A. Parretta

Angle-dependent reflectance measurements on photovoltaic materials and solar cells

Abstract Results of angle-dependent reflectance measurements on photovoltaic materials and solar cells are presented and discussed. The optical apparatus employs an integrating sphere of 40 cm diameter and it is suitable for working at variable wavelengths and incidence angles of the light beam. Directional/hemispherical reflectance measurements can be performed and both the diffuse and the total components of the reflected light can be obtained. Samples of any size can be measured by this apparatus, including small prototype solar cells (

Effects of solar irradiation conditions on the outdoor performance of photovoltaic modules

Abstract Energetic losses, relative to the standard conditions of testing, in photovoltaic modules in outdoor operation, were analyzed and the role of the optical effects is discussed. The following four loss effects were estimated: (a) reflection of unpolarized light, (b) spectrum, (c) intensity of the light and (d) temperature of the module. Four independent models were used to describe these four losses. The models were validated by the experimental data of an outdoor measurement campaign performed on 0° tilted modules at 41°N latitude in South Italy. Disagreement reaching 5% under clear sky conditions was found between theoretical predictions and experimental data for the instantaneous total loss. As a result of a critical analysis of the literature data on this subject, it could be explained by invoking the presence of a fifth loss mechanism: the polarization of the incident light. Final relative losses, due to the particular state of the incident sunlight, amount to about 7–8% of a total of 14–15%. Of these, 3% is due to the low irradiation level, 1–2% to the polarization of the skylight and 3% to the reflection of the incident light on the front cover of the module. The spectral effects are negligible (less than 1%). The remaining 7% loss is due to temperature effects on the module. All the loss data are reported as a function of the air mass (AM). The maximum operating efficiency is reached at AM≈1.5.

An investigation on the growth of thin chalcopyrite CuInSe2 films by selenization of Cu-In alloys in a box

Abstract The morphological and structural properties of copper-indium alloys selenized in a box by selenium vapours have been studied by varying the thermal cycle of the selenization process. In the first series of selenizations, equal metallic precursors were exposed for 30 min to the selenium vapours at different temperatures in the range 175–405°C The chalcopyrite CuInSe2 phase was found at temperatures as low as 250°C and as a single phase at T > 375°C. At low temperatures different copper selenides were found, like CuSe, Cu2 − xSe and CuSe2, which affected in a different way the composition and morphology of the film. Indium loss was detected in the 250–300°C range and was attributed to the evolution in the gas phase of the In2Se species. Short selenization cycles were also used with the purpose of identifying the chemical precursors of CuInSe2. The determination of the selenium content in the alloys at different temperatures allowed us to determine a 7 kcal/mol activation energy for the kinetics of selenium incorporation. Long selenizations at 450°C allowed us to obtain a large-grained, compact layer of chalcopyrite CuInSe2 which could be used for the fabrication of photovoltaic devices.

Optical efficiency of solar concentrators by a reverse optical path method

A method for the optical characterization of a solar concentrator, based on the reverse illumination by a Lambertian source and measurement of intensity of light projected on a far screen, has been developed. It is shown that the projected light intensity is simply correlated to the angle-resolved efficiency of a concentrator, conventionally obtained by a direct illumination procedure. The method has been applied by simulating simple reflective nonimaging and Fresnel lens concentrators.

Monitoring of concentrated radiation beam for photovoltaic and thermal solar energy conversion applications

Methods for evaluating the light intensity distribution on receivers of concentrated solar radiation systems are described. They are based on the use of Lambertian diffusers in place of the illuminated receiver and on the acquisition of the scattered light, in reflection or transmission mode, by a CCD camera. The spatial distribution of intensity radiation is then numerically derived from the recorded images via a proprietary code. The details of the method are presented and a short survey of the main applications of the method in the photovoltaic and thermal solar energy conversion field is proposed. Methods for investigating the Lambertian character of commercial diffusers are also discussed.

Method for measurement of the hemispherical/hemispherical reflectance of photovoltaic devices

Abstract A novel method and a relative apparatus, which allow the study of the reflectance properties at diffuse light of photovoltaic (PV) devices, are presented. The optical apparatus is provided with a 40 cm diameter integrating sphere (IS) which works like a lambertian source of diffuse light and, at the same time, collects diffuse light reflected by the sample. The reflectance measurements are based on the fact that the illumination intensity of the IS depends, besides on the input light power, also on the average reflectivity of its internal surface and then on the reflectance properties of any sample held against one of its apertures. The hemispherical/hemispherical reflectance, R hh , of the sample under test is obtained by measurements of light irradiance inside the sphere in correspondence with the sample and with a selected number of standards of diffuse reflectance. The described method was used to optically characterise prototype PV modules (to be used in buildings), realised by encapsulating different types of normal and “gridless” monocrystalline-Si (mono-Si) and multicrystalline-Si (multi-Si) cells with different colours. The reflectance of the optically active area of the prototype modules was measured and the optical effect of the grid evaluated. Also the optical loss at diffuse white light was found to be approximately 4–5% for the mono-Si cells and 6–9% for the multi-Si cells. The quantity R hh , when measured for very heterogeneous samples, such as multi-Si or poly-Si solar cells can be taken to represent the optical loss of the PV device operating outdoors, in the absence of sun tracking. This method, and the corresponding apparatus, can be extended to measurements on different plane surface samples.

Characterization of CPC solar concentrators by a laser method

In this paper we present a method of optical characterization of solar concentrators based on the use of a laser beam. The method, even though constrained by lengthy measurements, gives nevertheless interesting information on local mirror surface defects or manufacturing defects, like internal wall shape inaccuracies. It was applied to 3D-CPC-like concentrators and the measurements were supported by optical simulations with commercial codes. The method, simple to apply, requires just a laser to scan the CPC input aperture following a matrix-like path, at a controlled orientation of the beam. Maps of optical efficiency as function of the laser beam incidence angle are obtained by matching the CPC exit aperture with a photodetector with an efficient light trapping. The integration of each map gives the CPC efficiency resolved in angle of incidence, so curves of optical transmission (efficiency) as function of incidence angle can be drawn and the acceptance angle measured. The analysis of the single maps allows to obtain interesting information on light collection by the different regions of CPC input area. It reveals, moreover, how the efficiency of light collection depends on several factors like surface reflectivity, number of reflections of the single beam, local angle of incidence, local surface defects, and so on. By comparing the theoretical analysis with the experimental results, it is possible to emphasize the effects directly related to manufacturing defects.

A sky polarization compass in lizards: the central role of the parietal eye

SUMMARY The present study first examined whether ruin lizards Podarcis sicula are able to orientate using the e-vector direction of polarized light. Ruin lizards were trained and tested indoors, inside a hexagonal Morris water maze, positioned under an artificial light source producing plane polarized light with a single e-vector, which provided an axial cue. Lizards were subjected to axial training by positioning two identical goals in contact with the centre of two opposite side walls of the Morris water maze. Goals were invisible because they were placed just beneath the water surface, and water was rendered opaque. The results showed that the directional choices of lizards meeting learning criteria were bimodally distributed along the training axis, and that after 90 deg rotation of the e-vector direction of polarized light the lizards directional choices rotated correspondingly, producing a bimodal distribution which was perpendicular to the training axis. The present results confirm in ruin lizards results previously obtained in other lizard species showing that these reptiles can use the e-vector direction of polarized light in the form of a sky polarization compass. The second step of the study aimed at answering the still open question of whether functioning of a sky polarization compass would be mediated by the lizard parietal eye. To test this, ruin lizards meeting learning criteria were tested inside the Morris water maze under polarized light after their parietal eyes were painted black. Lizards with black-painted parietal eyes were completely disoriented. Thus, the present data show for the first time that the parietal eye plays a central role in mediating the functioning of a putative sky polarization compass of lizards.

Optical loss of photovoltaic modules under diffuse light

The optical behaviour of photovoltaic (PV) modules from different fabrication technologies was investigated under diffuse light by a novel characterization method. The optical apparatus, hemispherical/hemispherical reflectometer, allows both reflectance and transmittance measurements under an incident diffuse light which simulates the distributed outdoor irradiation from the sky or from the albedo. This paper presents only the configuration for reflectance measurements. The apparatus is provided with a single integrating sphere of 40 cm diameter, which acts as a lambertian source of diffuse light and spontaneously collects the diffuse light reflected by the sample in the front hemisphere. The hemispherical/hemispherical reflectance, R hh ; expresses the optical loss of the PV device under diffuse light, and is obtained by measurements of light irradiance inside the sphere in correspondence with the sample and with a selected number of standards of diffuse reflectance. The best optical performances, in terms of optical loss, were achieved by modules realized with blue mono-Si cells and having textured front surfaces in the cover sheets. r 2002 Elsevier Science B.V. All rights reserved.

Influence of Morphology and Structure of Cu/In Alloys on the Properties of CuInSe2

Thin CuInSe2 films were prepared by selenization of sputtered metal precursors. The surface morphology, phase composition and adhesion at the Mo/glass substrate of the ternary compound have been correlated to the structure of the precursor. Single phase, homogeneous, small grained (1 µm) CuInSe2 arises from precursors showing stable phases (Cu11In9 and In). An ordered vacancy compound (CuIn3Se5) is likely present in In-rich (In>27 at.%) films. Precursors with a high content of the metastable CuIn2 phase show a very poor adhesion to the substrate.