Light Pipes: An FPS Student Fellowship Research Project

Erin Owens, Eastern Michigan University

[During the summer of 2008, Erin Owens, a physics major at Eastern Michigan University, held a FPS Student Fellowship in Physics and Society. Fellowships consist of a stipend of up to $4000 and are awarded to undergraduate or graduate students in physics in support of projects that apply physics to a societal issue. (See http://www.aps.org/units/fps/awards/student-fellowship.cfm) Erin worked with Prof. Ernie Behringer of the physics department at EMU. – Ed.]

The objective of this project was to develop an onlineaccessible calculator which would estimate the cost savings that can be realized by installing a light pipe. In order to increase the likelihood that people will adopt new technology, an easy means to examine the costs and benefits of the technology should be available so the consumer can better make an informed decision about implementation. When it becomes apparent that money can be saved by implementing a mechanism which allows the utility of the sun as a renewable source of energy, more consumers will adopt this technology.

The fossil fuel resources used to support our standard of living are finite, and consequently their continued use is not sustainable. According to the 2007 Annual Energy Review by the U.S. Department of Energy, 86.2% of total U.S. energy was generated from a combination of petroleum, natural gas, and coal¹. These same sources furnished 70% of the total electric power generated in the U.S.¹ By increasing the energy efficiency of buildings, the use of fossil fuels for energy and electricity can be significantly reduced. Developing more efficient and affordable technologies to increase the 6.8% of total energy and 9% of electricity generated using renewable energy sources¹ should also be a major objective. If we can concurrently find ways to replace fossil fuel energy and electricity sources with sustainable sources, we can further reduce our level of dependence on fossil fuels. This two-tiered approach will prolong the practical exhaustion of fossil fuel resources. Taking action before fossil fuel costs become prohibitive as their resources are depleted will help us to maintain our standard of living.

Around 8.8% of total residential electricity use, and up to 30% of total commercial electricity use goes to lighting^2,3. One sustainable way to reduce electricity consumption for lighting is to find ways to utilize sunlight. Ironically, the buildings we use to protect us from the weather also largely block this energy source, but modifications can be made to buildings that allow sunlight to illuminate interior spaces from above (daylighting). In addition to conserving energy and reducing electricity costs, daylighting has been associated with the improved performance of elementary-school students on tests⁴.

Figure 1: Light Pipe Schematic

Light pipes allow sunlight to be transmitted into a building to illuminate interior spaces. A light pipe consists of a collector, a tube, and an emitter that is usually fitted with a diffuser to improve light quality [Figure 1]. Standard light pipes are coated internally with reflective material, which allows the pipes to transmit sunlight using internal reflection⁵. This allows installation in buildings with large roof-ceiling separations that are less amenable to skylights. Integration of light pipes, especially in conjunction with artificial lights and dimmers controlled by sensors, can significantly reduce the amount of energy used for lighting⁶. However, current light pipe designs do have some disadvantages. They are less effective in cloudy weather as clouds obstruct incident sunlight from the rooftop collector⁵. Additionally, light pipes can still allow some heat transfer as well as condensation, which can increase the heating or cooling load for the space⁵.

The longer the pipe is, and the more bends that it has, the more times the light is internally reflected before reaching its intended target. Each time the light is reflected within the light pipe, the transmitted intensity decreases. One way to counter this effect is by increasing the width of the light pipe, so the light travels further down the pipe between reflections. But even with a very reflective surface, unless the sun is aligned with the axis of the light pipe to reduce the number of reflections, intensity and efficiency will be diminished. Newer, more complex light pipe designs include a “sun-tracking” feature to address this issue, allowing the collector to effectively follow the direction of incident sunlight and thus increase efficiency⁵. The drawback is that the added complexity to increase efficiency results in more expensive designs.

In order to determine the light output that can be expected from a light pipe, Jenkins and Muneer developed models both for straight pipes and for pipes with bends^7,8. They use their straight pipe model to calculate the luminous flux inside of a light pipe for a given external illuminance. This flux also depends on the transmittances of the collector, the pipe, and the diffuser as well as the cross-sectional area and aspect ratio of the pipe, and is used to predict the internal illuminance at a given point below the diffuser. This model is the basis of an online calculator I am writing that will calculate the amount of light that will be emitted into an area by a straight light pipe. I decided not to consider pipes with bends on the rationale that they have more variables than most users would be likely to know about for their particular installation.

The online calculator will use the average daily illumination data from the National Solar Radiation Database as the initial value for the external illuminance⁹. The user will be able to select the nearest weather station in his or her state from a drop-down menu in the calculator. The user can also specify the aspect ratio of the light pipe, the vertical distance that the light will be traveling in his or her specific room application, and typical time duration for light use. Using the Jenkins and Muneer model for straight pipes as described above as well as the user’s specifications, the external illuminance is used to determine a predicted internal illuminance⁷. The annual cost savings per year is then estimated by converting this internal illuminance to kilowatt-hours avoided and multiplying by an average cost per kilowatt-hour. Having a predictive tool of this nature is important so that an estimate can be made of the energy and cost savings that can be realized by installing light pipes.

As disproportionate consumers of the Earth’s finite fossil fuel energy resources, Americans must conserve their remaining energy resources and begin increasing the use of sustainable sources of energy. It is in our best interest in the long term to use sustainable energy sources to power our economy. Using light pipes to supplement artificial lighting of our interior spaces is one way that we can begin to reduce our dependence on fossil fuel energy resources.

References

1. “Energy Information Administration (EIA) - Annual Energy Review.” 23 June 2008. Energy Information Administration - EIA - Official Energy Statistics from the U.S. Government. 26 Feb. 2009 <http://www.eia.doe.gov/emeu/aer/pecss_diagram.html>

2. “U.S. Household Electricity Uses: A/C, Heating, Appliances.” 14 July 2005. Energy Information Administration - EIA - Official Energy Statistics from the U.S. Government. 26 Feb. 2009 <http://www.eia.doe.gov/emeu/reps/enduse/er01_us.html>.

3. G. Oakley, S. B. Riffat, and L. Shao. “Daylight performance of lightpipes.” Solar Energy, 69(2), 89-98 (2000).

4. Lisa Heschong, Roger L. Wright, and Stacia Okura. “Daylighting impacts on human performance in school.” Journal of the Illuminating Engineering Society, 31(2), p. 101 (2002).

5. “Daylighting: Skylights and Light Pipes.” Oikos: Green Building Source Energy Efficient Construction Environmentally Responsible Building Green Building Materials. 9 April 2007. <http://oikos.com/library/eem/skylights/>.

6. Leon R. Glicksman. “Energy Efficiency in the Built Environment.” Physics Today, 61(7), p. 35 (2008), 2008.

7. David Jenkins and Tariq Muneer. “Modelling light pipe performances – a natural daylighting solution.” Building and Environment, 38(7), 965-972 (2003) .

8. David Jenkins, Tariq Muneer, and Jorge Kubie. “A design tool for predicting the performances of light pipes.” Energy and Buildings, Issue 5, May 2005, Pages 485-492.

9. “NSRDB: 1961 - 1990.” Renewable Resource Data Center (RReDC) Home Page. <http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/>.

This contribution has not been peer refereed. It represents solely the view(s) of the author(s) and not necessarily the views of APS.