PEOPLELukas Koester

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Lukas Koester

Post-Doc-Researcher
Institute for Renewable Energy

T +39 0471 055 728

lukas.koester@eurac.edu

https://orcid.org/0000-0003-2083-3416

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About

Performance and reliability of solar photovoltaic power plants.
Advanced inspection techniques for photovoltaic modules in lab and field.

Lukas Koester is postdoc researcher and lab technician in the photovoltaic energy system group of the institute for renewable energy, where he leads the team of performance and reliability of PV systems. After reaching his master of science in physics at the University of Düsseldorf (Germany) in 2020, he conducted a PhD in Energy and Nuclear Science and Technology with Politecnico di Milano, working at Eurac Research. The PhD thesis entitled "Multispectral imaging and correlation of image signatures of PV failures with electrical signatures" shows improvements in electroluminescence and daylight photoluminescence inspection techniques for large-scale PV plants, investigates the temperature-related behavior of the luminescence signal intensity, and uses the intensity-to-voltage dependency to detect PV module performance. In August 2025 he became teamleader of the performance and reliability of PV systems team.

Research gate:
https://www.researchgate.net/profile/Lukas-Koester-3

LinkedIn:
https://www.linkedin.com/in/lukas-koester-a40b66199/

Scientific Publications

Journal article2025

Redefining failure detection in PV Systems: a comparative study of GPT-4o and ResNet's computer vision in aerial infrared imagery analysis

Gallmetzer S, Sondoqah M, Turri E, Koester L, Louwen A, Moser D

Sandra Gallmetzer, Lukas Koester

Project: PNRR NEST - Network 4 Energy Sustainable Transition

The rapid growth of the solar photovoltaic industry underlines the importance of effective operation and maintenance strategies, particularly for large-scale systems. Aerial infrared thermography has become an essential tool for detecting anomalies in photovoltaic modules due to its cost-effectiveness and scalability. Continuous monitoring through advanced fault detection and classification methods can maintain optimal system performance and extend the life of PV modules. This study investigates the application of advanced artificial intelligence methods for fault detection and classification comparing the performance of GPT-4o, a multimodal large language model, and ResNet, a convolutional neural network renowned for image classification tasks. Our research evaluates the effectiveness of both models using infrared images, focusing on binary defect detection and multiclass classification. ResNet demonstrated advantages in terms of computational efficiency and ease of implementation. Conversely, GPT-4o offered superior adaptability and interpretability, effectively analysing multimodal data to identify and explain subtle anomalies in thermal imagery. However, its higher computational requirements limit its feasibility in resource-limited settings. The results highlight the complementary strengths of these models and provide valuable insights into their role in advancing automated fault diagnosis in photovoltaic systems.

DOI Source

DOI: 10.1051/epjpv/2025010

Journal article2024

Large‐Scale Daylight Photoluminescence: Automated Photovoltaic Module Operating Point Detection and Performance Loss Assessment by Quantitative Signal Analysis

Koester L, Louwen A, Lindig S, Manzolini G, Moser D

Lukas Koester

Project: EU H2020 TRUST-PV: Increase Friendly Integration of Reliable PV plants considering different market segments

Daylight photoluminescence (DPL) is a relatively novel imaging technique utilized in photovoltaic (PV) system inspection, using the sun as excitation source. Filtering the luminescence signal from the strong sun irradiation is its main challenge. Images acquired at two different operating points (OPs) of the module, allow subtraction of the background radiation while maintaining the luminescence signal. A DPL-ready inverter, which is able to toggle between manually selectable OPs of connected PV modules, is presented in this work. Synchronization of image acquisition and OP switching becomes particularly challenging if the camera is applied to unmanned aerial vehicles. To overcome this challenge, an algorithm is developed to identify OP switches in a set of images taken in the field by investigating image intensities. Further, by working out the detailed dependencies of the signal recorded during DPL, the temperature coefficient of photoluminescence intensity is derived theoretically, and its impact on quantitative inspections. The potential field application of DPL images to identify performance loss in PV modules is investigated by two approaches: recording the signal intensity of images over time and comparing the signal intensity of different PV modules in one image. For both approaches, their hypothetical applicability is shown experimentally.

DOI Source

DOI: 10.1002/solr.202300676

Journal article2023

Evaluating the effects of photovoltaic module heating during electroluminescence inspection

Koester L, Vallarella E, Louwen A, Lindig S, Moser D

Lukas Koester

Project: EU H2020 TRUST-PV: Increase Friendly Integration of Reliable PV plants considering different market segments

The application of electroluminescence imaging of photovoltaic modules increased in the last years, due to the reliable and detailed identification of degradation and failures. In future plants the time-consuming connection of power supplies could be overcome by use of inverters with bi-directional functionality, allowing backpowering of connected module strings directly. Temperature influences the open-circuit voltage of photovoltaic modules and must therefore be considered during backpowering. This work investigates the heating due to backpowering of photovoltaic modules of different types during electroluminescence inspection. The temperature increase until saturation is estimated by energy balance calculations and experimentally verified to be around 20 °C, with resulting voltage drops of up to 3 V. Further, these changes have an effect on the recorded luminescence intensity: a decrease of the electroluminescence signal intensity between beginning of backpowering and reaching saturation temperature is shown. For application of the results to a real-world scenario, the electroluminescence window of an electroluminescence-ready inverter is introduced, giving the boundaries of current and voltage that can be supplied. Combined with a simulation of the dark current–voltage curves of a connected photovoltaic module string, the electroluminescence inspection possibilities are visualized. Finally, the applicability of this heating phenomenon for snow melting is discussed.

DOI Source

DOI: 10.1051/epjpv/2023002

Conference proceedings article2022

Detailed Performance & Degradation Assessment of different PV Technologies with more than 10 years lifetime

Lindig S, Louwen A, Koester L, Astigarraga A, Moser D

Alexander Astigarraga, Lukas Koester

In this paper, performance losses and correlated degradation of five different PV systems representing five different PV technologies are investigated. Here, we describe the overall performance loss of a PV system in greater detail to better understand which underlying reasons trigger the performance decline of the investigated PV systems. The plants were installed in 2010, and almost 12 years of system operation are considered. Based on best practices developed by the IEA PVPS Task 13, the linear performance loss rates of all PV systems have been calculated. The results are correlated with corresponding nonlinear performance losses, voltage and current evolution, I-V curves and infrared and luminescence images captured in the field. Overall, it was shown that crystalline-based PV systems experience a lower overall performance loss compared to thin-film systems. The losses in crystalline systems stem from cell mismatches within modules and gradually developing degradation modes such as snail trails and cell cracking. The thin-film systems under investigation are subject to higher performance loss rates, (most) likely triggered by a degradation of the active PV material. Furthermore, glass-glass thin-film modules are additionally exposed to physical stresses by the mounting clamps leading to cracked glass, which may jeopardize the mechanical integrity of the modules.

DOI Source

DOI: 10.4229/WCPEC-82022-3DO.18.1

Journal article2022

Review of photovoltaic module degradation, field inspection techniques and techno-economic assessment

Koester L, Lindig S, Louwen A, Astigarraga A, Manzolini G, Moser D

Alexander Astigarraga, Lukas Koester

Project: EU H2020 TRUST-PV: Increase Friendly Integration of Reliable PV plants considering different market segments

Considering the relevance of photovoltaic technology in the power generation system, degradation and failure of photovoltaic modules are becoming particularly relevant. To adopt and coordinate preventive measures or actions, defects must be understood, detected and their economic impact evaluated. The variety of different degrading effects are categorized and the most significant ones as well as in-field characterization methods are described in detail. This information is summarized in a matrix showing signatures of important defects using different inspection methods and stating related power losses. The development of economic assessments is shown, resulting in a cost-based failure modes and effects analysis with the recently developed cost priority number. How the economic impact of a defect is estimated by the cost priority number is shown in three use cases, namely cell cracks, short circuit bypass diode and potential induced degradation. For each case, the fixing costs are evaluated in comparison to defect related downtime costs. Ultimately, to rank the usefulness of the calculations, influences beyond the financial factor are discussed.

DOI Source

DOI: 10.1016/j.rser.2022.112616

Conference proceedings article2022

Technical Considerations Resulting from Photovoltaic Module Heating During Electroluminescence Inspection

Koester L, Vallarella E, Louwen A, Lindig S, Moser D

Lukas Koester

Project: EU H2020 TRUST-PV: Increase Friendly Integration of Reliable PV plants considering different market segments

Electroluminescence imaging experienced uprising use in the last years. In future photovoltaic plants the time-consuming connection of power supplies to photovoltaic module strings could be overcome by use of inverters with bi-directional functionality, allowing backpowering of connected module strings directly. During backpowering, the limits of the power supply must be considered. As temperature influences the open-circuit voltage, it is a crucial parameter to remain in these limits. The EL inspection itself impacts the solar cells temperature, as they warm up during the process, resulting in a shift of open circuit voltage. This work investigates the heating of a hetero junction photovoltaic module during electroluminescence inspection. The saturation temperature is estimated by energy balance calculations and experimentally verified to be around 20°C, with resulting voltage drop above 2 V. The importance of this is not only to verify that the measurement is within the limits of the power supply but also to consider changes of the recorded luminescence intensity due to the temperature, shown to have a decrease of up to 4 %.

DOI Source

DOI: 10.4229/WCPEC-82022-4DO.4.4

Conference proceedings article2021

Quality Assurance of the Photovoltaic Power Plants Installation Stage - a Complementary Strategy Based of Photoluminescence and Steady-State Thermography

Koester L, Astigarraga A, Lindig S, Louwen A, Antinori M, Moser D, Manzolini G

Alexander Astigarraga, Lukas Koester

Project: EU H2020 TRUST-PV: Increase Friendly Integration of Reliable PV plants considering different market segments

Procurement, installation and operation are different phases of a photovoltaic (PV) plant life cycle. Each of these phases come with different risks. These risks and their possible effects are shown in this paper with particular focus on PV modules, explained along the real construction of a PV plant in northern Italy. The quality is tested during all phases; before procurement to ensure module quality, after transport and installation to detect the impact of wrong handling, and during the operational phase. Electroluminescence, current-voltage curve measurements, infrared thermography and daylight photoluminescence were performed. Electroluminescence and current-voltage curve measurements were carried out during procurement and after transport and installation to evaluate the impact of module mishandling. The damage wrong handling of a PV module can cause is shown. During the first year of operation, two infrared thermography inspections were performed to assess the health status of the PV plant and to detect possible further degradation modes, which have not been noticed before. The development of hot spots over half a year is analyzed and severe cell cracks were found as root causes, detected through daylight photoluminescence imagery. Further, the applicability of daylight photoluminescence and the detectability of degradation modes in images of two varying distances is investigated.

DOI

DOI: 10.4229/EUPVSEC20212021-5DO.2.2

Conference proceedings article2020

4CO.4.2 Development of Daylight Photoluminescence Technique for Photovoltaic Modules and Investigation of Temperature Dependency

Koester L, Astigarraga A, Lindig S, Moser D

Alexander Astigarraga, Lukas Koester

Daylight Photoluminescence (dPL) is a relatively novel imaging technique using the principle idea of photoluminescence with the sun as excitation source. The difficult process of filtering the luminescence signal is described by means of implementing a dPL technique at EURAC Research in Bolzano, Italy. The described approach uses modern inverter or maximum power point tracker to alter the working point of modules, which allows imaging of two different images, whose subtraction results in the luminescence signal. Further, the impact of temperature on the luminescence signal is investigated. For this, the signal intensity is measured with dPL for different temperatures under controlled conditions. The impact that temperature has on different factors influencing the luminescence intensity is described. Finally, the impact of temperature on dPL in practical use is calculated from the results, presenting a first attempt to calculate the temperature coefficient of signal intensity in the dPL image.

DOI Source

DOI: 10.4229/EUPVSEC20202020-4CO.4.2

Conference proceedings article2020

5DO.3.6 PV4.0: Combining Asset Management, PV Measurement Data and the Cost Priority Number Method in a Digital Infrastructure for PV Plant O&M

Louwen A, Koester L, Lindig S, Astigarraga A, Moser D

Alexander Astigarraga, Lukas Koester

In this work we present the ongoing PV4.0 project, which aims to develop a cost-effective PV plant monitoring and intervention system. By linking PV plant measurement data to a digital ticketing and asset management system, and additionally including the recently developed Cost-Priority-Number method [1], a tool will be developed that applies the concepts of Industry 4.0 to the PV sector. The developed tool aims to improve O&M by prioritizing maintenance activities, and streamlining and further digitalizing the workflow of technicians. Fault detection algorithms are developed based on machine learning approaches like clustering and outlier detection to automatically detect and classify anomalies in PV system output and consequently create maintenance tickets that specify a further course of action. By applying automatic failure detection, the aim is to decrease the time required for failure detection and intervention, in order to minimize revenue losses. By analysing historical and real-time PV plant data and historical failures in operation, the potential for predictive monitoring will be investigated. The digital tools will furthermore produce valuable statistics on PV plant failure modes within each analysed PV plant, and for PV plants in general.

DOI Source

DOI: 10.4229/EUPVSEC20202020-5DO.3.6

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Conference proceedings article2021

Quality Assurance of the Photovoltaic Power Plants Installation Stage - a Complementary Strategy Based of Photoluminescence and Steady-State Thermography

Koester L, Astigarraga A, Lindig S, Louwen A, Antinori M, Moser D, Manzolini G

Alexander Astigarraga, Lukas Koester

Project: EU H2020 TRUST-PV: Increase Friendly Integration of Reliable PV plants considering different market segments

DOI

DOI: 10.4229/EUPVSEC20212021-5DO.2.2

Lukas Koester

About

Scientific Publications

Redefining failure detection in PV Systems: a comparative study of GPT-4o and ResNet's computer vision in aerial infrared imagery analysis

Large‐Scale Daylight Photoluminescence: Automated Photovoltaic Module Operating Point Detection and Performance Loss Assessment by Quantitative Signal Analysis

Evaluating the effects of photovoltaic module heating during electroluminescence inspection

Detailed Performance & Degradation Assessment of different PV Technologies with more than 10 years lifetime

Review of photovoltaic module degradation, field inspection techniques and techno-economic assessment

Technical Considerations Resulting from Photovoltaic Module Heating During Electroluminescence Inspection

Quality Assurance of the Photovoltaic Power Plants Installation Stage - a Complementary Strategy Based of Photoluminescence and Steady-State Thermography

4CO.4.2 Development of Daylight Photoluminescence Technique for Photovoltaic Modules and Investigation of Temperature Dependency

5DO.3.6 PV4.0: Combining Asset Management, PV Measurement Data and the Cost Priority Number Method in a Digital Infrastructure for PV Plant O&M

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Scientific Publications

Redefining failure detection in PV Systems: a comparative study of GPT-4o and ResNet's computer vision in aerial infrared imagery analysis

Large‐Scale Daylight Photoluminescence: Automated Photovoltaic Module Operating Point Detection and Performance Loss Assessment by Quantitative Signal Analysis

Evaluating the effects of photovoltaic module heating during electroluminescence inspection

Detailed Performance & Degradation Assessment of different PV Technologies with more than 10 years lifetime

Review of photovoltaic module degradation, field inspection techniques and techno-economic assessment

Technical Considerations Resulting from Photovoltaic Module Heating During Electroluminescence Inspection

Quality Assurance of the Photovoltaic Power Plants Installation Stage - a Complementary Strategy Based of Photoluminescence and Steady-State Thermography

4CO.4.2 Development of Daylight Photoluminescence Technique for Photovoltaic Modules and Investigation of Temperature Dependency

5DO.3.6 PV4.0: Combining Asset Management, PV Measurement Data and the Cost Priority Number Method in a Digital Infrastructure for PV Plant O&M