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Energy Express

  • Editor: Christian Seassal
  • Vol. 22, Iss. S1 — Jan. 13, 2014
  • pp: A28–A34
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Experimental measurements of a prototype high concentration Fresnel lens CPV module for the harvesting of diffuse solar radiation

Noboru Yamada and Kazuya Okamoto  »View Author Affiliations


Optics Express, Vol. 22, Issue S1, pp. A28-A34 (2014)
http://dx.doi.org/10.1364/OE.22.000A28


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Abstract

A prototype concentrator photovoltaic (CPV) module with high solar concentration, an added low-cost solar cell, and an adjoining multi-junction solar cell is fabricated and experimentally demonstrated. In the present CPV module, the low cost solar cell captures diffuse solar radiation penetrating the concentrator lens and the multi-junction cell captures concentrated direct solar radiation. On-sun test results show that the electricity generated by a Fresnel lens-based CPV module with an additional crystalline silicon solar cell is greater than that for a conventional CPV module by a factor of 1.44 when the mean ratio of diffuse normal irradiation to global normal irradiation at the module aperture is 0.4. Several fundamental optical characteristics are presented for the present module.

© 2013 Optical Society of America

1. Introduction

Concentrator photovoltaic (CPV) modules are a promising technology for highly efficient solar energy conversion. Recently, cell conversion efficiency greater than 44.4% was achieved for a multi-junction solar cell [1

1. Sharp Corporation, “Sharp develops concentrator solar cell with world's highest conversion efficiency of 44.4%,” http://sharp-world.com/corporate/news/130614.html (2013).

] and a module efficiency of 34.9% was achieved for a CPV module [2

2. Amonix Inc, “Amonix achieves world record for PV module efficiency in test at NREL,” http://amonix.com/pressreleases/amonix-achieves-world-record-pv-module-efficiency-test-nrel (2013).

]. Recent advances have made the cost of CPV devices competitive with flat photovoltaic (PV) modules in regions with high direct normal irradiation (DNI). In contrast, the use of CPV in regions of medium DNI is not cost effective since conventional high-concentration CPV modules cannot utilize diffuse solar radiation due to the narrow acceptance angle of the concentrator, limiting the potential market for CPV technology. On the other hand, the current drastic increase in the installation of megawatt solar power plants (which predominantly use fixed, flat PV modules) in regions of medium DNI tend to result in the lack of cheap land for further deployment. In such cases, one needs to maximize the electricity generated in a limited installation space as much as possible.

Recently, Benítez et al. [3

3. P. Benitez, J. C. Miñano, and R. Alvarez, “Photovoltaic concentrator with auxiliary cells collecting diffuse radiation, US patent application publication,” Pub. No.: US 2010/0126556 A1 (2010).

] invented the concept of a high concentration CPV module that harvests diffuse solar radiation, which may provide one solution to the abovementioned problem, and expand the potential market for CPV modules into regions of medium DNI. Such a CPV module could also contribute to maximize solar energy conversion efficiency of a photovoltaic system as the maximum theoretical efficiency of photovoltaic conversion is achieved through the concentration of sunlight. Therefore, concentrating direct (beam) solar radiation to a high-efficiency, high-cost solar cell while simultaneously capturing diffuse solar radiation by a low cost solar cell with little or no concentration in a single module is of interest. Such a module will also be of interest when solar energy utilization is in higher demand. To date, such a CPV module has not yet been experimentally demonstrated.

In this study, we implemented this concept using a prototype Fresnel-lens-based CPV module with an additional low cost solar cell and experimentally demonstrated system performance. The improvement factor of the present module relative to a conventional CPV module is modeled and compared with experimental results. In addition, the optical characteristics of a Fresnel lens-based CPV module are illustrated by a ray-tracing simulation.

2. Optical configuration of a CPV module for harvesting diffuse solar radiation

Fig. 1 Optical configuration of a high concentration CPV module harvesting diffuse solar radiation [3].
Figure 1 shows the optical configurations of a CPV module for harvesting diffuse solar radiation through an additional low-cost cell, as described in a patent document [3

3. P. Benitez, J. C. Miñano, and R. Alvarez, “Photovoltaic concentrator with auxiliary cells collecting diffuse radiation, US patent application publication,” Pub. No.: US 2010/0126556 A1 (2010).

]. The geometrical concentration ratio for the low-cost cell should be close to unity in order to fully capture diffuse radiation. Although other optical configurations can be conceived from this example, here we have selected a Fresnel-lens CPV module because Fresnel lenses are currently the most commonly-used type of concentrator for high-concentration CPV modules [4

4. P. Benítez, J. C. Miñano, P. Zamora, R. Mohedano, A. Cvetkovic, M. Buljan, J. Chaves, and M. Hernández, “High performance Fresnel-based photovoltaic concentrator,” Opt. Express 18(S1), A25–A40 (2010). [CrossRef]

6

6. V. D. Rumyantsev, “Solar concentrator modules with silicone-on-glass Fresnel lens panels and multijunction cells,” Opt. Express 18(S1), A17–A24 (2010). [CrossRef] [PubMed]

]. Test modules based on Fresnel lenses are easily fabricated by installing an additional low cost cell. Although the test module in this study is a single-lens module, a practical multiple-lens-array module could in principle be constructed according to the schematic shown in Fig. 2.
Fig. 2 Image of Fresnel-lens CPV module with additional low-cost solar cells.

3. Outdoor experiments

3.1 Setup

3.2 Experimental results

Fig. 4 Time variation of the measured Pmax for the test module (outdoor test).
Figure 4 shows a stack chart of the measured time variation of Pmax for the test module on August 22 (clear sky case) and September 18 (partially cloudy case), 2013. Here, Pmax represents the power at the maximum power point. In the figures, the potential electricity generation by the present CPV module (here after termed as “CPV+ module” to distinguish this design from conventional CPV modules) is estimated as the sum of Pmax of the triple-junction cell and the Si cell, while that of a conventional CPV module is estimated as only Pmax of the triple-junction cell. The built-in Si cell boosts electricity generation even when the triple-junction cell does not generate electricity due to the nearly zero DNI. For a clear sky, where the mean diffuse-to-total ratio γ = (GNI − DNI)/GNI = 0.18 and the mean DNI = 848 W/m2, the potential electricity generation by the CPV + module is greater than that for a conventional CPV module by a factor of 1.19. For a partially cloudy sky (γ = 0.64, DNI = 318 W/m2 on average), this factor increases up to 37.3.

Fig. 5 Relationship between short circuit current ratio of the built-in Si cell in the test module to the reference Si cell mounted on the same two-axis solar tracker. Short circuit current of the reference cell is corrected by γ.
Figure 5 shows the relationship between the ratio defined by ISC_built-in / (ISC_ref × γ) and DNI. ISC_built-in and ISC_ref are the measured short circuit currents of the built-in Si cell and that of the reference Si cell, respectively. Thus, this ratio expresses the light intensity received by the built-in Si cell relative to that of the reference Si cell. Note that ISC_ref is corrected by a factor of γ so that (ISC_ref × γ) approximately represents the short circuit current of the reference Si cell under diffuse solar radiation exposure only. The plotted points were obtained from a 6 day experiment for ten DNI-bins ranging from 0 to 850 W/m2 with a bin size of 50 W/m2. The ratio is 0.84 when DNI is nearly zero but GNI – DNI is non-zero, i.e. the optical efficiency for incoming diffuse solar radiation to the built-in Si cell is 16% less relative to that of the reference Si cell. This reduction could be mitigated in an actual lens array module because the mirrors used in the test module result in losses from imperfect reflection. Interestingly, the ratio increases up to over 1.5 as DNI increases. This implies that a portion of the concentrated beam radiation, which is supposed to be captured by the concentrator cell, leaks onto the built-in Si cell. Further discussion accompanied by an optical simulation is presented in section 4.2.

4. Simulations

4.1 Fundamentals of electricity generation improvement

The experimental results indicate a significant increase in potential electricity generation using a CPV + module. This advantage primarily depends on the diffuse-to-total ratio γ, the optical efficiency, and the cell conversion efficiency of the low-cost and concentrator solar cell. Here, we derive the relationship between these factors. The module conversion efficiency of the CPV + module based on GNI is given by:
ηCPV+=ηopt_CPVηcell_CPVDNI+ηopt_PVηcell_PV(GNIDNI)GNI,
(1)
where ηopt_CPV and ηcell_CPV represent the optical efficiency of the concentrator cell component based on a DNI and the cell conversion efficiency of the concentrator cell, respectively. ηopt_PV and ηcell_PV represent the optical efficiency of the low-cost solar cell component based on GNI at the module aperture and the cell conversion efficiency of the low-cost solar cell, respectively. On the other hand, the module conversion efficiency of a conventional CPV module based on GNI is given by:
ηCPV=ηopt_CPVηcell_CPVDNIGNI.
(2)
Assuming that ηopt_CPV and ηcell_CPV in Eq. (1) are identical to those in Eq. (2), i.e. the low-cost cell component does not interfere with the concentrator cell component, we obtain the improvement factor f from:
f=ηCPV+ηCPVηCPV=ηopt_PVηcell_PVηopt_CPVηcell_CPVGNIDNIDNI.
(3)
Equation (3) is then re-written as follows:
f=γτ(1γ),
(4)
where,
τ=ηopt_CPVηcell_CPVηopt_PVηcell_PV,
(5)
γ=GNIDNIGNI.
(6)
γ is the diffuse-to-total ratio at the module aperture. τ represents the ratio of conversion efficiency of the concentrator cell to that of the low-cost solar cell in the CPV + module. Using these equations, we conducted a parametric characterization.

Fig. 6 Improvement factor vs. diffuse-to-total ratio for τ = 1.0, 1.5, 2.0, and 3.0.
Figure 6 shows the characteristic curves of the improvement factor. The ratio γ depends on the atmospheric conditions at the installation site, e.g. the yearly mean value for a middle DNI region such as Tokyo (Japan) is γ ≅ 0.4, while that for a high DNI region such as Phoenix (USA) is γ ≅ 0.2. The ratio τ depends on the type of solar cell and optical system. Here, four τ values in the range 1 ≤ τ ≤ 3 were used. For larger γ, a larger improvement factor is obtained because the amount of diffuse radiation captured by the low-cost cell increases compared to that for direct radiation concentrated onto the concentrator cell. Furthermore, the larger τ results in a smaller improvement factor because the electricity generated by the low-cost cell is low compared to that generated by the concentrator cell. Figure 6 also shows statistical f values for seven γ-bins ranging from 0.2 to 0.7 with a bin size of 0.1, which were obtained from a 6 day experiment. The plotted points are consistent with the curve for τ = 1.5. For τ = 1.5, an improvement factor f of 44% is expected for an annual mean γ ≅ 0.4, whereas an f of 17% is expected for γ ≅ 0.2.

Fig. 7 Simulation of the boosted generated electricity ΔP vs. the diffuse-to-total ratio.
In the Eq. (4) and Fig. 6, the amount of incident solar energy, which generally decreases as γ increases, is not considered. Here, Fig. 7 shows a simulation of the boosted electricity generation defined as ΔP = GNI (ηCPV+ηCPV). We assumed that GNI is a linear function of γ and determined by the slope and intercept of a linear fit to commercial yearly irradiation data (Meteonorm) within 20 locations including Phoenix and Tokyo. We also assumed ηopt_CPV × ηcell_CPV = 0.8 × 0.38 = 0.304 and ηopt_PV × ηcell_PV = 0.9 × 0.2 = 0.18 as a current realistic efficiency (τ = 1.69). The resulting curve shows a peak at approximately 0.4 where ΔP reaches approximately 150 kWhr/m2/year. At this peak γ, the electricity generation of the present CPV module is greater than that of a conventional CPV module by a factor of 1.4. The electricity generation is also greater than that of a tracked flat PV by factor of 1.41. Therefore, the present module is advantageous in the middle DNI regions but not in the high and low DNI regions. Obviously, the peak shifts to the right-hand side (larger γ) as τ decreases and vice versa. In terms of cost effectiveness, the present system could be feasible when the additional cost for extra solar cells is compensated by reduction in other costs, e.g., land cost.

4.2 Ray-tracing analysis for the effect of sunshape

5. Conclusions

References and links

1.

Sharp Corporation, “Sharp develops concentrator solar cell with world's highest conversion efficiency of 44.4%,” http://sharp-world.com/corporate/news/130614.html (2013).

2.

Amonix Inc, “Amonix achieves world record for PV module efficiency in test at NREL,” http://amonix.com/pressreleases/amonix-achieves-world-record-pv-module-efficiency-test-nrel (2013).

3.

P. Benitez, J. C. Miñano, and R. Alvarez, “Photovoltaic concentrator with auxiliary cells collecting diffuse radiation, US patent application publication,” Pub. No.: US 2010/0126556 A1 (2010).

4.

P. Benítez, J. C. Miñano, P. Zamora, R. Mohedano, A. Cvetkovic, M. Buljan, J. Chaves, and M. Hernández, “High performance Fresnel-based photovoltaic concentrator,” Opt. Express 18(S1), A25–A40 (2010). [CrossRef]

5.

K. Araki, T. Yano, and Y. Kuroda, “30 kW concentrator photovoltaic system using dome-shaped Fresnel lenses,” Opt. Express 18(S1), A53–A63 (2010). [CrossRef]

6.

V. D. Rumyantsev, “Solar concentrator modules with silicone-on-glass Fresnel lens panels and multijunction cells,” Opt. Express 18(S1), A17–A24 (2010). [CrossRef] [PubMed]

7.

J. Jaus, A. W. Bett, H. Reinecke, and E. R. Weber, “Reflective secondary optical elements for Fresnel lens based concentrator modules,” Prog. Photovoltaics 19(5), 580–590 (2011). [CrossRef]

8.

A. Neumann, A. Witzke, S. A. Jones, and G. Schmitt, “Representative terrestrial solar brightness profiles,” Trans. ASME J. Sol. Energy Eng. 124(2), 198–204 (2002). [CrossRef]

9.

D. Buie, A. G. Monger, and C. J. Dey, “Sunshape distributions for terrestrial solar simulations,” Sol. Energy 74(2), 113–122 (2003). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(220.1770) Optical design and fabrication : Concentrators
(220.2740) Optical design and fabrication : Geometric optical design
(350.6050) Other areas of optics : Solar energy
(220.4298) Optical design and fabrication : Nonimaging optics

ToC Category:
Solar Concentrators

History
Original Manuscript: October 2, 2013
Revised Manuscript: November 6, 2013
Manuscript Accepted: November 11, 2013
Published: November 18, 2013

Citation
Noboru Yamada and Kazuya Okamoto, "Experimental measurements of a prototype high concentration Fresnel lens CPV module for the harvesting of diffuse solar radiation," Opt. Express 22, A28-A34 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S1-A28


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References

  1. Sharp Corporation, “Sharp develops concentrator solar cell with world's highest conversion efficiency of 44.4%,” http://sharp-world.com/corporate/news/130614.html (2013).
  2. Amonix Inc, “Amonix achieves world record for PV module efficiency in test at NREL,” http://amonix.com/pressreleases/amonix-achieves-world-record-pv-module-efficiency-test-nrel (2013).
  3. P. Benitez, J. C. Miñano, and R. Alvarez, “Photovoltaic concentrator with auxiliary cells collecting diffuse radiation, US patent application publication,” Pub. No.: US 2010/0126556 A1 (2010).
  4. P. Benítez, J. C. Miñano, P. Zamora, R. Mohedano, A. Cvetkovic, M. Buljan, J. Chaves, and M. Hernández, “High performance Fresnel-based photovoltaic concentrator,” Opt. Express18(S1), A25–A40 (2010). [CrossRef]
  5. K. Araki, T. Yano, and Y. Kuroda, “30 kW concentrator photovoltaic system using dome-shaped Fresnel lenses,” Opt. Express18(S1), A53–A63 (2010). [CrossRef]
  6. V. D. Rumyantsev, “Solar concentrator modules with silicone-on-glass Fresnel lens panels and multijunction cells,” Opt. Express18(S1), A17–A24 (2010). [CrossRef] [PubMed]
  7. J. Jaus, A. W. Bett, H. Reinecke, and E. R. Weber, “Reflective secondary optical elements for Fresnel lens based concentrator modules,” Prog. Photovoltaics19(5), 580–590 (2011). [CrossRef]
  8. A. Neumann, A. Witzke, S. A. Jones, and G. Schmitt, “Representative terrestrial solar brightness profiles,” Trans. ASME J. Sol. Energy Eng.124(2), 198–204 (2002). [CrossRef]
  9. D. Buie, A. G. Monger, and C. J. Dey, “Sunshape distributions for terrestrial solar simulations,” Sol. Energy74(2), 113–122 (2003). [CrossRef]

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