The multimode interference inside a multimode fiber is used to design a tunable filter by utilizing a moving mirror in front of the fiber facet. Theoretical model is used to produce design curves for the center wavelength, filter bandwidth, tuning range, and the scanning range of the mirror. Experimental setups were performed to verify the theoretical results. A trade-off between the mirror scanning range and filter linewidth is found. Overall performance is observed to be better for smaller sections of multimode fibers, despite the difficulties encountered in handling (cleaving and splicing.)
V-trough concentrators are usually implemented in low-concentration ratios photovoltaics for its low-cost and ease of manufacturing. The three-dimensional (3-D) configuration using four mirrors offers higher effective concentration ratios than the conventional two-dimensional v-trough concentrators. However, the skew rays hitting the photovoltaic cell in the 3-D case leads to an increase in the intensity non-uniformity and the formation of hot spots. One method to smooth incident rays on a receiver aperture is to add a diffuser in the light path such that the light incident on a diffuser is scattered in all directions following Lambertian distribution. In this work, we present a compound concentrator based on 3-D v-trough embedding a transmission diffuser to improve the illumination uniformity and reduce the hot spots. A concentrator with input and output apertures of 7.5 cm and 5 cm, respectively, and a length of 4.8 cm is proposed. Using ray tracing, the optimal position of diffuser inside the concentrator was located at a distance of 4 cm from the input aperture. The tilting angle effect on the effective concentration and the illumination non-uniformity were analyzed. The illumination nonuniformity is decreased by 33 %, while the effective concentration ratio is decreased by 17 %. The structure was realized using low-cost commercial flat mirror segments and tracing paper diffuser and was used for solar collection in a normal incidence case. The illumination profile of the cell was recorded before and after installing the diffuser showing an improvement of 41 % in non-uniformity and reduction in the effective concentration by 22 % only.
Two-dimensional V-trough concentrators are widely used for photovoltaic applications since they can be implemented using low-cost fabrication method thanks to the flat surfaces. However, there is a trade-off between the effective concentration ratio and the concentrator ability to collect and concentrate the light uniformly from wide acceptance angle. The concentrated light uniformity is important to avoid the formation of hot spots on the solar cell that reduce the cell efficiency or causing damage. In this work, we present a novel design methodology and a set of optimal designs for three-dimensional V-trough concentrators maximizing the uniformity of collected light and acceptance angle at a higher concertation ratio shown to be C2 with respect to the two-dimensional case with a concentration ratio of C. A theoretical model is presented and compared to non-sequential ray tracing showing a good agreement. The non-uniformity is calculated as the ratio between the standard deviation and the mean of the value intensity on the cell. The acceptance angle is calculated based on the edge ray principle. The analysis shows that three-dimensional V-troughs suffer from slightly higher non uniformity than two-dimensional ones for the same acceptance angle. Example of the results show that the nonuniformity for C = 1.5 is increased from 0.29 to 0.39 and for C = 2.5 is increased from 0.18 to 0.22. The proposed designs can be used for wide acceptance angle concentration and tracking-free solar cells.
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