1. Introduction
Driven by ambitious national green energy targets and agreements, conventional power systems are currently witnessing an accelerated modernization, where renewable energy power resources are being implemented in power plants as an alternative to fossil fuel electrical generators [
1]. In addition, other local constraints imposed by governments, such as the Saudi Vision 2030, to reduce greenhouse gas emission have made renewable energy sources (RESs) attractive power sources in the electricity production sector.
RESs are characterized by their low operation costs and carbon dioxide “CO
2” emissions, making the energy they produce more environmentally friendly; however, these resources face massive technical and economic challenges, such as frequency instability, voltage deviation, and output power uncertainty. These issues contribute to reducing RESs’ feasibility and reliability, especially when utilized for stand-alone applications [
2,
3,
4]. Consequently, governments and energy lanners and developers should reconsider the technical challenges and different cost aspects of the widespread deployment of RESs. For example, one of the most important key influences on PV power sources is the existence of dust on the surface of PV panels. Dust and any other accumulated objects have a major impact on a PV system’s performance and cost effectiveness. Accumulated soiling over PV panels could reduce the PV system efficiency by between 8 and 12% per month, as concluded in [
5]. Moreover, daily or unoptimized processes for cleaning PV panels reduce the electrical feasibility of the PV system in addition to increasing the operational and maintenance costs [
6,
7,
8].
The studies conducted in [
9,
10] used actual annual collected PV data to compare with present PV output power in order to detect the presence of dust. Kelebaone T. et al. examined the use of light sensors to operate PV cleaning units when the light passing through the PV panels was less than 20% of the atmospheric sunlight [
11]. Studies in [
12,
13,
14,
15] relied on collected PV power data to estimate the impact of dust on PV panels and then model the effect of dust on PV performance. Russell K. Jones et al. in [
16] used a long-term observation on average soiling rates to propose an optimal schedule for PV panel cleaning in central Saudi Arabia. Another methodology of detecting dust was introduced in [
17], where the PV output voltage and current are monitored to operate the washing unit when the output power is less than 50% of the rated power during the daytime. Researchers in [
18,
19] investigated the feasibility of imaging process technology to detect dust on PV panels. An approach based on optical imaging and the routine measurement of aerosols was also explored in [
20] to obtain PV panels’ dust. Moreover, the study utilized mathematical models in order to normalize the calculated panel efficiency, whereas other influencing factors, such as solar radiation, ambient temperature, and inverter efficiency, were not initially accounted for.
The relation between the PV output voltage and the particle size of soiling was investigated in [
21]. The study showed that as the size of the soiling particles covering the panel gets smaller, the output voltage of the panel decreases linearly, thus negatively affecting the PV panel’s efficiency. Hence, the use of detecting cameras or photodiode sensors, as used in the study, might be not sufficient where the size of the contaminated particles could not be measured precisely.
Based on the aforementioned dust detection techniques, it is noticeable that the use of cameras, sensors, or other detection elements to measure the dust on PV panels inevitably poses more issues and costs, as the additional devices are considerably expensive and need to be cleaned and calibrated in addition to requiring access to electricity on the top of the PV panels.
Figure 1 summarizes a comparative study on PV cleaning units after an intensive review of the above literature. The cost and size in the comparison were estimated, relying on some commercially available detection elements, whereas the reliability and simplicity were estimated based on the required controller circuits and lifetime of the basic utilized components. All results were normalized to be a percentage of each individual comparative aspect. It should be noted that the comparison in
Figure 1 is not applicable for any detecting and cleaning approach; however, it is valid for the strategies in the literature, i.e., [
9,
10,
11,
16,
17,
18,
19], as they compared detection systems with no additional imaging or sensing devices.
Therefore, this present study proposes an intelligent computational system to detect the dust level on PV panel surfaces without integrating any external imaging, measuring, or monitoring devices. The innovative aspect of this work is its contribution in reducing the cost and complexity of PV cleaning units at any location and under any PV system specifications. The importance and feasibility of the proposed system comes from the ability of operation from the first day after installing the PV system and providing results in real-time manner. The analysis of this work mainly relies on an estimation model of solar radiation along with an expert artificial-intelligence (AI)-based system [
22]. The detailed analysis procedures are as follows:
Estimating the solar radiation energy;
Obtaining the output power for a specific PV system;
Injecting an estimated air temperature to the process;
Scaling and calibrating the estimated PV power;
AI computational process to analyze the PV performance.
The output decision of the proposed methodology is fed to the attached cleaning system to choose the optimal time and level of cleaning. Different cleaning devices, such as robotic systems and automated water pumps, were discussed and analyzed in previous studies [
23,
24,
25]. Furthermore, modern techniques of PV panel cleaning were recently proposed, such as utilizing drones for getting rid of dust over the panels’ surfaces, as in [
26]. Due to the fact that this study focuses only on detecting the dust on PV panels and optimizing the time and level of the cleaning process, the physical cleaning tools will not be discussed further in this paper.
The rest of the paper is organized as follows:
Section 2 introduces the mathematical solar irradiation model and the methodology of estimating the PV output power for different PV system size and orientation.
Section 3 discusses the strategy of detecting the dust over the surfaces of the PV panels, while the utilization of the AI system for the computational and logical processes is explained in
Section 4. Acquisition and illustration of actual PV data is discussed in
Section 5, and an evaluation of the derived solar irradiation model is shown in
Section 6.
Section 7 investigates the feasibility of the proposed AI system using actual PV data. Significant deliverables and conclusions for this study are discussed in
Section 8.
3. Correlation of Dust to the PV Output Power
It is well-known that the produced power by PV panels is affected negatively by the accumulation of dust on the surface; however, there are no determined formulas to describe this relationship specifically. Due to the fact that this relationship is necessary to complete the construction of the intelligent detection tool in this work, the PV output power versus the accumulated dust was formulated through an intensive survey study on the performance of PV panels in the presence of accumulated dust [
19,
20].
Figure 5 shows two PV panels with different cleaning conditions.
From the previous conducted studies [
10,
13], the average generated power (in W) for a normalized size of PV system under different levels of dust concentration (in g/m
2) was collected and listed in
Table 2.
In order to extract the dust versus PV power relationship, the data in
Table 2 were plotted, as shown in
Figure 6, and the formula was generated. It is worth mentioning that the PV output power was converted to its percentage values (considering the maximum power in
Table 2 as the rated power) to generalize the extracted formula so it could be applied for any size and any combination of PV panels and arrays.
From the dotted black curve shown in
Figure 6, we could generate the relationship between accumulated dust and reduction in PV panels’ output power. This step was accomplished with the help of the curve fitting tool in MATLAB software. Furthermore, in the interest of obtaining a very precise fitting formula, the third-degree polynomial type of curve fitting was carefully chosen, and the final fitting expression was established, as expressed in the following equation.
It can be concluded from Equation (4) that the negative impact of the contaminated dust on the PV output power is more severe at the dust weight of 0.25 g/m2 to 0.5 g/m2 (where the PV output power reduced by 20% to 60% of its rated power). This verifies the importance of this study, as the need for the panel to be washed arises only after a specific amount of dust. The next section discusses the intelligent system methodology.
4. Artificial-Intelligence-Based Prediction Model
The main objective of this paper is to give optimized decisions on the cleaning of PV panels, using a fewer number of external measuring elements, with the help of an intelligent computational engine. Hence, this section demonstrates the methodology of gathering the entire discussed knowledge to generate a reliable and feasible processing unit. An expert artificial intelligence system was utilized for this aim. The flowchart in
Figure 7 shows the mechanism of the expert system, where the computational algorithm, along with the prediction and analyzing knowledge, interacts with the information entered by the end user to generate suitable decisions.
Recently, expert system (ES) and expert control system (ECS) techniques are utilized for renewable energy systems in order to enhance the operating and control decisions of non-expert users [
30,
31,
32]. For example, the expert artificial intelligence system can be used in the pitch control of wind turbines for improved system performance [
31]. The proposed expert system in the aforementioned study was applied to recognize the pattern of the generated power of the wind system in order to apply the predictive model. Second, the AI system must collect all required data from the interfaced user to analyze the wind system output power to determine suitable control decisions and deliver them to the end user via the user interface.
Figure 8 illustrates the control schematic diagram of that study.
Figure 9 virtually shows the processing flow of the proposed expert artificial intelligence system. First, the user must enter the time, location of interest, and the private PV system specifications. Second, the processing unit estimates the output power of the PV system after including the ambient temperature parameter. Third, the resultant information is calibrated and up/down-scaled to the actual PV output production. This step is necessary to take into account the unprovided input information, such as the type of PV panels, performance of the DC-DC regulator, and the panel highest from the sea level.
The calibration process is designed to use the actual data collected from the first day of installation to identify the exact PV performance. Eventually, the production of the PV system (in kW) is analyzed with the help of the calibrated predicted results in order to detect the existence of accumulated dust.
The computational analysis shown in
Figure 9 is responsible for intelligently analyzing the behavior of the PV output power in order to discover the indicators hiding behind each performance change. For instance, the fluctuation and intermittence in the PV power could indicate cloudy weather, shading, a temperature rise, or dust. In addition, the degradation in PV performance might be due to manufacturing causes, hardware issues, the PV life span, contaminated dirt, or any other possible causes. Consequently, the analysis should differentiate among the aforementioned causes and recognize the indication of accumulated dust.
Of course, determining the exact problem is difficult, since the number of inputs to the processing unit are kept few, and the impacts on the PV performance are considerably high. However, since the scope of this study focuses on detecting the existence of dust, the objective is possible and can be achieved.
The key factor in recognizing the accumulated dust is the fact that the PV power has a special pattern when it operates under several levels of dust conditions. The PV performance with dust has a cumulative dwindling pattern, and it is repetitive every day. In addition, the percentage of power reduction is related to the amount of dust in a way that can be logically differentiated from other impacts by analyzing the trending behavior and the moving average of the PV output power performance. It is well-known that the degradation in PV output power is caused by several influential factors, such as shading, clouds, raised temperature, and dust. However, the nature of the impact of each factor has its own distinguishable characteristics.
For example, if the PV power, during a normal ambient temperature, is fluctuating up and down wildly, and its moving average follows the same estimated trend of the output power, as in
Figure 10a, then the impact in this case is considered as cloudy weather. On the other hand, in the event that the moving average of the PV output power is dropping with a rate of change similar to the change of the altitude angle, as in
Figure 10b, then the shade effect is considered in this case. The rate of change of the altitude angle is considered here due to the fact that the increase/decrease in the shaded area over the PV panel is directly related to the sun trajectory. The failure of the PV system can be distinguished by the sharp degradation in the power, as in
Figure 10c, whereas the effect of dust can be detected by the downward trending of the output power with a low-rate descent of the moving average, as in
Figure 10d. After detecting the effect of dust, the amount (weight) of the contaminated soiling is determined using Equation (4). If more than one factor is detected, such as dust with cloud or dust with shade, the order of cleaning will not operate in this case due to the unfeasibility of the cleaning process.
The time window of the process is designed to be 24 h, starting from 12:00 PM, based on an intensive analysis. The ultimate outcome of the processing operation can be classified into two possible decisions: either “No dust” and showing results or “existence of dust” and commanding for panel cleaning.
5. Acquisition and Illustration of Actual PV Data
In order to validate the study feasibility, actual field PV data were used for this purpose. The realistic PV measurements of clean and dusty PV panels were collected from the Sustainable Energy Technologies Center (SET), King Saud University, Riyadh, Saudi Arabia [
33]. The SET center has a completed test set to investigate the impact of dust on PV panels. The dust impact was evaluated by measuring and analyzing the performance of the PV panels under different dust level conditions. A set of four PV panels, with disparate cleaning time intervals, was used in that study. Based on the cleaning strategies, the collected data can be broken down into four main categories:
PV data with daily cleaned panels;
PV data with panels cleaned every week;
PV data with panels cleaned every month;
PV data with panels not cleaned for a year.
All data were collected in 2020 for 12 months. The tilt angle is 24 degrees toward the south. The SCADA system collects open voltage, short circuit current and temperature every single minute (1 min sampling rate). The validity of the proposed system was investigated by processing the four sets of panel data in order to evaluate the ability of the AI system to take the right course of action. The AI system here is responsible for detecting the cause of the PV power degradation as well as determining whether the cleaning command is feasible or not.
Figure 11,
Figure 12 and
Figure 13 show the number of selected PV measurements during certain days. In
Figure 11, the data for every panel are shown during a sunny day. As observed, the panel output power for the daily cleaned panel is more than the output power for the other panels, where the accumulated dust is inversely related with the panel output efficiency. In this case, the dust detection process is obvious, and the outcome from the AI system must be the cleaning order when the amount of dust exceeds the allowable limit (0.15 g/m
2).
Figure 12 shows the collected data during a partly cloudy day. In this case, the PV panel produced a fluctuated output power during the morning due to the presence of clouds. Since the timeframe of the computational process is 24 h, the AI system must be able to differentiate the amount of dust despite the presence of clouds for part of the day and then give the order to the cleaning unit when it is feasible.
Figure 13 illustrates the PV data during cloudy weather for the entire day, and it is obvious that the AI system must not operate the cleaning unit since the panels will not be able to capture the sun light for that day.
6. Evaluation of the Estimating Model
In the interest of validating the proposed assessment tool, an evaluation analysis was conducted to test the outcomes of the sun energy predictor. Three days were selected carefully (beginning, middle, and end of the year) to cover all possible sun angle conditions.
Figure 14 and
Figure 15 show the sun energy estimation for the Riyadh region on 23 February, 27 May and 22 November 22.
Figure 14 shows the estimated sun energy on a flat surface, while the results in
Figure 15 are for a surface with a 24-degree tilt angle and 10-degree azimuth angle toward the south. These predicted curves and those for all remaining days will be utilized to estimate the PV output power and then detect the dust level with the help of the AI system.
It is worth mentioning that the power scale of the estimated sun energy is in W/m2; hence, it is necessary to normalize it (in scale of 100%) to be compatible with any size of PV system. In addition, the sun energy data must be clipped to the same time interval of the actual data, as some unwanted real PV data were rejected due to technical errors during data collection.
7. Evaluation of the Proposed AI System
The feasibility of the proposed AI system, shown in
Figure 9, is investigated by combining all required inputs and observing the ultimate system outputs. The detection unit is responsible for assigning the level of the accumulated dust on PV panels as well as sending a command signal to the cleaning system at the correct time.
Table 3 lists the parameters of the studied PV system.
Different weather conditions on different days were considered to test the detection unit under all possible consequences. The output results from the AI unit are shown in
Figure 16 and
Figure 17. The analysis outcomes of the AI unit are delivered to the end user via the interactive user interface, as illustrated in
Figure 9.
The outcome figures,
Figure 16 and
Figure 17, show only three plots for the PV system: the estimated delivered power, actual delivered power with cleaned panels, and actual delivered power with uncleaned panels, while the proposed system can virtually accommodate any type of PV power data. The reason for not showing all collected data is to not repeat the previously shown figures as well as to explain the basic process with obvious graphs. However, the completed analysis for all available data, including weekly and monthly cleaned panel data, are illustrated in
Table 4.
The final decision of the AI system not only relies on the comparison between the estimated and actual PV output power, but also depends on a deep intelligent computational analysis where the dust versus PV power, as in Equation (4), is one of the main key detection factors. For instance, on 27 May (and for the monthly cleaned panel), the order was not given for cleaning due to the existence of clouds; however, on the same day (and for the annually cleaned panel), the decision for cleaning the panel was suggested, where the AI system detected a high level of dust despite the presence of clouds. Moreover, despite the fact that the dust level on 22 November exceeded 0.15 g/m2, the dust detection unit did not order cleaning due to the presence of heavy clouds throughout the day. In other words, the proposed system is responsible for giving the order for cleaning when it is logically feasible.
The validity process is not limited to the days shown in
Table 4; however, it has been conducted for more than 300 days throughout the year of 2020. The days in
Table 4 were carefully selected to clearly demonstrate the outcomes of the proposed technique during different seasons and weather conditions in order to ensure the feasibility of the proposed PV cleaning technique.
8. Conclusions
This paper proposes an artificial-intelligence-based prediction model (AIPM) in order to detect the amount of dust accumulated on PV panels; consequently, it operates the attached cleaning units using an optimal strategy. Unlike the use of cameras, sensors, power datasets, and other detection elements, this paper attempted to determine the dust level by utilizing the expanded knowledge on solar irradiation models and logical/intelligent computational analysis. The expert artificial intelligence (AI) computational system is used in this study for a high level of data processing and to accommodate more input/output data. The feasibility of the proposed dust detection strategy was investigated using actual field data during all possible weather conditions. The results proved the ability of the detection unit for commanding the cleaning system at the optimal time as well as the capability of determining the dust level.
The proposed strategy contributes by simplifying the attached dust detection unit in terms of lower cost and higher usage flexibility. In addition, the beginning of use time for the proposed system is considerably faster than other actual-data-based methodologies. On the other hand, the proposed system must be investigated with different PV case scenarios in future work in order to fulfill the accuracy and reliability requirements. The allowable dust level in this study is 0.15 g/m2 (corresponds to 12% PV power reduction) during a sunny day, while it is dependent on the intelligent computational analysis during cloudy and high-temperature weather conditions.