Optical Characterization of Nonimaging Focusing Heliostat First version published: NON-IMAGING, FOCUSING HELIOSTAT Solar Energy, Vol. 71, No. 3, pp. 155–164, 2001 Faculty of Electrical Engineering, University of Technology Malaysia, Malaysia Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Malaysia Irfan ullah Department of information and communication engineering Myongji university, yongin, south korea Copyright © solarlits.com

Contents Introduction Methodology Sun-tracking Design of heliostat Results Conclusions

Objective Desiging a nonimaging focusing heliostat Reflective area of heliostat: 25 m 2 Slant range: 25 m Mirrors: 1×1, 9×9, 11×11, 13×13, 15×15, 17×17 and 19×19 achieving high concentration Nonimaging Heliostat : “Energy of beams without a specific optical image” Primary tracking Heliostat

Introduction Heliostat must focus sunlight to central receiver Traditional heliostats have astigmatic aberration To remove astigmatic aberration Use small independent moveable mirrors Ideal case: (2 x m x n) motors for (m+n) mirrors NIFS: (m + n - 2) motors Latest: Only three motors New approach with same reflective surface area 1×1, 9×9, 11×11, 13×13, 15×15, 17×17 and 19×19 arrays of concave mirrors Astigmatic aberration

Method Two functions simultaneously Primary tracking (sun-tracking) Secondary tracking (focusing sunlight toward receiver) L : Horizontal distance H Z : offset distance of the reflector from the plane θ : Incident angle δ : Declination angle Φ : latitude ω : hour angle φ : facing angle of heliostat λ : Target angle of heliostat Local coordinate system Two dimensional array of mirrors master slave

Method cont’d.. Coordinate system attached to the local heliostat reference frame. Direction cosines of OS Coordinate system attached to earth reference frame. Three rotation transformations Direction cosines of CS

Method cont’d.. Relates the sun’s position in the heliostat-based coordinate system and earth-based coordinate system Primary tracking

Method cont’d.. H x : perpendicular distance between center of heliostat and center line of row where mirror is located H y : perpendicular distance between center of heliostat and center line of column where mirror is located Two dimensional array of mirrors Mirrors’ tilted angles required in local movement 8.11x10 9 rays are traced Image size, w, produced by square mirror with curvature, r, and focal length (L) = r/2 D : mirror dimension θ : incident angle β : angle subtanded by sun To reduce astigmatism 1)Use small mirror 2)Effective limit for size of mirror

Results The angular movement σ versus the incidence angle θ of the sun for L=540 m and different value of H as 0.4 m, 0.8 m, 1.2 m, and 1.6 m. In the graph, σ is rather small with maximum value less than The angular movement γ versus the incidence angle of the sun for L=40 m and different value of H as 0.4 m, 0.8 m, 1.2 m, and 1.6 m. For θ less than 80 o, γ is smaller than 7 o.

Results cont’d.. Specifications used in the simulation of solar flux distribution Focal length = 25 m Reflective area = 25 m 2 Size should be bigger than cm Used size : 26.5 x 26.5 cm 19x19 arrays

Results cont’d.. For 19×19 array (each with the size of 26.5 cm × 26.5 cm and the focal length of 25 m) provided that θpreset = 60° and L = 25 m. For 1×1 array, focal length of 25 m and target distance of 25 m.

Results cont’d.. Maximum solar concentration ratio versus incident angles Ratio of aberrated-to-ideal image area versus incident angles Maximum solar concentration (solid line) and its corresponding ratio of aberrated-to-ideal image area (dot line) versus incident angles For 19 × 19 array Aberrant image / Ideal sun image

Results cont’d.. Spillage loss versus receiver size for the case of 19 × 19 array Intercept efficiency versus average concentration for the case of 19 × 19 array Spillage loss is defined as “the percentage of solar irradiation falling beyond the boundary of the square receiver”.

Conclusions High precision sun-tracking Achieved highest solar concentration With incident angles from 0 to 70 degrees Can be used for single stage large solar furnace Receiver size and reflector size plays an important role in concentration

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