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The ZHAW IEFE is involved in the performance research of power optimizer systems, which is funded by the Swiss Federal Office of Energy. The ZHAW PV shading simulation tool was compared to commercial tools, which show forecasts of additional yield by power optimizer systems with percentages in the double digits (PVSyst: 7.2% | PVSol: 14.6 %) relative to the SINV system for the heavy shading case. The reason for this is the use of the manufacturers’ data-sheet values, whereas the effectively measured power optimizer efficiency is generally 1.0 to 2.5% lower in points relevant for real-life operation. Finally, according to the results a list of performance-based recommendations for the application of the different PV systems was formulated.

Partially shaded rooftop photovoltaic systems operate with non-uniform conditions within the photovoltaic array, which lead to power losses. Module-level power electronics can potentially improve the performance of such photovoltaic systems. However, the potential performance increase compared to standard string inverter systems depends on the site-specific details of the installation. To investigate this, power optimizer and string inverter efficiency measurements were conducted in the ZHAW indoor laboratory. With these results, simulations were performed for a module-level power electronics system, for which power optimizers were used at every module, and a standard string inverter photvoltaic system. As a performance comparison, the P370 power optimizer and 3500H inverter was used for the module-level power electronics system and partial shading by a chimney was considered. For the standard string inverter system, the string inverter SUN2000- 3.68KTL-L1, without the use of module-level power electronics was chosen. The results of the annual simulations showed a gain of the module-level power electronics system between 0.9 % and 1.4 % (14-modules) or -0.2 % and 0.8 % (13-modules), depending on the position of the chimney. Furthermore, the shading adaption efficiency, a method of quantifying the annual performance for shading situations by applying weightings to a few indoor measured performance values of power electronic components, is described.

Video of the talk is available at: https://youtu.be/NlLg1MOyvWg, In the last decade decentralized Modul Level Power Electronics (MLPE) equipment has gained tremendous market share due to the potential to operate each Photovoltaic (PV) module in their optimum power point even in partial shading condition. The total losses of the group of decentralised DC/DC converter combined with the coupled centralised DC/AC inverter not always offer an advantage to the standard String Inverter System (SINV), like in the unshaded moments of high-power operation of a PV system at noon. The customer expects a clear answer about the quantified gain in annual power of a roof top system either operated by MLPE or SINV. Today, even the experienced planner is not able to elaborate these numbers in an economic efficient way. This is the case due to a lack of complex geometrical data of the shading obstacles and absence of software tools which are able to simulate the MLPE and SINV by calculating the shade of each solar cell in all PV modules together with an appropriate loss model of all used power electronic components. Up to know no standards exist to measure the set of MLPEs in the lab and the manufactures have not proposed detailed loss models up to now, whereas only max efficiency number of the MLPE are stated in their data sheets. This paper shows that detailed loss measurements performed in the lab, provided up to three percent higher losses of the MLPEs in the relevant operation area commonly used through a year of operation. It is recommended to use a very narrow range of numbers of MLPE in the string for high efficiency power conversion, due to the fact, that losses increase by 1.5% if the input/out voltage ratio of MLPE differ 5% from unity. A concept is presented to estimate the final so-called shading adaption efficiency which is based on the efficiency measurement of the MLPE in the indoor lab at a few operation points and by using weighting factors. Thus, the comparison of the shading adaption efficiency is given, either for different MLP

In the last decade, very few papers with outdoor measurement test results were published, which demonstrated a benefit of module-level power electronics (MLPE) higher than the measurement uncertainty. Due to the expected small difference in efficiency between conventional string-inverter based and MLPE systems, indoor measurements are required to exclude natural variation of solar irradiance in outdoor tests. Therefore, indoor efficiency measurements were performed on MLPE. Afterwards, the indoor measured efficiency of the single PV module DC/DC power optimizers were multiplied with the DC/AC inverter measurements. The final system efficiency with the maximum average value resulted in 94.78 % and with the power weighted efficiency in 94.37 %. Accordingly, if mismatches are neglected, the analysed MLPE system is expected to yield approximately 2.82 % less energy than a comparable stringinverter based system in unshaded conditions. However, in the case of shading, the tested system with heavy-shading orthogonal to the cell strings was estimated to yield approximately 3.5 % more energy on a clear-sky day in March in Winterthur, Switzerland. The performances determined in the tests are significantly lower than the efficiencies provided by the datasheet of the power optimizers (around -2.29 %). However, they are still presumed to provide significant additional yields for PV systems with medium- and heavy-shading conditions.

​© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., This paper is devoted to the problem of voltage limits violations in low-voltage networks with high share of photovoltaic power plants and provides a vision and comparison of different technologies based on their advantages and disadvantages and classification of technical methods for performing cost effective control in the various distribution grids categories. It was carried out based on the proposed methodology, which covers a wide range of technical solutions, to have a clear comparative analysis between different solutions. Technical and economical comparison between the results gained from the load flow calculations in Matpower and the additional functionalities provided from the OpenDSS were used to classify smart grid solutions for various low voltage distribution grids categories and to generate the ranking matrix.

A first wireless PV module prototype is presented. The energy transfer from the solar cells to the string cable is done using the inductive power transfer technology. A half-bridge LLC resonant converter is designed for the DC/AC conversion. The wireless module consists of 60 half-cells and an integrated planar coil. The resonant converter is not yet integrated into the module, but it can be connected externally to the planar coil. The energy is transferred from the primary coil to the secondary planar coil placed outside of the PV module on top of the primary coil. An active rectifier is connected to the secondary coil and it feeds the DC system cable. The first measurements yielded in an efficiency of 88.2 % including the resonant converter, the inductive power transmission and the active rectifier efficiencies. The corresponding output was 207 W.

Distributed Energy Resources (DERs) equipped with standardized, interoperable, grid-support functionality have the capability to provide a range of services for power system operators. These requirements have been recently codified in the 2018 revision of the American DER interconnection and interoperability standard, IEEE Std. 1547, as well as the revised Canadian interconnection standard, CSA C22.3 No. 9. Currently, the IEEE standards committee is drafting a new revision of the IEEE Std. 1547.1 test standard, which outlines the test procedures for certifying equipment compliant to IEEE Std. 1547. In addition, it is often referenced as a test standard in CSA C22.3 No. 9. This draft test standard has not been fully exercised yet to identify mistakes, redundancies, and/or implementation challenges. In this work, an international community of research laboratories developed open-source IEEE Std. 1547.1 test scripts. The scripts are used to evaluate grid-support functions – such as constant-power-factor, volt-var, volt-watt, and frequency-watt functions – of several DER devices to the draft standard, EEE1547.1. Sample test results are presented and discussed, and recommendations are offered to improve the draft standard during the balloting process.

The low-voltage distribution grid in Dettighofen, Germany was analysed regarding load flows and the voltage behaviour because of decentralised photovoltaic (PV) power generation. High load flow fluctuations of 400kW within 25 min were observed in the more critical western part of the grid. During the night, the average base and the peak load obtained from the medium voltage grid was 55.8 and 150 kW, respectively. On certain days, the transformer (400 kVA) fed with rated power into the medium-voltage grid. The relative voltage rise on the transformer was between 2% and 3%. A critical situation was found at a distribution box with two long stubs and a total PV installation power of 99.3 kWp. The load flows caused a relative voltage rise of 4.5% on a day with clear sky, and there was a voltage peak slightly above 110% on a day with high fluctuating irradiance. In a second step, load flow simulations with NEPLAN were performed to analyse three different voltage reducing measures. The relative voltage rise could be reduced by 3% with an active power curtailment ratio of 70% together with reactive power control (cosφ of 0.95 and cosφ of 0.9 at p>13.8kW). Peak shifting with battery storage leads to the same reduction if the power that exceeds 40% of the rated PV power is shifted. The average costs of limiting the active power to 70% are 1.45 ct/kWh compared with €3.11/kWh for peak shifting with commercial available lead-acid batteries. This is due to the low number of storage cycles resulting in high costs.

Minute by minute outdoor measurements of five commercial PV modules including multi c-Si, HIT, a-Si/μc-Si, CIS, CdTe over one year were used to develop a model for predicting the individual module temperatures. The simulated module temperatures over a whole year in minute interval correspond to the outdoor measured values within a RMSE of 1.5°C. Input of the model includes irradiance measurements, which are extracted from the measured short circuit current ISC, the measured wind speed and the ambient temperature. The measured energy weighted module temperature for Zurich was found to be between 33.3°C and 38.0°C for the different module types with a maximum difference of 0.8°C relative to the simulated average module temperature. Due to the high module temperature, the annual average efficiency was reduced relative to the STC values by only 2.6% (HIT) and 5.6% (CIS). Another reduction of the STC efficiency could clearly be attributed to the efficiency at low ISC values which classify low irradiance values. The value is a decrease of the annual energy output of 4.7% and an increase of 1.1%. A further loss analysis was performed to separate spectrum related losses resulting in a relative difference of 2% for the tandem compared to the multi c-Si module.

A new LED based photovoltaic module flasher is presented to perform nominal power measurements at 1000W/m2 within a flash period of 10ms, together with low light measurements. Additionally performance tests of the typical 3 bypass diodes of a standard 6” crystalline silicon PV module are performed during three flasher periods within totally 2 seconds. Thus the flasher setup (2.2m x 1.5m x 0.1m, weight 50kg, including power electronics) is placed in a distance of a about five centimeters direct onto the PV module, which remains mounted in the same position as it is operated in the outdoor PV plant. The LED flasher is equipped with 2400 single blue LEDs and 1440 IR LEDs. The total flasher area is divided into twenty individually controlled subsections with an area of 0.13m2 each. During one flasher period different values of intensity and color composition are able to be controlled independently. Two LED colors are used at 455nm and 850nm. A standard crystalline module is typically divided in three sub-module areas, protected by one bypass diode for each sub-module. Thus the intensity of these three submodule areas relative to each other is varied. During a single flash up to four different intensity patterns are applied. The complete measurement system was successfully qualified by test sequences performed on standard modules, equipped with different defect bypass diodes. The fast method may also be applied as a new module end test in a production line as well as a new PV field test method performed within a few seconds by the mobile LED flasher.

Bifacial solar modules promise an increased energy yield, compared to systems with standard, monofacial panels, and also offer new opportunities with regard to the installation. One particular approach is the vertical mounting of PV modules, which is reported to be an effective measure to avoid soiling or dust deposition and is an option to obtain a broadened energy generation profile. In spite of the general interest in this type of installation, the amount of published data is very limited, especially with regard to arrays, for which pronounced shading effects can be expected. In this work we present an analysis of the energy yield and the respective losses for arrays of vertically mounted bifacial solar modules with varied installation conditions.

With decreasing subsidies for PV energy fed into the grid other economic incentives are needed to sustain the growth of PV power. An effective solution is to increase self-consumption for grid-connected, residential PV systems by means of generating hot water by the use of a heat pump. This widely used concept of thermal energy storage has been adapted for a facade element with integrated PV modules for the first time. The so called Swiss PV Wall System (SPWS) contains all elements needed in the façade, behind the PV modules with a power of 1 kWp, like a PV inverter, a heat pump and a hot water tank. Simulation showed that virtually 100% of the PV energy can be used either by the heat pump to produce hot-water or the household’s electricity needs. A functional model of the SPWS was realized and first measurements showed that the heat demand of a one family household may be covered for the majority of the months during the year. Economic analyses showed that the cost per kWh heat is at about 0.215 CHF and thus lower than standard solar thermal and some fossil solutions.

PV systems using bifacial solar modules have shown an increased energy harvest compared to systems with standard, monofacial panels. Due to bifaciality of the modules, the tilt angle, the installation height, the distance between the modules and in particular the ground reflectance have a significant influence on the energy harvest. For bifacial module arrays reflective elements in between the rows might be an option in order to achieve a low light concentration resulting in a further increased energy harvest. A simulation of such conditions is hardly possible; measurements at real systems would require considerable effort. In this work we present results obtained by a measuring setup with rotating mini modules. In particular the influence of the ground reflection was studied by using various grounds and reflecting elements with different shapes. Prior to the analysis of the reflector elements the measurement setup is described and the assignability of results from the miniaturized test array to a corresponding large scale system is demonstrated.

The measurement of the spectral photo current of stacked solar cells using semiconductor materials with a lower bandgap material as the bottom cell and a higher bandgap material as the top cell, like the a-Si/uc-Si tandem solar cell, is needed to conduct high accurate nominal power measurements. The prediction of the annual energy yield of tandem cell concepts also depends on the current matching of the top and bottom cell at standard STC spectra, not only on the STC nominal power. Thus the customers have to know if the optimal current matching is reached at STC AM 1.5 spectra or at a spectrum shifted more to the red or to the blue. The spectral photo current measurement is a standard on the small square centimeter cell area, especially in research labs, mostly using monochromatic light and colored bias light. In this paper we report the successful measurement results of the photocurrent of large area square meter sized tandem modules at wavelength between 400 and 1100nm using 50nm bandpass filters and blue and infrared LED bias light. Additionally a new measurement method is successfully introduced to measure changes of the short circuit current of a tandem modules using a dynamic changing LED bias light together with STC flasher light during a few milliseconds or without flasher light. Analyzing the characteristics of the measured tandem module current within a few milliseconds it can be clearly seen if the module is top or bottom limited, even the degree of mismatch is able to be characterized precisely. Thus the later analysis has the potential to be implemented as a standard measurement procedure within the end test of a tandem module production line. Even existing standard flasher may be adapted, without changing the existing flasher light source, by mounting the additional LED bias light source to perform different characteristics of dynamic bias light.

Eine hohe Rentabilität von PV-Anlagen setzt eine hohe Qualität der Solarmodule voraus. Eine Minderleistung von Modulen musste bisher aufwendig in einem Labor verifiziert werden. Der neue Portable LED-Flasher ermöglicht nun eine preisgünstige und effiziente Nennleistungsbestimmung von Solarmodulen vor Ort.

There is an increased energy yield for n-type bifacial solar modules as compared with n-type monofacial modules. However, the benefit of bifaciality depends on several factors, such as module height, ground reflectivity, distance between modules, etc. This work enriches the understanding of the correlation between laboratory measurements and outdoor performance of bifacial modules. Indoor measurements of monofacial and bifacial modules, containing n-type bifacial cells of similar technology, were performed at PI Berlin (Germany). Their outdoor performance was monitored in El Gouna (Egypt), where the yield in terms of Wh/Wp was compared, and in Winterthur (Switzerland), where the influence of module height and ground reflectivity was studied. In El Gouna, an average yield gain of 22.3% in terms of Wh/Wp was measured for a bifacial module as compared with a monofacial module, over the first eight months of 2014. A maximum average monthly gain of 25.6% was reached in the month of August. A peak power output of 426 W was registered. In Winterthur, the application of a white foil beneath a bifacial module showed a gain of 7.6% in power output divided by the incident irradiance on the front side, as compared with a gravel covered ground. The power/irradiance gain was 9.7% when the module was raised from 0 cm to 64 cm above the ground level.