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Passive Solar Lighting

For most people, the term solar lighting conjures up images of simple photovoltaic cells on the tops of LED lamps in gardens and along walkways. While that is one form of solar lighting, it is not the only one. For thousands of years, sunlight was the only means humans of lighting anything adequately. While light bulbs may let modern humans work long into the dark of night, making the best use of sunlight when it is available can help save energy, increase lifespan of a light bulb, and decrease waste.

Daylighting

The term refers to the use of natural light to illuminate the interior of a structure. While the implementation of daylighting can be accomplished in a number of ways, including hi-tech fiber optics, there are still some general principles that hold in any application.

The first principle is orientation. Just as with passive heating and cooling, orientation for light is important. Fortunately, orientation for light is not as critical as that for heating and cooling because these interests can often compete with one another. Spaces that will be used during the day should be oriented to account for not only light, but also for heat gain (positive and negative) and glare.

The second principle is that natural sunlight can be harsh and, in many cases, too bright. Diffusion is the key to filtering out some of the harshness of natural lighting. It also plays a double role of ensuring that the entry of sunlight does not overheat a space.

The third principle is color. Light and dark colors play different roles in solar design. Dark colors are generally used to increase the thermal mass of structure so that it absorbs more heat energy from light. Light colors, on the other hand, are useful for reflecting light. They do not heat up, but they reflect and diffuse light. The diffusion properties make light colored walls excellent for indirect lighting. Sunlight from windows that is too harsh for practical use can be reflected into a living space to create comfortable, diffuse lighting.

Light Shelves

One of the impediments to lighting in the summer is that the equator-facing side of the house gets the most light, but also the most heat. Allowing direct entry of sunlight through windows would cause too much heat to enter. The solution to the heat is to build overhangs that block direct sunlight in the summer, but allow it to enter in the winter when the sun is lower on the horizon. Of course, this means that little light will enter that side of the building and may be inadequate to meet needs.

The answer to the problem is light shelves. These are nothing more than light colored or slightly reflective shelves that are placed outside of a window and extend beyond the shadow of the overhang. They reflect light, but not heat back through the window and onto the ceiling of the room. A lightly colored ceiling will then reflect and diffuse the sunlight throughout the room.

Light Tubes

Light tubes are basically reflective tubes that channel light from the roof of a structure to an interior ceiling. They come in a number of configurations, but they often have a capped top to admit light, a reflective coating, and an interior diffuser that creates soft light. It is also possible to get them without diffusers for spot lighting. Some of the high end models use fiber optics to carry the light. Light tubes have the advantage of transporting little heat along with the light.

Hybrid Solar Lighting

Oak Ridge National Laboratory in Tennessee has created an interesting alternative to fluorescent lights using fiber optics. Much lithe the light tubes discussed above, exterior sunlight is collected and transport to the interior of a building via large-diameter fiber optic cable. Specially modified fixtures using transparent rods are then connected to the optical fibers. They look like fluorescent lights, but use no electricity and are provide soft, diffuse light during daytime hours. What makes these hybrids is that as the sunlight fades, electricity is applied to create artificial light. Special sensors keep the light level nearly constant at all times.

Smart Glass

Smart glass is another high technology application of basic solar lighting principles. This type of lighting uses electrical voltage to change the opacity and reflective properties of a pane of glass. It may seem that the application of electricity would move this type of lighting from passive to active, but this is a case where the distinction becomes fuzzy. Because the light itself is not being manipulated, this is categorized as passive, though an argument for active can certainly be made.

The electric charge that changes the properties of the glass is not continuous. It is a single burst to change and then another burst to change the glass back. A constant flow of electricity is not needed, so these are very energy efficient devices.

Smart glass comes in several different technologies including electrochromic, suspended particle devices, and polymer dispersed liquid crystal devices. Only electrochromic technology currently allows for glass that can be transparent, opaque, and reflective. The other technologies allows for transparent and opaque only.
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