1. Introduction
PV generators are one of the most important renewable energy sources today. In fact, considering the power production, they are now the third most important behind hydro and wind energy generation [
1]. PV generators can be used for small productions, such as domestic installations, or in high power plants. Their growing importance can be seen through the worldwide cumulative installed capacity, with a verified growth between 1995 and 2018, from 0.6 GW to 512 GW [
2]. Most of the production is ensured by high power plants. Considering the 2018 value of the cumulative installed capacity, 180 GW were related to the utility-scale plants.
One of the problems associated with PV generators is the low efficiency [
3], so it is extremely critical to ensure the maximum possible production. There are several factors that influence the production of the electrical energy from a PV panel, some being factors associated with the solar irradiation level, like the temperature of the PV panel, the wind velocity and the dust on the panels [
4,
5]; some of these factors are interrelated. For example, irradiance and dust have the same effect since if there is dust on the PV panel surface the irradiation is reduced. The same applies for wind and temperature, as when wind velocity increases, the temperature of the PV panel is reduced. However, to ensure the maximum solar irradiation, a fundamental aspect is related to the position between the PV modules and the brightest point in the sky. To ensure these conditions, several aspects should be considered, like the PV panels’ geographic location and the position of the sun [
6]. In this way, the use of trackers associated with the PV panels play a very important role. Since this component has the ability to track the best position of the PV panels in relation to the sun, its use allows an important boost of the produced energy to be obtained [
7]. Depending on the geographic location, the increase in the total yield that can be obtained by the sun tracking system can achieve values of up to 40% [
8]. So, many solar power plants have adopted their use. There are two types of solar trackers: the single-axis and the dual-axis. The single-axis solar tracker panels rotate around a fixed axis [
9]. In the case of the dual-axis trackers, the solar panels will move around two axes (
Figure 1). Due to this, the solar panels can move in any direction and the best position of those panels in relation to the sun can always be achieved [
10]. So, the increase in the yearly specific yield (generated power per unit of area) using solar tracking with the right tilt angle and direction are becoming more and more important. Some studies analyzed the positioning features of polycrystalline (p-Si), monocrystalline (m-Si) and amorphous silicon (a-Si) modules relative to the visible focus point of a reference concentrator photovoltaic module, under real meteorological conditions, using a dual tracking system [
11,
12]. These studies showed that the performance insensitivity thresholds of m-Si, p-Si and a-Si modules depended on the direction of the changes.
Solar trackers require the use of several components and systems, such as motors, motor drive, controllers and sensors [
13]. Generally complex strategies of tracking with chips of microprocessors as control platforms are used [
14]. Analyzing the operation and maintenance power plant reports it was assessed that the tracker systems were the major reason for the underperformance of the most significant PV power plant systems [
15]. Therefore, a fault in one of these components will affect the correct position of the PV panels in relation to the sun, which will severely affect the optimum harness of energy that is possible to obtain from those panels. In this way, the existence of a fault detection system related to the PV panels’ tracker is fundamental to avoid important losses. A study about this problem in a real PV power plant (with an installed peak power of 2.15 MW) was able to verify that this problem is frequent [
16]. In this case trackers with failed fuses were detected, and through the use of an algorithm proposed in that study, single fuse repair events were found quicker (5.31 days).
Due to the importance of PV systems in the context of renewable energy sources production, many works have addressed the fault diagnosis of such systems. However, most of them are focused on the parts of the PV systems that are not related to the trackers. On the other hand, most of the works are not related to the image processing. Nevertheless, the use of image processing has been an important tool in many systems. Examples of this are in agriculture [
17], medical imaging [
18], forensic dentistry involving the automatic identification of individuals based on their dental records [
19], surface defect detection [
20] or satellite imagery [
21]. Image processing is also present in different areas of engineering, such as electrical engineering, with the detection faults found in electric motors [
22], electrical capacitance tomography [
23] or in the classification of solder joints in surface-mount devices [
24]. It can also be found in chemical engineering, with fault detection and isolation of the Tennessee Eastman process [
25] or in the ceramic and tile industry with surface defect detection [
20]. Although in reduced numbers when compared with other approaches, image processing has also been used in solar energy. For example, it is widely used in thermal solar energy to calibrate the field of heliostat and to identify any fault in their orientation, compared to the optimum position [
26,
27]. Regarding the area of PV systems it is also used in several applications. One of the aspects in which this approach is used is for the detection of shadows in PV panels [
28]. Another use is related to obtaining cloud cover indices. In this way, a method in which sky images and image processing are used to obtain forecasting models for direct normal irradiance is presented in [
29]. Another interesting application is the one in which thermographic cameras are used for PV inspection [
30,
31]. One aspect related to these photovoltaic systems is that the PV panel is operated at its maximum power point (MPP). However, shadows could affect the algorithms for this operation. In [
32] a global maximum power peak was proposed in which an optical camera to obtain the image of the PV panel was used to estimate the required irradiances. Another aspect related to these systems, especially related to the trackers, is the need for a sensor to direct the panels towards the sun. Thus, in [
33], a position sensor based on camera and image processing was developed. Other studies explored the use of image processing for the detection of faults or degradations in PV systems. One of the parts of the system is related to the PV panel itself. In fact, with the passing of time the panels will degrade, also reducing their efficiency. Several methods appear to detect this degradation. In [
34], a method in which the condition of the PV panels is verified through infrared images was presented. Another aspect related to faults of the PV panel, is the appearance of hot spots in their surface. Several works have proposed to detect those hot spots also through infrared images and image processing [
35]. Regarding the detection of faults in PV panels’ trackers, in [
36] a method based on image processing was proposed for the first time. The method is based on the concept that if one PV module is not aligned with the others, then there is a fault associated with the tracker of that PV module. With that approach, the inclination of the panels was determined using statistic moments and by the use of a line joining the centroids of two cells belonging to that PV module. However, that approach presents some limitations, since in some conditions it fails to detect the tracker under fault.
This paper is organized as follows:
Section 2 focuses on the proposed machine learning method that allows detection of a fault in PV system trackers. This method uses a more simplified and non-conventional approach, namely, through the use of images and image processing algorithms, avoiding in this way the use of other types of sensors and a wide range of data. The image processing algorithms determine the points of all the cells of the PV modules and the assignment of each point to the correspondent module are described. The use of the PCA algorithm that uses the coordinates of the points of each module to determine their orientation is also innovative and presented in this section. First, the points of the PV cells detected by the image processing algorithms are clustered in each of the existing PV modules in the image. Then, the position (coordinates) of those points is used in the PCA based algorithm to determine the orientation of the PV module. In
Section 3, several tests are presented and discussed with the objective to analyze the efficiency of the proposed method and the fast and reliable determination of the inclination. Finally, conclusions are drawn in
Section 4.
4. Discussion
The developed image-processing algorithm based on the PCA was implemented to diagnose faults in solar trackers panels with two axes. Since the PCA is determined using the points that belong to the PV cells when partial occlusion in a panel occurs, there are the remaining points that allow the correct calculation of the slope. It is what happened in the first case study, where the left and middle panels had some reflection in their PV cells (
Figure 6). In this situation, although the obtained slope values are not the same for the three panels, they are within the tolerance margin that allows for inferring that there is no fault in the trackers. It should be noted that in the three case studies the angular positioning of the panels was not performed with precision, thus justifying the differences in the values of slope obtained for panels that apparently appear to be completely aligned. The processing time of the proposed method depends on the number of PV cell points used in the PCA calculation. The computational time, when the down-sampling is higher than 1:40, is reduced when compared with the one obtained by the method presented in [
36] which was 0.681 ms.
Figure 15 and
Figure 16 show the points in each panel, used in the principal component of each module, using a down-sampling of 1:160 and 1:240, respectively. It can be seen that the direction of the three panels in both situations are practically unchanged showing that even by sharply reducing the number of points used in the calculation of the PCA, the orientation obtained by the proposed method is quite the same. One aspect that was possible to verify, is that the proposed method is able to handle images that are incomplete due to reflections. This is an important aspect, since it was verified that in the other method used for the recognition of the PV panel, erratic slope values were obtained.
Another aspect is related with the implementation of this procedure in an automatic way in a large PV plant. So, regarding this aspect, there are several possibilities. However, a practical possibility is through the implementation of a structure in which a camera (with mobility capability) or cameras are strategically located in order to take a picture of all panels.
Regarding the angle that this procedure can detect with feasibility to detect any failure, the resolution depends of the defined threshold value. In this case, since the slope of the panels was implemented manually, there are some errors that do not appear in a real system. So, in a real system the angles of panels of the healthy trackers will practically present the same value, by which the resolution can be lower than 1 degree, or lower than 0.01 deviation index (DI). However, since this precision is not needed, one recording every 15 min is more than enough, and higher angles (or deviation indexes) are recommended. In these particular case studies, considering that in healthy conditions they do not have exactly the same slope, since they were aligned manually, there was a maximum error of 4.4 degrees, or under the point of view of the deviation index 1.40.
5. Conclusions
This paper focused on an algorithm to diagnose faults in solar tracker panels with two axes. This algorithm can be associated with a camera that acquires photographs. Thus, with the purpose of obtaining a fast diagnosis of a fault in the trackers where the PV modules are installed, a new method is proposed, using an artificial vision process. In this way, the identification of the fault is realized through a pattern recognition process applied to the PV modules’ photographs. The use of photographs for the detection of these types of faults presents an important advantage since it allows a fast fault detection ensuring maximum renewable energy production. On the other hand, this approach also means avoiding the acquisition of large amounts of data, as well as extra sensors. The new image processing approach uses principal component analysis (PCA) to process images. However, instead of using the PCA to reduce the data dimension, as is usual, in this case it was proposed to use it to determine the slope of an object. In this way, in the context of several PV modules, it will be possible to identify the PV module(s) with a different slope from the majority. Besides the identification of the PV modules and their slope in a photograph, a deviation index that can be used to discriminate the panel(s) under fault is also proposed. So, with the proposed approach, several benefits are obtained, such as, avoiding the use of a wide range of data and specific sensors, fast detection and reliability, even when there are incomplete parts of an object in an image. It was also possible to verify that the PCA enables the correct determination of the slopes of a PV panel even with down sampling. To test the proposed approach several case studies with and without fault trackers were used. A comparison with other method based on a pattern recognition approach was also realized. From this comparison, it was possible to conclude that the proposed approach obtains a more effective detection, even in a situation in which the images are incomplete due to reflections.