Robert P. Kenny

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.

Spectral mismatch in calibration of photovoltaic reference devices by global sunlight method

The state of the art for calibration of photovoltaic reference devices has been reviewed recently. In the global sunlight method the total irradiance is measured by a pyrheliometer and a shaded pyranometer while the device is held at 25 °C. The method is supposed to have high repeatability even if the spectral irradiance is not measured, provided that the atmospheric conditions are selected carefully. Here we investigate seasonal variation of this method by calculating the spectral mismatch for crystalline silicon reference solar cells for spectra measured at air mass of 1.5 and the defined reference spectral irradiance. A seasonal variation is found with negligible mismatch for certain periods but significant deviations for other periods. Based on this a modified global sunlight method using global natural sunlight for the calibration of photovoltaic reference devices is proposed, using measured spectral irradiance and applying mismatch correction.

Estimating PV performance over large geographical regions

We present a method for estimating the performance of PV modules over large geographical regions. It uses solar radiation and temperature geospatial data that are modeled in a geographical information system. We employ a relation for the conversion efficiency of crystalline silicon cells at varying irradiance and temperature to obtain values for the loss of efficiency at any location. The method has been applied to the European subcontinent. Results indicate that the relative efficiency of crystalline Si modules varies across Europe with values ranging from 87 to 97% of the conversion efficiency at STC (highest in the North and in mountainous areas).

Characterization of a Pulsed Solar Simulator for Concentrator Photovoltaic Cell Calibration

The upgrade of a large area xenon lamp pulsed solar simulator to a 1500X pulsed solar simulator is presented in detail. Spectral match, spatial non-uniformity and temporal instability are analyzed according to the existing international standards, resulting in a Class CAA solar simulator: ongoing improvements to meet Class AAA requirements are discussed. The procedure for the calibration of a set of c-Si reference cells equipped with neutral density filters for calibrated measurements at high intensity is also presented. The paper finally highlights the importance of the spectral match to the standard AM1.5d (direct beam) spectrum over a wider range than described in the available standards for solar simulator classification.

Performance Measurements of CIS Modules: Outdoor and Pulsed Simulator Comparison for Power and Energy Rating

A method for energy yield prediction under development in this laboratory has proved to be accurate for crystalline Si modules, however, in the determination of an energy rating of CIS modules, it was found to be difficult to accurately predict the module performance based on simulator measurements. The Energy Rating (ENRA) procedure is based on determining the maximum power point Pmax as a function of the irradiance and the module temperature using standard pulsed simulator measurements. Long term outdoor measurements have been performed to test the methods accuracy. It has been observed that the power measured on the simulator is in general lower than that measured outdoors and this is a significant cause of the difficulties in the energy predictions. The short circuit current has not been found to be significantly different, rather the difference is mainly due to lower open circuit voltage and fill factor. It is known that pre-conditioning CIS modules by light soaking can increase these parameters, and a study has been made of the short term effect of light soaking on the module. It has been found that the light soaking does increase the power measured on the simulator, but the light soaking effect relaxes very quickly, and it is therefore important to measure the module immediately following light soaking

From an existing large area pulsed solar simulator to a high intensity pulsed solar simulator: Characterization, standard classification and first results at ESTI

The development and the characterization of ESTIs new high intensity pulsed solar simulator for concentrator photovoltaic (CPV) cells is presented, as the upgrade of an old large area pulsed solar simulator. The new setup is currently able to reach intensities as high as 1500 suns over a testing area of few centimeter diameter. Classification according to the international standard IEC 60904-9 and the ASTM 927-09 standard is given, resulting in a Class CAA solar simulator. Class C spectrum does not affect the characterization of single-junction CPV devices, thanks to the use of proper reference cells to reduce the spectral mismatch. Nevertheless, a better classification is needed to allow the current-voltage characterization of multi-junction CPV cells. The improvement is ongoing, but a better current balance is already shown when proper spectrally selective filters are used.

Electrical characterization of concentrator PV cells: A comparison between outdoor testing under direct solar radiation and indoor measurements on a high intensity solar simulator

The European Solar Test Installation (ESTI) laboratory and the Department of Electrical and Computer Engineering of the University of Cyprus (UCY) have shared their exploratory research on indoor/outdoor characterization of concentrating photovoltaic (CPV) cells and modules. The activity is performed within the 5-year APOLLON project, a co-financed activity of the VII research framework program of the European Commission. In this work we present results on the indoor characterization of c-Si and III–V group CPV cells by means of the new high intensity pulsed solar simulator developed at ESTI and the outdoor measurement under concentrated irradiance that has been carried out at UCY to corroborate the indoor results.

Outdoor characterization of Luminescent Solar Concentrators and their possible architectural integration on a historically relevant site in Milan (Italy)

Luminescent Solar Concentrators (LSCs) typically consist of transparent plastic slabs, doped with luminescent species: incident light is partially absorbed and internally re-radiated by luminescence towards the edges of the slab, where photovoltaic cells are placed to convert light into electricity. Interest in LSCs has recently been raised thanks to improvements in module efficiency. A LSC is able to collect both direct and diffuse light and it is therefore very promising as a potentially cheap residential concentrating system. Outdoor characterization of various LSC prototypes has been performed at the outdoor solar field of the European Solar Test Installation in Ispra (VA), Italy. Results enable a first estimate of the annual energy yield of a hypothetical building integration of LSCs, given the annual average irradiance and the geometry of the installation. In this work, we also present a possible architectural integration concept in the historic environment of the Roman Empire archaeological site of Milan.

SOPHIA CPV MODULE ROUND ROBIN: POWER RATING AT CSOC

In the frame of the European project SOPHIA a concentrator photovoltaic (CPV) module measurement round robin has been initiated. The round robin includes measurements of four CPV modules at seven different test laboratories located in Europe. IV curves of the modules are measured with different measurement equipment under various climatic conditions. The aim of this activity is to perform at each site a rating of the modules at concentrator standard operating conditions CSOC according to IEC 62670-1. The outcome of the round robin is intended for direct feedback to the current draft standard IEC 62670-3 “Concentrator Photovoltaic (CPV) Performance Testing - Performance Measurements and Power Rating”. The paper discusses initial results from the first three partners that have already finished the measurements up to now.

High Quality Measurements of the Solar Spectrum for Simulation of Multi-Junction Photovoltaic Cell Yields

Photovoltaic cells that incorporate several active junctions in electrical series are demonstrating everhigher laboratory efficiencies. However, their sensitivity to variations in the solar spectrum can reduce their operating efficiency in the field. To assess how these variations can affect energy yield requires a detailed and reliable spectral irradiance dataset covering a typical meteorological year for any potential deployment site. This paper describes an attempt to produce such a dataset through a combination of measurements and simulation. A dedicated measurement system has been assembled to collect environmental data alongside direct normal spectral irradiance measurements utilising charge-coupled device array spectroradiometers. The calibration and validation procedures for this system are presented, and the final calibration uncertainty of the system is judged to be ±5.4% (k=2). The SMARTS atmospheric radiative transfer model is used to provide another means of measurement validation. Furthermore, SMARTS is then used to generate long-term irradiance data covering several months. A comparison of the measurements and simulations has shown that SMARTS is a useful tool for measurement validation. However, the present environmental inputs to the model produce long-term data that underestimates the occurrence of very bluerich spectra compared to measurements.