G. Friesen

Energy rating of PV modules: comparison of methods and approach

The Matrix and Performance Surface Methods of Energy Rating are related techniques under development for the determination of electrical yield that is intended to be a more useful predictor of performance for installers than Wp alone. Here a power matrix or performance surface as a function of irradiance and ambient temperature P(Gi; Ta) is linked to a climatic condition occurrence matrix N(Gi, Ta) for a particular location. The use of just two independent variables has the advantage of simplicity but it is important to evaluate the possible cost of reduced accuracy due to the exclusion of other variable parameters such as Air Mass for example. The power matrix may be determined by outdoor or indoor measurements, and the complete matrix may also be extrapolated from a reduced data set using models of cell behaviour thereby reducing measurement time. The present paper relates solely to crystalline Si and reports on the comparison of outdoor measurements at two sites, and their further comparison with indoor measurements.

Capacitance effects in high-efficiency cells

Abstract Frequently transient measurement techniques lead to capacitance effects which complicate the accurate measurement of the performance of high efficiency solar cells. The photo-current-response measurement (PCR), developed at ESTI, offers a tool for the investigation of these capacitance effects. This paper describes the theory of capacitance effects and the diffusion capacitance as experimental results achieved by the PCR-method. The theory shows that the diffusion capacitance is strongly dependent on the minority carrier diffusion length and lifetime. In the future the PCR-method could be used for the determination of this solar cell parameter. We show, using monochromatic light pulses, that the induced diffusion capacitance charge (Qdiff) is exponentially dependent on the bias voltage and linearly dependent on the light intensity. Finally, the capacitance effect is made clearly visible by the generation of the current-voltage characteristic from PCR-measurements.

Impedance model for CdTe solar cells exhibiting constant phase element behaviour

Abstract Different equivalent circuit models were used to fit the impedance spectra of CdTe solar cells at fixed d.c.-voltages and the resulting C – V (capacitance–voltage)-curves and C – f (capacitance-frequency)-curves were analysed. A simplified equivalent circuit model, which consists of a parallel resistor R p and capacitor C in series with a resistor R s , does not give a good fit to the experimental data and is not capable of simulating the dispersive trend. Also the commonly assumed model for CdTe, which consist of two sub-circuits ( R p and C ) does not allow the simulation of the measured impedance spectra. As both models are only consisting of frequency-independent circuit elements they can not be used to describe the frequency dispersion of thin film CdTe solar cells. A phenomenological description of the capacitance behaviour was obtained by replacing the capacitor of the first model by a frequency-dependent non-ideal capacitor, a so-called constant phase element (CPE). This element is described by the two CPE-parameters ( T and P ). A correlation of these parameters to the capacitance dispersion and to the real cell capacitance are presented and the physical phenomena responsible for the capacitance dispersion are discussed.

Investigation of CdTe solar cells via capacitance and impedance measurements

Abstract Two CdTe solar cells with very different IV-characteristics were analysed using impedance spectroscopy (IS) and capacitance–voltage (CV) measurements at different temperatures and under dark and illuminated conditions. The two samples were distinguished by different durations of the nitric–phosphoric (NP) etching process applied before back contact deposition. The cell with a short etching time (30 s) shows a strong roll over in the forward bias region, and the cell with the surface that was etched for a longer time (3 min) showed a very strong decrease of the roll-over and a better fill factor. The results show that the etching process dramatically changes the CV characteristics, the resulting CdTe effective doping concentration and that this change can be correlated to different equivalent circuits obtained by the fitting of the impedance spectra of the two samples.

The Effects of Solar Cell Capacitance on Calibration Accuracy When using a Flash Simulator

A model for investigating the dynamic behaviour of solar cells, which allows the simulation of the measurements process with flash solar simulators, is presented in this paper. This model is then applied to the simulation of measurement processes currently employed by common solar simulator types. It is shown that the cell capacitance, the series resistance and cell area can all influence the measurements, giving rise to transient errors, especially in short flash solar simulators. It is also shown that point distribution as well as scan time also has significant importance

Photovoltaic Performance Measurements in Europe: PV-Catapult Round Robin Tests

Two sets of modules have been sent around to different testing installations across Europe, one set to laboratories performing indoor calibrations and one set to laboratories performing outdoor power and energy ratings. The results show that for crystalline and poly-crystalline devices, a very good agreement between laboratories has been achieved. A lower agreement between laboratories has been achieved for thin film devices and further need for research is identified

Fast and accurate methods for the performance testing of highly-efficient c-Si photovoltaic modules using a 10 ms single-pulse solar simulator and customized voltage profiles

Performance testing of highly efficient, highly capacitive c-Si modules with pulsed solar simulators requires particular care. These devices in fact usually require a steady-state solar simulator or pulse durations longer than 100?200?ms in order to avoid measurement artifacts. The aim of this work was to validate an alternative method for the testing of highly capacitive c-Si modules using a 10?ms single pulse solar simulator. Our approach attempts to reconstruct a quasi-steady-state I?V (current?voltage) curve of a highly capacitive device during one single 10?ms flash by applying customized voltage profiles?-in place of a conventional V ramp?to the terminals of the device under test. The most promising results were obtained by using V profiles which we name ?dragon-back? (DB) profiles. When compared to the reference I?V measurement (obtained by using a multi-flash approach with approximately 20 flashes), the DB V profile method provides excellent results with differences in the estimation of Pmax?(as well as of Isc, Voc?and FF) below ?0.5%. For the testing of highly capacitive devices the method is accurate, fast (two flashes?possibly one?required), cost-effective and has proven its validity with several technologies making it particularly interesting for in-line testing.

Guidelines for PV Power Measurements in Industry

The mission of the JRC is to provide customer-driven scientifi c and technical sup-port for the conception, development, implementation and monitoring of EU policies. As a service of the European Commission, the JRC functions as a refer-ence centre of science and technology for the Union. Close to the policy-making process, it serves the common interest of the Member States, while being inde-pendent of special interests, whether private or national.

Characterization of CPV cells on a high intensity solar simulator: A detailed uncertainty analysis

The authors modified a large area pulsed solar simulator to a high intensity pulsed solar simulator, moving the test measurement plane to various distances close to the xenon lamp. Measurement results reported in this work have confirmed the r−2 dependence of the total irradiance on the cell-to-lamp distance, up to the maximum measured intensity at 3000X. Classification of the solar simulator based on the international standard IEC 60904–9 is reported. Though the standard procedure for current-voltage measurements of non-concentrating cells prescribes a calibrated reference cell to detect the total irradiance independently, a “self-reference” method is usually preferred at high intensities by the CPV community. In this work the authors apply a detailed uncertainty analysis to both the procedures. The result highlights the pros and contra of the two methods, giving a useful tool in the pre-normative work for the preparation of norms and standard procedures for terrestrial CPV cells characterization.

Accuracy of Energy Prediction Methodologies

In the current market, the specific annual energy yield (kWh/kWp) of a PV system is gaining in importance due to its direct link to the financial returns for possible investors who typically demand an accuracy of 5% in this prediction. This paper focuses on the energy prediction of photovoltaic modules themselves, as there have been significant advances achieved with module technologies which affect the device physics in a way that might force the revisiting of device modelling. The paper reports the results of a round robin based evaluation of European modelling methodologies. The results indicate that the error in predicting energy yield for the same module at different locations was within 5% for most of the methodologies. However, this error increased significantly if the nominal nameplate rating is used in the characterization stage. For similar modules at the same location the uncertainties were much larger due to module-module variations