European Solar Test Installation Research Articles

Using the equivalent cell temperature for power matrix calculations

Performing measurements and compiling a power matrix (IEC 61853-1) is a useful tool for illustrating the energy production of a PV module at different levels of irradiance and temperature. At the European Solar Test Installation (ESTI) a steady-state solar simulator can be used to determine these matrices. The steady-state solar simulator irradiates the PV module from the front side, which heats the module to the desired temperature. However, this causes a temperature difference between the front surface of the module and the rear, where the temperature sensor is attached. This temperature difference, between the measured module temperature (Tm) and the temperature of the cell junction, gives rise to errors. In this paper, a correction procedure according to the determination of the equivalent cell temperature (ECT) of photovoltaic devices by the open-circuit voltage method in IEC 60904-5 is proposed and evaluated. In this study the difference between the Tm and the ECT was determined to about 1 °C to 3 °C, resulting in a deviation of the Pmax value in the power matrix up to about 1.5%, for a monocrystalline PV module.

Recent Progress in Perovskite Tandem Solar Cells.

Tandem solar cells are widely considered the industry's next step in photovoltaics because of their excellent power conversion efficiency. Since halide perovskite absorber material was developed, it has been feasible to develop tandem solar cells that are more efficient. The European Solar Test Installation has verified a 32.5% efficiency for perovskite/silicon tandem solar cells. There has been an increase in the perovskite/Si tandem devices' power conversion efficiency, but it is still not as high as it might be. Their instability and difficulties in large-area realization are significant challenges in commercialization. In the first part of this overview, we set the stage by discussing the background of tandem solar cells and their development over time. Subsequently, a concise summary of recent advancements in perovskite tandem solar cells utilizing various device topologies is presented. In addition, we explore the many possible configurations of tandem module technology: the present work addresses the characteristics and efficacy of 2T monolithic and mechanically stacked four-terminal devices. Next, we explore ways to boost perovskite tandem solar cells' power conversion efficiencies. Recent advancements in the efficiency of tandem cells are described, along with the limitations that are still restricting their efficiency. Stability is also a significant hurdle in commercializing such devices, so we proposed eliminating ion migration as a cornerstone strategy for solving intrinsic instability problems.

Implementation of the IEC TS 60904-1-2 Measurement Methods for Bifacial Silicon PV Devices

Bifacial crystalline Silicon (Si) photovoltaic (PV) devices are attracting considerable interest from manufacturers and the market since they can enhance the performance in comparison with traditional monofacial PV devices. Technical specification IEC TS 60904-1-2 was published in 2019 and proposes several characterization methods for bifacial PV device testing based on single-side, double-sided and natural sunlight illumination. This article analyses the advantages, disadvantages, the suitability and the feasibility of the different methods and compares the electrical performance obtained by the proposed approaches at the European Solar Test Installation for the testing of bifacial Si devices. Deviations in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> below 0.9% for the different testing methods under test was obtained except for the double-source approach with a rear reflector (2.59%) where the measurement of the rear irradaince influence the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <u>P</u> </i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> .

A European proficiency test on thin‐film tandem photovoltaic devices

AbstractA round‐robin proficiency test (RR PT) on thin‐film multi‐junction (MJ) photovoltaic (PV) cells was run between 13 laboratories within the European project CHEETAH. Five encapsulated PV cells were circulated to participants for being tested at Standard Test Conditions (STC). Three cells were a‐Si/μc‐Si tandem PV devices, each of which had a different short‐circuit current ratio between the top junction and the bottom one; the remaining two cells were single‐junction PV devices made with material representative of the individual junctions in the MJ cells. The RR PT's main purpose was to assess the capability of the participating laboratories, in terms of employed facilities and procedures, to test MJ PV devices. Therefore, participants were requested to perform STC measurements of all cells according to their own procedure, which might not include external quantum efficiency measurements. The European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) provided the reference calibrations against which the participants' results are compared. ESTI made also a verification of the cells performance at STC at the end of the RR PT, in order to allow a comparison between the initial stable state at which the cells were calibrated (just before circulation) and the one they had reached at the end of the RR PT. The overall results of the RR PT are here presented and discussed together with some aspects of MJ PV testing that emerged as not adequately applied or largely missing. Their full implementation is expected to improve the consistency of future results.

Uncertainty budget assessment of temperature coefficient measurements performed via intra-laboratory comparison between various facilities for PV device calibration

The performance of PV devices at different operating temperatures is important for determining the energy they produce once deployed. Therefore current–voltage characteristics are measured over a range of typical operating temperatures and the temperature coefficients of the main electrical performance parameters are extracted. They are the temperature coefficient α of short-circuit current, β of open-circuit voltage and δ of maximum power. The European Solar Test Installation has recently upgraded its capability to determine these temperature coefficients. The existing two setups were improved to be able to cover extended temperature ranges and two further setups and respective measurement procedures were made operational. For all setups revised uncertainty estimates were calculated. Here the setups and procedures are presented, together with the measurement uncertainty for the three temperature coefficients. Measurements were made on seven PV devices and their consistency compared based on uncertainties. It was found that all setups are fully consistent. The advantages of the different setups are also discussed.

Traceable calibration of photovoltaic reference cells using natural sunlight

At the European Solar Test Installation (ESTI) photovoltaic (PV) reference cells are calibrated traceably to SI units via the World Radiometric Reference (WRR) using natural sunlight. The Direct Sunlight Method (DSM) is described in detail and the latest measurement results and an updated uncertainty budget are reported. These PV reference cells then provide a practical means for measuring the irradiance of natural or simulated sunlight during the calibration of other PV devices.

Uncertainty Propagation of Spectral Matching Ratios Measured Using a Calibrated Spectroradiometer

The international standard IEC62670-3 (International Electrotechnical Committee) “Photovoltaic Concentrators (CPV) Performance Testing—Part 3—Performance Measurements and Power Rating” sets the guidelines for power measurements of a CPV device, both in indoor and outdoor conditions. When measuring in outdoor conditions, the acquired data have to be filtered a posteriori, in order to select only those points measured with ambient conditions close to the Concentrator Standard Operating Conditions (CSOC). The most stringent requirement to be met is related to the three Spectral Matching Ratios (SMR), which have all to be within the limit of 1.00 ± 0.03. SMR are usually determined by the ratio of the currents of component cells to monitor the outdoor spectral ratio conditions during the CPV device power measurements. Experience demonstrates that obtaining real world data meeting these strict conditions is very difficult in practice. However, increasing the acceptable range would make the entire filtering process less appropriate from a physical point of view. Given the importance of correctly measuring the SMR, an estimation of their associated measurement uncertainties is needed to allow a proper assessment of the validity of the 3% limit. In this study a Monte Carlo simulation has been used, to allow the estimation of the propagation of uncertainties in expressions having the and integral form. The method consists of applying both random and wavelength correlated errors to the measured spectra and to the measured spectral responses of the three CPV cell junctions, according to the measurement uncertainties of the European Solar Test Installation (ESTI). The experimental data used in this study have been acquired during clear sky conditions in May 2016, at ESTI’s facilities in Ispra, northern Italy (45°49′ N 8°37′ E).

Evolution of measured module characteristics versus labelled module characteristics of crystalline silicon based PV modules

This work outlines the evolution of measured module parameters (power Pmax, short circuit current Isc, open circuit voltage Voc) vs labelled module parameters of crystalline silicon PV modules over a number of decades at the European Solar Test Installation (ESTI) at Ispra, Italy. Manufacturers' tolerance in nameplate power has changed over the years with many claiming ±10% in the 80s and 90s. The declared tolerance over recent years has been reduced to ±5% then ±3% and today values of −0/+3% are quite common. To know the declared tolerance of modules in an operational system is useful additional information when investigating the aging of PV systems, when the original module power may either be unknown or missing with the passage of decades. Therefore, researchers have often to rely on the labelled power for the starting point of the system. Measurements at the ESTI laboratory have been performed since the early 1980s. A comparison of those modules which incorporated either a nameplate (or an associated datasheet, flash report or online information) to their measured output parameters (Pmax, Isc and Voc) has been carried out. The impact of these differences from an economic point of view has been also analysed. This work presents some selected results from a set of 992 crystalline silicon based modules, extracted from various time periods from 1982 to 2014.

Long-term soiling of silicon PV modules in a moderate subtropical climate

The results of 28 silicon-based PV modules which were installed from 1981 to 1985 in a free rack configuration in the outdoor test facility of the European Solar Test Installation (ESTI) and dismantled at the end of 2014 without cleaning were analysed. The system was composed of modules with two series-connected laminates mounted in a single frame produced by the same manufacturer but using different cell layouts and cover glasses (plain glass or textured glass). The effects of long-term soiling on the PV module performance for more than 30years of outdoor exposure in a moderate subtropical climate and the influence of different cleaning methods from manual cleaning to the use of high pressure water washing were investigated. The influence of the cover glasses and the mismatch due to the particular manufacturing design were also analysed. It was observed that a manual cleaning was effective at improving the output of all the module types. However, additional high pressure water spraying on plain glass modules showed no further improvement, but showed small improvements on the textured glass modules. Overall improvements in Pmax after cleaning ranged from 3.5% to 19.4%, with an average value of 9.8% and an average improvement in Isc of 6.7% were obtained.

Improved conversion efficiencies of thin-film silicon tandem (MICROMORPH™) photovoltaic modules

Increased electrical power generated from a thin-film silicon (TF-Si) photovoltaic device can lead to a reduced cost of electricity production that will support the mass adoption of this technology as a renewable energy source. Extracting the highest conversion efficiency from ‘champion’ large area TF-Si modules has been the focus of development at TEL Solar AG, Trübbach. The layer deposition process adjustments and further module technology improvements that led to a significant increase in the absolute stabilized module conversion efficiency of large area (1.43m2) tandem MICROMORPH™ modules centered first on obtaining high quality amorphous TF-Si deposited materials for the top cell. This was integrated with microcrystalline TF-Si material for the bottom cell that was deposited under conditions close to the transition point between the amorphous and microcrystalline growth regimes. In an optimized solar cell design the TF-Si materials were then combined with effective light management technologies and an improved module layout. The end result of a world record large area (1.43m2) stabilized module conversion efficiency of 12.34% was certified by the European Solar Test Installation (ESTI). The main technology contributions in the device design for this breakthrough result that generated more than 13.2% stabilized efficiency from each equivalent 1cm2 of the active area of the full module are described.

Reduction of uncertainties for photovoltaic reference cells

This paper presents the latest implementation of the ESTI reference cell set at the European solar test installation. The set consists of five photovoltaic reference cells used to measure solar irradiance. Each cell has been calibrated multiple times by primary methods. The stability of the set has been maintained and monitored over a long time period (up to 20 years). Taken together this allowed a rigorous determination of the assigned calibration value for each cell based on the weighted average, together with its uncertainty. This has led to unprecedented low uncertainties of 0.23% (k = 2), which are at least a factor of two smaller than achievable with any single primary calibration. Using these cells for calibrating secondary reference cells with a dedicated set-up leads to uncertainties for these secondary calibrated cells which is of the same order as the best primary calibration methods.

Record 12.34% stabilized conversion efficiency in a large area thin‐film silicon tandem (MICROMORPH™) module

AbstractMass‐adoption of thin‐film silicon (TF‐Si) photovoltaic modules as a renewable energy source can be viable if the cost of electricity production from the module is competitive with conventional energy solutions. Increased module performance (electrical power generated) is an approach to reduce this cost. Progress towards higher conversion efficiencies for ‘champion’ large area modules paves the way for mass‐production module performance to follow. At TEL Solar AG, Trübbach, Switzerland, significant progress in the absolute stabilized module conversion efficiency has been achieved through optimized solar cell design that integrates high‐quality amorphous and microcrystalline TF‐Si‐deposited materials with efficient light management and transparent conductive oxide layers in a tandem MICROMORPH™ module. This letter reports a world record large area (1.43 m2) stabilized module conversion efficiency of 12.34% certified by the European Solar Test Installation; an increase of more than 1.4% absolute compared with the previous record for a TF‐Si triple junction large area module. This breakthrough result generates more than 13.2% stabilized efficiency from each equivalent 1 cm2 of the active area of the full module. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Statistical functions and relevant correlation coefficients of clearness index

This article presents a statistical analysis of the sky conditions, during years from 2010 to 2012, for three different locations: the Joint Research Centre site in Ispra (Italy, European Solar Test Installation – ESTI laboratories), the site of National Renewable Energy Laboratory in Golden (Colorado, USA) and the site of Brookhaven National Laboratories in Upton (New York, USA). The key parameter is the clearness index kT, a dimensionless expression of the global irradiance impinging upon a horizontal surface at a given instant of time. In the first part, the sky conditions are characterized using daily averages, giving a general overview of the three sites. In the second part the analysis is performed using data sets with a short-term resolution of 1 sample per minute, demonstrating remarkable properties of the statistical distributions of the clearness index, reinforced by a proof using fuzzy logic methods. Successively some time-dependent correlations between different meteorological variables are presented in terms of Pearson and Spearman correlation coefficients, and introducing a new one.

Power rating of photovoltaic modules including validation of procedures to implement IEC 61853‐1 on solar simulators and under natural sunlight

ABSTRACTThis paper describes the measurement of photovoltaic module performance over a range of temperatures and irradiances according to the international standard IEC 61853 Part 1. The purpose of this work is to assess the reproducibility of power matrix measurements obtained using two methods specified in the standard: under natural sunlight with a tracker, and with a solar simulator. A comparison of results using the third principal method (under natural sunlight without tracker) is also summarised for completeness. The same measurement techniques have been employed to measure four modules of different technologies, namely mono and poly crystalline Si, CdTe and CIS. The method used to vary the irradiance in the natural sunlight with tracker and solar simulator approaches is based on un‐calibrated mesh filters. The uniformity and effect on spectrum of the mesh filters have been studied, and the impact of these on the measurements estimated. Measurements from all methods are compared over as much of the ranges as possible. The results show that for all modules, the reproducibility is within the estimated measurement uncertainty. The suitability of the different methods is discussed in light of the results and the limitations of the various methods as applied to different modules technologies. On the basis of the results, parts of IEC 61853 Part 1 will be introduced into the ISO 17025 laboratory accreditation at the European Solar Test Installation (ESTI). Copyright © 2013 John Wiley &amp; Sons, Ltd.

Polycrystalline silicon PV modules performance and degradation over 20 years

ABSTRACTThe presented paper reports the results of the experimental work performed at the European Solar Test Installation, using an array of 70 polycrystalline silicon photovoltaic (PV) modules by the same manufacturer. After almost 20 years of continuous outdoor exposure, the modules were subjected to a comprehensive indoor test plan; in particular, electrical performance measurements were performed, together with a detailed visual analysis. It was also possible to perform a comparison between final and initial data (in particular IV characteristics): module average performance decay is 4.42% for the whole period. Degradation mechanisms, together with their effect on module lifetime, were also analyzed. Results of such a measurement exercise clearly show how PV device reliability over decades can guarantee safe investments, for the benefit of all PV users and stakeholders. The authors are currently installing the modules for further 20 years of outdoor exposure. Copyright © 2012 John Wiley &amp; Sons, Ltd.

A validation study of photovoltaic module series resistance determination under various operating conditions according to IEC 60891

ABSTRACTThis paper presents a validation study on series resistance determination of photovoltaic modules on the basis of the IEC 60891 edition 2.0 (2009) normative standard. A selection of different technologies has been investigated, ranging from polycrystalline silicon to thin film modules. Their I–V characteristics have been measured at the European Solar Test Installation under various operating conditions with respect to module temperature and irradiance. The aim of this research work is twofold: firstly, it contributes to the quantitative assessment of the effect of the various calculation parameters on the overall precision of the series resistance; secondly, it provides information about the dependence of series resistance on module performance over a wide range of irradiance and module temperature. A comparison with the previous calculation procedure from the first edition of the IEC 60891 (1987) has also been performed in order to understand how the differences between the two methods can affect the final results. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Characterization of high‐efficiency c‐Si CPV cells

AbstractConcentrating photovoltaics (CPV) represent an emerging market at its early stage for terrestrial application with large growth expectation and considerable research activity on new cell production has been conducting recently. In response to a fast growing demand for traceable and independent measurements on CPV cells and systems the European Solar Test Installation (ESTI) has started a research program on indoor/outdoor devices characterization. This paper focuses on two essential components of device calibration. Calibration requires traceable calibrated reference devices: for this purpose reference c‐Si cells equipped with neutral density filters of various optical densities have been used at ESTI for checking device linearity at different concentration ratios. Also essential is the implementation of common procedures and intercomparison of test results at different test laboratories. Procedures and experimental setups for concentrating c‐Si cell characterization at both ESTI and at the UTTP FOTO Laboratory of ENEA are described and results of an intercomparison are presented and discussed. Electrical performance measurements have been performed at high intensities on three different setups: a recently developed 1500× pulsed solar simulator at ESTI and two modified solar simulators capable of achieving up to 250× concentration at ENEA. Reported results show agreement within measurement uncertainties of the electrical parameters. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Cu(In,Ga)Se2 solar cell grown on flexible polymer substrate with efficiency exceeding 17%

AbstractDevelopment of a Cu(In,Ga)Se2 thin film solar cell on a polyimide film with a conversion efficiency of 17.1%, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Seasonal variations on energy yield of a‐Si, hybrid, and crystalline Si PV modules

AbstractThis paper presents a study of long‐term outdoor performance of a‐Si and hybrid modules mounted in the same location over several years. The modules were also characterized indoors using standard measurement methods employing pulsed solar simulators at the European Solar Test Installation (ESTI). The present study is intended to contribute to future standards on energy rating by presenting a common procedure for correcting the outdoor performance measurements to standard test conditions and comparing the resulting module performance at real and laboratory conditions. A seasonal variation in output, higher in the summer and lower in the winter, suggests that the module performance improves due to annealing when the module temperature is higher. The total output energy per month for these two technologies and a reference c‐Si technology is also presented. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Analysis and mitigation of measurement uncertainties in the traceability chain for the calibration of photovoltaic devices

This paper describes the traceability chain for photovoltaic devices and the measurement methods employed to perform the various transfer steps. The measurement uncertainties are analysed in detail based on the accreditation of the European Solar Test Installation (ESTI) for the calibration of photovoltaic devices. The various contributions to the overall uncertainty are critically analysed for various traceability chain options. A major contribution is the uncertainty in the calibration of the primary reference device. The overall measurement uncertainty is reduced using the ESTI reference cell set compared to the traceability from the world photovoltaic scale. For the maximum power of photovoltaic modules, the expanded combined uncertainty is reduced from ±2.6% to below ±2%. Recommendations are made on the scope for further reduction of uncertainty and for the best calibration strategy for various PV technologies.