93. REACTIVE SELF-TRACKING SOLAR CONCENTRATION
Name: Katherine A Baker
Grad Year: 2012
Concentrator photovoltaics collect incoming sunlight over a large aperture and use reflective or refractive optics to focus it to small photovoltaic cells. Minimizing the required photovoltaic material in a large panel allows cost-effective use of highly efficient but expensive multijunction cells. However, high concentration systems are necessarily limited in angular acceptance by conservation of radiance, requiring precise large-scale two-axis mechanical tracking. Previous work described the design and fabrication of a planar micro-optic concentrator, where incoming sunlight is focused through millimeter pitch lenslets onto mirrored micro-prisms, which deflect it into guided modes of a slab waveguide where it propagates toward photovoltaic cells. This enables an efficient high concentrator system with a compact geometry as well as avoiding the complexity of multiple PV cells, but with the typical CPV limitation of low angular acceptance. By incorporating a material with a nonlinear response to sunlight into the planar micro-optic solar concentrator, we can reduce or eliminate the need for mechanical tracking without losing concentration. This nonlinear material serves as a cladding layer between the waveguide and a continuous pattern of mirrored prisms. A large, slow, and localized increase in index of refraction in response to focused sunlight enables that sunlight to pass through a high-index region to the coupling features while disperse coupled light totally internally reflects off a low-index cladding. This high index region must move with the focused sunlight over the day. Optical simulations indicate that a 128x geometric concentrator designed with such a material can have 85% optical efficiency for on-axis light, and accept 83% of total annual incident energy if mounted on a low-precision one-axis tracker. To reach this performance, the index of refraction of the illuminated reactive material must increase by 0.3. One possible material is a colloidal suspension of high index particles in a low index fluid. Through dielectrophoresis, the fluid will increase particle density and average index in the presence of a localized space-charge field, which can be optically induced through a thin photovoltaic or externally biased photoconductor. Preliminary experimental measurements using aqueous polystyrene nanoparticles are promising: a 0.033 local and reversible increase in index in response to a 2V rms 60 Hz square wave signal, indicating a 2.3x increase in particle density. A similar physical response using optimized materials can plausibly meet the necessary index change. A reactive tracking approach is a promising solution for reducing mechanical tracking requirements of high concentration PV systems.