Reliability of photovoltaic modules:from indoor testing to long-term performance prediction

A fundamental challenge with solar energy is improving the reliability, and increasing the lifetime, of photovoltaic modules. Typically, photovoltaic module manufacturers guarantee 80% of the nominal power of their modules for 25 years, but this type of guarantee is not based on a deep comprehension of the real degradation mechanisms of the modules. Moreover, the current challenge is to extend module lifetimes up to 30-40 years. For these reasons, accurate predictions of the lifetime of modules are needed. The primary goal of this dissertation is to develop predictive models to evaluate the lifetime of photovoltaic modules, taking into account the climates in which they operate. We consider the technology of crystalline silicon (c-Si) modules, which largely dominate the field. In Chapter 1 we identify two main degradation mechanisms, namely, potential-induced degradation (PID), and interconnection failures, that are particularly of interest as they can occur non only in harsh environments such as desert or tropical regions, but also in temperate climates. The main topic of this dissertation is the PID failure mechanism. Chapter 3 offers an introduction to the topic of PID for conventional p-type c-Si modules. In Chapter 4 we develop a lifetime model for PID. Such model is based on empirical equations obtained from accelerated tests in laboratory. In particular, we introduce a dependency on voltage that allows to perform PID prediction at string level, and analyze in detail the regeneration mechanism under irradiance. The result is a set of equations that describe the main phases of the evolution of PID as a function of the stress parameters. In Chapter 5, this model is applied to predict the evolution of PID for devices operating outdoors, considering four locations with different climates. One difficulty in applying our model to outdoor conditions arises from the fact that stress levels vary continuously according to the meteorological conditions, while our model was developed from indoor testing at constant stress conditions. We overcome this issue by using a mathematical method based on the concept of equivalent time. Moreover, suitable thresholds on the weather conditions are set to properly simulate the different phases of PID. In Chapter 6, we employ accelerated stress tests to investigate which strategies (selection of materials and/or module architectures) would allow us to manufacture PID-free modules. Chapter 7 is devoted to the second degradation mode under study in this dissertation, namely, disconnection failures. We validate experimentally a model developed in LT-SPICE that simulates the performance of a string with disconnected ribbons. In summary, this thesis proposes a combination of accelerated test sequence and simulations that allow to predict, for a given location, the effect of PID on the module power output. We are confident that this methodology could be applied in general to other degradation mechanisms, thereby allowing an improvement of the prediction of photovoltaic modules reliability in different climate conditions

Promoting a Sustainable Diffusion of Solar PV Electricity in Africa: Results of the CODEV Project

In this work we present the result of a collaboration between the Polytechnic Schools of Dakar (ESP) and Lausanne (EPFL) on the testing and monitoring of solar photovoltaic modules. The collaboration has involved the exchange of knowledge, methodologies and data, and, in particular, the analysis of the aging of PV modules exposed to the hot semiarid climate of Dakar for eight years. With the aim of promoting a focus on quality and reliability, the long-term goal of the collaboration would be to set-up a testing laboratory for PV modules and systems in Dakar and a training center. The testing laboratory will be working in close collaboration with the University and should potentially have a “lean” and easy-replicable structure. The implementation of a third-party institution able to assess independently the quality of components and support system developers and installers in the design, commissioning and maintenance of PV projects is crucial to promote a “sustainable” diffusion of solar electricity in Africa, particularly when considering the residential and commercial/industrial rooftop PV market segment. By minimizing risk, focus on quality should promote a virtuous cycle leading to: (1) mitigation of financing costs of solar projects, therefore, considerably reducing the overall costs of this technology, (2) increase positive perception and awareness about PV

Modeling potential-induced degradation (PID) in crystalline silicon solar cells: from acceleratea-aging laboratory testing to outdoor prediction

We present a mathematical model to predict the effect of potential-induced degradation (PID) on the power output of c-Si modules in different climates. For the experimental part, we manufacture mini-modules made of two c-Si p-type cells, and use accelerated ageing laboratory testing performed at different combinations of stress factors (temperature, relative humidity, and voltage). By modeling the effect of each stress factor in a step-wise approach, we obtain a model for the PID at constant stress conditions, which agrees well with models that can be found in the literature for full-size modules. Our model is obtained complementing existing models by introducing a term that describes a linear dependence of module’s power degradation on the magnitude of the applied voltage. Since in field installations PV modules are connected in strings and exposed to different potential – and, therefore, stress – levels, this latter term is needed to approach real field conditions. Finally, we present the first attempts to model PID outdoor degradation in different climate conditions based on the proposed model and on the indoor-determined coefficients for the devices tested. The outdoor prediction model makes use of Typical Meteorological Year (TMY) data for a specific location

One-type-fits-all-systems: Strategies for preventing potential-induced degradation in crystalline silicon solar photovoltaic modules

In this work, we investigate the relationship between potential-induced degradation (PID) and the bill of material used in module manufacturing. We manufacture samples with different combination of materials, using two types of solar cells (conventional vs PID-free c-Si cells), two types of ethylene-vinyl acetate (EVA) films with low/high resistivity, and two types of backsheets with, respectively, low/high breathability properties, and subject the mini-modules to extended PID testing. Our results clearly indicate that, when using a breathable polymeric backsheet, to have a "PID-free" module the combination of PID-free cells and high-resistive EVA encapsulants is recommendable. The use of a conventional c-Si cell in combination with a high-resistive EVA encapsulant is still more effective than the use of PID-free cells in combination with low-quality EVA. Further, our results initially show that the breathability properties of the backsheet have apparently no influence on PID degradation. A second set of experiments using sandwich structures with increased resistance properties to water ingress (ie, glass and backsheets with barrier layers as rear covers and an edge sealant), however, indicates that preventing or reducing the diffusion of moisture in the encapsulant layer plays a role in further mitigating the impact of PID. This finding is supported by simulations of moisture ingress in the sandwich structures. Finally, we show that the use of a glass rear cover-compared with a polymeric backsheet-does not contribute in worsening the PID effect. On the contrary, by reducing moisture ingress in the front encapsulant layer, it delays the occurrence of PID

In-situ Determination of Moisture Diffusion Properties of PV Module Encapsulants Using Digital Humidity Sensors

We conducted a long-term (>6500 h) in-situ PV module humidity monitoring experiment, where digital humidity sensors encapsulated in different encapsulants (EVA, TPO, POE and ionomer) were exposed to different humidity conditions in a climatic chamber. From the acquired data, humidity ingress parameters (moisture diffusion coefficient and solubility) of used materials are extracted. Furthermore, first promising results have been obtained by an ongoing experimental work, where we obtained a correlation between the sensors' RH readings and the water mass concentration in the EVA encapsulant. Such a calibrated monitoring method, capable to measure humidity content in different PV modules using various encapsulants, represents a very cost effective and versatile technique for laboratory and field testing applications

35 years of photovoltaics: Analysis of the TISO-10-kW solar plant, lessons learnt in safety and performance-Part 2

The TISO-10-kW plant, installed in Lugano (Switzerland) in 1982, is the first grid-connected PV plant in Europe. In a joint publication (part 1), we presented the results of the electrical characterization performed in 2017-after 35 years of operation-of the 288 Arco Solar modules constituting the plant. Power degradation rates were different among modules and two groups could clearly be distinguished: group 1, with a remarkably low mean degradation rate of -0.2% per year, and group 2, with a mean degradation of -0.69% per year. After 35 in a temperate climate, approximately 70% of the modules (considering a +/- 3% measurement uncertainty) still exhibit a performance higher than 80% of their initial value. In this paper, when possible, we attempt at correlating module performance losses to specific failure mechanisms. For this sake, an extensive characterization of the modules was performed using visual inspection, IV curve measurements, electroluminescence, and infrared imaging. We remarkably find that module degradation rates are highly correlated to the aging pattern of the encapsulants used in module manufacturing. In particular, a specific formulation of the encapsulant (PVB), which was used only in a minority of the modules (approximately 10%), leads to degradation rates of -0.2% per year, which corresponds to a loss in performance below 10% over 35 years. Potential safety threats are also investigated, by measuring the frame continuity, the functionality of the bypass diodes, and the module insulation. Finally, we discuss how the analysis of a 35-year-old PV module technology could benefit the industry in order to target PV module lifetimes of 40+ years

35 years of photovoltaics: Analysis of the TISO-10-kW solar plant, lessons learnt in safety and performance-Part 1

The TISO-10-kW solar plant, connected to the grid in 1982, is the oldest installation of this kind in Europe. Its history is well documented, and the full set of modules has been tested indoors at regular intervals over the years. After 35 years of operation, we observe an increase in the degradation rates and that the distributions of modules' performances are drastically changing compared with previous years. Two groups of modules can be observed: (a) group 1: 21.5% of the modules show a very modest degradation, described by a Gaussian distribution with mean yearly power degradation of only -0.2%/y. (b) Group 2: 72.9% of the modules form a negatively skewed distribution with a long tail described by mode (-0.54%/y), median (-0.62%/y), and mean (-0.69%/y) values. In earlier years, decreases in performances could strongly be correlated to losses in fill factor (FF). After 35 years, the situation changes and, for a subset of modules, losses in the current (Isc) are superimposed to losses in FF. The reasons for this will become clearer in part 2, where we will present results of a detailed visual inspection on the whole set of modules and will focus on safety aspect too. We conclude that, after 35 years of operation in a temperate climate, approximately 60% (similar to 70% if considering a +/- 3% measurement uncertainty) of the modules would still satisfy a warranty criteria that module manufacturers are presently considering to apply to the technology of tomorrow: 35 years of operation with a performance threshold set at 80% of the initial value

35 years of photovoltaics: Analysis of the TISO‐10‐kW solar plant, lessons learnt in safety and performance—Part 1

The TISO‐10‐kW solar plant, connected to the grid in 1982, is the oldest installation of this kind in Europe. Its history is well documented, and the full set of modules has been tested indoors at regular intervals over the years. After 35 years of operation, we observe an increase in the degradation rates and that the distributions of modules' performances are drastically changing compared with previous years. Two groups of modules can be observed: (a) group 1: 21.5% of the modules show a very modest degradation, described by a Gaussian distribution with mean yearly power degradation of only −0.2%/y. (b) Group 2: 72.9% of the modules form a negatively skewed distribution with a long tail described by mode (−0.54%/y), median (−0.62%/y), and mean (−0.69%/y) values. In earlier years, decreases in performances could strongly be correlated to losses in fill factor (FF). After 35 years, the situation changes and, for a subset of modules, losses in the current (Isc) are superimposed to losses in FF. The reasons for this will become clearer in part 2, where we will present results of a detailed visual inspection on the whole set of modules and will focus on safety aspect too. We conclude that, after 35 years of operation in a temperate climate, approximately 60% (~70% if considering a ± 3% measurement uncertainty) of the modules would still satisfy a warranty criteria that module manufacturers are presently considering to apply to the technology of tomorrow: 35 years of operation with a performance threshold set at 80% of the initial value.JRC.C.2-Energy Efficiency and Renewable

Quantifying and modeling the impact of interconnection failures on the electrical performance of crystalline silicon photovoltaic modules

Failures in the metallic interconnections are among the main degradation modes for photovoltaic modules. Fatigue accumulation due to thermomechanical stresses can result in deterioration of the solder joints or in broken ribbons. In this paper, we first quantify experimentally how the module performance is affected when one or more cell interconnect ribbons are cut or disconnected. For this purpose, we manufactured a set of minimodules, composed by six monocrystalline silicon cells with three bus bars connected in series. Cells were encapsulated in a glass/backsheet construction, employing a polymeric (ETFE) backsheet that can be easily opened. We then sequentially cut one or more ribbons. We observe that the power loss strongly depends on the ribbon's position with respect to the cell (external or central ribbon). In a second step, we implemented an electrical model in LT‐SPICE where the solar cell is composed by three subcells (as the number of cell bus bars) and show that this model is able to reproduce the experimental results with a good accuracy. We then use the model to demonstrate that these results are directly transferable to the case of large‐area modules composed of 60 or 72 cells. Finally, we analyze the case when the disconnections are randomly distributed in the module. As a first approximation, a module with 10% of disconnections has a Pmax variation between −1.34% and −2.75% in average, while 20% of disconnections lead to a Pmax variation in the range of −2.83% and −5.64%