A Predictive Thermal Model of Heat Transfer in a Fiber Optic Bundle for a Hybrid Solar Lighting System

Abstract

Hybrid lighting systems distribute natural sunlight to luminaires in office or other retail buildings in order to provide natural lighting that can impact employee productivity, morale, and even sales. In some situations, these systems may also result in a significant reduction in energy consumption by reducing both the lighting energy and the cooling load that is associated with conventional lighting systems. A key component of a hybrid lighting system is the fiber optic bundle (FOB) that transmits the light from the collector to the luminaire. The FOB consists of many small plastic optical fibers in a close-packed array. The thermal failure of these FOBs when exposed to concentrated sunlight has motivated the development of a thermal model that can be used to understand the behavior of these systems. Thermal management is necessary due to the concentrated incident solar radiation on the face of the fiber optic bundle and the low melting point temperature of the plastic optical fiber.

A predictive thermal model of heat transfer in a fiber optic bundle for a hybrid solar lighting system has been developed in order to better understand and manage the thermal loading associated with the concentrated solar radiation on the face of the FOB. Experiments were carried out on an instrumented FOB section exposed to illumination energy in a controlled environment. The experimental results provide information regarding the characteristics of the thermal loads that result from the radiation that is incident on the pores between the fibers as well as the effective, anisotropic thermal conductivity associated with the complex structure that makes up the FOB. It was found that the radiation incident on the FOB face contributed to the thermal loading in two ways: radiation incident on the face of the plastic fibers contributed a low level of volumetric generation within the FOB related to the transmission loss while ii radiation incident on the air gaps between plastic fibers contributed a volumetric generation concentrated near the face of the FOB.

The experimental results were used to specify the thermal loads and equivalent parameters required for a more detailed, multidimensional finite element model (FEM) of the FOB and its support structure. This FEM is used to understand the transient behavior of the FOB and evaluate alternative thermal management strategies.