A curve Fresnel lens is developed as secondary concentrator for solar parabolic troughs to reduce the number of photovoltaic cells. Specific measurements and optical tests are used to evaluate the optical features of manufactured samples. The cylindrical Fresnel lens transforms the focal line, produced by the primary mirror, into a series of focal points. The execution of special laboratory tests on some secondary concentrator samples is discussed in detail, illustrating the methodologies tailored to the specific case. Focusing tests are performed, illuminating different areas of the lens with solar divergence light and acquiring images on the plane of the photocell using a CMOS camera. Concentration measurements are carried out to select the best performing samples of curve Fresnel lens. The insertion of the secondary optics within the concentrating photovoltaic (CPV) trough doubles the solar concentration of the system. The mean concentration ratio is 1.73, 2.13, and 2.09 for the three tested lenses. The concentration ratio of the solar trough is 140 and approaches 300 after the introduction of the secondary lens.
Optical characterization and practical experimentation on manufactured components are key elements to address and ameliorate the production process and to select the best performing samples. When the optical component presents a particular geometry of radiation collection, the tests need to be tailored to examine it. Furthermore, specific measurement procedures and customized set-ups can study peculiarities or interesting aspects of the examined samples. Optical tests are frequently used to identify defective portions of the realized samples to correct manufacture.
The tested component is a novel secondary optics especially designed and implemented for an existing concentrating photovoltaic (CPV) trough. It is a cylindrical prismatic lens of Fresnel type [
The linear parabolic reflector (primary mirror) acts only in the direction transversal to the solar trough, so the entire focal line must be covered by a cell row [
The working principle of the cylindrical Fresnel lens (CFL) is that it transforms the focal line, concentrated by the linear parabolic mirror, into a series of focal points (Figure
The cylindrical Fresnel lens (CFL) reconcentrates the radiation on a square photocell.
The research started with the optical design of the secondary component [
The solar installation is not a classical linear trough but is an innovative device provided with a two-axis tracking system that permits to follow the sun and to keep the receiver always aligned. Therefore, the insertion of a secondary element does not require to modify the original tracker.
In practice, the curve Fresnel lens was specifically designed for a trough, whose parabolic mirror focuses the light over an articulated absorber. The focal image is a rectangle, and it is concentrated on a row of squared photovoltaic cells [
The receiver of the solar trough is made of a tube covered by a photocell row and is inside a glass tube. This is the receiver before the insertion of the CFLs.
The next phase was the practical implementation of the cylindrical Fresnel lens. Successive series of samples were manufactured, improving the quality of the production by adjusting the optical realization parameters. To check the optical quality of the realized samples, they were optically characterized in laboratory. The test procedures were especially tailored taking into account collection geometry and particular shape of the curve Fresnel lens. Some of these tests and measurements are illustrated in the next sections, presenting as exemplificative results the data measured on three lenses of the last production of CFLs.
Based on the optical design of the cylindrical Fresnel lens (CFL), developed as secondary optics for parabolic troughs, some samples were implemented by molding production. The research proceeded with the optical characterization of the samples by means of tests to evaluate the accuracy of the realized shape of the CFLs. These optical tests are useful both to control the progresses achieved by the successive series of mass production and to select the best performing samples among the realized optical components.
The examined solar trough uses a parabolic trough concentrator (PTC) and a series of cylindrical Fresnel lenses that concentrate solar radiation in two orthogonal directions. Figure
As can be seen from Figure
Perspective view of the CFL. The red arrow indicates the longitudinal axis (tube axis) and the blue arrow the transversal axis of the trough.
(a) Top view of the lens: the red segment represents the CFL image plane. (b) Side view of the lens: the green lines are the rays reflected by the trough.
The next sections present tailored methodologies and results of laboratory tests on prototypes of these cylindrical Fresnel lenses. The purpose of the optical tests is to evaluate the quality of the production process by testing the optical behavior of the samples in terms of concentration factor and image quality. To perform these tests, a set-up has been developed to separately illuminate different lens sectors in order to analyze their optical features.
The test [
A solar divergence collimator [
The beam was used directly to illuminate the CFL sample. A vertical diaphragm (Figure
Top view of the lens. Lens and sensor rotate around the same axis that coincides with the CFL axis.
To do this, the CFL and detector are rotated around the same axis by placing them on a rotator. The detector is placed in the PV cell plane, so it is rotated together with the lens, as visualized in Figures
The light strip rays form an angle
The light strip rays form an angle
Figures
For these tests, the sensor is a CMOS camera Vector International BCi4-6600 High Resolution USB. The camera is mounted on an x-y translation support, separate from the lens rotation stage, which allows scanning a vertical plane. This is because the size of the image formed by the CLF is greater than the size of the camera sensor (CMOS image sensor 7.74 × 10.51 mm). In this way at each rotation of the lens, the camera had to be realigned with the lens.
An additional translator is added to vary the distance D (Figure
The procedure used to test the CFL samples is as follows:
The diaphragm is adjusted to generate a 5 mm wide beam on the lens. The lens is aligned in a central position ( The sensor is aligned to the lens, and three images, upper edge, center, and bottom edge of the image, are captured by moving the camera with the x-y translator along the red arrow direction in Figure For each lens, acquisitions of the center and edge of the image were made at angles
The laboratory set-up was also simulated with the Zemax optical simulation program, using the CFL optical design. The measurement layout is reproduced in Figure
Zemax simulation of the layout of the laboratory set-up.
As can be seen from simulations, the CFL should generate an image of about 10 mm in length on the PV cell (about the same size as the long side of the CMOS camera sensor). The image should have a good uniformity of lighting inside it and quite sharp edges. In fact, the laboratory tests show that
the images produced by the analyzed lenses have no sharp edges and extend to a width much greater than the cell size; the lighting has a maximum in the central part and it gradually falls towards the lateral zones; the different lens sectors should, by symmetry, produce the same image on the camera, unless small differences are due to the inclination of the lens with respect to the diaphragm plane; instead, the images of the lateral sectors (
The results, as measured imaged and simulations, are illustrated for three selected samples of the last CFL production, named lens_A, lens_B, and lens_C. They are able to create (in the image plane at a suitable distance) images with better optical characteristics with respect to the previous productions. Figures
Images at
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The tests performed in laboratory are done only on the secondary lens, so the concentration due to the parabolic trough is absent. When the lens is mounted on the parabolic trough, the rectangles in Figures
Figures
Considering lens_B, Figures
The principal test concerns the angular scan, so the images measured varying the
On the other hand, some measurements varying the
Again, choosing lens_B, Figures
The main requirements concern the concentration factor and uniformity of illumination over the cell area. The concentration should be the around the value that maximizes the photovoltaic conversion of the PV cell. The collected light should be uniformly distributed over the cell area to have a good functioning of the cell. If the photocell is unevenly lighted, it does not work well and can disturb the whole cell line or even cause damage.
The three examined samples are compared in Figures
The comparison with the irradiance maps shows a general agreement between the acquired images and the results of the Zemax ray-tracing simulations.
A quantitative assessment of the solar concentration factor was effectuated by laboratory measurements. Due to the peculiarity of the system geometry, it was chosen to calculate the concentration factor
The quantity
In conclusion,
The set-up for the measurements is similar to the layout previously described (Figures The sensor used in this case is a Hamamatsu S6337-01 silicon photodiode, with 18 × 18 mm active area. A mask with a square aperture of 10 mm side was placed in front of the sensor, to limit the active area to the size of the photovoltaic cell. The center of the lens and center of the sensor are aligned with the beam direction. The detector does not rotate solidly with the lens but is placed on a fixed support and is constantly facing in direction perpendicular to the beam, as in Figure
The procedure used for the quantitative measurements of The aperture width of the diaphragm is set to 3 mm. The lens-receiver distance is set to An angular scan is performed between The lens was removed, and point (3) was repeated to capture the signal The procedure was repeated for
The results of the measurement of the concentration factor
Concentration factor
|
|
|
|
|
|
---|---|---|---|---|---|
40 | 1.56 | 1.67 | 1.77 | 1.84 | 1.87 |
35 | 1.51 | 1.64 | 1.74 | 1.83 | 1.86 |
30 | 1.46 | 1.58 | 1.69 | 1.80 | 1.84 |
25 | 1.45 | 1.53 | 1.62 | 1.74 | 1.79 |
20 | 1.42 | 1.51 | 1.59 | 1.68 | 1.71 |
15 | 1.38 | 1.46 | 1.54 | 1.62 | 1.65 |
10 | 1.36 | 1.45 | 1.51 | 1.56 | 1.58 |
5 | 1.35 | 1.44 | 1.49 | 1.54 | 1.57 |
0 | 1.35 | 1.44 | 1.49 | 1.54 | 1.56 |
−5 | 1.34 | 1.42 | 1.48 | 1.54 | 1.56 |
−10 | 1.36 | 1.45 | 1.51 | 1.57 | 1.60 |
−15 | 1.39 | 1.47 | 1.55 | 1.61 | 1.65 |
−20 | 1.42 | 1.52 | 1.59 | 1.67 | 1.72 |
−25 | 1.46 | 1.56 | 1.65 | 1.74 | 1.80 |
−30 | 1.52 | 1.63 | 1.73 | 1.83 | 1.87 |
−35 | 1.58 | 1.69 | 1.81 | 1.89 | 1.89 |
−40 | 1.65 | 1.76 | 1.84 | 1.90 | 1.90 |
Concentration factor
|
|
|
|
|
|
---|---|---|---|---|---|
40 | 1.96 | 2.03 | 2.07 | 2.07 | 2.05 |
35 | 1.98 | 2.05 | 2.09 | 2.10 | 2.06 |
30 | 1.98 | 2.06 | 2.11 | 2.12 | 2.08 |
25 | 2.00 | 2.07 | 2.11 | 2.11 | 2.09 |
20 | 2.02 | 2.11 | 2.15 | 2.13 | 2.10 |
15 | 2.00 | 2.11 | 2.16 | 2.16 | 2.13 |
10 | 1.98 | 2.07 | 2.13 | 2.13 | 2.10 |
5 | 1.97 | 2.06 | 2.10 | 2.10 | 2.08 |
0 | 2.00 | 2.09 | 2.15 | 2.15 | 2.13 |
−5 | 2.03 | 2.13 | 2.19 | 2.19 | 2.16 |
−10 | 2.04 | 2.13 | 2.18 | 2.17 | 2.13 |
−15 | 2.02 | 2.11 | 2.17 | 2.17 | 2.15 |
−20 | 2.03 | 2.13 | 2.18 | 2.17 | 2.14 |
−25 | 2.02 | 2.11 | 2.15 | 2.15 | 2.12 |
−30 | 2.01 | 2.08 | 2.13 | 2.13 | 2.10 |
−35 | 1.99 | 2.07 | 2.12 | 2.12 | 2.07 |
−40 | 2.00 | 2.07 | 2.10 | 2.09 | 2.05 |
Concentration factor
|
|
|
|
|
|
---|---|---|---|---|---|
40 | 1.88 | 1.98 | 2.02 | 2.03 | 1.97 |
35 | 1.90 | 1.98 | 2.03 | 2.05 | 2.01 |
30 | 1.91 | 1.99 | 2.04 | 2.05 | 2.02 |
25 | 1.92 | 2.00 | 2.04 | 2.06 | 2.01 |
20 | 1.91 | 2.00 | 2.04 | 2.05 | 2.00 |
15 | 1.88 | 1.98 | 2.02 | 2.05 | 2.01 |
10 | 1.87 | 1.97 | 2.01 | 2.01 | 1.98 |
5 | 1.87 | 1.95 | 1.99 | 2.00 | 1.96 |
0 | 1.92 | 2.02 | 2.06 | 2.08 | 2.03 |
−5 | 1.95 | 2.06 | 2.10 | 2.11 | 2.07 |
−10 | 1.97 | 2.07 | 2.12 | 2.14 | 2.09 |
−15 | 2.01 | 2.11 | 2.15 | 2.14 | 2.10 |
−20 | 2.03 | 2.13 | 2.16 | 2.14 | 2.09 |
−25 | 2.05 | 2.13 | 2.16 | 2.15 | 2.09 |
−30 | 2.05 | 2.12 | 2.14 | 2.14 | 2.11 |
−35 | 2.03 | 2.10 | 2.15 | 2.18 | 2.08 |
−40 | 2.05 | 2.14 | 2.16 | 2.15 | 2.10 |
Only the concentration ratio of the secondary lens is measured, because the laboratory set-up does not reproduce the parabolic trough focalization. The value of
The results of concentration factor
The graphs in Figures
Concentration factor
Concentration factor
Concentration factor
The mean values of concentration factor are reported in Table
Mean values of the concentration factor
Lens |
|
|
|
|
|
---|---|---|---|---|---|
Lens_A | 1.44 | 1.54 | 1.62 | 1.70 |
|
Lens_B | 2.00 | 2.09 |
|
2.12 | 2.10 |
Lens_C | 1.95 | 2.04 | 2.08 |
|
2.04 |
Some considerations can be done to conclude the concentration factor assessment.
The theoretical concentration factor, calculated with the formulas described in Section The concentration factor should remain constant along the entire angular profile of the CFL. In fact, only lens_B shows such behavior. The behavior of the three CFL samples is not the same, although the measurements have been carried out with the same procedure. Actually, the trend of
Test results show that the idea of adding a secondary system to perform a concentration on the longitudinal axis is both feasible and convenient. The lens construction solution, however, did not dare to get the expected performance for the CFL. The study has also provided accurate guidance on the emerging criticalities and possible solutions that will necessarily take into account the lens construction technique.
The purpose was to develop a secondary optics for a concentrating photovoltaic trough that increases the solar concentration and reduces the photovoltaic cell number. The proposed cylindrical Fresnel lens (CFL) focuses in the direction where the trough does not concentrate: in practice, it transforms the focal line into a series of focal points.
Some lens prototypes were produced based on the CFL optical design. Tailored tests were developed to optically characterize and check the quality of the manufactured CFL samples [
The image analysis gives a qualitative assessment of the CFL quality: every image measured on a lens indicates the level of uniformity of sun concentration, which is fundamental to correctly exploit the solar cells. The behavior of each examined lens was simulated using a ray-tracing software (Zemax), and the simulated CFL image is in general agreement with the measured image, but some exceptions are presented.
Quantitative measurements assessed the concentration factor
The trough concentration ratio before the introduction of the secondary optics was 140; since the CFL doubles it,
The optical performance improves with the successive CFL productions. The three selected samples reach a higher concentration factor and create images with better optical characteristics with respect to the previous products. In spite of this, they do not meet the requirements in terms of concentration and lighting uniformity on the cell plane (in fact the
The study and the tests have made possible to detect the criticality of the first CFL prototypes, and they provide some precise indications of the possible actions to be implemented, also considering a rethinking of the lens construction processes adopted, which can be considered as the main responsible for the loss of optical lens performance.
The authors declare that they have no conflicts of interest.