Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1, pp.01-12 https://doi.org/10.46904/eea.23.71.1.1108001 1 Importance of Preventive Maintenance in Solar Energy Systems and Fault Detection for Solar Panels based on Thermal Images Alexandru-Ionel CONSTANTIN1, Gabriela IOSIF1, Rareș-Andrei CHIHAIA1, Dorian MARIN*1, Gafireh Umut ABU SHEHADEH2, Mehmet KARAHAN3, Bilgin GERIKOGLU3, Stefan STAVREV4 1 Institutul Național de Cercetare-Dezvoltare pentru Inginerie Electrică (INCDIE) ICPE-CA, Splaiul Unirii, Nr. 313, Sector 3, 030138, Bucharest, Romania, Atahan Arge Turizm, Ehlibeyt Mahallesi, Tekstilciler Cad. Bayraktar İş Merkezi. 17/A Kat: 9, No.:33 Balgat, 06520, Çankaya, Ankara, Türkiye 3 Susurluk Mesleki ve Teknik Anadolu Lisesi, Sultaniye Mahallesi Yeni Sanayi 1. Sok. No 2/B 10600 Susurluk/ Balıkesir, Türkiye 4 EGLA Consulting Oy, Juuritie 7, 03100 NUMMELA, Finland 2 * Corresponding author Abstract The article presents the importance of renewable energy in reducing the potential dangers of global warming and climate crises, related to a proper maintenance in solar energy systems in the context of increase in global energy consumption which generates an excess of greenhouse gases. The operation and maintenance activities of photovoltaic systems represent key aspects for obtaining the profitability of investments and ensuring their viability and reliability. Currently, the procedures applied mainly refer to simple techniques such as visual inspection and scheduled maintenance strategies. Also, the types of faults that can occur in photovoltaic panels can be detected by thermography (single hot spot, multiple hot spots, activation of the bypass diode and a higher temperature of the junction box) and are therefore presented with their characteristics and consequences. In the last part of the article, a thermal imaging processing software based on artificial intelligence technology is proposed for use for the preventive maintenance, in order to detect the photovoltaic (solar) panels with faults to be repaired or changed to increase the efficiency of the system. The software will be used to develop an innovative maintenance and repair curricula for the departments related to solar energy in vocational and technical education schools in order to better predict and prevent malfunctions in solar energy systems. Keywords: Solar Energy, Photovoltaic system, Preventive maintenance, Thermography, Fault detection Received: 2 March 2023 How to cite this paper: CONSTANTIN A-I., IOSIF G., CHIHAIA R-A., MARIN D., ABU SHEHADEH G. U., KARAHAN M., GERIKOGLU B., STAVREV S., “Importance of Preventive Maintenance in Solar Energy Systems, Fault Detection for Solar Panels based on Thermal Images”, in Electrotehnica, Electronica, Automatica (EEA), 2023, vol. 71, no. 1, pp. 01-12, ISSN 15825175. 1. Introduction Energy is among the most important components of economic activities, production processes and daily life. Being determinant in both the economic growth and development indicators of countries, energy is a scarce resource. World resources have been consumed to obtain energy and the damaged natural environment in the last two centuries [1]. A great part of the energy produced in the world is obtained from fossil fuels such as oil, coal and natural gas. The damage to the natural environment is not limited to the production of energy from fossil fuels. Use of these resources also results in high carbon emission rates in the atmosphere, which is the primary cause of climate crisis. Threat caused by the production and use of fossil resources is getting closer to an irreversible point, which threatens all the gains of humanity. For this reason, it becomes very critical to turn to renewable resources. The energy needs of the rapidly increasing population and developing industry cannot be met with limited resources, and the gap between energy production and consumption is increasing. It is estimated that global energy consumption will be twice the amount of energy consumed in 1998 by 2035 and three times in 2055 [2]. This has led the world to seek alternative energy sources. The search for alternative energy has gained great momentum and studies on renewable energy have begun to increase. 2 Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 Renewable energy sources are mainly grouped as “solar”, “wind”, “geothermal”, “hydraulic”, “biomass”, “wave” and “hydrogen” energies. It can be said that the sun is the main source of most of these types of energy and has a direct or indirect effect on them. The 1950s were a turning point in terms of search for renewable energy resources. In 1954, Bell Labs demonstrated the first practical silicon solar cell [3], and the New York Times reported on a breakthrough in solar photovoltaic (PV) technology that “may mark the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams–the harnessing of the almost limitless energy of the sun for the uses of civilization” [4]. Researchers discovered that silicon transistors, the building blocks of computers, can generate electricity when exposed to sunlight. However, in the same year, nuclear power generation costs began to decline, and throughout the 1950s, extensive R&D support for nuclear energy emerged in the United States. Solar energy has therefore been overshadowed by nuclear energy since its inception. With the increase in the world population, the demand for clean and cheap energy increased steadily in the last decades. The negative consequences of nuclear energy and the high investment costs also caused the popularity of this energy source to decrease. The damage caused by fossil fuels to the environment and the depletion of this resource have led people to seek alternative energy sources (solar, wind, hydraulic, geothermal, wave, biomass). Solar energy is among the important clean energy sources and has great growth potential. With the development of technology and the decrease in investment costs [5], solar energy has become one of the most prominent sources among alternative energy sources in recent years. 1.1. Use of solar energy in the World The current situation of energy consumption worldwide is given in [6]. The Global energy consumption, which was approximately 156 Exajoules in 1965, increased by approximately 3.75 times to 584 Exajoules in 2019. The share of renewable energy in total energy, which followed a nearly constant course from 1965 to the beginning of the 2000s, was around 6% in the mentioned period. A serious upward trend has started since 2003, and in parallel with this upward trend, the share of renewable energy has increased to approximately 11.4 % in 2019. The amount of installed capacity from 2000 to 2019 has increased more than 3 times for total renewable energy sources. This increase is approximately 500 times in the installed capacity of solar energy. This increase is also an indication that solar energy has come to the fore as a serious option [7]. In the last 20 years, the installed solar power in the world has increased with an annual growth rate of 33.2 % and reached 710 GW installed power by the end of 2020. While the global solar energy installed power was recorded as 940 GW at the end of 2021 with the record installations added last year, it increased to the "TW" level as of May 2022 [8]. For a sustainable future, it is predicted that the installed solar power will exceed 9 TW in 2050. Solar energy can be converted into different forms of energy with a wide range of applications. For example, thermal energy is produced with the help of collectors made of materials that absorb heat. Photovoltaic electricity is obtained by capturing solar radiation by a photovoltaic cell system and converting it directly into electricity. This electricity is either used directly or stored in special batteries or fed into the national grid [9]. 1.2. Role in fight against climate change In the past two centuries, primary energy uses have undergone a long-term shift from conventional fuels to coal, from coal to oil and natural gas. Currently, 80 % of the available primary energy supply is based on liquid fossil fuels [10]. Energy use, which still relies heavily on fossil fuels, will continue to increase significantly in the coming decades. It is estimated that the use of crude oil and natural gas will increase by 30% and 53.2 %, respectively, and global energy consumption will grow by 48 % in 2040. This trend has the potential to cause more greenhouse gases and then serious environmental problems with its effects on the climate [11]. The goal of reducing the potential dangers of global warming and climate crises and making tomorrow more “sustainable” has become one of the priorities of world economies. An unprecedented effort is being made to control the emission of carbon dioxide gas, the number one cause of global warming. Countries are increasing their commitments to move towards the common net zero target and to eliminate their carbon footprints, and they support this with various plans. The diffusion and transfer of climate-friendly energy technologies has become a constant topic in international climate negotiations and major conferences. Developed countries, especially the European Union (EU), are trying to take concrete steps to use renewable energy sources more. Starting from the late 1980s, studies have been carried out under the leadership of the United Nations and international organizations to reduce the negative impact and pressure of people on the climate system. As a result of these studies, the United Nations Framework Convention on Climate Change (UNFCCC) was established in 1992 and the Kyoto Protocol (KP) was established in 1997 with wide participation. While the UNFCCC and KP brought legal regulations to limit and reduce anthropogenic greenhouse gas emissions, on the other hand, they have become Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 increasingly active in international emission trade, technology, and capital movements. The comprehensive report published in October 2018 by the Intergovernmental Panel on Climate Change (IPCC), the world's leading organization in the assessment of the effects of climate change, made it clear that the climate crisis should be addressed urgently. The IPCC warns that if we do not want climate change to have even more devastating consequences on people and the world, we should not rise above or at least 1.5 °C above preindustrial levels. The report reveals the enormous differences between the 1.5 °C and 2 °C scenarios. We should try to limit the global average temperature increase to 1.5 °C (Paris United Nations (UN) Climate Change Conference (21st Conference of Parties - COP21)). The IPCC states that the only way to keep warming below 2 degrees is that the world is completely carbon-free by the end of this century, without consuming any coal, oil, gas. This increases the importance of solar energy, which is a more stable, sustainable, and environmentally friendly energy source. Since the rays coming from the sun do not emit any harmful gas, they create a healthy and safe energy potential. In this direction, sun rays are absorbed today with special systems; can be stored and used directly. 2. The importance of preventive maintenance and repair There is a major downside to renewable energy sources, which are gaining more attention as a result of environmental concerns, demands for security in energy supply, and the need for greater independence in fuel imports. The cost of energy produced from renewable sources is still higher than the cost of energy from conventional plants. Although the importance of solar energy systems in combating climate change is known, it has not yet become widespread in the world at the desired level. The economic life of Solar Power Plants is considered to be over 25 years. The efficiency of a facility that will produce energy for 25 years will change over time. Solar panels can be damaged over time, due to weather conditions, temperature changes, pollutants, and UV rays. The main factors affecting the efficiency of Solar Energy Systems are the materials used, labour and maintenance / repair services. In order for Solar Power Plants to reach the promised operating performances, to be long-lasting and to avoid vital and financial losses, maintenance and repairs should be done professionally. The maintenance and repair of solar panels should be carried out by experts in their fields, using professional measuring devices calibrated in accordance with international standards. The equipment to be used during the controls must comply with the standards, and the personnel must have the necessary qualifications. Developing innovative approaches in the training of personnel 3 who will work in the maintenance and repair of solar energy systems will contribute to the protection of the environment and the fight against climate change. Solar energy systems are long-lasting but highly costly investments. Malfunctions that may occur not only reduce the service life of the systems, but also cause revenue losses during the recovery process. Therefore, it is extremely important to prevent the occurrence of the malfunction as well as to eliminate the malfunction in a short time. However, such an early warning system has not yet been developed. The energy demand in the world continues to increase every year. Therefore, energy efficiency and the ability to use renewable energy sources are one of the ways to meet the increasing energy needs. Carrying out the maintenance and repairs of solar energy systems without malfunctions will both reduce maintenance costs and eliminate the energy loss caused by the failure of the systems, thus contributing to energy supply. Thanks to preventive maintenance and repair services, the maintenance and repair costs of the facilities will also be reduced. Recently, there is an increased interest regarding how to evaluate the quality and performance of photovoltaic modules and their service lifetime. Thus, the reference definition of the defective PV panel was considered from Subtask 3.2: Review of Failures of Photovoltaic Modules [12]. A PV panel is considered defective if its power has irreversibly degraded under normal operating conditions or creates a safety problem. A purely cosmetic problem that has no effect on the power or safety in operation is not considered a defect of the PV module. A defect of the photovoltaic module is relevant to warranty terms only when it occurs under normal operating conditions. A problem that is caused by the misuse of the module or generated by the local environment in which the module operates, is not considered to be a defect. Dirt on the module or faults caused by lightning strikes are not considered defects. The dirt problem has to be treated by the operator, and lightning strike is considered a natural phenomenon which can occur if the solar park is not suitably protected and for which the module is not designed to withstand. However, defects due to heavy snow load are considered defects of the module, if stated in the technical specification that the product can operate under these conditions. On their entire service life, photovoltaic modules are subjected to mechanical stresses, solar radiation, humidity, heat, snow, hail, salt fog, acid rains, dust, wind, abrasive particles, etc. All these external causes, when acting on PV modules made with incompatible materials, produce defects and/or accelerated degradation of their output power. It is absolutely normal that these causes affect all photovoltaic modules, but the degradation of power, for those manufactured with 4 Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 compliant materials, must be under 0.7 %/year. The main phenomena that occur in photovoltaic modules and which lead to their failure and/or premature aging are listed below [13]: — Power degradation; — Corrosion of electrical contacts; — Cells breaking; — Interrupting cell connections; — Delamination of encapsulation; — Air bubbles formation inside the encapsulation; — Changing the colour of the encapsulated foil and/or the back sheet; — Snail trails (dark cell traces due to cell cracking); — Back sheet deformation; — Degradation of ribbon welding on the PV cell; — Burning encapsulation and film due to electric arc or hot spot; — Bypass diode failure; — Breaking the glass; — Degradation of the anti-reflex layer of the glass; — Degradation of the adhesive that secures the connection box. Maintenance deficiencies can be easily identified in old photovoltaic parks. One of the common shortcomings is related to the growing vegetation in the immediate vicinity of the panels. The most common situations encountered is due to the plants that grow under the panels and seeking the sunlight, come out above the PV panels generating hot spots. The hot spot phenomenon produces not only a decrease in the string power of the shaded panel but also its irreversible degradation by changing the colour of the EVA film and destroying the structure of the silicon crystal [14]. The hot spot generated by the plant that grew under the panel can reach 107 oC and will destroy the PV cell eventually. Hot-spot heating occurs when there is one low current solar cell in a string of at least several high short-circuit current solar cells. If the operating current of the overall series string approaches the short-circuit current of the "bad" cell, the overall current becomes limited by the bad cell. The extra current produced by the good cells then forward biases the good solar cells. If the series string is short circuited, then the forward bias across all of these cells reverses biases the shaded cell. Hot-spot heating occurs when a large number of series connected cells cause a large reverse bias across the shaded cell, leading to large dissipation of power in the poor cell. Essentially, the entire generating capacity of all the good cells is dissipated by the bad cell. The enormous power dissipation occurring in a small area results in local overheating, or "hot spots", which in turn leads to destructive effects, such as cell or glass cracking, melting of solder or degradation of the solar cell [15]. A photovoltaic (PV) plant is essentially an electrical power system with very few elements impacted by regular ageing and damage. However, overstraining due to high temperatures and electrical overloads may be substantial in the case of inverters, switches and other system components. In addition, the components exposed to weather fluctuations require continuous supervision in order to avoid premature damage. Preventive Maintenance (PM) It represents the service which, by scheduled system interventions, avoids the accidental fall of the essential components related to the photovoltaic system. Constant monitoring, associated to a Condition-based maintenance (CBM) program, is indispensable to providing a highPerformance Ratio (PR). In the frame of maintenance contracts, intensive monitoring of the operating parameters and technical conditions regarding the photovoltaic installation is carried out. All the gathered data is stored in the monitored system database and is used as a constant resource for periodic reports on the performance of the photovoltaic system. Each maintenance activity is recorded in the operating and maintenance report of the supervised PV plant and transmitted to the customer monthly. All the activities focus on cost efficiency and safety regarding the operation of photovoltaic installations. Preventive (or planned) maintenance (PM) It includes routine inspections and equipment maintenance, defined by technical specifications, with a fixed frequency established by equipment type, environmental conditions, and warranty terms. The purpose of PM maintenance is to prevent unnecessary damage and production losses (power losses). This approach is becoming increasingly popular due to the fact that it significantly reduces the probability of unplanned withdrawing from service of the PV power plant. Optimization and moderate costs associated to PM activity comparing to the total cost will always be considered, thus avoiding unnecessary activities. The intervention usually begins with the visual inspection of the equipment, in particular of the PV modules. Some hotspots, like the ones caused by bird droppings or glass breakage can be visible to the naked eye. Still, there are some defects which are visible only by using the thermal camera, such as the poor connection between the cells, the activation of the bypass diode in the panel junction box, broken PV cell or damaged encapsulation film. Corrective maintenance It is the intervention that is typically performed following Preventive Maintenance (PM) finding when an inappropriate operation of an inverter is detected (for example, entering limiting /protection mode, with or without real justification, or entering thermal protection due to inadequate ventilation) or when relatively low power produced by a series of Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 5 modules is encountered (one of the string’s modules is provided by power losses). It is usually solved by adjusting/repairing (in case of inverters) or replacing/repairing the faulty module. The Corrective Maintenance is different from Reactive Maintenance. The latter involves interfering with a PV system or equipment when it is already out of service. In general, preventive maintenance (scheduled according to the contract) associated with the corrective maintenance which is described above apply where the monitoring equipment does not provide sufficient information to allow a Conditionbased maintenance program. Condition-based maintenance (CBM) It uses real-time data from PV power plants in order to anticipate faults and/or lower performance and prioritize activities maintenance and resource allocation. Intervention is performed if one or more indicators show that the equipment will fail, or the performance of the equipment deteriorates. For the most part, PV power plants are equipped with hardware and software equipment which offer real-time data on the state of the PV system such as: DC and AC (active and reactive) power, electric current and voltage at the inverters’ input and output of, PR (Performance Ratio), weather data (temperature, humidity, wind speed and solar irradiation) and other data depending on the complexity of the monitoring equipment. Such a system determines the state of a PV power plant (PV panels, inverters, connection boxes, cables, connectors, etc.), intervening when necessary (i.e, decrease of PR, the current /power of an inverter drops inappropriately compared to the other inverters, etc.). The intervention consists in performing specific parameter measurements in the area where a disturbance has been identified, on the DC side comprising the area of PV panels along with cables, connectors and Stringer Boxes or the alternating AC side with inverters, cables, and connection boxes. Both universal instruments (voltmeter, ammeter, and ohmmeter) as well as equipment specific to PV system measurements are used. The measurements that determine the status of PV panels must be performed according to the SR EN 61829:2016 – “Photovoltaic (PV) array. On-site measurement of current-voltage (IV) characteristics”, according to the SR EN 60904-1, Art. 5 –“Measurement of the current-voltage of photovoltaic devices in natural light” and according to SR EN 60904-1, Art. 7 – “Measuring the currentvoltage of photovoltaic devices in pulsed solar light”. The Current-Voltage curve diagram is presented in Figure 1 under typical faults. Figure 1. I-V curve diagram under typical faults Figure 1 shows the I-V curve diagram that changes when specific defects are encountered. The interpretation of the IV characteristics, can determine the degradation state of a PV panel (by comparison to the initial parameters), the failure of a cell, an interrupted or degraded connection of the connectors or cables, or, more seriously, the effect of Potential Degradation Induced (PID). A complete analysis of a PV module also involves electroluminescence (field test) for crack detection or PID effect. Reactive maintenance It is performed after the equipment has ceased to work. It is opposed to the Preventive Maintenance (PM), which faces a pre-established program. Reactive Maintenance (also known as "Maintenance of Malfunction") is limited to bringing the equipment to its normal operating condition after being defective. Faulty equipment is replaced or repaired by replacing defective parts/components in accordance with the service contract specifications. Emergency repairs cost 3 times to 9 times more than the planed repair, so maintenance plans that are based on Reactive Maintenance are generally the most expensive. This type of maintenance is expensive due to the fact that failures occur accidentally during production (instead of preprogrammed maintenance interruptions) while the period and cost of spare parts supply is relatively high. Special transport is needed and also the maintenance personnel are often needed to work overtime in order to complete the equipment repair. 3. Fault detection with the thermographic method The biggest disadvantage of photovoltaic systems is their high installation costs and low efficiency. This affects the return time of the investment cost. If the installed power of the plant is not obtained as scheduled for the entire operation life, then the expected efficiency is not met, causing financial losses. In order to prevent these efficiency losses, installations must be made and monitored regularly in accordance with international standards without compromising system security. 6 Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 Some precautions are taken, and tests are carried out during the installation phase in order to have power plants operating at maximum efficiency. Performing these tests with due diligence will increase the efficiency of the power plants. The aim of this study is to detect the problems encountered or to be encountered in photovoltaic systems with thermography method beforehand and to ensure that the facility operates at maximum power for the maximum period of time by taking the necessary precautions. In order to maintain the long-term reliability of solar modules and maximize power output, faults in the modules must be diagnosed at an early stage. High PV cell temperature due to staining can damage cell encapsulation and cause secondary degradation, both of which will cause permanent damage to the PV panel. The design and development of two hotspot reduction techniques using a simple, inexpensive and reliable method. Photovoltaic systems are power plants with high investment costs. Attention should be paid to the investment cost of the power plant, the payback period and the life of the facility. Power losses that occur frequently in power plants affect efficiency negatively. The most frequent issues which cause serious efficiency drops in panels are related to shading losses, electrostatic discharge, micro-crack failure, assembly error and hot spot defects. 3.1. Errors due to shading Panels in photovoltaic enterprises consist of modules connected in series and parallel. A panel consists of cells connected in series and parallel to each other. It has been observed that it varies between 0.5 V and 0.6 V depending on the solar radiation falling on the cells. The voltage does not change regardless of size. The purpose of connecting in series is to achieve the appropriate voltage range. In this case, the current remains constant. The cells are connected in series. If we accept the cells as 1 Ampere; Four cells provide a voltage of 2 Volts and a current output of 1 Ampere. Shading a cell causes power losses in power outputs as much as the number of parallel connections. Shading one panel in a series of panels connected in series significantly reduces the power output of the entire array. Preventing these shadow losses is achieved with proper engineering designs. In some cases, shading problems cannot be avoided. In this case, bypass diodes are used in cell connections. It provides current bypass when the cells are shaded and prevents the array from experiencing significant power loss [16]. As a result of a vertically mounted panel shadowing the lower cells due to the angle of the sun, the entire panel is disabled. If the panel is mounted horizontally, there will be power loss only in the shaded part of the parallel array connected in series as a result of shadow falling according to the sun angle. However, in this case, it covers a lot of surface area, especially in the fields. This requires careful power analysis and feasibility preparation. This problem is encountered significantly in facilities. When shading analysis is made with the help of a thermal camera, panels with or without power output due to temperature difference are easily detected, as can be seen in Figure 2. Figure 2. Shading Detection with Thermal Camera Figure 2 represents an aerial view of a PV power plant captured with a thermal camera by a solar panel inspection drone. It can be seen that the PV panels from one of the strings have higher temperature than the neighbouring ones. These phenomena observed in the picture is due to partial shading of the PV panels from the string. During overheating the panel supplies reduced power and adequate measures must be taken. 3.2. Crack errors In photovoltaic panels, cells are damaged and micro-cracks occur due to faults made during production, transportation, stocking, or assembly stages. If these cracks are not detected before commissioning the plant, it will cause serious problems and efficiency losses. Cells reaching high temperatures due to these cracks cause solder melting, thermal fatigue, rupture of contact wires and rapid aging in solid cells. This event adversely affects efficiency and safety [17]. Figure 3 presents a thermal image of a PV panel with microcracks. Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 7 Figure 3. Thermal camera assist microcrack detection. As shown in Figure 3, the microcracks can be easily detected by thermography method due to the temperature difference comparatively with the rest of the panel. 3.3. Bubble formation error Bubbling defect is an error that occurs during adhesion between glass, encapsulant, active layers and backing layers. Delamination error in the optical path and interfaces leads to optical reflection and subsequent loss of current. Some of the reasons for this problem are: — Improper preparation of the cell surfaces, — foil portions of the residual flux backing layer during soldering, — poor adhesion of the back encapsulating layer can be counted [18]. As a result, attention should be paid to the fact that the modules are designed and manufactured in accordance with the environmental conditions (humidity, temperature change, air pollution, dusting, etc.) of the region where they will operate. It is one of the most common problems in the lamination process. It can be easily detected by thermography method due to the temperature difference. Figure 4. A bypass activated diode PV panel Figure 4 represents PV panel with a bypass diode activated that causes the temperature increase of the last two cells columns on the right side. The activation of the bypass diode is due to a specific cell defect(bad connection). 3.5. Hotspot Error Another error is the hot spot occurrence. If the current produced by the cells connected in series inside the solar panel is lower than the other cells, the panel may switch to the load state and the voltage will change its polarity [20]. In such case, since the solar panel works under load, the temperature of the cell starts to rise and may even start to burn. This situation is called the Hot Spot Effect, represented in Figure 5. 3.4. Diode faults Another frequent fault of the panels is related to the diode’s operation. The diodes used in photovoltaic modules are activated in case of shading. When the condition that causes the diode to activate disappears, the diode switches to cut-off mode. When the diode is under reverse biasing conditions, a small leakage current flow through it. The leakage current of some diodes increases considerably with temperature and consumes a certain amount of power when reverse biased [19]. Therefore, it can stay at high temperatures by not providing sufficient cooling. If the diodes remaining at high temperatures do not have sufficient strength, they may fail. It can be easily detected by thermography method due to the temperature difference, as can be seen in Figure 4. Figure 5. PV panel with single cell hot-spot The hot spot fault shown in Figure 5, also can be easily detected by thermography method due to the temperature difference between one cell and the others. 3.6. Water Entering the System Another fault is the ingress of water between the polycarbonate and the solar cell. It occurs as a result of the cavities remaining in the coating stage and the penetration of water molecules by the 8 Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 heated air, or as a result of water leaking into the cell during the rain, cleaning stage. If it is not detected beforehand and the necessary changes are not provided, it can lead to serious power losses and overheating due to missing cells in the serial arrays [21]. It can be easily noticed with the help of the thermal camera and the temperature difference. However, detailed investigation is required. In Figure 6, the PV panel has an increase temperature in the area corresponding to the junction box. 4. Innovative approaches in preventive maintenance The recent development of software applications allowed the analysis of PV panels defects by thermal imaging in a quick and reliable way to evaluate the degradation of the panels. The software needs to be able to differentiate between the types of faults that can be detected with a thermal camera for a PV panel and to give some information related to the probable cause of the fault and some information for maintenance and repair: − Junction box - the PV panel has the junction box at a higher temperature than normal operating ones. It is recommended to perform a visual inspection of the junction box for any deformation, else IV tracing can provide details of the electrical impact caused if any due to this fault. − Single and multicell hot spots - arise due to cell mismatch in the modules. The root cause could be external (glass crack, non-uniform soiling, object shadows) or internal cell mismatch (manufacturing defects). Perform a visual inspection of the modules. If no external factor is observed, IV tracing of the modules can reflect the root cause of the hotspot; Some single/multi cell hotspots can be caused by foreign object fallen over the cells. It is recommended to clear the objects as the cell temperature can rise above 100°C absolute degrading the cell performance and back sheet quality. − Bypass activated diode - check for any visible fallen object or heavy soiling that could result in bypass diode activation. If no visible signs of fault cause, cell mismatch, solder interconnect issues could have caused the substring isolation. It is recommended to perform IV tracing of the modules indicating the operating point at which the diode bypasses the substring. 1/3rd of module power is lost when one bypass diode is activated. Note that the VOC(open circuit voltage) might not necessarily be 1/3rd lower than rated. Bypass diode activation can only be identified by comparing the temperature of the substrings (the columns with cells of the PV panels) or by IV curve measurements. The same cannot be verified by measuring the module VOC in all the cases. Figure 6. A junction box PV panel As shown in Figure 6, the temperature increase of the junction box reaches about 57 °C. This increase does not significantly affect the operation of the panel and does not represent an activation of the bypass diode, because all the cells of the panel have the same temperature field. The temperature increase can be due to a week connection between the bus bar and cable connector which has an increased electrical resistance. The severities of the faults can be defined as: − Minor/uncritical - junction box, in most cases it is not necessary to change the PV panel, only if the junction box is deformed; − Medium/uncritical – single cell hot spot and multicell hot spots, the output power of the panel is between 1 and 4 % lower for each single cell hot spot than the output power of a healthy PV panel, therefor if the panel has more than one hot pot need to be changed; − Major/critical - bypass activated diode, in this case the PV panel needs to be changed, because the output power of the panel is around 40 % lower than of a healthy one. 5. Proposed detection and classification approach by thermal imaging software analysis There are four basic types of damage that occur on PV panels that can be identified using thermal imaging, such as: single hotspot, multiple hot spots, junction box connection issues that causes excessive heat, bypass diode activation (<2 %, less common). The following approach is suitable in order to detect and classify the types of panel damages. At first, thermal camera image data must be divided into several training categories, with equal number of images into each category. The training images need to be at least 100 samples. A typical division of training /testing set is 80/20. In addition, several training / evaluation rounds, using data cross validation is necessary. Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 In this manner, we make our trained model less dependent on the data and more robust towards the data structure. For the actual training, some pre-processing is required. Then, calibration of the images by rotating, skewing, and transforming them is needed, so that they are the same size and orientation. In the next step, the transformation of the colour space to grayscale is performed. The reason for that step is to reduce the sensitivity of the training images. One possible solution is to use the IS2 thermal info, however not all thermal cameras support it. From IS2, we get a matrix with the thermal values per pixel, per channel. The use of grayscale (after PCA) reduces the dimensionality and use those matrixes for training. The thermal panel data is labelled into different categories. In addition to the above mentioned 4 damage types, we can add a 5th category – healthy panels. An example of healthy PV panel is presented in Figure 7. Figure 7. Healthy PV panel In Figure 7, it can be observed that the temperature field of the cells is relatively the same and the temperature of the junction box is with normal operating limits. Based on the aspects stated before, two main approaches can be considered: − KNN, Linear model (linear discrimination), short training time [22]; − convolutional Neural networks (CNN), 3-4 layers. (long training time) [23]. Also, a low-cost and efficient method for photovoltaic (PV) plant quality surveillance can be successfully used that combines technologies such as an unmanned aerial vehicle (UAV), thermal imaging, and machine learning so that systematic inspection of a PV farm can be performed frequently. Most emphasis is placed on using deep neural networks to analyse thermographic images [24]. 9 6. Vocational education and preventive maintenance Vocational education prepares students to work as a technician or to take up employment in a skilled craft or trade. Applying PV technologies requires highly qualified technicians to install, repair and maintain them. National markets have been growing faster than the qualified PV installers force can satisfy. Furthermore, the interested parties (manufacturers, developers, investors) seek skills certification and quality assurance in all phases of a PV installation (design, installation, and maintenance). The shortage of qualified workforce will increase the need for vocational education and preventive maintenance in the field of PV power plants. Thus, multidisciplinary projects are required for training and certification scheme, creating a qualified installer workforce, by supporting the European Photovoltaic industry in addressing the need for skilled technicians [25]. Such applications like those described in this paper for identifying and troubleshooting defects of PV panels by thermal imaging is suitable to be integrated in a learning curriculum to ensure modern means of training and education for the technicians who will be involved in the maintenance of photovoltaic panels and PV power plants. This new and upgraded curricula provides comprehensive training, curriculum guidance and student-cantered lesson plans for best results in transferring the knowledge to keep up with the current trends in the engineering field of PV plants maintenance. The expected impact of using such software applications will contribute to the development of preventive solar energy maintenance and repair technologies both in the academic community and in private raising awareness on the importance of proper operation of PV plants in order to avoid any losses caused by defects that occurred over time and were not properly monitored. 7. Conclusions The importance of renewable energy sources is increasing due to the continuous increase in energy needs, the decrease in fossil fuels and their harm to the environment. Considering the problems such as dusting, shading, micro-cracks that occur during transportation, which are not considered in solar power plants, will enable more efficient power plants to be connected to the grid. One of the biggest problems in photovoltaic systems is the heating problem. Among the reasons for this, are the errors in the diodes and the hotspot effect. With this heating problem, the prevention of fires that may be caused by excessive current flow to the inverters will prevent the extra costs that will occur in the power plants and increase the efficiency. The use of thermography method in finding these problems is one of the most effective methods in 10 Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 detecting these problems, which are not considered but cause large yield losses as a result. Using application for defects identification, modules that may have problems are detected in a short time, healthy data is obtained, and more accurate estimations are made in recycling time with less lossy power outputs. In order to develop and popularize the thermography method used to find these problems, it is necessary to process the thermal camera images and present the Hot Spot errors to the end user by developing software. Thanks to this software, errors and malfunctions that may occur can be detected quickly. Developed software should be given as vocational training in schools providing vocational and technical education that train solar energy maintenance and repair technical personnel. Technical personnel who have the skills to use this software will be able to detect Hot Spot faults earlier in the businesses they will work with, thanks to the software. In this way, the formation of photovoltaic dumps in the coming years can be prevented. Efficiency losses can be kept to a minimum by intervening at the initial stage before problems occur. Major power plant fires and accidents are prevented. [13] [14] [15] [16] [17] [18] [19] 8. Bibliographic References [1] Ümit Koç, Solar Energy and Economic Growth, Journal of Research in Economics, Politics & Finance, 2021, 6(2): 515-533, p. 515. 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Tsoutsos, Theocharis & Tournaki, Stavroula & Gkouskos, Z. & Charalambous, A. & Maxoulis, Christos. (2011). Vocational Training and Certification of PV Installers. The European initiative PVTRIN. Funding Sources Disclaimer: “The European Commission support for the production of this publication does not constitute an endorsement of the contents which reflects the views only of the authors, and the National Agency and Commission cannot be held responsible for any use Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 which may be made of the information contained therein”. Co-funded by the Erasmus+ programme of the European Union, EU programme for education, training, youth and sport, Key Action: Partnerships for cooperation and exchanges of practices, Action Type: Cooperation partnerships in vocational education and training, Project identifier: 2021-1-RO01-KA220-VET000025733 with the Project Title: Smart Innovative Maintenance and Repair in Solar Energy. Authors’ Biographies Alexandru-Ionel CONSTANTIN was born in Bucharest, Romania on September 24, 1989. He received a BSc in 2012 and a MSc in 2014 in Computer Science in Electrical Engineering from the Faculty of Electrical Engineering, University “Politehnica” of Bucharest, Romania. He is a First Stage Researcher (R1) at the National Institute for Research and Development in Electrical Engineering ICPE-CA Renewable Sources and Energy Efficiency Department /Photovoltaic System Laboratory. He was a team member or key person in 10 research projects, from which, 3 international projects, 38 scientific papers (of which 26 Web of Science indexed), 2 patent applications. E-mail: [email protected] Web of Science ResearcherID: AAD-5556-2019 ORCID: 0000-0003-0050-4500 Gabriela IOSIF was born in Bucharest, Romania, on January 29, 1976. Since 2014, she has a PhD in science communication. Currently, she is a Counsellor and a First Stage Researcher (R1) at INCDIE ICPE-CA, Bucharest (Romania). Now she is the project manager of the Erasmus project “Smart Innovative Maintenance and Repair in Solar Energy (SIM – RISE)”. Her research interest concern: electrical engineering, new sources of energy. E-mail: [email protected]. Rareș-Andrei CHIHAIA was born in Bucharest, Romania, on September 30, 1986. He is an Established Researcher (R3) at National Institute for Research and Development in Electrical Engineering ICPECA, Romania. His work focuses on the use of renewable energy sources, research, development, and design of innovative products for energy harvesting from renewable energy sources. He has a PhD in Civil Engineering (Innovative solutions used for developing small hydropower plants with reduced environmental impact) graduated in 2015. He also has experience in small scale experiments on dedicated test stands involving water flow, electrical generators, or PV panels. Professional experience: 6 research projects (as project responsible), team member or key person in more than 25 research projects, from which, 4 international projects, 53 scientific papers (of which 18 Web of Science indexed), 7 patent applications and 3 awarded patents. E-mail: [email protected] Web of Science ResearcherID: D-3184-2018 ORCID: 0000-0002-5084-3212 11 Dorian MARIN was born in Cluj-Napoca, Romania, on June 22, 1970. He has a PhD in Electrical Engineering from University “Politehnica” of Bucharest received in 2010. He is a Leading Researcher (R4) at National Institute for Research and Development in Electrical Engineering ICPE-CA, Romania. He is the head of the Photovoltaic System Laboratory. His work focuses on the use of renewable energy sources, research, development, and design of innovative products for energy harvesting from renewable energy sources. Professional experience: 5 research projects as project director/responsible, team member or key person in more than 25 research projects, from which, 4 international projects, 58 scientific papers (of which 18 Web of Science indexed), 4 patent applications and 2 awarded patents. E-mail: [email protected] Web of Science ResearcherID: AAK-4351-2021 Gafireh Umut ABU SHEHADEH was born in Türkiye. She graduated from Ankara University, Faculty of Agriculture, Agricultural Engineering in 2016. She completed her master’s degree in the same field. During her undergraduate education, she prepared a dissertation on the Nutritional and Technological Properties of Milk Except Cow's Milk. She has studies on Packaging Techniques of Fermented Dairy Products, Lysozyme and Lactorerrin, Natural Antimicrobial Substances of Milk. She gave a seminar on the Use of Casein Nanoparticles as Biocarriers during her Master's Degree. She has been working as Agricultural Project Manager at Atahan Arge for 3 years now. E-mail: [email protected] Mehmet KARAHAN was born on April 24, 1976 in Balikesir, Türkiye. He graduated from Marmara University Technical Education Faculty in 1999. In 1999, he started to work as a Technical Teacher in the Field of Electrical and Electronics at Susurluk Vocational and Technical Anatolian High School. He is a specialist teacher on Electrical and Electronics Technology and School Principal at Ministry of National Education Susurluk Vocational and Technical Anatolian High School, Susurluk / Balıkesir / Turkey. He has 24 years of professional experience, including 16 years as a teacher and 8 years as an administrator. In this process, in the Field of Electrical and Electronic Technologies and Renewable Energy Technologies. He gave Vocational Training to the students on the subjects of Electricity Production, Electric Installations Installation, Electric Panel Installations, Electronic Circuits, Project Drawings, Energy Production Distribution Centres, Wind and Solar Power Plants Installation and Operation, Solar and Wind Power Plants Failure Maintenance - Repairs. In this process, he took part in 3 Transnational and 3 National Projects in the Fields of Renewable Energy and Electrical and Electronic Technologies. E-mail: [email protected] Bilgin GERIKOGLU was born in Türkiye. He graduated from Marmara University Technical Education Faculty in 1992. He started to work as a Technical Teacher in the Field of Machinery and Design Technology in Ordu in 1992 and at Susurluk Vocational and Technical Anatolian High School in 1999. He is a Machinery and Design Technology Teacher and Deputy Principal at Ministry of National Education Susurluk Vocational and Technical Anatolian High School, Susurluk / Balıkesir / Turkey 12 Electrotehnica, Electronica, Automatica (EEA), vol. 71 (2023), nr. 1 He has 31 years of professional experience, including 23 years as a teacher and 8 years as an administrator. In this process, in the Field of Machinery and Design Technologies and Renewable Energy Technologies. He gave Vocational Education to the students on the subjects of machine design - production, Hydraulic - Pneumatic, Automatic Control Systems, Basic Electricity Courses, Project Drawings, Wind and Solar Power Plants Installation and Operation, Solar and Wind Power Plants Failure - Maintenance - Repairs. In this process, he took part in 3 Transnational and 4 National Projects in the Fields of Renewable Energy and Machinery and Design Technologies. E-mail:[email protected] Stefan STAVREV was born in Bulgaria. He has a PhD in Computer Science (Design and architecture of software systems for gamebased learning) graduated in 2022. He is a Senior Lecturer at Plovdiv University “P. Hilendarski”, Department of Software Technologies, Bulgaria. His work focuses on the use of artificial intelligence for creating realistic computer simulations, machine learning and computer vision. He also has experience in serious games development, STEM projects and game-based learning. He is a contract-based employee of EGLA Consulting Oy. Professional experience: 7 research projects (as project responsible), from which, 1 international project, 22 scientific papers (of which 2 Web of Science indexed, 6 in SCOPUS), and more than 40 independent citations. E-mail: [email protected] Web of Science ResearcherID: Q-2653-2019 ORCID: 0000-0003-3499-101X
