Inhomogeneous shading on the PV generator leads to disproportionately high losses. As the potential of PV generation on roofs or façades is to be increasingly utilised in the coming decades, these cases will occur more frequently. The aim here is to provide an overview of the challenges and state-of-the-art technical solutions for partial shading. Current developments in PV engineering show that maximum performance lies in the combination between optimised module placement, the use of modules that are tolerant of shading and optimised power electronics. Shortly after the discovery of the solar cell, blocking or bypass diodes were used to solve the inhomogeneous currents of groups of solar cells arranged in series or parallel wiring. Even today, they are still the most efficient and robust solution for the majority of common shading PV applications. Due to the very high rated outputs of the solar modules and the presence of only three bypass diodes, high temperatures can occur on a locally shaded solar cell. This forces heat outputs of up to 200W or 100W in the butterfly module connection through the associated activated bypass diode, which must be dissipated by the most shaded cell. If additional small-area defects occur in this affected solar cell, hotspot peak temperatures can occur, which can lead to permanent damage to the module or the risk of fire. However, in order to prevent a third of the module output being lost in this case, four or more bypass diodes are now used in so-called shadow-tolerant PV modules. With a higher number of bypass diodes per module area, it is also possible to selectively bypass smaller, less efficient areas of the module, which leads to an increase in the module yield. The hotspot effects can also be comprehensively and robustly prevented by the small number of solar cells per bypass diode, provided the bypass diode is properly designed. The first manufacturers are beginning to place these shade-tolerant PV modules on the markets. Today, planners can also select different power electronics systems for the next step in system integration towards grid feed-in, i.e. the connection of the individual modules in the string. This is the classic series connection of all modules in the string to the input of the DC/AC string inverter (SINV), which leads to the highest yields for weak and medium shading. This applies, for example, to light shading with a chimney or a ventilation pipe, where no more than one tenth of the modules in the string are reached by the shade at the same time during the six hours around midday, even when using standard modules with only three bypass diodes. (see Table 1) With medium to heavy shading, the widely used DC/DC converters directly on the PV module (MLPE), often also called power optimisers, can be used profitably. However, the combination of shade-tolerant PV modules with conventional SINVs can often deliver comparable annual yields. However, if the optimisers are also used behind each module even with weak shading (allMLPE), they deliver less yield in total than the simple SINV, as their own DC/DC losses then have a negative impact compared to simple connectors. This only becomes apparent if the MLPE manufacturers' data sheet claims of 99% efficiency are not viable. The published measurements carried out in independent laboratories over the last four years are listed in this report, which suggest that losses are around 2% higher. As the differences in yield between the power electronics variants SINV and MLPE are usually less than four per cent in annual yield for light to medium shading, the above-mentioned real MLPE efficiency at the specific operating points plays the decisive role in planning the most efficient system. However, as the commercial PV software planning tools currently use these MLPE manufacturer specifications which are over estimated, no meaningful system comparison can be expected for these shading categories. In this report the results of annual simulations performed by some sophisticated simulation tools that take these real MLPE losses into account are discussed.

Partially shaded photovoltaic systems operate with nonuniform 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 is site specific. To investigate this, power optimizer and string inverter efficiency measurements are conducted in the ZHAW indoor laboratory. With these results, simulations are performed for a module‐level power electronics system with power optimizers at every module and a standard string inverter rooftop photovoltaic system. As a performance comparison, the P370 power optimizer and 3500H inverter are used for the module‐level power electronics system and partial shading by a chimney is considered. For the standard string inverter system, the string inverter SUN2000‐3.68KTL‐L1, without the use of module‐level power electronics, is chosen. The results of the annual simulations show 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.

PV carports are an ideal solution to produce electricity locally and sustainably, while taking advantage of the dual use of the space. On average, large carports have a power output of 0.17 kWp/m2. The average costs amount to 2'100 CHF/kWp. The price per kWp decreases slightly when the power clearly increases. The costs of the PV modules are lower than the costs for the carport's foundation and supporting structure, which represent generally between 30 % and 60 % of the total costs. The amount of steel is estimated to 155 kg/kWp and the quantity of concrete required is approximately 650 kg/kWp. The specificities of the terrain naturally have an impact on these values. A carport parking space emits 690 kgCO2-eq during the construction phase and an electric vehicle (EV) 11.2 kg-Co2-eq/100 km on average. Innovative projects include retractable roofs to offers the potential to reduce the amount of material used in the mounting structure, due to storing the PV modules in a safety box during heavy wind or snow load. The use of wood instead of metal, and screws anchor instead of concrete, as well as the addition of greenery, greatly reduces total CO2 emissions.

Main Authors: Roger H. French1, Laura S. Bruckman1, David Moser2, Sascha Lindig2, Mike von Iseghem3, Björn Müller4, Joshua S. Stein5, Mauricio Richter6, Magnus Herz7, Wilfried van Sark8, Franz Baumgartner9, 1Case Western Reserve University, USA 2Eurac Research, Bolzano, Italy, 3EDF, France, 4Fraunhofer ISE, Freiburg, Germany, 5Sandia, USA, 63E, Belgium, 7TUV Rheinland, Germany, 8Utrecht University, The Netherlands, 9Züricher Hochschule für Angewandte Wissenschaften, Switzerland

The IEA PVPS Task 13 group, experts who focus on photovoltaic performance, operation, and reliability from several leading R&D centers, universities, and industrial companies, is developing a framework for the calculation of performance loss rates of a large number of commercial and research photovoltaic (PV) power plants and their related weather data coming across various climatic zones. The general steps to calculate the performance loss rate are (i) input data cleaning and grading; (ii) data filtering; (iii) performance metric selection, corrections, and aggregation; and finally, (iv) application of a statistical modeling method to determine the performance loss rate value. In this study, several high‐quality power and irradiance datasets have been shared, and the participants of the study were asked to calculate the performance loss rate of each individual system using their preferred methodologies. The data are used for benchmarking activities and to define capabilities and uncertainties of all the various methods. The combination of data filtering, metrics (performance ratio or power based), and statistical modeling methods are benchmarked in terms of (i) their deviation from the average value and (ii) their uncertainty, standard error, and confidence intervals. It was observed that careful data filtering is an essential foundation for reliable performance loss rate calculations. Furthermore, the selection of the calculation steps filter/metric/statistical method is highly dependent on one another, and the steps should not be assessed individually.

The reliability, durability and lifetime performance of passivated emitter, rear cell (PERC) modules used in real-world PV power plants is a critical challenge underlying the rapid adoption and bankability of these PERC cells, whose high efficiency help reduce the levelized cost of electricity (LCOE). We propose a degradation-science study of PERC module degradation pathways, benchmarking them relative to known degradation mechanisms and pathways of the incumbent aluminum back surface field (Al-BSF) modules exposed to real-world and accelerated exposure conditions.

Annual degradation rates of PV modules are important in the yield prediction. For a high-quality PV module, these rates are lower than the measurement uncertainty of a nominal power measurement performed in todays most advanced certified photovoltaic reference laboratory. Therefore, the analysis requires a well thought out methodology that can compare the data relative to each other or relative to an unused module stored in the dark on an annual base. Over the past 10 years, several multi c-Si and HIT modules have been accurately monitored in a string and single module setup by an outdoor performance measurement system. Additionally, all modules have been dismantled and measured using an indoor flasher measurement system once every year. With this unique measurement setup, the annual degradation rates of multi c-Si modules and HIT modules are quantified based on three different analysis methodologies. The multi c-Si modules showed an average annual degradation rate of 0.18% +/- 0.06% and 0.29% +/- 0.06% measured by the outdoor and indoor system, respectively. The indoor analysis of the HIT modules yielded an average annual degradation of 0.26% +/- 0.05%. That corresponds to half of the degradation observed by the outdoor analysis method. Further evaluations of the performance ratio PR confirmed the results gained by the indoor methodology. The comparison of the standard PR with a temperaturecorrected PR'STC for both technologies showed that the benefit of the lower temperature coefficient of the HIT technology is eliminated by its worse low light behaviour.

: Several mono- and bifacial mc-Si PV modules were installed at a location 2500 m above sea level in the Alps and grouped in six segments with different inclinations 30°, 70° and 90°. The PV modules and the meteorological conditions are monitored minutely and compared to a 30° tilted PV module installed in the urban region of Zurich. During the analysis period between October 2018 and September 2020, the yield loss due to snow coverage was mini-mal. The highest loss was 2.3 % (2018/19) and 3.6 % (2019/20) for the 30° inclined PV modules evaluated by the introduced snow coverage model based on electrical and weather data. The two segments with a 30° inclination showed a 20.9 % to 27.2 % higher yield than the PV module installed in the urban region, mostly produced in the winter season. The bifacial alpine energy yield is about twice as high as that of the urban PV module from November to May. The highest yields of 1800 Wh/Wp in 2018/19 and 1696 Wh/Wp in 2019/20 was measured at the 70° tilted bifacial PV module without losses due to row shading as it is expected in PV plants.

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 mediumand heavy-shading conditions.

Grid codes around the world are requiring grid-support functions (GSFs) and standardized interoperability interfaces for distributed energy resources (DERs) to address the rapid increase of renewable energy. However, these new GSFs need to be assessed to ensure the desired power and communication capabilities exist in the field. The IEEE 1547.1 standard outlines the conformance test procedures for DER devices and is currently undergoing a major revision to align it with IEEE 1547-2018. Once it is published (anticipated in mid-2020), GSFs in commercial PV inverters in USA and Canada will be certified to the IEEE 1547.1 conformance test procedures. Several international research laboratories are collaborating to develop a versatile open-source DER testing platform that performs automated testing of DER devices. This community of laboratories is developing open-source IEEE Std. 1547.1 test scripts to lower barriers to DER vendor internal equipment evaluations, ease product compliance testing at certification laboratories, and provide research institutions a tool to study DER behaviors. In this work, test scripts were used for test verification of GSFs, including limit active power, constant reactive power, active power-reactive power (watt-var), and prioritization of GSF response for several DER devices. Sample test results for these DER GSFs and test protocol recommendations are presented in this paper.

Potential investors in bifacial PV systems are still deterred by the uncertainty of energy yield predictions. Validation of the prediction methods and establishing an accuracy range is therefore a central prerequisite concerning the bankability of the bifacial technology. Because of the sensitivity on multiple additional factors compared to standard monofacial installations, the yield simulation of a bifacial PV array is far more complicated. The available simulation tools for bifacial systems still have to prove their reliability, also for predicting the effect of varying ambient and installation conditions. Moreover, bifacial devices enable new types of applications, such as vertical installations. In this work, we compare the results of simulation tools developed at ISC Konstanz and ECN.TNO, as well as the output of the commercial PVsyst simulation software, to measured data. The measured data is obtained by a test rig that carries out a continuous and automated variation of the tilt angle. The test rig is not a single stand-alone module but an array in order to include shading effects of real extended systems. In addition, the measurements focus on a central module, which represents the conditions of a typical device in an array. Days with differing light intensity and share of diffuse radiation were chosen to determine the impact of the insolation conditions on the simulation accuracy. General trends with regard to the sensitivity to tilt and insolation conditions are analyzed in order to evaluate the quality of state-of-the-art bifacial energy yield simulations.

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.

Dependent on the specific conditions flat roofs can be well suited for the installation of large photovoltaic systems in urban areas. For urban designers also other aspects, such as the insulation of buildings, cooling, air purification and water retention play an important role besides the ecological energy generation. The combination of photovoltaics and roof greening can therefore be an interesting fusion. It combines the advantages of a green roof with the local electrical energy production at the place of consumption. However, using a conventional photovoltaic system with tilted modules in south or east-west direction on a green roof causes problems, as typical low tilt angels and high ground coverage rates result in an almost complete coverage of the roof surface. Plants, growing in between the covered areas provoke undesirable shading of the collector surface. Only a frequent maintenance procedure, complicated by dense PV system layouts, can avoid a reduction of the energy yield in the course of time. Vertically mounted specially designed bifacial modules are an option to realize photovoltaic power generation in combination with a functional green roof at low maintenance costs. In this paper, we report on the layout and the energy yield of a corresponding system. Custom-made bifacial modules with 20 cells were produced and vertically installed in landscape orientation. The narrow layout of the modules lowers the wind load and reduces the visibility. The enhanced power in the morning and evening of vertically east-west installed modules can additionally lead to higher self-consumptions rates. Despite having some shading and undergrounds with albedo factors of less than 0.2, the bifacial installation with a rated power of 9.09 kWp achieved a specific yield of the 942 kWh/kWp in one year (11.08.2017-10.08.2018). This is close to typical values of 1000 kWh/kWp achieved for south-facing PV systems in the same region. The impact of the greening on the albedo and the system performance is investigated in more detail with two smaller sub-systems. The energy yields of the two bifacial sub-systems are compared to a monofacial, south facing reference module. The use of silver-leaved plants in this system resulted in higher albedo values and a more resilient roof greening.

Photovoltaic (PV) inverters increase the line voltage in Distribution Grids (DG) by active power feed in. Today, modern PV inverters are also able to feed in reactive power to mitigate the above voltage rise. The favoured, cost effective implementation is the control of reactive power feed in according to the instantaneous measured line voltage. The stability of this decentralised Q(U) PV inverter closed-loop control is mandatory and analysed in this work. The DG operator must guarantee the voltage limits given in the regulatory framework. This is challenging due to fast changing solar irradiance, load flows and the interaction of an overlying automatic voltage control-loop of a connected substation. The performed tests in the AIT SmartEST laboratory resulted in very stable operation even at small Time Constants (TC) below 5 seconds of the Q(U) control parameter. As one test scenario, an abrupt rise of solar irradiance immediately followed by load drop is realized by use of the PV and load emulators. The PV inverter reduces the resulting voltage rise by increasing its reactive power, depending on the Q(U) control time constant. It was found that even at smaller Q(U) time constants than the typical applied values of 5 or 10 seconds no sign of instability arises. It is recommended to the DG operator to apply TC of 1 or 2 seconds of Q(U) control to minimise the duration of overvoltage condition during the transient voltage adjustment. Applying irradiance conditions of a typical cloudy day in the lab test yields 45% of the time the line voltage was above a given grid voltage limit, while applying Q(U) at TC of 1 second there was no occurrence of overvoltage. In detail at Q(U) TC setting of 20 seconds the overvoltage arises 3.4% of the total period and only 0.3% at TC 5 seconds. Only stable operation conditions were found including the automatic voltage control of the transformer sub-station at a typical setting of 10 seconds delay time of that sub-station control setting. Summarized, in combination with that delay time setting the smaller Q(U) time constant of the PV inverters below 5 seconds are beneficial due to the minimised overvoltage time.

Several international research laboratories are collaborating under a Smart Grid International Research Facility Network (SIRFN) project to develop certification procedures for advanced distributed energy resources (DER). To effectively evaluate interoperability and grid-support functionality in DER equipment, test permutations across the full range of modes and parameters are required. It is impractical to complete these experiments manually so the project team is working to develop a software tool, associated abstraction layers, and hardware drivers to execute the experiments autonomously using the same opensource test logic. This software can then be programmed to complete interoperable DER certification experiments at DER vendor facilities, certification laboratories, or research institutions. By sharing the codebase with all institutions, barriers to adoption steadily decrease. To demonstrate the approach, Underwriters Laboratories 1741 Supplement A volt-var and specified power factor test results from multiple laboratories are presented and compared.