This Small Business Innovation Research (SBIR) Phase I project investigates a novel optical element (OE) that uses liquid crystal (LC) in thin panels to steer sunlight. Any technology using focused sunlight must account for the motion of the sun. For decades this has been accomplished by physically moving the focusing optics and/or the target (a photovoltaic cell, for instance) so that the system is always aligned with the sun. This project develops a simple, motion-free tracking system where the refractive index of the LC is continuously varied with a low-voltage signal to keep the focused sunlight on the target as the sun moves throughout the day. The technology is applicable to any concentrated solar application. The project goals are to optimize design elements of the OE with respect to materials configuration and manufacturing technique, and to build prototypes for lab and field-testing. Phase 1 will address three interconnected design issues. These are 1) maximizing solar throughput of the device by eliminating unwanted reflections from various interfaces, 2) maximizing the acceptance range of solar incidence angles, and 3) lowering the cost of the finished device for commercialization.
The broader impact/commercial potential of this project will be its effect on the economics of the energy sector, national energy security, and global climate change. The unique beam steering technology developed in this project will make on-site energy production more economically viable, which will have the immediate effect of reducing the amount of power drawn from the grid. This will lower the demands on centralized power plants, the vast majority of which produce electricity by burning fossil fuels, and on the aging transmission line infrastructure. Reducing the amount of electricity consumed in the U.S. will improve national energy security not only by strengthening the economy, but also by reducing the amount of fossil fuel the U.S. must purchase offshore. Reducing the amount of fossil fuel burned to make electricity will reduce the carbon footprint of the U.S., further mitigating the effect of global climate change.
Direct use of solar energy to light interior space is far more efficient than generating electricity from sunlight using photovoltaic panels and then turning the electricity back into light with LEDs or fluorescent bulbs. This project develops an Optoelectronic Solar Tracking Device that is a simple, motion-free tracking concentrator that eliminates all the negative aspects of current electro-mechanical trackers and can be easily coupled to fiber-optic daylighting to provide 100% of the building’s lighting requirements between 8 AM and 4 PM. In this Phase I program we completed tasks for the manufacturing of an optoelectronic solar tracking device. We tested several methods of producing a transparent conductive film with a microprism microstructure, a key component of the device. We have had varying success with these methods, and we have determined several criteria for the microprism structure that facilitate its use in our steering device. We have also identified and tested several Liquid Crystal (LC) materials that are more suited to our device than those currently used for standard displays, resulting in better performance and reduced manufacturing cost. We have modeled the design of secondary optical elements to further extend the usable range of solar trackability. With regard to these tasks, we have established the following: 1. The embossing process yields a smooth surface on the scale of the desired microprism geometry, but near-surface polymer chain alignment interferes with the desired liquid crystal alignment. Furthermore, our experience with embossing thus far is that it disrupts the electrical continuity of the pre-coated surface. 2. Casting microprisms via a liquid-resin vehicle onto carbon nanotube (CNT)-coated film has significant adhesion problems associated with it because of the surface properties of the proprietary CNT topcoat used by the CNT manufacturer. 3. Coating the non-conductive microprism surface with a conductive material remains the best alternative. We have identified an application method that does not affect the microprism geometry and that leaves a suitable surface for liquid crystal alignment. 4. We have identified a family of LCs that are inexpensive and offer improved steerer performance due to their large anisotropy and their close index match to acrylic. 5. We have designed custom secondary optical components that double the usable light collection time over the course of a day. 6. We have explored the use of half-wave plates for controlling the polarization of light within the device.
