Optical Characterization of Nonimaging Focusing

HeliostatFirst version published:NON-IMAGING, FOCUSING HELIOSTATSolar Energy, Vol. 71, No. 3, pp. 155–164, 2001Faculty of Electrical Engineering, University of Technology Malaysia, Malaysia

Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Malaysia

Irfan ullahDepartment of information and communication engineeringMyongji 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 m2

• 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

HZ : 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 heliostatLocal coordinate system

Two dimensional array of mirrors

masterslave

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..

Hx : perpendicular distance between center of heliostat and center line of row where mirror is locatedHy : 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

• 8.11x109 rays are traced

• Image size, w, produced by square mirror with curvature, r,

and focal length (L) = r/2D : mirror dimension θ : incident angle β : angle subtanded by sun

To reduce astigmatism1)Use small mirror2)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 1.28.

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 80o, γ is smaller than 7o.

Results cont’d..

Specifications used in the simulation of solar flux distribution

• Focal length = 25 m

• Reflective area = 25 m2

• Size should be bigger than 23.25 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 imagearea (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

Thanks ?