System testing for the fresnel lens-based optical

System Testing for the Fresnel-lens-based Optical Concentrator for
Photovoltaic (CPV) Solar Energy Harvesting
A. Beltran-Gonzalez1
, G. Garcia-Torales1
, M. Strojnik1,2
, J. Milton-Garduno3
, G. Veroone3
1
Departamento de Electronica, Universidad de Guadalajara, Av. Revolucion 1500,
C. P. 44840 Guadalajara, Jalisco, Mexico
garcia.torales@academicos.udg.mx, anuar_beltran@hotmail.com, mstrojnik@gmail.com
3
MIXBAAL, Anillo Periférico Sur, Manuel Gomez Morín 7980, Interior 2-e,
Santa María Tequepexpan, Tlaquepaque, Jalisco, Mexico
ABSTRACT
We designed, developed, fabricated, and tested an opto-electronic system to test alignment of CPV solar system
modules that is portable and robust to implement as a step in the assembly line. In addition to the components used
in systems employed previously, we implement a thin prism in four orientations in a plane normal to optical axis of
the unit under test. Its advantage is robustness against its positioning and orientation errors.
Key terms: renewable energy, solar concentrators, CVP, Fresnel lens, homogenizer prism, alignment, testing
1. INTRODUCTION
This paper describes an improved and simple method for the alignment of the primary and secondary optics of a
CPV (concentrator for a photovoltaic energy generation), based on the Fresnel lens. Such a module is illustrated
schematically in Fig. 1. The alignment of the secondary (prism) to the primary optics is performed during assembly.
The novel alignment method can be (and has been)
installed for the use in the manufacturing process. It
may easily be automated for testing alignment during
the assembly process. Thus, a module may be tested
and its performance demonstrated during an
automatic quality control step during the fabrication
in the assembly line.
The map of the worldwide solar irradiation, shown in
Fig. 2, illustrates that Mexico and the southwester
USA receive highest amounts of radiation. Direct
solar spectrum G173; spectral response of a triple-
junction device, weighted by its external quantum
efficiency (EQE); and spectral transmission of the
PMMA (the Fresnel lens) are shown as a function of
wavelength in Fig. 3.
The sensitivity of the MJ solar cell, incorporating
three cells along vertical direction (shown in Fig. 4),
is between 300 nm to 1800 nm.
Fig. 1. One unit of a solar power generator,
incorporating a single photocell, a radiation
homogenizing element above it, and a positive
optical lens (Fresnel lens here) to concentrate the
radiation. It is concentrator photovoltaic (CPV).
_________________
2
Centro de Investigaciones en Optica, AP 1-948, CP 37000, Leon, Gto., Mexico
Optical Measurement Systems for Industrial Inspection X, edited by Peter Lehmann, Wolfgang Osten,
Armando Albertazzi Gonçalves Jr., Proc. of SPIE Vol. 10329, 103294P · © 2017 SPIE
CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2272512
Proc. of SPIE Vol. 10329 103294P-1
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/03/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

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Spectral transmissivity PMMA
1 .
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- 0.8 E
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e
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' 0
400 600 800 1000 1200 1400 1600
Wavelength, nm
Fig. 2. Map of worldwide solar radiation. Direct normal irradiance (DNI) for the whole Earth in cartographic
coordinates. The lighter color indicates the increasing amount of radiation. [http://www.dlr.de/tt/Portaldata
/41/Resources/dokumente/institut/system/projects/reaccess/]. Map created and map layout by DLR 2008 [http://
www.dlr.de]. Data based on NASA SSE 6.0 data set for a 22-year period (July 1983 – June 2005) [http://
eosweb.larc. nasa.gov/sse/].
Fig. 3. Direct solar spectrum G173; spectral response of a triple-junction device weighted by its external quantum
efficiency (EQE); and spectral transmission of the PMMA, the material often used for the Fresnel lens as a function
of wavelength. Because the optical system as a minimum includes two components, with two surfaces each, the
transmission of optical system is about 80 % (82 %). This value deteriorates in location because of dust
accumulation and aging. © Springer International Publishing Switzerland 2015, P. Pérez-Higueras and E.F.
Fernández (eds.), High Concentrator Photovoltaics, Green Energy and Technology, DOI 10.1007/978-3-319-15039-
0_1.(From 11).

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400 600 8(0 1000
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1200 1400 1600 1800
(nm)
top cell
middle cell
contact
window la er
top cell
BSF
tunnel junction
window la er
middle cell
BSF
tunnel junction
buffer
bottom cell
contact
Fig. 4. Three cells form the MJ solar cell, laid along vertical direction, corresponding to three principal regions of
solar irradiation. Their spectral sensitivity ranges from 300 nm – 1800 nm. The radiation of increasing wavelength
passes through the top layers. A potential candidate for the solar cell might be Q4 2013, made of four compounds,
GaInP/GaAs/GaInAsP/GaInAs/. A similar cell may be acquired from the Fraunhofer institute. It is designed for solar
concentration of about 300 (297) and features a nearly 50% efficiency (47.7 %). © Springer International Publishing
Switzerland 2015, P. Pérez-Higueras and E.F. Fernández (eds.), High Concentrator Photovoltaics, Green Energy and
Technology, DOI 10.1007/978-3-319-15039-0_1.(From 11).
2. THEORY
We using the experimental arrangement showed in Figure 6a. We assume that camera is tilted with resect to the
opto-mechanical system of the CPV unit. It is possible to obtain the difference between the center of the entrance
pupil of the camera in the first surface, D, and the center of the Fresnel lens, c2. We designate it ∆1.
(1)
Here, the distances DC, Dc2 and Ec1 are vectors and u
is another vector parallel to the plane not showed.
(2)
(3)
(4)
We employ a thin prism, with parameters illustrated
in Fig. 5, that may be rotated about some axis, but it
itself does not need to be precisely aligned.
Fig. 5. A wedge prism has a small apex angle α to
deviate the ray incident normal to its first surface (red
arrows).

,L=24cm
m
.L=24cm,
rm
CO
L=24cm
Now we have two unknown quantities, the displacement ∆ and angle A1. L1 is the distance between the camera and
the plane of the Fresnel lens. L2 is the distance between the Fresnel lens and the first plane of the refractive prism. L3
is the distance between the Fresnel lens and the surface of the photovoltaic cell.
Other quantities of interest are denoted in Fig. 6. They are: c1 is the center of the first plane of the refractive prism;
c2 is the center of the Fresnel lens; B is the center of the entrance pupil of the camera; A1 is the angle between the
camera axis and the perpendicular reference axis; C is the point of intersection of the reference axis and the center of
the Fresnel lens; D is the point of intersection of the camera axis and the center of the Fresnel lens; E is the point of
intersection of the camera axis and the first surface of the refractive prism; and L is the distance between the Fresnel
lens and the first plane of the refractive prism.
Next, we consider the special case that the camera axis is pointing to the center of the Fresnel lens (see Fig. 6b). We
place a wedge prism between the pupil of the camera and the Fresnel lens. We have a new angle A2 that is now
equal to A1+a. a is the deviation angle introduced by the wedge prism. Then Eq. 1 is modified.
(5)
(a) Measurement setup (b) Misalignment scenario 1 (c) Misalignment scenario 2
Fig. 6.(a) Schematic diagram of a single module under test. The testing system of the solar concentrator module
consists in the image acquisition system, the Fresnel lens, the refractive prism, and the solar cell with the isolated
electrodes and the heat spreader. The diagrams (b) and (c) may be used for the determination of the centers of the
Fresnel lens and the homogenizer prism, respectively. (b) The deviation of the introduction of a wedge prism is
considered. (c) The deviation upon the introduction of a wedge prism rotated by180o
. We also show the system
parameters. Abbreviations for the symbols used in the proposed testing system are: c1 : Center of the first plane of
the refractive prism; c2: Center of the Fresnel lens; B: Center of the entrance pupil of the camera; A1: Angle between
the camera axis and the perpendicular reference axis; C: Intersection point of the reference axis and the center of the
Fresnel lens; D: Intersection point camera axis and the center of the Fresnel lens; a E: Intersection point between
the camera axis an de the first plane of the refractive prism; and L: Distance between the Fresnel lens and the first
plane of the refractive prism.

x xi
i i
X x x
Now we have a system of equations with three unknowns, ∆1, A1 and a. If we rotate the wedge prism by 180° the
new angle will be A3= A1-a, as illustrated in Fig. 6c. Now we have a system of three equations with three
unknowns: this system can indeed be solved. With this method, we also eliminated the need for the calibration of the
camera position and orientation. We only require a rough alignment at the beginning; then all the differences
between centers may be calculated.
3. SIMULATIONS
Figure 7a illustrates what happens with two rays, one denoted o and the other x, potentially corresponding to the axis
of the Fresnel lens and the top surface of the homogenizer prism. As we saw in Fig. 5, the ray each time deviates by
a small angle in the direction of bigger thickness of the thin prism. In the case of a well-aligned prism, the separation
of ray o and ray x remains the same upon passing through the prism. When the prism is rotated by 180o
, the rays are
displaced by the same amount, while their relative separations from each other remain unchanged.
Figure 7b illustrates what happens with two rays, one denoted o and the other x potentially corresponding to the axis
of the Fresnel lens and the top surface of the homogenizer prism, the same as in Fig. 7a when the prisms are tilted
and displaced transversally. The ray each time deviates by a small angle in the direction of bigger thickness of the
thin prism, independently of the prism misalignment in position or in angle. In the case of a well-aligned prism, the
separation of ray o and ray x remains the same upon passing through the prism. When the prism is rotated by 180o
degrees, the rays are displaced by the same amount, while their relative separations from each other remain
unchanged. Thus, the use of thin prism is insensitive to the prism alignment errors in either the position or in angle.
(a) (b)
Fig. 7. The effect of introducing the wedge prism on two rays. In part (a), the wedge prisms are perfectly positioned
in space and aligned in angle. In part (b), the wedge prisms are imperfectly positioned in space and misaligned in
angle. The robustness of the method is shown for two angles, one by 180o
larger than the other. When the first angle
is zero (90 o
), and the second one is 180 o
(270 o
), the ray separations remain the same, and they are displaced toward
the thick part of the prism. Two orthogonal deviations with the same results are obtained with angle values in
parenthesis. The position of the center is displaced laterally compensating for any potential lateral errors.
In Figures 8 and 9 we present some interesting results. We performed simulations of the displacement of the first
surface of the homogenizer prism with respect to the center of the Fresnel lens. We calculate of amount of
displacement between the respective centers, after each of the four rotation increments of the wedge prism by about
90o
. The origin of coordinates is set at the center of the Fresnel lens. These simulations explain how possible errors
in the wedge prism position and orientation have no effects on the alignment measurement. They are compensated
when the prisms are placed in two orthogonal orientations. The compensation is accomplished even when the center
of the wedge prism has lateral displacement and angular misalignment with respect to the first surface of the
homogenizer prism.

o
V

O.
T
u
41F,

t
//e
////ll/If
"///
0
r
///II
(a) If the system is aligned, four magnitudes of
displacement that correspond to each increment in
rotation angle are identical.
(b) The differences in displacement maintain their
magnitudes and directions, even when the 90°
rotations of the wedge prism are not set accurately.
We perform measurements along two the orthogonal
direction (0, 90, 180, and 270 degrees). Measurement
sets at 0 and 180 degrees are orthogonal to those at
90 and 270 degrees.
(c) Small separations [see distance x-(0,0)] between
axes may be detected near the axis.
(d) Large separations [see distance x-(0,0)] between
axes may be detected with this method.
Fig. 8. We performed simulations of the displacement of the first surface of the homogenizer prism with respect to
the center of the Fresnel lens. We calculate of amount of displacement between the respective centers, after each of
the four rotation increments of the wedge prism by about 90o
. The origin of coordinates is set at the center of the
Fresnel lens.

(a) The system is not aligned. The four magnitudes of
displacement are compensated even the tilt in the
wedge prism. The magnitudes of vectors A, B, C, and
D are outside the circle within which the
displacements are permitted.
(b) Because of the tilt in the wedge prism, only
magnitudes of vectors A and C are outside the circle
within which the displacement are permitted.
Fig. 9. We simulated the displacement of the first surface of the homogenizer prism with respect to the center of the
Fresnel lens. We calculate of amount of displacement between the respective centers, after each of the four rotation
increments of the wedge prism by about 90o
. The origin of coordinates is set at the center of the Fresnel lens. This is
presented in parts (a) and (b).
4. EXPERIMENTAL SETUP
Figure 10 illustrates the fixed experimental setup with a CPV module inserted, designed for the placement into the
production line. We are not showing the camera output into the PC on which the algorithm and software has been
implemented, or the display. The four rotation angles of the wedge prism are accomplished with blue round arrow,
while the red arrow indicates how the prism is inserted into the optical path. The dimensions are shown to illustrate
that the optical system under test is nearly f/1.
In the upper part of the test module, a camera is positioned approximately on the axis of the Fresnel lens, at a
distance that ensures acquisition of the lens surface and the two surfaces of the homogenizer prism. The camera
captures as a reference the Fresnel lens and the associated software calculates the center of the lens, before the
wedge prism is inserted. The wedge prism is mounted on a mechanical arm that brings the prism into the field of
view of the camera. This arm also rotates the prism four times by 90 degrees. BK7 glass is used as a prism material;
its angle of deviation θd = 2°, and the prism apex angle is θw = 3° 52 ̕.

Camera --t
Fresnel
Wedge
8 Cm prism
Lens
24,48 cm
Homogenizer prism
29,48 cm
5 cm I
L__J
N
l'
1
Fig. 10. Experimental arrangement to test the
alignment of a single solar concentrator unit.
Dimensions and positions of the elements that
constitute the inspection system are indicated. Each
CVP module has a Fresnel lens, a prism
homogenizer, and a MJ photovoltaic cell, shown in
Fig. 1. The Fresnel lens has an area with dimensions
of 25cm by 27cm, a focal length of 24.5 cm and
provides a demagnification. The area on first
(second) surface of the homogenizer prism is 2 cm by
2 cm (1 cm by 1 cm). Its height is 5 cm. The MJ solar
cell is a triple cell (see Fig. 4) glued onto the bottom
at second surface of the prism. The testing fixture
and the associated measurement instrumentation is
built in such a way that each unit may be easily
inserted and taken out of the apparatus, as a part of
the fabrication assembly line. The test apparatus is
enclosed inside a case of anodized aluminum.
(a) Simulated images. (b) Experimental images.
Fig. 11. The images of the Fresnel lens (those in the top are captured without the wedge prism) and two surfaces of
the homogenizer prism: after four 90° rotation increments of the wedge prism. The square indicates images of the
upper (larger) and lower (smaller) surfaces of the homogenizer prism, further explained in Fig. 14. When both prism
surfaces are aligned with the Fresnel lens, squares have the same axis, i.e., the separation between their edges is the
same for all four sides, and they are centered on the coordinate origin.

DiNer ente-of -center si
X-0.01 inm1
V-0.254mm$
RESULTS:
Differences surface 1
X= 1.17 mm ; Y= 0.96 mm
Differences surface 2
X= 1.17 mm ; Y= 0.96 mm
Angular difference
AX= 0.099; AY= 0.096
F
MDL
i
/
/F-wnz-I
T
LLDZ
1
5. RESULTS
Figure 11 presents a sequence of images to determine the displacements in the centers of optical surfaces in the CPV
module. Experimental results in graphic form and as a numeric printout obtained with the inspection system are
shown in Figs. 12 and 13. The squares in the images are further explained with the assistance of Fig. 14.
(a) Difference between the Fresnel lens center
(the reference) and the camera.
(b) Image with the lines that indicate the center of the
first and the second surface of the homogenizer
prism, with respect to that of the Fresnel lens.
Fig. 12. Experimental results in graphic form and as a numeric printout obtained with the inspection system.
Fig. 13. Example of the resulting
printed values obtained in the
case of Fig. 12. Note that the
angular error in the prism
orientation is also calculated.
The module may be accepted
upon a single glance.
Fig. 14. The top part shows the
dimensions of the upper and lower
surface of the homogenizer prism.
When the prism is aligned with the
Fresnel lens, both squares have the
same axis, i.e., the separation
between their edges is the same for
all four sides. In order to capture
images of both surfaces of the
homogenizer prism simultaneously,
we use the MJ cell as a transducer.
We apply the voltage to its
electrodes making the solar cell
glow. This way it functions as a
radiation source, illuminating both
surfaces of the homogenizer prism,
captured here photographically.

6. CONCLUSIONS
We designed, developed, fabricated, and tested an opto-electronic system to test alignment of CPV solar system
modules that is portable and robust to implement as a step in the assembly line. In addition to the components used
in systems employed previously, we implement a thin prism in four orientations in a plane normal to optical axis of
the unit under test. Its advantage is robustness against its positioning and orientation errors.
REFERENCES
[1] Juan C. Miñano, Pablo Benítez, Pablo Zamora, Marina Buljan, Rubén Mohedano, and Asunción Santamaría,
"Free-form optics for Fresnel-lens-based photovoltaic concentrators," Opt. Express 21, 494-A502 (2013).
[2] Fabian Duerr ; Hugo Thienpont, "Freeform optical design of an XY-zoom beam expander," Proc. SPIE 9889,
98890Y (2016).
[3] C.-F. Chen Chih-Hao Lin, Huang-Tzung Jan, Yun-Ling Yang, "Design of a solar concentrator combining
paraboloidal and hyperbolic mirrors using raytracing method," Optics Communications 282(3), 360–366
(2009).
[4] K.K. Chong F.L. Siaw, C.W. Wong, G.S. Wong., "Design and construction of non-imaging planar concentrator
for concentrator photovoltaics system," Renewable Energy 34, 1364–1370 (2009).
[5] Gonzalo Paez , Marija Strojnik, Jaime Sandoval Gonzalez, Jesus Castrellon-Uribe, P. Vacas-Jaques, Guillermo
Garcia-Torales, "Prism system to control wavefront tilt and position in vectorial shearing interferometer,"
Proc. SPIE 4369, San Diego, 680 (2001).
[6] G. Garcia-Torales, M. Strojnik, and G. Paez, "Risley prisms to control wave-front tilt and displacement in a
vectorial shearing interferometer," Appl. Opt. 41, 1380-1384 (2002).
[7] M. Strojnik, G. G. Torales, G. Paez, "Vectorial shearing interferometer," Proc. SPIE 3744, 529-539 (1999).
[8] J. Sandoval, G. Paez, M. Strojnik, "Opto-mechanical design of a prism rotator," Proc. SPIE 4486, 170-180
(2001).
[9] G. García-Torales, G. Paez, M. Strojnik, "Simulations and experimental results with a vectorial shearing
interferometer," Opt. Eng., 40 (5), 767-773 (2001).
[10] M. Strojnik, G. García-Torales, "Vectorial shearing interferometer," Appl. Opt., 39 (28), 5172-5178 (2000).
[11] P. Pérez-Higueras and E.F. Fernández (eds.),[ High Concentrator Photovoltaics, Green Energy and
Technology], Springer International Publishing, Switzerland, 13 and 27 (2015)
[12] DLR," SOLAR-JET," DLR, 29 April 2014,
http://www.dlr.de/tt/Portaldata/41/Resources/dokumente/institut/system/projects/reaccess/ (April 15 2017).
[13] Paul W. Stackhouse, Jr. Ph.D., "Surface meteorology and Solar Energy," NASA Langley, 12 April 2016,
http://eosweb.larc.nasa.gov/sse/(April 17 2017).

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