A solar lighting system based on a design developed by Oak Ridge National Laboratory is currently being built at Utah State University. The system has several important attributes that make it unique: it delivers sunlight directly instead of converting it into electricity and then into artificial light; it captures direct rather than diffuse sunlight, which means it can deliver more light than traditional systems; and it delivers full-spectrum sunlight, which can benefit both plant and human life. The system could be of even greater value if it were fully independent, generating enough electrical power to run its solar tracking without drawing electricity from the host building's grid.
The lighting system consists of a two-axis solar tracking system, a 1m parabolic mirror to concentrate sunlight, an elliptical secondary mirror to filter the infrared (IR) and re-direct the visible, and a fiber-optic bundle for transmission (see Figure 1). An infrared-photovoltaic (IR-PV) array placed behind the elliptical mirror could be used to generate enough electrical power to run the solar tracking system. However, the IR and visible light concentrated by the primary and secondary optics are highly symmetric and not uniform at or near the focal plane. This means that some cells in the IR-PV array or fibers in the fiber-optic bundle would receive more energy than others, causing the array to output only minimal power.
Figure 1. The solar lighting system concentrates rays directed by primary and secondary mirrors to full-spectrum output through a fiber optic bundle. NI: Non-imaging. IR: Infrared
To evenly distribute the energy and maximize power output, optical models of the system were built using TracePro®, a geometrical raytracing program used for optical systems analysis. Based on work done by O'Gallagher, et al.,1 non-imaging (NI) devices were investigated for use in energy distribution. Different NI device configurations were tested by varying their shape and length.2–4
The IR-PV array was not exactly square in shape, so rectangular, octagonal, silhouette, and round NI tubes were investigated. The results showed that a rectangular (almost square) NI tube produced the best balance between flux uniformity and total energy delivered to the array by breaking up the rotational symmetry of the concentrated flux (see Figure 2). A prototype was built and tested with positive results (see Figure 3). Temperature measurements across the array showed the flux was uniform, and the array was able to generate more than enough power to run the tracking system.
Figure 2. IR rays enter the NI tube, reflect off the sides as they travel down it, then strike the surface of one cell in the IR-PV array. (The secondary mirror and other rays have been removed for clarity.) IR-PV: Infrared photovoltaic.
Figure 3. A reflective NI tube and IR-PV array are configured for testing.
A refractive NI device has also been designed for use with the fiber optic bundle. It transmits the visible light reflected by the secondary mirror while dispersing it evenly over the bundle's entrance region, ensuring that each fiber will receive an equal amount of light. Optical tests were performed to compare round- and square-shaped NI tubes under various optical-system-misalignment conditions. The square NI tube was shown to be more robust and efficient at spreading the light over the fiber bundle entrance. These tests identified an optimal length of NI tube for this particular configuration: one made of quartz NI tube has been ordered and will be tested this summer.
Solar energy concentrated by a parabolic mirror is not suitable for directly illuminating an IR-PV array or a fiber-optic bundle, since the concentrated energy is not uniform across a plane at the focal zone. NI devices have been shown to be effective optics for uniformly distributing the concentrated flux over a plane. Future work will involve investigating different materials for construction of the NI tube, as well as possibly looking at different configurations for use with the fiber optic bundle.