Experimental Study on the Effect of Dust Deposition on a Car Park Photovoltaic System with Different Cleaning Cycles

Abstract

For a decade, investments in solar photovoltaic (PV) systems have been increasing exponentially in the Middle East. Broadly speaking, these investments have been facing tremendous challenges due to the harsh weather in this particular part of the world. Dust accumulation is one the challenges that negatively affects the performance of solar PV systems. The overall goal of this paper is to thoroughly investigate the effect of dust accumulation on the energy yield of car park PV systems. With this aim in mind, the paper presents scientific values for further research and opens the horizon for attracting further investments in solar PV systems. This study is based on a real PV system in the Sultanate of Oman and considers different cleaning cycles for 16 months (from 29 July 2018 to 10 November 2019). Furthermore, four different PV groups were assessed, and the system was monitored under different cleaning frequencies. In general, it was found that dust accumulation has a significant impact; under 29-day, 32-day, 72-day, and 98-day cleaning cycles, the average percentages of energy loss due to soiling were 9.5%, 18.2%, 31.13%, and 45.6%, respectively. In addition, the dust effect has a seasonal variation. The study revealed that dust accumulation has a more negative impact during summer than during winter. During summer, the energy losses due to soiling were 8.7% higher than those during winter. The difference was attributed to different environmental conditions, with high humidity and low wind speed being the main factors that worsen the impact of dust during summer. Based on the findings of this research, a monthly cleaning program is highly recommended in the city of Muscat.

1. Introduction

Dust can be considered to be small solid particles with a diameter of less than 500 µm. Dust appears in the atmosphere from various sources, such as dust particles transported with the wind, sandstorms, sand crusher factories, the construction of buildings, vehicle movements, volcanic eruptions and pollution due to various industries. In addition, dust can include minute pollens, fungi, bacteria, and vegetation [1,2,3,4].

Dust can affect the performance of photovoltaic (PV) solar modules in two ways: (1) through the accumulation of dust on the top surface of the PV modules; and (2) dust in the ambient air surrounding the PV system. In both cases, the dust will affect the efficiency the PV modules negatively because the dust particles will absorb or disperse part of the solar radiation. In fact, dust and soiling cannot be avoided; however, the impact on the PV modules can be eliminated by using different cleaning techniques.

Figure 1 summarizes some factors that can influence dust settlement. These factors are related to dust properties such as size, shape, and weight. In addition, some factors are related to the tilt angle, glazing characteristics, site, wind velocity, and ambient temperature and humidity [1,2,3,4].

The impact of dust on solar systems has been presented and discussed in several studies. For example, an experiment using artificial dust to study the effect of the presence of dust in an indoor lab under constant irradiance was done in reference [5]. Two types of artificial dust were used in the experiment: dried mud and talcum powder, to represent the dust accumulation. The module’s output voltage and current were measured during the experiment. As a true control, a clean PV panel without plastic covering was used, and to represent other conditions, three different plastic sheets were used. The experiment’s results indicated that the presence of dust reduced the generated power by as much as 18%. The experiment also showed that the effect of dust on the PV system under greater irradiation reduced slightly but not negligibly.

In a study conducted in California, USA, the impact of dust deposition on solar PV systems was quantified for 108 days during the summer dry period [6]. The soiling losses were found to be 0.21% per day, and a high decrease in efficiency was observed. A decrease in site efficiency of 1.6% was found. The decrease in efficiency during the summer was due to dust accumulation (there was no cleaning of the panels until the rain started in the fall, which led to a restoration of PV plant efficiency). It was observed that the efficiency significantly increased during the rain period, meaning that soiling was the main cause of the reduction in efficiency. The study also estimated the effect of one annual washing of the panels, showing a system efficiency increase of around 1.75%.

In an experiment conducted in reference [7], the influence of dust accumulation on PV panel performance was studied by measuring I–V characteristic curves of modules with different soiling conditions. Two identical panels were exposed to the same factors and varying quantities of dust deposition. The experiment was first conducted indoors by simulating the sunlight by halogen lamps and then conducted outdoors as well; the resulting I-V curves were plotted and analyzed. The study showed that dust deposition affects mainly the short-circuit current; however, the influence on the open-circuit voltage was found to be very minimal. The effect of dust deposition on the efficiency and power losses was found to be around 5–6% of the maximum possible power output of clean photovoltaic modules. Further, it was observed that the dust deposition impacts the cell’s operating temperatures of the modules.

In another study presented in reference [8], the performance of PV modules was tested over time by examining the I–V characteristic curve every 10 minutes in an outdoor testing facility run by the Fraunhofer Institute for Solar Energy Systems (ISE) to investigate the long-term durability of solar systems. The module was left uncleaned, and only the irradiation sensors were cleaned. The effect of soiling and partial shading on the system efficiency was investigated. The results showed a 20% reduction in efficiency within 5 months. The I–V curves showed partial shading by soiling. After some months, the original efficiency was re-established because the dust was completely washed off by rain.

The study in reference [9] focused on finding a model for estimating the cleaning frequency for PV systems. The study was conducted in desert areas and considered different parameters such as the tilt angle, the concentration of the dust in the ambient air, and the average diameter of the dust particles. The proposed model in the paper adopted a cleaning criterion based on accumulated dust density with 2 g/m².This level of dust density, based on their experience, causes a 5% decrease in power output in silicon PV modules. Thus, the model indicates that the PV modules need to be cleaned when the deposited dust density equals 2 g/m². Based on adopted criterion, the study recommended a 20-day cleaning frequency for PV modules in a desert climate.

The effect of the tilt angle on dust accumulation and hence on the output power of the PV system was investigated in reference [10].

Eight PV modules were installed on a rooftop at different tilt angles, namely 45°, 30°, 20°, and 15°, and then the performances of cleaned and dusty PV modules were analyzed and compared. The reduction in generated power was found to depend on the tilt angle. For 10 months of dust accumulation, the reductions in the generated power were found to be 25.5%, 31%, 38%, and 43% for tilt angles of 45°, 30°, 20°, and 15°, respectively.

Many studies also concentrate on other factors that influence PV module efficiency losses due to dust. In reference [11], a study was conducted to study the relationship between the optical characteristics of the deposited dust particles and the efficiency loss of PV panels. The studies revealed that the total transmission decreases linearly with the dust mass deposited per unit area. In a study conducted in Doha-Qatar, a cost-effective design for a PV-monitoring system was presented [12]. The designed system was examined under several climate conditions for five months. The study showed the effects of temperature, humidity, and ambient dust on the PV panels’ efficiency in Doha. The study revealed that due to dust deposited on the PV panel surface, a 30% drop in the system’s efficiency occurred after five months.

In reference [13], the effect of dust from the main active dust sources in Western Asia was discussed. The study revealed that the effect of dust on the PV system depends on the location of the PV site relative to the active dust source. The study also indicated that the effect of soiling on PV performance is highly affected by the wind and humidity.

In reference [14], an experiment was conducted to analyze the effect of the dust’s physical properties on PV performance in Northern Oman. The dust was collected from different locations in Northern Oman, namely Muscat, Burka, Sohar, Liwa, Al-Wasta, Al-Buraimi, and Saham. The experiment was conducted with a small-scale PV module (106 Wp), with the effect of dust being evaluated from the observed reduction in current, voltage, and power of the module. The study revealed that the physical properties, and hence the dust effect, depend on the dust source (location). The dust collected from Saham and Sohar has a stronger negative impact on the performance of PV systems than the dust from Mascut, Barka, and Liwa. In addition, the study revealed a 35–40% reduction in generated power due to the dust and recommended a three-month cleaning frequency.

The most recent relevant studies are presented in references [15,16]. In reference [15], a study was carried out in Lebanon. The PV modules were cleaned between 15 June and 30 September 2018. The energy produced was compared with the energy produced in the same period in 2017, when the PV modules were uncleaned. The study found that cleaning would increase the average generated power by 32.27%. In reference [16], a study was conducted in the United Arab Emirates for a five-month period. The results revealed that under real weather conditions, dust accumulation can decrease the generated power by as much as 12.7%.

The deposition of dust on the PV modules is a function of several factors such as dust properties (weight, size, and shape), wind speed and direction, the tilt angle of the panel and the surface smoothness, and environmental factors (rain, humidity, dew, and dust storms). The causes of dust deposition on the PV modules and the effect of dust deposition on optical characteristics, electrical characteristics, and thermal characteristics of the PV module were discussed in references [17,18,19,20,21,22]. The effect of solar irradiation, dust deposition, and sand storm on the performance of crystalline modules installed in a Saharan environment was analyzed experimentally [23]. It was reported that the PV power output and short circuit current were inversely proportional to the dust accumulation. The output power of dusty module was reduced by 5.71% after two weeks of outdoor exposure (without cleaning). This value increased to 8.41% after eight weeks of outdoor exposure, as compared to the clean module. In reference [24], it was reported that the output power of the monocrystalline soiled panel reduced by approximately 8–12% per month compared with a clean panel in a solar test facility in Tehran. One panel was cleaned every day, while the other remained unclean for 45 days. The effect of dust and temperature on the PV power output and efficiency was reported in reference [25]. The reduction in efficiency was found to be dependent on the deposited dust mass and on dust properties, and followed a non-linear trend.

A 51-day field study was conducted in the coastal desert environment of Doha, Qatar [26]. The main finding of the study shows that the wind speed dominated the deposition and rebound of dust particles.

In reference [27], a case study investigated the dust accumulation in a desert area in the city of Madinah, Saudi Arabia. The study revealed that 60 days of dust accumulation caused a 28% reduction in PV output power.

Broadly speaking, the literature has addressed dust’s impact on PV performance under different conditions, such as tilt angles, time of the year, and different climate locations. However, these studies were conducted either on a few PV modules or relatively small scale systems; in addition, the majority of the studies presented short term investigations either for ground mounted PV or through an indoor experiment. Nevertheless, the current study used a larger-scale system on a real operating car park system. The practical testing took place over a long-term period (16 months). In other words, this long-term study was conducted to investigate the effects of dust deposition with different cleaning frequencies on an actual operating PV system installed on a car park located in Oman at the Sultan Qaboos University (SQU) campus. The study provides decent details in term of the studied period and frequency of the cleaning. The span of the study was 16 months (from 29 July 2018 to 10 November 2019). The cleaning was done on the 29th-day, 32nd-day, 72nd-day, and 98th-day during different semesters. The findings in this study provide a constructive reference and guidelines for suitable cleaning frequencies for PV systems in Oman, and for other similar countries in the area. A further contribution is accomplished by a non-destructive analytical technique (X-ray Fluorescence (XRF)) to determine the elemental composition of the accumulative dust. The results show that Silicon oxide (SiO₂), Sodium oxide (Na₂O), Calcium oxide (CaO), Magnesium oxide (MgO), Aluminum oxide (Al₂O₃), and Ferric oxide (Fe₂O₃) are the major dust components. This XRF analysis is significant for further studies to develop coating materials that can mitigate the accumulation of dust.

The remaining part of this paper is organized as follows. Section 2 provides the system details, cleaning setup, and the methodology. Analysis and results, including the environment effect and dust components analysis, are discussed in Section 3. Section 4 concludes the work.

2. System Details, Cleaning Setup, and Methodology

Solar PV systems are vital and strategic solutions for promoting the electricity sector in Oman. The country is located in an area with a tremendous amount of solar radiation. As a result, Oman can be considered as a good candidate for harnessing solar energy. Figure 2 shows the global horizontal irradiation (GHI) [26]; the average annual sum is 2200–2300 kWh/m². In September 2017, the Council of Financial Affairs and Energy Resources approved a target indicating that the minimum renewable energy contribution must be 10% by 2025 in the main interconnected system of Oman. In 2018, Tanfeeth increased this target to 11% by 2023. Since then, many renewable energy projects, mainly involving solar energy, have been installed in the Sultanate.

SQU’s energy vision aims to utilize renewable energy resources to achieve net-zero energy consumption and minimum carbon emissions within the Muscat municipality. To this end, SQU installed a small-scale PV project with 84 kWp as a pilot seed for a large-scale project that was initiated during 2020.

2.1. System Details

The monitored PV system is a car park PV project located at SQU campus, consisting of 333 solar PV panels with a tilt angle of 5°. The PV panels used are polycrystalline solar panels and have a 15.33% conversion efficiency. The module dimensions are 156 mm × 156 mm, with a rated power of 255 Wp. The panels are grouped and connected into four 20 kW on-grid inverters. All inverters are connected to the monitoring system (data manager), in which all the data are stored. Figure 3 shows the top view of the site location, and Figure 4 shows the single-line diagram of the PV system. The PV modules are grouped into four groups, while each group is connected to an inverter.

2.2. Cleaning Setup

The PV modules are grouped into four groups.

Table 1. Cleaning schedules for the monitored PV system.

	Group #4	Group #3	Group #2	Group #1
29 July 2018
29 August 2018
29 October 2018
13 November 2018
13 December 2018
01 May 2019
01 July 2019
05 August 2019
10 November 2019

The green background indicates the cleaning occurrence/data for the PV modules.

shows the date of scheduled cleaning for these groups (with green highlights). The PV group connected to Inverter #4 was cleaned once during the test period, Inverter #1 was cleaned every cleaning cycle, and Inverter #2 and Inverter #3 were cleaned alternatively. The highlights indicate cleaning occurrence, with the cleaning process conducted at the end of the assigned day. The monitored period was from 29 July 2018, to 10 November 2019.

For the sake of the comparison and calculation, the generated alternating current (AC) energy from each inverter (group) is normalized by its actual connected PV modules (i.e., represented in kWh/kWp). Further, the percentage of loss, due to cleaning, is calculated using Equation (1), where the percentage of loss refers to the cleaned group (cleaned panels are taken as a reference):

$$\% \mathrm{Loss}=\frac{{\mathrm{E}}_{\mathrm{CLEANED}}-{\mathrm{E}}_{\mathrm{UNCLEANED}}}{{\mathrm{E}}_{\mathrm{CLEANED}}}\times 100\%$$

(1)

where E_CLEANED and E_UNCLEANED are the generated energies (kWh) by the cleaned modules and uncleaned modules, respectively.

3. Analysis and Results

3.1. Analysis of the First Cleaning Cycle

Figure 5 shows the variation of daily produced energy (normalized yield in kWh/kWp) by the inverters for different days during the month of August before and after the first cleaning process. The first cleaning process was performed for the modules connected to inverter#1, as mentioned in Table 1. All inverters show the same performance and similar energy production before the cleaning day. On 30 August (when the PV modules connected to Inverter #1 were cleaned), the yield from Inverter #1 increased. For August 30, the day just after the cleaning, the energy produced by uncleaned modules was 4.03 kWh/KWp while the energy produced by cleaned modules was 4.98 kWh/KW. Figure 6 shows the percentage yield loss for few days before and after the cleaning. The average percentage of loss in yield for the first cleaning cycle, which is for 32 days from 29 July until 30 August, was found to be 18.2%.

3.2. Analysis of the Second Cleaning Cycle

In this cleaning cycle, both PV groups connected to Inverter #1 and Inverter #2 were cleaned. The second cleaning process was performed on 10 October. For this period, the dust effect was evaluated for a 73-day period (from 30 July until 10 October). There was a clear difference between the output of the cleaned modules (Inverter #1 and Inverter #2) and that of the uncleaned ones (Inverter #3 and Inverter #4). Figure 7 and Figure 8 show the variation of daily produced energy (normalized yield in kWh/kWp) caused by the inverters and the average percentage of loss in yield for the second cleaning cycle a few days before and after the second cleaning. The percentage of loss was found to be around 31.13%.

3.3. Analysis of the Third Cleaning Cycle

Unplanned cleaning of PV modules occurred due to heavy rain that occurred on 1 November 2018. This event reset the planned cleaning cycles (all the modules were cleaned) and started a new one. On 13 November, both PV groups connected to Inverter #1 and Inverter #3 were cleaned and on 13 December 2018 PV groups connected to both Inverter #1 and Inverter #2 were cleaned. For this cycle, the dust effect was evaluated for the PV groups connected to Inverter #3 for 30 days (the period from 13 November until 13 December), while the evaluation period was 42 days for the groups connected to Inverter #4 (the period from 1 November until 13 December).

The average energy yields after the cleaning for Inverter #1, Inverter # 3, and Inverter #4; were 3.336, 3.019, and 2.908 kWh/kWp, respectively. Figure 9 shows the reduction percentage in the yield between Inverter #1 (Inverter #1 is selected here as a reference) and Inverter #4, with Inverter #4 kept uncleaned for 40 days It is found that the reduction is approximately equal to 12.8%. Figure 10 further shows the reduction percentage in the yield between Inverter #1 and Inverter #3 for few days after the cleaning, with Inverter #3 kept uncleaned for 29 days. It is found that the reduction is around 9.5%.

3.4. Analysis of the Fourth Cleaning Cycle

On 1 May 2019, all the modules were cleaned by heavy rain. Figure 10 shows the normalized energy (kWh/kWp) produced a few days before and after this cleaning. Since then, all the modules were left uncleaned until 2 July, a period of approximately 60 days. On 1 July, the modules connected to Inverters #2, #3, and #4 were cleaned, and those connected to Inverter #4 were left uncleaned. As observed from the figure, Inverters groups 2, 3. and 4 showed higher values compared with Inverter #4 because the latter was left uncleaned. Figure 11 shows the percentage yield loss for selected days for this cleaning period (approximately 60 days). The finding in this cycle shows that the average energy loss due to dust accumulation on Inverter #1 was approximately 30%.

3.5. Analysis of the Fifth Cleaning Cycle

On July 16, the module groups of Inverter #1 and Inverter #2 were cleaned, while the module groups of Inverter #1 and Inverter #3 were cleaned on 4 August 2019. This arrangement allowed the evaluation of the dust effect for two different periods: the first period for evaluating the PV modules connected to Inverter #4 for almost 34 days and the second period for evaluating the PV modules connected to Inverter #2 for almost 19 days. The key findings are depicted in Figure 12 and Figure 13. For the 34-day period, the reduction in the energy yield of the Inverter #4 modules reached as high as 14.8%. For the 19-day period, dust accumulation on the Inverter #2 modules resulted in a reduction in the energy yield of 9.33%, as presented in Figure 13.

3.6. Analysis of the Sixth Cleaning Cycle

On November 10th, all PV modules were cleaned. Figure 14 shows the variation of the yield for the groups a few days before and after the cleaning day. A comparison was made for each inverter group before and after cleaning. The average energy yield was 1.6329 kWh/kWp and 3.00 kWh/kWp before and after cleaning, respectively. The calculated average energy losses due to dust was found to be as high as 45.6%. That percentage represents an evaluation of effect of the dust accumulation of the yield for a 98-day period.

3.7. Dust Deposition Summary and Weather Correlation

Figure 15 summarized the main findings of this study. Where the dust accumulation effect was evaluated for seven different periods. It is noticed there is a change in dust accumulation from years; for example, in July/August 2018 and July/August 2019, the dust impact was 18.2% and 14.8%, respectively. This variation could be attributed to weather changes. The key observation is that the impact of dust accumulation was smaller during winter than during summer. A period of 29 days in winter (November/December 2018) resulted in a 9% reduction, whereas a period of approximately 32 days in summer (July/August) resulted in a 18.2% reduction, which indicates that in a summer season the dust accumulation effect increases by as much as 8.7% compared to same dust accumulation period in winter. Looking more closely at these results, the impact of dust accumulation was found to depend on the season. This variation was fundamentally attributed to deviation in weather conditions between the seasons.

Figure 16 shows the variation in weather conditions: wind speed, ambient temperature, and relative humidity during August and November in the year 2019. The data samples were obtained from a meteorological station located at SQU for 30 days in August and 30 days in November. The data was collected with a sample rate of three samples per minute. Obviously, the temperature and average humidity are higher in August than in November. As the PV location is close to the sea, it is expected to observe such a weather profile.

The increase of humidity contributes to more dust settlement on the surface of modules, with dust settlement due to coagulation also increasing as the humidity increases, which exacerbates the effect of dust and leads to a higher reduction in generated power. Further, dust accumulation on PV modules influences the cell operating temperatures. It is reported in the literature [7] that a dusty panel was noted to be operating at 1–2 °C higher temperatures than a clean panel for the same light incidence, and hence reduced the amount of generated power. Under these conditions, the dust effect on the PV performance in the summer months is magnified compared to in the winter months.

Wind speed also has an impact of on dust accumulation [29,30]. This can be observed from Figure 15, which shows that wind speed during the day is higher compared in November than that in August. Therefore, it is expected with the increase of wind speed, which is considered as natural cleaning process, the dust accumulation reduces and hence has a positive impact on the output power of PV modules. These results also have been reported in the references [29,30].

4. Dust Chemical Composition Analysis

The chemical composition of dust particles is essential to identify its original sources [31]. The obtained information from this test is useful for assessing the particle adhesion degree between the dust particles and panel glass surface. This might guide the PV operators to select the appreciate cleaning methods [32,33].

A laboratory non-destructive analytical technique (X-ray Fluorescence (XRF)) analysis was carried out on the dust components for some samples of the accumulated dust. The sample were taken and tested on 30 June 2019. The test shows that the dust particles contain several compounds and elements. The ratio of each compound is depicted in Figure 17. It was found that quartz (SiO₂), Sodium oxide (Na₂O), and Calcium oxide (CaO cement and gypsum materials) are the dominant elements in collected dust samples. They accounted for approximately 75% of the dust particle contents.

The analysis also shows the presence of other compounds such as Magnesium oxide (MgO), Aluminum oxide (Al₂O₃), and Ferric oxide (Fe₂O₃). The existence of the car park close to the building and streets exposed the PV panels to suspended particles on the atmosphere from building materials and automobile smoke.

5. Conclusions

This paper has extensively discussed the impact of the long term of dust accumulation. It was conducted under different cleaning cycles, on a real solar PV system at Sultan Qaboos University in Oman. This system was monitored for a total of 16 months. The key finding is that under 29-day, 32-day, 72-day, and 98-day cleaning cycles, the average percentages of energy loss due to soiling were 9.5%, 18.2%, 31.13% and 45.6%, respectively.

Furthermore, the effect of dust varies between the seasons; the study reveals that dust has a more negative impact during summer than during winter. During summer, soiling loss increased by an average of 5.3% (in 2019) to 8.7% (in 2018) compared to same cleaning period in winter. The difference was attributed to different weather conditions. With the combined effect of a higher humidity and low wind speed in the summer months, the dust decomposition problem becomes more severe. Based on the finding in this research, the recommended cleaning frequency varies depending on the season. In general, it is recommended to have 30-day cleaning cycles in order to limit the soil loss to less than 10%. The dust analysis using non-destructive analytical technique shows that Silicon oxide (SiO₂), Sodium oxide (Na₂O), and Calcium oxide (CaO) make up the majority of dust elements.

It is worth mentioning that as the study was conducted on a car park PV system. A ground installation PV system might require more frequent cleaning cycles. Further, the system studied is located in Muscat at the SQU campus, where the dust does not appear to be as harsh as in open fields. Therefore, more cleaning cycles might be required for PV systems located close to deserts or in locations that are more prone to dust.