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## What are Concentrator photovoltaics (CPV)??

Wikipedia : Concentrator photovoltaic systems employ curved reflectors such as lenses and mirrors to focus incoming sun rays onto the solar cells to harvest solar energy with more efficiency measured as watt-peak Wp. They are often equipped with single or dual-axis solar trackers and cooling systems that promote dual-way power generation. Based on the intensities measured in number of suns, CPV systems are classified as Low concentration PV, High concentration PV, Medium concentration PV and Luminescent solar concentrators.

This idea of concentrating sun's radiation dates back to 212 B.C.The famous Greek inventor Archimedes used mirrors, later called as burning mirrors, to set enemy ships at blaze. Concentrators/reflectors use principles of optics (focal point) to concentrate sunlight onto absorbers/Solar cells.

#### K. G. T. Hollands, "A concentrator for thin-film solar cells," Solar Energy, vol. 13, no. 2, pp. 149–163, May 1971. doi: 10.1016/0038-092X(71)90001-6

• Considerations : V-Trough reflector as side walls with solar cells at the base; Seasonal tracking alone is considered but not diurnal tracking; Axis along east-west direction; Used thin-film polycrystalline cadmium Sulphide cells.
• Assumptions: Side walls to be perfectly specular, gray surfaces; restricts the trough geometries studied to those where, with the solar beam normal to the base, two conditions are met: (a) the base is uniformly irradiated; (b) no ray suffers more than one reflection.
• Aim: To determine yearly average direct-beam concentration factor for any incidence angle, opening angle and side-wall reflectance.
• Findings: Concludes that total yearly mean concentration factors of the order of 2 are possible with V-trough concentrators.
• Imp concepts: Calculations on Solar geometry (N-S & E-W).
• Limitations: Side- walls are ideal specular reflectors.

#### R. M. Swanson, "The promise of concentrators," Prog. Photovolt: Res. Appl., vol. 8, no. 1, pp. 93–111, Jan. 2000. doi: 10.1002/(SICI)1099-159X(200001/02)8:1<93::AID-PIP303>3.0.CO;2-S

• Aim: To address the issue of why concentrator systems have not gained a significant market share.
• Information: 1. Provides overview of active concentrator developments in major universities and labs in US, UK and Japan. 2. Surmises advantages of concentrator technology, its barriers and suggestions to overcome them along with recommendations for future developments.

#### D. A. W. B. Dr. Simon P. Philipps and D. S. K. Kelsey Horowitz, "Current Status of Concentrator Photovoltaic (CPV) Technology," Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany & National Renewable Energy Laboratory NREL in Golden, Colorado, USA, TP-6A20-63916, Sep. 2015.

• Aim: To Summarize the status of the concentrator photovoltaic (CPV) market and industry as well as current trends in research and technology. This report is intended to guide research agendas for Fraunhofer ISE, the National Renewable Energy Laboratory (NREL), and other R&D organizations.
• Review: 1. Clearly distinguishes between CPV strengths and weaknesses. 2. Focusses on market and industry aspects- levelized cost of electricity (LCOE) studies of current available CPV technologies. 3. Overview of research and technological developments in Fraunhofer Institute and NREL. 4. Serves as reference for stakeholders in the CPV industry and research.

#### A. K. Pandey, V. V. Tyagi, J. A. Selvaraj, N. A. Rahim, and S. K. Tyagi, "Recent advances in solar photovoltaic systems for emerging trends and advanced applications," Renewable and Sustainable Energy Reviews, vol. 53, pp. 859–884, Jan. 2016.10.1016/j.rser.2015.09.043

• Aim: To provide a comprehensive review on the solar photovoltaic (SPV) systems especially BIPV, CPV & PV/T and their recent advances along with emerging applications in the present and future scenario.
• Review: 1. Provides estimates on energy consumption over past few decades to near future. 2. Provides abridged version of background on PV but focusses more on recent advances in the PV technology and compares numerous cell configurations in accordance to their efficiencies. 3. Gives insights of various PV materials like crystalline, thin-film, Concentrated PV, Hybrid, Organic and Polymer materials etc. 4. Discusses deeply the applications of PV technology with particular case studies which extends even for space based solar technology. 5. Provides recommendations wherein to concentrate the more R&D.

## Low concentration photovoltaics (LCPV)

Low concentration PV systems can be illuminated with intensities less than 20 suns[1] which can be varied up to 100 suns. LCPV systems eliminate the need of complex cooling systems and are often facilitated with booster reflectors. LCPV systems doesn't require active tracking mechanisms due to wide acceptance angles.[2] These can sufficed with single-axis tracking system yet maintaining 35-40% increased power output. The reflected radiation incident on these modules depends on the clearness of the index of the location[3][4] and thus they are more effective when installed where direct radiation is a significant percentage of the global radiation (South Europe, Northern Africa, Southern states of the USA, etc.).

Measuring Intensity in Suns: Intensity of sunlight illuminating on PV cells are measured as 'Suns'. 'One Sun' is the amount of energy drawn to an object openly exposed out on a cloudless day which is approximately 100 watts per square foot.


## Concept of Reflectors-Concentrators

The efficacy of incorporating reflectors or concentrators for PV cells or modules is to intensify the incident radiation i.e., to increase incident radiation per m^2. They increase the output efficiency leading to reduction of capital costs. The main features of reflectors are high reflectance, low scattering and low degradation i.e., loss of reflectance over time.

Link directly to Understanding solar concentrators


#### H. Tabor, "Stationary mirror systems for solar collectors," Solar Energy, vol. 2, no. 3–4, pp. 27–33, Jul. 1958. doi:10.1016/0038-092X(58)90051-3

• Aim: To provide with studies that proves tilting of solar concentrators along with usage of mirrors for concentrating radiation is more efficient.
• Considerations:The mirror has the form of a cylindrical parabola with the cylindrical axis mounted horizontally east-west.
• Findings: Complete stationary mirror cannot provide any useful concentration while tilting the solar collectors with varying seasons can yield more efficiencies; The maximum optical concentration of 3 is obtained at a minimum angle of acceptance of 15-17 deg for the mirrors. Employing an auxiliary side mirror for second stage concentration can increase this concentration power to about 4. One cannot apply a second stage of optical concentration to the double-sided receiver.
• Imp concepts:An angle in solar geometry termed the EWV altitude is defined, and its variation with time and season is shown.This indicates the necessary acceptance angle of a stationary mirror system for solar collectors. Solar geometry studies. Geometrical studies of cylindrical parabola.

#### A. Rabl, "Solar concentrators with maximal concentration for cylindrical absorbers," Applied Optics, vol. 15, no. 7, p. 1871, Jul. 1976. doi: 10.1364/AO.15.001871

The differential equation is derived that describes the reflector of an ideal two-dimensional radiation concentrator with an absorber of arbitrary convex shape. For the special case of an absorber with circular cross section, the equation can be solved in closed form if suitable coordinates are used. The effect of absorption at the reflector is considered, and formulas are presented for determining the attenuation of radiation on its passage from aperture to absorber.

#### A. Rabl, "Comparison of solar concentrators," Solar Energy, vol. 18, no. 2, pp. 93–111, 1976.doi: 10.1016/0038-092X(76)90043-8[1]

• Analyses the geometric concentration ratio of different types of concentrators.
• Concludes that there is a nonuniformity of the flux density distribution on the absorber.

#### A. Rabl and R. Winston, "Ideal concentrators for finite sources and restricted exit angles," Applied Optics, vol. 15, no. 11, p. 2880, Nov. 1976. doi: 10.1364/AO.15.002880

Design procedures for ideal radiation concentrators are described which are applicable to finite sources and/or restricted exit angles. Finite sources are relevant for second stage concentrators which collect and further concentrate radiation from a primary focusing element (mirror or lens) in a manner similar to the field optic element in a telescope. Restricting the exit angle is useful for improving the optical efficiency of solar collectors by eliminating grazing angles of incidence of the absorber. It also serves to extend the useful range of angular acceptance values available from solid dielectric concentrators that function by total internal reflection. Concentrators of this type can be used to construct highly efficient radiation traps (spectrally selective filters).

#### D. P. Grimmer, K. G. Zinn, K. C. Herr, and B. E. Wood, "Augmented Solar Energy Collection Using Various Planar Reflective Surfaces: Theoretical Calculations and Experimental Results," Los Alamos Scientific Lab., N.Mex. (USA), LA-7041, Apr. 1978

The use of planar reflective surfaces can substantially improve the performance of both active and passive solar collectors. The results of theoretical calculations and experimental tests on the use of different types of flat reflective surfaces to increase the collection of solar energy by flat collectors are presented. Specular, diffuse, and combination specular/diffuse reflective surfaces are discussed. A computer model has been generated to describe surfaces as a combination of specular- and diffuse-like reflectivities. The reflective properties of a given surface can be measured in the laboratory as a function of incident and reflected angles. Predictions of system performance were made for various collector/reflector configurations and compared with the performance of an optimally oriented collector without a reflector.

#### R. W. Stacey and P. G. McCormick, "Effect of concentration on the performance of flat plate photovoltaic modules," Solar Energy, vol. 33, no. 6, pp. 565–569, 1984. doi: 10.1016/0038-092X(84)90012-4

The effect of low concentration ratios on the performance of passively cooled conventional photovoltaic modules has been investigated. Peak power outputs of up to 140 W per square metre of module area have been obtained with single crystal modules of high cell packing factor using a 2.2X plane mirror concentrator. Both cell temperature and series resistance losses are found to be important in limiting module efficiency. Performance simulations indicate that the use of a 4X concentrator with polar axis tracking will increase annual peak output by a factor of 3.2 over that of a fixed flat plate module.

#### G. Smestad, H. Ries, R. Winston, and E. Yablonovitch, "The thermodynamic limits of light concentrators," Solar Energy Materials, vol. 21, no. 2–3, pp. 99–111, Dec. 1990. doi: 10.1016/0165-1633(90)90047-5

To concentrate the light, photons from a larger area are collected and directed to a smaller area. Some devices use geometrical optics, or a change in index of refraction to increase the illumination on a surface above the incident solar level. Other systems use a frequency or Stokes shift to increase the illumination of light at one photon energy at the expense of another. Presented is a unification of the ideas and principles developed for the various classifications of concentrators. Equations are developed that describe the limits of concentration in geometrical and fluorescent systems. Concentration is shown to be a function of the index of refraction, angular collection range, as well as the frequency shift. Applications of the ideas involve the understanding of diffuse radiation concentrators and the solar powered laser.

#### R. P. Friedman, J. M. Gordon, and H. Ries, "New high-flux two-stage optical designs for parabolic solar concentrators," Solar Energy, vol. 51, no. 5, pp. 317–325, 1993. doi:10.1016/0038-092X(93)90144-D

A new two-stage optical design for parabolic dish concentrators that can realistically attain close to 90% of the thermodynamic limit to concentration with practical, compact designs was presented. For comparison, the parabolic dish-plus-compound parabolic concentrator secondary design, at this rim angle, achieves no more than 50% of the thermodynamic limit. A new secondary concentrator is tailored to accept edge rays from the parabolic primary, and incurs less than one reflection on average. It necessitates displacing the absorber from the parabola's focal plane, along the concentrators optic axis, toward the primary reflector, and constructing the secondary between the absorber and the primary. The secondary tailored edge-ray concentrators described here create new possibilities for building compact, extremely high flux solar furnaces and/or commercial parabolic dish systems.

#### B. Perers and B. Karlsson, "External reflectors for large solar collector arrays, simulation model and experimental results," Solar Energy, vol. 51, no. 5, pp. 327–337, 1993.doi: 10.1016/0038-092X(93)90145-E

A model for the calculation of incident solar radiation from flat- and CPC-shaped external reflectors onto flat plate solar collector arrays has been developed. Assuming an infinite length of the collector/reflector rows, the basic calculations of incident radiation in the collector plane from the reflector become very simple. The incident radiation onto the collector, including corrections for shadowing and lost radiation above the collector, can then be calculated using 2-D geometry. The diffuse radiation is assumed to be isotropic. The incidence angle for the solar radiation from the reflector onto the collector is in most cases higher than the incidence angle for the radiation directly from the sun. Therefore the incidence angle characteristics of the collector glazing and absorber become more important in this application. Equations for the incidence angles for diffuse and beam radiation are provided. An annual performance increase of over 30%, 100–120 kW h/m2, has been measured for aged (four operating seasons) flat reflectors in the Swedish climate. With a CPC-shaped reflector and new reflector materials, a performance increase of up to 170 kW h/m2 is not unrealistic. This means that the collector and ground area requirement can be reduced by more than 30% for a given load.

#### S. Hess and V. I. Hanby, "Collector Simulation Model with Dynamic Incidence Angle Modifier for Anisotropic Diffuse Irradiance," Energy Procedia, vol. 48, pp. 87–96, 2014. doi:10.1016/j.egypro.2014.02.011

One constant collector parameter, independent from slope or weather conditions is considered. The simulation model introduced considers the varying anisotropy of sky radiance. To create realistic distributions, the approach of Brunger and Hooper is used. Three possible modes were demonstrated. The model is applied to a stationary, double-covered process heat flat-plate collector with one-sided CPC booster reflector (RefleC). The collector shows a biaxial and asymmetric IAM for direct irradiance. It is found that, compared to anisotropic modeling, the simplified isotropic model is undervaluing the annual output of this collector by 13.7% for a constant inlet temperature of 120 °C in Würzburg, Germany. An annual irradiation distribution diagram shows that this is due to an underestimation of diffuse irradiation from directions with high direct irradiation. It is concluded that isotropic modeling of diffuse irradiance can be expected to significantly undervalue the annual output of all non-focusing solar thermal collectors. Highest relevance is found for high collector slopes, complex IAMs and at low-efficiency operation. The optimal collector slope is almost not affected. Accuracy of existing models can be increased by applying Mode 2.

#### V. P. Anand, M. M. Khan, E. Ameen, V. Amuthan, and B. Pesala, "Performance improvement of solar module system using flat plate reflectors," in 2014 International Conference on Advances in Electrical Engineering (ICAEE), 2014, pp. 1–4. doi: 10.1109/ICAEE.2014.6838547

The energy output of Si-cell modules is very low due to their low power conversion efficiencies of approximately 15%. Reflectors are used to improve the power output of PV modules, by increasing the effective capture area. The performance of the solar panel with reflector depends mainly on three parameters namely length, tilt angle and reflectivity of reflector. A model system to analyse the effect of reflector parameters on the overall power output is developed. Also a simplified mathematical model was developed which is capable of estimating the optimum tilt angle for a particular reflector length, and the optimality of the tilt angles predicted by this model was verified using the above mentioned experimental setup. Finally, the suitability of various materials for use as reflectors was studied using the setup. Interestingly, paper based reflectors like bond paper and thermocole showed excellent results with increase in power output of more than 60%. Also, the cost per watt of the system is minimal when we use aluminium foil as the reflector at optimum tilt angle.

#### S. Hess,"Stationary booster reflectors for solar thermal process heat generation," SASEC, 2015

The performance of a flat-plate collector with glass-foil double cover is compared to that of the same collector with a one-sided external CPC booster reflector (RefleC-collector) for process heat generation up to 150 C. Efficiency curve measurements of both collector variants are compared to state-of-the-art simulations and the new model calculates significantly higher additional gains of the reflectors.Monitoring results of one reference year for the overall system performance as well as for the additional gains by the booster reflectors are provided. It is shown that the stationary booster reflectors highly increase the efficient operation temperature range and also the annual energy gain of the double covered flat-plates.

### V-trough solar concentrators

#### J. Freilich and J. M. Gordon, "Case study of a central-station grid-intertie photovoltaic system with V-trough concentration," Solar Energy, vol. 46, no. 5, pp. 267–273, Jan. 1991. doi:

Our presentation is a case study of an installed, central-station (no storage), utility-intertie photovoltaic (PV) system in Sede Boqer, Israel (latitude 30.9°N). The nominally 12 kW peak PV system is comprised of 189 polycrystalline silicon modules mounted on inexpensive, one-axis north-south horizontal trackers with V-trough mirrors for optical boost. The power conditioning unit operates at a fixed voltage rather than at maximum power point (MPP). The primary task in analyzing the installed system was to investigate the cause of measured power output significantly below the design predictions of the installers, and to recommend system design modifications. Subsequent tasks included the quantitative assessment of fixed-voltage operation and of the energetic value of V-trough concentration and one-axis tracking for this system. Sample results show: (i) fixed-voltage operation at the best fixed voltage (BFV) can achieve around 96% of the yearly energy of MPP operation; (ii) the sensitivity of the yearly energy delivery to the selection of fixed voltage and its marked asymmetry about the BFV; (iii) the influences of inverter current constraints on yearly energy delivery and BFV; and (iv) how the separate effects of tracking and optical concentration increase yearly energy delivery.

#### N. Fraidenraich, "Design procedure of V-trough cavities for photovoltaic systems," Prog. Photovolt: Res. Appl., vol. 6, no. 1, pp. 43–54, Jan. 1998.doi: 10.1002/(SICI)1099-159X(199801/02)6:1<43::AID-PIP200>3.0.CO;2-P

The combination of photovoltaic (PV) systems with V-trough cavities has been identified as an attractive option to reduce, in the short time scale, the prices of the PV electrical energy. In places of good radiation level, the output energy of these devices can be almost doubled, compared to PV flat-plate fixed systems. Additionally, V-trough cavities are simple to manufacture and can be used with conventional (1-sun) solar cells. In this work we present a design procedure for V-trough cavities used in combination with PV generators. The main design requirements are: uniform illumination on the plane of the PV module, within a finite interval of incidence angles; minimum cost of energy; and heat dissipation by natural, passive means. The V-trough cavities depend on two parameters. We obtain a first analytical relation between the concentration ratio (C) and the V-trough angle (ψ) for concentrators with uniform illumination at the absorber. The region of minimum cost of the V-trough PV ensemble yields a second relation. Then, a unique pair of cavity parameters, satisfying the above criteria, is found. A design example of a V-trough cavity for the city of Recife, PE, Brazil, is presented.

#### J. Bione, O. C. Vilela, and N. Fraidenraich, "Comparison of the performance of PV water pumping systems driven by fixed, tracking and V-trough generators," Solar Energy, vol. 76, no. 6, pp. 703–711, 2004. doi: 10.1016/j.solener.2004.01.003

Photovoltaic pumping systems with solar tracking, coupled to low concentration cavities, have been proposed as a viable alternative to reduce the final cost of the pumped water volume. V-trough concentrators are particularly appropriate for photovoltaic applications since, for certain combinations of the concentration ratio (C) and vertex angle (Ψ), they provide uniform illumination on the region where the modules are located. Water pumping systems are only operational when the irradiance is larger than a minimum irradiance level (IC). Solar tracking increases the average collected irradiance (Icoll) and, for a system operating with a given critical irradiance level (IC), it is verified that the smaller the relationship (IC/Icoll), the larger the useful energy. Thus, the gain, in terms of pumped water volume, provided by solar tracking systems, can be larger than the gain in collected solar radiation. The combination of both devices, tracking and concentration provides an additional increase of the benefits resulting from the use of solar trackers. By means of the "Utilizability Method", we estimate the long-term gains of pumped water volume, for tracking systems, with and without concentration, against fixed systems. The long-term water volume has been calculated using the characteristic curve of a tested PVP system with a tracking V-trough concentrator. Results show that, for the climate of the city of Recife (PE-Brazil), the annual pumped water volume of the tracking system is 1.41 times the value obtained with the fixed system. In that case, the gains observed for the collected solar energy were around 1.23. For the PVP system with tracking V-trough concentrator the annual benefits for pumped water volume are around 2.49, while for collected solar radiation we found 1.74. The annualized cost of the cubic meter of pumped water has been estimated for the three configurations. Results show a cost reduction of the order of 19% for the tracking system and of 48% for the concentrating system, when compared to the fixed configuration.

#### C. S. Sangani and C. S. Solanki, "Experimental evaluation of V-trough (2 suns) PV concentrator system using commercial PV modules," Solar Energy Materials and Solar Cells, vol. 91, no. 6, pp. 453–459, Mar. 2007.doi: 10.1016/j.solmat.2006.10.012

V-trough photovoltaic (PV) concentrator systems along with conventional 1-sun PV module is designed and fabricated to assess PV electricity cost ($/W) reduction. V-trough concentrator (2-sun) system is developed for different types of tracking modes: seasonal, one axis north–south and two axes tracking. Three design models based on these tracking modes are used to develop the V-trough for a 2-sun concentration. Commercially available PV modules of different make and types were evaluated for their usability under 2-sun concentration. The V-trough concentrator system with geometric concentration ratio of 2 (2-sun) increases the output power by 44% as compared to PV flat-plate system for passively cooled modules. Design models with lower trough angles gave higher output power because of higher glass transmissivity. PV modules with lower series resistance gave higher gain in output power. The unit cost ($/W) for a V-trough concentrator, based on different design models, is compared with that of a PV flat plate system inclined at latitude angle (Mumbai, φ=19.12°).

#### N. Martín and J. M. Ruiz, "Optical performance analysis of V-trough PV concentrators," Prog. Photovolt: Res. Appl., vol. 16, no. 4, pp. 339–348, Jun. 2008. doi: 10.1002/pip.817

This paper proposes a method for the analysis of the optical losses that take place inside PV concentrators, which is useful in the design of such systems. The study is focused in V-trough concentrators with two-axis tracking. Those are low concentration systems that use nearly conventional flat PV modules. Optical losses are shown to depend on the cavity angle, the mirrors spectral and angular reflectance and the surfaces dirtiness. Final effective concentration ratio and relative cost should consider all these analysed factors. This will help in the search of the most efficient solution in each case.

#### C. S. Solanki, C. S. Sangani, D. Gunashekar, and G. Antony, "Enhanced heat dissipation of V-trough PV modules for better performance," Solar Energy Materials and Solar Cells, vol. 92, no. 12, pp. 1634–1638, Dec. 2008. doi: 10.1016/j.solmat.2008.07.022

A concentrator photovoltaic (PV) module, in which solar cells are integrated in V-troughs, is designed for better heat dissipation. All channels in the V-trough channels are made using thin single Al metal sheet to achieve better heat dissipation from the cells under concentration. Six PV module strips each containing single row of 6 mono-crystalline Si cells are fabricated and mounted in 6 V-trough channels to get concentrator V-trough PV module of 36 cells with maximum power point under standard test condition (STC) of 44.5 W. The V-trough walls are used for light concentration as well as heat dissipation from the cells which provides 4 times higher heat dissipation area than the case when V-trough walls are not used for cooling. The cell temperature in the V-trough module remains nearly same as that in a flat plate PV module, despite light concentration. The controlled temperature and increased current density in concentrator V-trough cells results in higher Voc of the module.

Page data
Type Literature review Akhila Reddy Gorantla 2016 CC-BY-SA-4.0 175 No main image Akhila Reddy Gorantla (2016). "Kirigami design approach to LCPV reflectors". Appropedia. Retrieved September 25, 2022.
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