylights are often used for daylighting. Skylights admit more light per unit area than windows, and distribute it more evenly over a space. They are therefore a good choice when daylight is being used to illuminate a space. The optimum number of skylights (usually quantified as ‘effective aperture’) varies according to climate, latitude, and the characteristics of the skylight, but is usually 1-10% of floor area. The thermal performance of skylights is affected by stratification, i.e. the tendency of warm air to collect in the skylight wells, which in cool climates increases the rate of heat loss.
The amount of light skylights deliver peaks around midday, when the additional light and heat it provides is least needed. Some skylight designs use domed or pyramidal shapes along with prismatic or other light-redirecting glazings to achieve more even light levels through the course of a day. Poorly constructed skylights may have leak problems and single-paned ones may weep with condensation. Using skylights with at least two panes and a heat reflecting coating will increase their energy efficiency.

Clerestory windows

Another important element in creating daylighting is the use of clerestory windows. These are high, vertically-placed windows. They can be used to increase direct solar gain when oriented towards the south in the Northern Hemisphere, and towards the north in the Southern Hemisphere. When facing toward the sun, clerestories and other windows may admit unacceptable glare. In the case of a passive solar house, clerestories may provide a direct light path to north-side (in the northern hemisphere; south-side in the southern) rooms that otherwise would not be illuminated. Alternatively, clerestories can be used to admit diffuse daylight (from the north in the northern hemisphere) that evenly illuminates a space such as a classroom or office.
Often, clerestory windows also shine onto interior wall surfaces painted white or another light colour. These walls are placed so as to reflect indirect light to interior areas where it is needed. This method has the advantage of reducing the directionality of light to make it softer and more diffuse, reducing shadows.

Light tubes

Light tubes or light pipes are used for transporting or distributing natural or artificial light. In their application to daylighting, they are also called solar tubes, solar pipes, daylight pipes, or solar light pipes.
Generally speaking, a light pipe or light tube may refer to:

a tube or pipe for transport of light to another location, minimizing the loss of light;
a transparent tube or pipe for distribution of light over its length, either for equal distribution along the entire length (see also sulfur lamp) or for controlled light leakage.

[b]Light tube with reflective material/[b]

Also known as a ‘tubular skylight’, this is the oldest and most widespread type of light tube used for daylighting.
A round tube lined with highly reflective material leads the light rays through a building, starting from an entrance-point located on its roof or one of its outer walls. A light tube is not intended for imaging (in contrast to a periscope, for example), thus image distortions pose no problem.

The entrance point usually comprises a dome (cupula), or alternatively a diamond-shaped light collector, which has the function of collecting and reflecting as much sunlight as possible into the tube.

Light transmssion efficiency is greatest if the tube is short and straight. In longer, angled, or flexible tubes, part of the light intensity is lost. To minimize losses, a high reflectivity of the tube lining is crucial; manufacturers claim reflectivities of their materials, in the visible range, of up to 98 to almost 99.5 percent.

At the end point (the point of use), a diffuser spreads the light into the room.

To further optimize the use of solar light, a heliostat can be installed which tracks the movement of the sun, thereby directing sunlight into the light tube at all times of the day as far as the surroundings´ limitations allow, possibly with additional mirrors or other reflective elements that influence the light path. The heliostat can be set to capture moonlight at night.

Optical fiber

Optical fibers are well known as fiberscopes for imaging applications and as light guides for a wide range of non-imaging applications. In the latter context, they can also be used for daylighting: a solar lighting system based on plastic optical fibers was in development at Oak Ridge National Laboratory in 2004; the system was installed at the American Museum of Science and Energy, Tennessee, USA, in 2005,

and brought to market the same year by the company Sunlight Direct.
A similar system, but using optical fibers of glass, had earlier been under study in Japan.

In view of the usually small diameter of the fibers, an efficient daylighting set-up requires a parabolic collector to track the sun and concentrate its light.

Optical fibers intended for light transport need to propagate as much light as possible within the core; in contrast, optical fibers intended for light distribution are designed to let part of the light leak through their cladding.

Transparent hollow light guides

A prism light guide was developed in 1981[13] and has been used in solar lighting for both transport and distribution of light. A large solar pipe based on the same principle has been set up in a narrow courtyard of a 14-floor building of a Washington D.C. law firm in 2001, and a similar proposal has been made for London. A further system has been installed in Berlin & nbsp.
The 3M company developed a system based on optical lighting film and developed the 3M light pipe,which is a light guide designed to distribute light uniformly over its length, with a thin film incorporating microscopic prisms, which has been marketed in connection with artificial light sources, e.g. sulfur lamps.

In contrast to an optical fiber which has a solid core, a prism light guide leads the light through air and is therefore referred to as hollow light guide.

Fluorescence based system

In a system developed by Fluorosolar and the University of Technology, Sydney, two fluorescent polymer layers in a flat panel capture short wave sunlight, partucularly ultraviolet light, generating red and green light, respectively, which is guided into the interior of a building. There, the red and green light is mixed with artificial blue light to yield white light, without infrared or ultraviolet. This system, which collects light without requiring mobile parts such as a heliostat or a parabolic collector, is intended to transfer light to any place within a building. By capturing ultraviolet the system can be especially effective on bright but overcast days; this since ultraviolet is diminished less by cloud cover than are the visible components of sunlight.

Solar and hybrid lighting systems

Solar light pipes, compared to conventional skylights and other windows, offer better heat insulation properties and more flexibility for use in inner rooms, but less visual contact with the external environment.
In the context of seasonal affective disorder, it may be worth consideration that an additional installation of light tubes increases the amount of natural daily light exposure. It could thus possibly contribute to residents´ or employees´ well-being while avoiding over-illumination effects.

Compared to artificial lights, light tubes have the advantage of providing natural light and of saving energy. The transmitted light varies over the day; should this not be desired, it can be combined with artificial light in a ‘hybrid’ set-up.

Some artificial light sources are marketed which have a spectrum similar to that of sunlight, at least in the human visible spectrum range, as well as low flicker. In some cases, their spectrum varies dynamically such as to mimick the changes of natural light over the day. Manufacturers and vendors of such light sources claim that their products can provide the same or similar health effects as natural light. When considered as alternatives to solar light pipes, such products may have lower installation costs but do consume energy during use; therefore they may well be more wasteful in terms of over-all energy resources and costs.