Thermal Management of the Polymethylmethacrylate (PMMA) Core Optical Fiber for Use in Hybrid Solar Lighting

This work is part of a team effort to develop a hybrid solar lighting (HSL) system that transports day light from a paraboloidal dish concentrator to a luminaire via a large core polymer fiber optic. For example, the luminaire can be a device to distribute sunlight into a space or it can be a device that is a combination of day lighting and florescent lighting. In this project, the sunlight is collected using a one-meter paraboloidal concentrator dish with two-axis tracking. The secondary mirror consists of eight planar-segmented mirrors that direct the visible part of the spectrum to eight fibers (receiver) and subsequently to eight luminaires. This results in about 8,200 lumens incident at each fiber tip. This paper is concerned with predicting the radial and axial temperature distribution in the entrance region of the fiber in light transmission. Thermal management is especially important because when PMA-core optical fibers are used in light transmitting systems, the core of the optical fiber is prone to thermal degradation due to the heating effect of the infrared spectrum. Thermal degradation of the fiber core can cause fiber aging, reduce the life-time of the fiber from years to months, increase the attenuation in the short visible region (in the 400 ∼ 470 nm) and can soften the PMA core making it susceptable to damage. In order to ensure the thermal degradation free operation of plastic optical fibers, an economical and viable solution has to be found. The solution should keep the light loss at minimum at the expense of its implementation. Several filtering techniques have been investigated to minimize the effect of IR portion of the solar spectrum at the fiber entrance tip surface as well as the whole-spectrum absorption of solar energy inside the fiber. According to the analysis results, the use of fused quartz glass attachment was proven to be cheaper, more cost effective and feasible among other proposed solutions. Depending on the mirror/filter specifications, the first 8 mm of the fiber was found to be critical based on 5 m/s air speed across the lateral surface of fiber. Modes of heat transfer taken into account included thermal conduction, convection and radiation on the first 5 cm of the polymethylmethacrylate (PMMA) core and Teflon-FEP cladding fiber. A case-basis comparison was made with the experimental result.