Know the effects of dirt on photovoltaic modules

What can dirt on a photovoltaic module cause and when is it necessary to clean it?

8 minute(s) of reading

This article aims to evaluate the effects of dirt deposition on the surface of photovoltaic modules, analyzing how this directly contributes to the reduction of energy generation. Let's address a common question in the market about how often photovoltaic modules should be cleaned. We will also analyze a case study developed by the Laboratory of Energy and Photovoltaic Systems at UNICAMP (State University of Campinas) and published at the Brazilian Congress of Power Electronics [1]. Dirt found on the surface of photovoltaic modules directly impacts energy generation. Dirt leads to a reduction in solar irradiation absorbed by the cells that make up the photovoltaic module, which in extreme cases can cause partial shading, hot spots and, consequently, thermal stress on cells that can contribute to the development of microcracks in cells. Faced with so many negative impacts from dirty modules, it is necessary to clean them at a reasonable frequency so that the system does not have its useful life reduced and so that system owners have a guaranteed return on investment. The rate of dirt deposition in photovoltaic installations varies according to the elements that make up the environment, such as the type of soil, relative humidity, the incidence of wind and rain, nearby vegetation and fauna, in addition to the proximity of regions with high car traffic or industrialized areas. Considering this entire spectrum of conditions, it is a great challenge to find a formula capable of covering all types of installations to assess the impacts of dirt and determine the appropriate frequency for cleaning. The main evidence that photovoltaic modules must go through a cleaning process is the reduction in energy generation. This problem can be detected through monitoring at the module or string level, whether with MLPE (module-level power electronics) type equipment or conventional inverters. If you notice a drop in generation, cleaning is necessary. In solar plants, despite monitoring, periodic cleaning of the modules is a mandatory item in the operation and maintenance process throughout the system's useful life. The first thing to do when you notice a noticeable reduction in energy generation is to check the cleanliness of the modules. Dirt is one of the main factors that negatively interfere with generation, as we have already said. To highlight the need to monitor energy generation for cleaning actions, two practical cases are presented below that demonstrate the effects of dirt deposition on photovoltaic modules.

Case study I

The first case study was carried out on a photovoltaic system installed near the Guanabara Social Integration Center – UNICAMP. This system is made up of 1 string of 12 modules of 190 Wp. The system was hit by heavy rain and winds, causing the modules to be subject to an intense coating of dirt on the surface. In order to verify the effects of accumulated dirt, an IV curve tracer was used to carry out measurements on the modules and string. To facilitate the study, the modules were numbered as shown in figure 1.

Figura 1: Numeração dos módulos que compõem a string fotovoltaica em teste — Figure 1: Numbering of the modules that make up the photovoltaic string under test

Initially, all moduli were measured under the condition of high dirt deposition on the surface. Module 4 presented the worst performance, with a power delivery of 152.9 W in STC, with a deviation of 19.5% in relation to its nominal power. Its low performance can be attributed to the fact that it has a different dirt pattern than the others.

Figura 2: Módulo fotovoltaico nº 4, com sujidade acentuada em relação aos demais e maior desvio de potência em relação ao valor nominal — Figure 2: Photovoltaic module nº 4, with severe dirt in relation to the others and greater power deviation in relation to the nominal value

This same module, after being properly cleaned, presented an STC power of 170.84 W (variation of 10.1%), presenting an increase of 12.09 W in relation to the previous condition, curiously becoming the module with the best performance among all 12 modules of the string. In Figures 3 and 4 it is possible to analyze the IV and PV curves of the same module in the two situations studied.

Figura 3: Curvas I-V do módulo sujo (35) e limpo (41) em condições de STC — Figure 3: IV curves of the dirty (35) and clean (41) module under STC conditions

Figura 4: Curvas P-V do módulo sujo (35) e limpo (41) em condições de STC — Figure 4: PV curves of the dirty (35) and clean (41) module under STC conditions

It should be noted that partial shading caused by the concentration of dirt in some parts of the module caused the maximum power point to shift. This effect has negative consequences for the system, as it ends up limiting the generation of other modules connected to it, requiring the modules to operate under current and voltage conditions below their respective maximums. Using this same set of modules, a segmentation of 2 strings was carried out, one dirty (modules 1 to 6) and the other clean (7 to 12), according to the numbering in Figure 1. To analyze the difference in OPC generation of the two strings the following IV and PV curves were obtained. The dirty string presented a maximum OPC power of 577.1 W, while the clean string developed a maximum power of 604.4 W. The difference of 27.3 W highlights the real need to carry out cleaning in situations where there is a very large discrepancy in relation to the expected generation. Agile system maintenance can be essential for generation to meet the demand expected by the system owner and, consequently, avoid questions and headaches for the company that supplied the photovoltaic system.

Figura 5: Curvas I-V da string limpa (25) e suja (26) em OPC — Figure 5: IV curves of the clean (25) and dirty (26) string in OPC

Figura 6: Curvas P-V da string limpa (25) e suja (26) em OPC — Figure 6: PV curves of the clean (25) and dirty (26) string in OPC

Case study II

In this second case study, an installation that experienced a sharp drop in generation will be analyzed. Figure 7 shows the dirty situation of the photovoltaic modules.

Figura 7: Módulos do sistema fotovoltaico em processo de limpeza — Figure 7: Photovoltaic system modules in the cleaning process

One way to identify that modules need cleaning is to monitor generation through the inverter supervisory system. Figure 8 shows the situations before and after the cleaning carried out on 13/05. Average generation jumped from 300 kWh daily to around 350 kWh after cleaning.

Figura 8: Monitoramento de geração fotovoltaica com o sistema supervisório do inversor — Figure 8: Monitoring photovoltaic generation with the inverter supervisory system

The generation of a photovoltaic system depends, above all, on the irradiance and operating temperature of the modules. The presence of dirt is directly linked to the reduction in irradiance received by the modules. Dirt can also, when deposited in a concentrated form in some points, cause cells to overheat.

Figura 9: A limpeza periódica é necessária para garantir o bom desempenho do sistema fotovoltaico — Figure 9: Periodic cleaning is necessary to ensure the good performance of the photovoltaic system

Conclusion

This article showed two cases in which the effect of dirt on the performance of photovoltaic modules is evident. In the first case we see a highly dirty situation. One of the modules had a high concentration of dirt and had its IV curve changed with the displacement of the maximum power point, which could impact the generation of the other modules connected in series. In extreme cases, the concentration of dirt can lead to overheating of the photovoltaic cells, which impacts generation performance and can reduce the useful life of the module or cause fires in extreme situations. In the second case analyzed, a system was found to have a considerable level of dirt. After cleaning, the system's daily generation increased noticeably. In the cases analyzed, two small photovoltaic systems were addressed. Periodic cleaning of photovoltaic modules is necessary in photovoltaic systems of any size. In solar plants, cleaning must be part of the operation and maintenance routine. Determining the frequency of cleaning must consider the reduction in energy generation, which can be verified by monitoring generation.

Reference

[1] EY Sakô, J. Lucas de Souza Silva, D. d. Bastos Mesquita, R. Espino Campos, HS Moreira and M. Gradella Villalva, “Concepts and Case Study of Mismatch Losses in Photovoltaic Modules,” 2019 IEEE 15th Brazilian Power Electronics Conference and 5th IEEE Southern Power Electronics Conference (COBEP/SPEC), Santos , Brazil, 2019, pp. 1-6, doi: 10.1109/COBEP/SPEC44138.2019.9065311.