Measure 8.3.1 Install skylights or light pipes.

Special Features of Light Pipes

The desire to gain the benefits of skylights without suffering their disadvantages created an interest in “light pipes.” As the name implies, light pipes convey light to locations within the building where it is needed. Many types of light pipes have been designed, but only a few are commercially available.

Some claims made for light pipes defy the laws of physics. A light pipe cannot deliver more light energy to the space than it collects on the outside of the building. At present, commercial light pipes do not concentrate sunlight from large exterior collectors into smaller pipes. Understand the principles of light pipes so that the installation will yield the results you expect. Huvco, LLC

Fig. 12 Rigid wall light pipe This unit has a reflector in the back of the dome to capture more daylight at low sun angles. A diffuser is installed at the ceiling. Fig. 13 Flexible wall light pipe This type is easier to install than rigid wall light pipes. However, light losses are greater because of the corrugated surface of the tube.

Light Loss from Reflection Inside the Pipe

The efficiency of fixed light pipes suffers from absorption that occurs when light is reflected from the walls of the pipe. Unless the sun is lined up with the axis of the pipe, the light is reflected repeatedly as it travels through the pipe. Even if the surfaces of the pipe have high reflectance, say 90%, a large fraction of entering light is lost with a few reflections. The light loss is proportional to the length-to-width ratio of the pipe. Therefore, efficiency is sacrificed if the pipe is long in relation to its width.

Simple Light Pipes

Most light pipes that are available commercially consist of an exterior transparent dome, a reflecting metal pipe, and a diffuser for installation at the ceiling level of the space. The pipe may be rigid or flexible. Flexible pipes are easier to install, but they suffer more light loss from increased reflection and scatter inside the pipe. Figure 12 shows a rigid light pipe, and Figure 13 shows a flexible light pipe.

Sun Trackers

A movable mirror or refracting system can be used to align the incoming sunlight with the axis of the light pipe, minimizing reflection losses. A light pipe with this feature is called a “sun tracker.” Sun trackers have been built commercially. Figure 14 shows a number of installed units, and Figure 15 shows a cross section of the unit. If mass produced, sun trackers could be relatively inexpensive. Their main limitation is that they lose effectiveness if the sky does not remain clear. The system is designed to collect light from the sun, which is a point source. The light reflecting apparatus gets in the way of the whole sky when the sun is obscured. Another disadvantage is the need for occasional maintenance.

Future Develoments

An ideal light pipe would have a large exterior collecting surface, it would funnel the light into a narrow conduit, and it would deliver the light wherever it is needed.

A small conduit is desirable to minimize heat loss and to make the light pipe easy to install. Funneling the light from a large collector into a small pipe requires a tracking mirror and a lens system. These do not have to be precision components. For example, light can be concentrated with flat fresnel lenses made of molded plastic. Both the tracking mirror and the lens system should be able to adapt to changes in sky conditions, from direct sunlight to a diffuse sky.

Fig. 14 Sun tracking light pipes The rotating head contains mirrors that reflect direct sunlight straight down the pipe, minimizing losses. However, sun trackers may be less effective than simple light pipes for collecting sunlight from a diffuse sky.

To make this economical, all the light has to be squeezed into a light pipe of small diameter. The small diameter is a major advantage in itself, but it involves complication at each end of the light pipe. This concept is presently used with high-intensity electric lamps as the light source, for special effects. Present light pipes of this type are too expensive for daylighting.

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These excerpts shows the level of detail and accuracy that you'll find in the Energy Efficiency Manual by Donald Wulfinghoff. This 1500-page manual offers the most comprehensive coverage of energy-saving measures for buildings ever assembled under one cover. For more information and to view the complete table of contents, find the Energy Efficiency Manual in the Oikos Bookstore.