Advanced PV Energy Rating, "PV-Enerate"

Abstract: Within the EURAMET ENG55 “PhotoClass” project, several characteristics of photovoltaic (PV) devices beyond their performance at Standard Test Conditions were investigated, including measurements at varying irradiance and temperature. Four groups of PV devices of different size and technology were prepared and corresponding round-robins were run between partner laboratories with substantially different facilities and methods – namely based on spectral or integral measurements. This paper presents the outcome of the four inter-laboratory comparisons dealing with temperature coefficient measurements of the short-circuit current of PV devices, from reference-cell size to full-size commercial modules of mainly several c-Si technologies, but also with some examples of CIGS and GaAs devices. The measurement results are compared via En number assessment, hence including measurement uncertainties. The main outcome of this measurement exercise is a very good agreement of all the laboratories although completely different approaches were applied. In some cases, laboratory measurement uncertainties are even considered rather conservative and could therefore be revised. Furthermore, a comparison between bare cells and commercial modules of the same technology is made, which may represent useful information for PV manufacturers.

Abstract: Bifacial photovoltaic (PV) modules can increase the performance with respect to traditional PV modules because both sides of the cells, front and rear, absorb solar radiation. To assess their performance and quality, PV modules are characterized using international standards. However, currently only a draft IEC technical specification exists for bifacial PV modules and research needs to be done in order to study the indoor performance testing conditions. One of the issues that need to be addressed is how to measure bifacial PV modules correctly and analyse the different testing approaches proposed. This work outlines the indoor performance testing of c-Si bifacial modules under different module mounting setups including open rack, a structure with baffles and 3 modules. For each mounting method a white reflective rear panel of several dimensions was placed at various distances behind the module as a potential approach for a double-sided illumination characterization method. Electrical performance is also studied with a single-side illumination method with a black rear panel. The rear irradiance measurements and non-uniformity are also studied and the performance measurements are validated with a single-side illumination method. Additional rear irradiance allows Pmax increment up to 20% under certain conditions. However, the rear irradiance non-uniformity needs to be improved in order to fulfil the current requirements of the draft technical specification.

Abstract: The determination of the spectral responsivity is an essential part of solar cell characterization. Since solar simulators only approximate the reference spectrum, a spectral mismatch correction has to be performed. This correction procedure requires spectral responsivity data. Apart from the complete differential spectral responsivity procedure, the IEC 60904-8 standard defines four simplifications. In this paper, we provide information on the variations in the spectral responsivity curves for these simplifications. We show that for nonlinear front junction cells, deviations predominantly occur at wavelengths above 700 nm and become largest around 1000 nm. While we found a maximum deviation of 30% for the simplification with lowest requirements in bias irradiance, all other simplifications yield deviations below 10%. For a nonlinear cell measured relative to a world photovoltaic scale reference cell using a class A solar simulator, this transfers to a deviation below 0.01% in the spectral mismatch factor. If one depends on the use of a simplification, we recommend using the multicolor approach. Even though the singlecolor approach might yield lower deviations, this approach requires knowledge about the maximum in the spectral responsivity, which is not generally known in advance of the measurement. Accepting a slightly higher deviation, the white bias approach is a recommendable alternative.

Abstract: The interest on the n-type crystalline silicon bifacial PV modules has increased in the last years since they can increase the performance in comparison to traditional PV modules. As the PV modules are generally sold according to the nominal power output and efficiency under standard test conditions STC, the lack of standards for bifacial modules leads each manufacturer to claim different amounts for the "added value" of the bifaciality that makes it difficult for customers to directly compare between bifacial manufacturers. In this work, we have compared the electrical performance of framed and frameless c-Si bifacial modules from 5 different manufacturers utilising rear covers of different reflectivity and different measurement methods such as a single-sweep flash simulator, the so-called multi-flash (MF) method, a largearea steady state sun simulator and under natural sunlight for the accurate measurement of the module parameters. It was observed that depending on the measuring method, some modules presented STC front power out of the tolerance range given by the manufacturer. The design of the module also plays an important role on the rear power contribution.