Designing solar collectors and optical fibers for daylighting
Within the framework of research on renewable energies, since 1997 the Solar Collectors Laboratory (SCLab) of the National Institute of Applied Optics (INOA) in Florence, Italy, has been studying optical systems that can concentrate sunlight on small surfaces. The work incorporates three lines of research: the first relates to optical systems for concentrating and transportating sunlight using optical fibers; the second concerns sunlight collectors that concentrate light onto photovoltaic (PV) cells; and the third involves optical systems for concentrating solar light coupled to various devices. Possible applications of these devices include heating, interior illumination of buildings, and PVs.1
For the Sunflowers project, the SCLab developed a solar collector made of plastic. In particular, the collector was designed for serial production that would allow a considerable reduction of costs. The collector is an aspheric lens made of polymethylmethacrylate (PMMA). The lens is narrow—only 14.9mm wide—and lightweight, weighing only 24g. We analyzed the characteristics and performance of the collector for interior illumination during the development of the Sunflowers project.2,3
We designed and built suitable mechanical systems to support and move the optical system. The equatorial structure includes two motors, one for the temporal axis and one for the declination axis. Optical head movement is performed by a double guiding system. The first part of the system provides the preliminary orientation. It is a passive system and drives the motors to correctly orient the head every day of the year. The second part enables fine positioning and adjustments using an optical sun pointer.
A demonstration of the Sunflowers system has been installed and tested to provide illumination of a museum room. The fundamental elements of this plant are solar collectors, optical fibers, and mechanical and electronic systems for sun tracking. Each collector is coupled to a fiber bundle made with a fusion procedure that does not introduce glue or any external material.
Museum lighting calls for an alternative illumination source that can be used in case sunlight is absent. This has been realized employing novel LEDs with very low supplying levels. But several fundamental requirements must be met. First, illuminance levels are dictated by the exhibit items. Moreover, the LED light must replicate light from the sun. Finally, the quality of the light (including uniformity and color) is important.
We examined light quality and color balance using photometric and colorimetric measurements to define the suitable filters for plastic fiber and LED light. The lighting color is yellow-orange, in agreement with the museum experts' recommendations. Additional tests have been carried out on lens production to evaluate collection efficiency and image size, examine optical treatments, and experiment with the effects of external agents and UV exposure.
The Sunflowers project developed a daylighting system from design to production, installation, and testing in working conditions. The Sunflower system exploits sunlight, using optical collectors and fibers, to illuminate building interiors. Wenamed the project Sunflowers for the ability of the solar collectors to track the position of the sun during the day. EverySunflower contains several solar collectors, each of which is coupled to an optical fiber. The Sunflower includes mechanical systems and electrical accessories for solar tracking.
The light focused by the solar collector can be used either for direct illumination or to accumulate power for lighting during the sun's absence. The first function has been explained. The second consists in focusing the solar light on a PV cell, which converts the light into electrical energy for illumination when the sun is not present.
Franco Francini received his degree in electronics engineering from Bologna University, Italy, in 1974. He is research director at the Istituto Nazionale di Ottica Applicata, CNR, and professor of optical metrology at the University of Florence. At present, he directs the Laboratory of Photometry and Lighting. His current research is devoted to developing optical measurement techniques applied to systems for industrial automation, and quality and process control.
