SOLARTRAN  IMPROVING THE PERFORMANCE OF LIGHT PIPES

A light pipe is a reflective cylinder between an opening at the roof  and an opening at the ceiling. The ratio of the length of the pipe L, to the diameter of the pipe D, is called the aspect ratio of the light pipe, aspect ratio = L/D. Long,  narrow pipes have a high aspect ratio and low elevation light makes many reflections when traveling the length of the pipe, see A below. At each reflection some light is absorbed and the remainder transmitted. The amount transmitted at each reflection is called the reflectance of the material of the pipe, symbol r. Typically r = 0.9 for light pipe reflecting material. If, as in A below, the light makes 22 reflections the overall pipe transmittance is T = r^N = 0.9²² = 0.10. Thus, generally, the transmittance of low elevation light, such as early morning sunlight, is poor. The transmittance of low elevation light can be greatly improved by fixing a laser cut panel at the edge of the pipe to intercept the light and redirect it down the light pipe as in B below. When the sun is high in the sky, as in C below, the transmittance of a pipe is high. Having a laser cut panel at the edge of the pipe intercepts some of this high sunlight and redirects it so that it makes more reflections, see D below. Thus the transmittance of high sunlight is reduced. The overall effect is to greatly improve the performance to low elevation light and partly reduce the performance to high elevation light. Also, see "How it Works" at : www.skydome.com.au/skylight_images/skylight_images_48.htm. 

SYSTEM EVOLUTION

Early redirection systems used a single laser cut panel fixed at an angle of about 35 degrees to the horizontal. The panel faced the equator to intercept and redirect winter sunlight. Due to the large intercepting area the system was very effective for winter sunlight but much less effective at other times of the year. Various attempts to track the panel  by rotating about a central vertical axis daily proved economically unfavourable for small diameter light pipes.

  To address this problem a technique of laser cutting followed by folding was used to fabricate self supporting multi-facetted redirecting systems. The one shown to the left is 6-facets and is designed to self-lock into a 250 mm or 10 inch light pipe. These proved to be very economic and effective at all times of the year at redirecting low elevation light, giving increases of sunlight transmission by up to a factor of three times at low sun elevation. It should be noted that light pipes collect both direct sunlight and diffuse skylight. Diffuse skylight transmittance is reduced to about 80% and as a pipe collects varying amounts of direct sunlight and diffuse skylight the overall transmittance is a combination of the two components. The Australian rights to this technology have been transferred to Skydome Skylight Systems Pty Ltd. www.skydome.com.au/products_skytubes_home.htm.

Due to the combined effects of direct and diffuse light it is difficult to predict theoretically the performance of light pipes in actual conditions. For this reason performances are measured under real sky conditions. A typical measurement set up is shown to the left where a 3 m long, 250 mm diameter solid reflector pipe is being compared with a 3 m long, 400 mm diameter flexible reflector pipe. Pipe output is measured in calibrated light boxes. Measurements of performance are obtained in terms of pipe output in lumens as a function of time, solar elevation and ambient illuminance (global and diffuse) at solar elevations from 0^o up to 85^o. As the pipes and systems are rotationally symmetric results, as a function of time of day, can be extrapolated to any location.

Results available on request to ian@solartran.com.au.

 

A further improvement is to reduce the number of redirecting facets while maintaining the self-locking function. This works because low elevation sunlight occurs, during working hours, mainly in the winter half of the year when low sunlight is towards the Equator. The result is that the 200% -300% performance increase for low elevation light is maintained while the reduction of performance at high elevation is even smaller - about a 20% reduction in  high sunlight transmittance and a 10% reduction in overcast skylight transmittance.    

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