This paper presents theoretical and experimental optical evaluation and comparison of symmetric Compound Parabolic Concentrator (CPC) and V-trough collector. For direct optical properties comparison, both concentrators were deliberately designed to have the same geometrical concentration ratio (1.96), aperture area, absorber area, and maximum concentrator length. The theoretical optical evaluation of the CPC and V-trough collector was carried out using a ray-trace technique while the experimental optical efficiency and solar energy flux distributions were analysed using an isolated cell PV module method. Results by simulation analysis showed that for the CPC, the highest optical efficiency was 95% achieved in the interval range of 0° to ±20° whereas the highest outdoor experimental optical efficiency was 94% in the interval range of 0° to ±20°. For the V-tough collector, the highest optical efficiency for simulation and outdoor experiments was about 96% and 93%, respectively, both in the interval range of 0° to ±5°. Simulation results also showed that the CPC and V-trough exhibit higher variation in non-illumination intensity distributions over the PV module surface for larger incidence angles than lower incidence angles. On the other hand, the maximum power output for the cells with concentrators varied depending on the location of the cell in the PV module.
One of the greatest challenges facing the world today is breaking fossil fuel dependence and promoting the development of new and renewable sources of energy that can supplement and, where appropriate, replace the diminishing resources of fossil fuels. Solar energy is clearly one of the most promising prospects to these problems since it is nonpollutant, renewable, and available everywhere in the world although with varying intensity. However, solar electricity has not been utilised as much as it should have been due to low photovoltaic (PV) cell conversion efficiency [
Solar concentrators are generally categorized as refractive or reflective [
Extensive research in the design and testing of solar concentrators has resulted in several different concentrating solar collector designs such as flat planar [
The performance of the CPC and V-trough collector for PV applications depends, amongst other factors, on the optical efficiency and solar flux distribution along the PV module [
Theoretical characterisation of a concentrating system for PV application can be achieved by ray-tracing technique [
In this study two concentrators, symmetric 2-dimensional (2D) CPC and V-trough collector, were considered. For direct comparison, both collectors were deliberately designed to have the same geometrical concentration ratio, aperture area, absorber area, and maximum collector length. The CPC was truncated to reduce its height and to ensure that its geometrical concentration ratio is the same as that of a V-trough collector. Table
Geometrical dimensions of the 2D CPC and V-trough collector.
Parameter | CPC (before truncation) | CPC (after truncation) | V-trough |
---|---|---|---|
Acceptance half-angle (°) | 30.0 | 30.7 | 21.0 |
Trough angle (°) | [—] | [—] | 10.0 |
Concentration ratio | 2.0 | 1.96 | 1.96 |
Aperture width (mm) | 218.0 | 213.64 | 213.64 |
Receiver width (mm) | 109.0 | 109.0 | 109.0 |
Receiver length (mm) | 109.0 | 109.0 | 109.0 |
Maximum height (mm) | 282.4 | 212.07 | 296.72 |
Maximum length (mm) | 500.0 | 500.0 | 500.0 |
In order to evaluate theoretical optical performance of a 2D CPC and V-trough collector, ray-trace technique was used to determine the acceptance angle, optical efficiency, optical losses, solar energy flux distribution on the PV surface, and general characteristics of angular acceptance function as well as general optical efficiency. The ray-trace technique employed in this work is called “Eazee ray-trace program” developed by Zacharopoulos [
Parameters employed in the ray-trace software.
Parameter | Value |
---|---|
Refractive index of glass ( |
1.52 |
Refractive index of air ( |
1.00 |
Glass extinction coefficients (m−1) | 4.00 |
End loss coefficient | 0.99 |
Reflectivity of the reflector ( |
0.91 |
Absorptance of the PV cells | 1.00 |
For a specific set of inputs, a ray-tracing program randomly chooses the coordinates of the striking point for each ray on the aperture cover and determines the path taken by that ray through the aperture cover to the PV module. During the ray passage, the ray-tracing program calculates (for each ray) the point of intersection and the associated angles of incidence and refraction on the surface in question (aperture cover, reflector surfaces, or PV module). The program also calculates the solar energy absorbed and refracted by the reflector surfaces and the total solar energy absorbed by the PV module. Furthermore, the ray-tracing program calculates the energy flux distribution across the PV module as well as number of ray reflections on the reflector surfaces. The program computes all these values for a sufficient number of initial rays such that the averaged outcome of the rays is a fair representation of the simulated illumination.
For this study, the PV module was assumed to be a perfect absorber of the solar irradiation [
To determine the solar energy flux distribution along the PV module within the CPC and V-trough collector as well as the experimental optical efficiency, an isolated cell PV module was designed and fabricated as detailed in Paul et al. [
By using an isolated cell PV module as the absorber of the CPC and V-trough collector and measuring current yields from each of the cells it was possible to determine the amount of solar radiation reaching the PV module and its distribution. In this study, the experimental solar flux distribution along the PV module for each collector (CPC and V-trough) was determined using
Since the short-circuit current of a PV module, at a constant temperature, depends only on the absorbed solar radiation, the experimental optical efficiency
Since the PV module had 11 isolated cells, the total solar energy on the PV module (
The experimental optical efficiency and solar flux distribution characterisation of the fabricated CPC and V-trough collector were carried out at the roof of the Centre for Sustainable Technologies laboratory building, University of Ulster, UK (56° N, 5° W). The location is influenced by a maritime climate, with highest and lowest solar insolation received at June and December solstices, respectively. For this reason, the experiments were carried out during June and August. To avoid interruption of diffuse radiation, all the measurements presented in this work were carried out only on clear sunny days.
During the measurements, the CPC and V-trough collector (each fabricated by aluminium sheet reflector with reflectivity of 0.91) were aligned in the East-West direction (Figure
Experimental setup of the CPC at 10° incidence angle for outdoors testing.
The intensity of the solar radiation on the aperture of each concentrator was measured using Kipp and Zonen pyranometer (see Figure
The experimental solar flux distribution along the PV cell, for both collectors (CPC and V-trough), was examined for 0°, 10°, 15°, and 20° incidence angles using the short-circuit current method.
During incidence angles adjustment, each test unit (PV module at the aperture of a concentrator, CPC or V-trough collector) was covered to avoid overheating of the PV module. Furthermore, an air-fan was used to produce forced cooling and achieve uniform temperature across the PV module during the experimental test.
The way in which solar radiation is incident on the aperture of a PV concentrator is vital to the performance of a PV module. This is due to the fact that the position in which the rays strike the PV module directly or indirectly (after reflections) is primarily dependent on the incidence angle of the ray,
Ray-trace diagrams for the CPC and V-trough collector with solar radiation incident perpendicular to the aperture of the concentrators.
Comparison of number of rays reaching the PV module directly or after reflection(s) for the CPC and V-trough collector when solar radiation incident was perpendicular to the aperture of the concentrators.
Comparisons of angular acceptance, optical efficiency, and optical losses for the CPC and V-trough collector when solar radiation was incident perpendicular to the aperture of each concentrator.
Solar energy flux distributions along the PV module for the CPC and V-trough collector for solar radiation incident perpendicular to the aperture of each concentrator.
Since nonuniform flux distribution on a PV module in which cells are connected in series has significant negative effect on the electrical performance, thus the high solar flux at the edges of the PV module within the CPC (which is about 9 times than the least-illuminated area) will not contribute to an increase in power output. The reason is that when a PV module in which cells are connected in series is nonuniformly illuminated the current generated by the whole PV module is limited by the least-illuminated cell(s) [
Ray-trace diagrams for the CPC and V-trough collector with solar radiation incident at 5° from the perpendicular to the aperture of each collector.
Ray-trace diagrams for the CPC and V-trough collector with solar radiation incident at 10° from the perpendicular to the aperture of each concentrator.
Comparisons of angular acceptance, optical efficiency, and optical losses of the CPC and V-trough for solar radiation incident at 5° and 10°.
From Figure
Variations in ray reflections as a function of incidence angle and concentrator reflecting surface geometry.
Incidence angle (°) | CPC | V-trough | ||||
---|---|---|---|---|---|---|
No reflection (%) | One reflection (%) | Two reflections (%) | No reflection (%) | One reflection (%) | Two or more reflections (%) | |
0 | 52 | 44 | 4 | 52 | 48 | 0 |
5 | 52 | 46 | 2 | 52 | 48 | 0 |
10 | 52 | 48 | 0 | 52 | 44 | 2 |
25 | 28 | 72 | 0 | 10 | 40 | 40 |
27.5 | 24 | 76 | 0 | 2 | 40 | 58 |
Solar flux distributions along the PV module for the CPC and V-trough collector for solar radiation incident at 5° and 10° from the perpendicular to the aperture of the collector.
For the V-trough collector, it can be seen from Figure
The solar energy flux distribution analysis presented in Figure
Ray-trace diagrams for the CPC and V-trough collector with solar radiation incident at 25° from the perpendicular to the aperture of the concentrator.
Ray-trace diagrams for the CPC and V-trough collector with solar radiation incident at 27.5° from the perpendicular to the aperture of the concentrator.
Increasing the incidence angle to 27.5° (Figure
Comparisons of angular acceptance, optical efficiency, and optical losses of the CPC and V-trough for solar radiation incident at 25° and 27.5°.
As shown in Table
Solar energy flux distributions along the PV module for the CPC and V-trough collector for solar radiation incident at 15° and 20° from the perpendicular to the aperture of the concentrator.
As the incidence angle increases to 27.5°, 59% of all the accepted rays in the CPC are focused onto a very small portion at the edge of the right side of the PV module, giving a peak of solar concentration up to 48 compared with less than 2 for the V-trough collector. Low solar energy peak for the V-trough is due to multiple reflections and rejected rays. Due to high solar energy flux near the edge of the right side of the PV module for the CPC, an increase in local temperature and a significant reduction in conversion efficiency are expected at these incidence angles in real applications.
Variations in angular acceptance function with incidence angle for the CPC and V-trough collector.
Although both the CPC and V-trough have equal concentration ratios and aperture areas, they have difference angular acceptance function beyond their acceptance angle limits due to difference in heights and reflector surface geometry. It can be seen from Figure
Variation of optical efficiency with incidence angle for the CPC and V-trough collector.
Figure
Variation of experimental optical efficiency with incidence angle for the CPC and V-trough collector.
Figures
Experimental solar energy flux distributions along the PV module for the CPC and V-trough collector at 0° incidence angle.
Experimental solar energy flux distributions along the PV module for the CPC and V-trough collector at 10° incidence angle.
Experimental solar energy flux distributions along the PV module for the CPC and V-trough collector at 15° incidence angle.
Experimental solar energy flux distributions along the PV module for the CPC and V-trough collector at 20° incidence angle.
As shown in Figure
From Figures
To examine the effects of solar radiation incidence angle on the maximum power output of each cell, the PV module with and without concentrator was tilted manually to different incidence angles. However, for the purpose of this paper, only the variations in maximum power output for 0° and 20° incidence angles are presented. For each angle of incidence, a complete I-V curve measurement was recorded for each cell with and without concentrators. From the I-V curve, the maximum power output of each cell under each system was extracted. Figures
Comparison of maximum power output for cells with and without concentrator when solar radiation incident was perpendicular to the aperture of the test unit.
Comparison of maximum power output for cells with and without concentrator when the incident solar irradiance was 20° from perpendicular to the aperture of the test unit.
As the solar radiation incident angle increases to 20°, the highest power output (of about 500 mW) for the PV cells within the CPC was recorded at cell number 4 while the lowest power output of about 100 mW was measured at cell number 10. However, due to difference in reflecting surface geometry between the CPC and V-trough collector as well as the optical efficiency, the highest power output for the PV cells with a V-trough collector was about 250 mW at cell number 9 whereas the lowest power output of about 100 mW was recorded at cell number 1.
In this work, a detailed simulation optical analysis, experimental optical efficiency, and solar energy flux distribution along the PV module as well as the maximum power output with the CPC and V-trough collector have been presented. Results by simulation analysis showed that, in spite of both concentrators having the same concentration ratio and aperture area, the concentrators had different optical behaviour, except for the angular acceptance in the interval
Theoretical results indicated that the CPC and V-trough exhibit higher variations in non-illumination intensity distributions over the PV module surface for larger incidence angles. On the other hand, the maximum power output for the cells with concentrators varied depending on the location of the cell in the PV module as well as the reflecting surface geometry of a concentrator.
The author declares that there is no conflict of interests regarding the publication of this paper.