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Design and Performance Evaluation of Photovoltaic Systems with Automatic Dust Wiper in a Natural Dusty Environment

This work was carried out in collaboration among all authors. Author NAA designed the study and wrote the protocol

power points of the panels were also utilized for this work. Results obtained revealed that the accumulation of dust on polycrystalline solar panel adversely affects its power output and efficiency. From the results, it was also revealed that the average panel temperature of the photovoltaic system with the automatic dust wiping mechanism was 5.30 0 C lower than the other system without the mechanism. This lower panel temperature led to an increase of 16%, 32.5%, 43.40% and 43.37% in average voltage, average current, average power and average efficiency respectively over the dusty panel. It was demonstrated that solar panel efficiency plummets as panel temperature rises due to heat dissipation caused by the accumulation of dust.

INTRODUCTION
The photovoltaic (PV) cell is one of the most amazing mechanisms developed by man for the generation of electricity. Connecting the PV cells in series and parallel make up a solar panel which can be carefully assembled with other components to make up a photovoltaic system. The birth of the PV system for the generation of electricity was made possible through the PV effect which was discovered by Alexandre E. Becquerel in 1839 [1][2][3]. At present, PV systems are ranked as the most favourable and reliable source of energy in the middle east and Africa due to the abundant solar irradiation of the regions [4]. Due to the remarkable and steady improvements in PV technology, PV systems are no more confined to street lighting or household applications since it has been shown that they can be used to generate power on large scales in the range of mega and gigawatt [5]. PV systems possess many advantages (long-lasting, almost maintenance-free, minimum operational control and generating maximum electricity during periods of peak load demand) which makes them eye-catching for both small and large-scale investments [6][7].
The working principle of the PV system envelops features including charge controller topology, battery technology, solar panel technology and inverter design. Regardless of these remarkable features, PV systems are constantly influenced by atmospheric factors including temperature variations, ageing, wind, shading and dust [8][9][10].
The emergence of solar tracking technology enabled solar panels and PV systems in capturing maximum solar radiation which enhance the power output and efficiency [11][12].
Notwithstanding the enormous benefits and improvements that solar tracking technology adds to PV systems, temperature, dust and shade still affect their performance efficiency [13][14][15].
Particles with sizes less than 500 μm are classified as dust, and their deposition, composition and morphology depends on the characteristics of the location [16][17][18]. Dust is normally measured in micrometers and comprises of minute particles of various elements and sizes, which are transported by air currents and formed by the disintegration of solids into tiny pieces through crushing, grinding or impact among other ways [19][20]. Some particles cause crowding in the dust layer which results in adhesion to surfaces due to ionic charges, which inevitably causes significant increase in effort required to remove dust particles from surfaces [21][22]. The size of particles and the density of dust on the surface of solar panel influences the plummet in the performance of solar panels [23]. The accumulation of dust on the surface of a solar panel tends to trigger a significant rise in temperature due to the affected cell heating up while acting as a barrier to the generated photocurrent [24]. This phenomenon can lead to the birthing of hot spot that have the potential to cause severe damage to the solar panel [25]. The performance and conversion efficiency of solar panels is related to the size of the dust particles deposited on it. Dust made of very fine particles have minimum interparticle separation between them which makes it very difficult for light to pass through to reach the solar panel [26]; this cause a higher drop in performance efficiency of a solar panel when compared with dust of larger particle and larger interparticle separation [27].
The output power of a solar panel has been ascertained to drop by 85% due to sand, dust, algae-like substance and even small stones that accumulate on their surface, which means regular cleaning is a must if maximum performance from the panel is desired [28]. The shading, masking and coating due to deposition, accumulation and contamination can be classified into two categories: soft shade and hard shade [29]. Soft shades can be formed due to smoke, dust particles in the air hovering over the surface of the solar panel or a distant obstruction such as the diffused or dispersed shadow of a tree, adjacent building, towers, telephone poles or clouds which significantly reduces the amount of radiation reaching the solar cells. While hard shade is an obstruction such as fallen leaves, stones, bird droppings or a tree branch laying on the panel surface that can completely obstruct light from reaching the solar cells. [30].
Gholami et al. [31] carried out an experimental study about the impact of dust accumulation on a solar panel surface after 70 days without rainfall. Their findings revealed that the surface density of the accumulated dust increased up to 6.0986 g/m 2 , which triggered a 21.47% decrease in the output power. Said and Walwil [32] researched a dusty solar panel for 5 weeks without cleaning in Dhahran, Saudi Arabia. They realized that the short circuit current and output power dropped by 13% and 6% respectively. Kaldellis and Kapsali [33] studied how PV system outputs are affected by different air pollutants. They disclosed that the output parameters of PV systems are severely affected by dust deposition and accumulation on PV surfaces. While Sulaiman et al. [34] researched the impact of dust on the performance of a 50 W solar panel using artificial dust (mud and talcum) and clear plastic under constant radiation sources in a laboratory. Their results portrayed a massive drop of 50% and 18% for efficiency and maximum power respectively. Furthermore, their results show just a slight difference in the performance of the panel covered with mud and talcum. However, Darwish et al. [35] using different types of dust pollutants studied the impact of each pollutant on the performance of PV systems. They concluded that ash, silica and limestone are the pollutants that mostly influence the performance of PV systems. Mustafa et al. [36] investigated the impact of dust accumulation, bird dropping, shading effects and water drops on the performance of PV systems. From their analysis, it was revealed that dust accumulation caused the output power and efficiency to drop by 8.80% and 11.86% respectively. While about a 7.40% reduction in the output power was observed when the panel was covered with bird droppings. When the surface of the PV panels was exposed to water droplets, an improvement of at least 5.9% in the output power was observed. Jiang et al. [37] researched on the impact of airborne dust deposition on PV module performance. Data was obtained when the density of the accumulated dust reached 22 g/m 2 . Analysis of the obtained data revealed a 22% decrease in the short circuit current and a 6% reduction in the open circuit voltage. Andrea et al. [38] studied the effect of industrial dust deposition on PV module performance in Arusha Tanzania. Dust utilized in the study was obtained from fertilizer, gypsum, aggregate crusher and coal mine industries. Maximum module efficiency loss was observed to be 64%, 42%, 30%, and 29% for coal, aggregate, gypsum, and organic fertilizer dust, respectively. Their results also show that PV module performance depreciated with the increase in temperature owing to heat dissipation triggered by dust accumulation.
The effect of various types of shade, dirt and dust accumulation on solar panel surfaces have been investigated. As a matter of fact, there is a plethora of obtainable literatures on the effect of dust on the performance of photovoltaics, but a huge amount of the obtainable information is only valid to for a specific location or region and does not apply to PV systems with automatic cleaning/wiping mechanism. Generally, there is a shortage of pertinent information on how the performance of PV modules are hindered by dust deposition for a specific location in Nigeria that can be employed by engineers in the design and sizing of PV systems.
Deposition and accumulation of dust on PV module surface is inevitable. Solar panels were designed and manufactured to be optimally efficient in outdoor environments with their surface free from shade, dust and dirt. Since there is difficulty that comes with the regular cleaning of solar panels especially when they are installed on roof tops. Then the solution is to create and design a photovoltaic system that will always be free from dust and dirt irrespective of how dusty the environment might be. Thus the introduction of an automatic cleaning/wiping mechanism will not only ensure that the module surface is kept clean but also ensure that the system remains optimally efficient. The polysilicon technology is the dominant technology in the Nigerian market and is widely utilized for backup power in households and small business premises. Hence the need to develop a means so that it surface can be kept clean. Investigation on the performance of polysilicon PV systems with automatic cleaning/wiping mechanism is not yet investigated in the Nigeria's prospect.
The aim of this study is to experimentally investigate the degradation in performance efficiency of PV systems caused by the natural accumulation of dust and dirt. In achieving the objectives, an intelligent water timer coupled with relays that is linked to the dust wipers was employed. The intelligent water timer triggers the wiping mechanism once the preset times (6:30 and 13:00) for the PV module surface to be cleaned is reached. Once the preset time is reached, the cleaning/wiping mechanism activates and remains active for 90 seconds before shutting down automatically and reactivating again once the next preset times is reached. This study provides information that will enlighten user of how much energy their PV system will lost as dust and dirt keeps accumulating on solar panel surface.

MATERIALS AND METHODS
This section is about the materials used in the study, how they were connected and the steps followed in the course of measurement.

Materials Utilized in the Study
Two exact 130-watt polysilicon solar panels of the model AF-130W produced by Africell solar were utilized in this study: the output electrical characteristics of the module are revealed in Table 1. A digital high-precision intelligent photovoltaic panel maximum power point tracker (MPPT) (model WS400A) manufactured by Elejoy was used to track and determine the maximum power generated by the photovoltaic modules as shown in Fig. 1a. An intelligent digital programmable water timer (model ZG004) manufactured by Zhigarden and a digital relay timer (model THC-15A) manufactured by Gangbei were also deployed and can be seen in Figs. 1b and 1c respectively. Water hose, water sprinklers (Fig. 1d) and wipers were also employed. Solar batteries (Gel battery: 12 V, 100 A) and a digital charge controller were also utilized. A digital hygrometer (model KT-908 shown in Fig. 1e), a digital solar power meter (model SM206) manufactured by RZ (Fig. 1f) and a digital non-contact infrared gun thermometer (model GM320) manufactured by Aneng (Fig. 1g) were deployed for the tracking of the relative humidity, irradiance and temperature respectively at the surface of the solar panel. The range and accuracy of all the measuring instruments are displayed in Table 2.

Experimental Setup
The experiment was done in an outdoor environment in Calabar at a location close to the Calabar river (latitude 4 0 57 ' 38.6161" N and Longitude 8 0 18'58.482"). The solar panels were installed at an angle of 5 0 facing the north on a platform of one metre above sea level. One solar panel was installed with the cleaning/ wiping mechanism and served as the control, while the other was left unattended for dust to accumulate.

Fig. 1. Materials used for the experimental setup
For the dusty solar panel, connecting cables were connected from its output to the input of the intelligent MPP tracker from which its maximum power points were tracked and determined as displayed in Fig. 2. For the panel installed with the cleaning/wiping mechanism, connecting cables were connected from its output into a twoway switch. One output of the two-way switch leads to the intelligent MPP tracker from which its maximum power points were precisely tracked and determined. while the other output enters the input of the charge controller for smooth charging of the battery as shown in Fig. 3. The input of the programmable timer relay was connected to the battery while the output powers the wiper which wipes the surface of the solar panel. The input of the intelligent water timer was connected to a tap from a water tank, while the output led to the water sprinkler via a hose. Through the water sprinkler, water was directed to the surface of the solar panel.

Measurement Procedure
Data was acquired from the solar panels at 30 minutes' interval from 6:00 to 18:00 for 16 weeks. During data acquisition, data were taken from both panels simultaneously. The time of day was noted, while the power, current and voltage from the panels were ascertained with the aid of the intelligent MPP tracker. The humidity level, the surface temperature and the level of irradiance at the surface of the panels were measured through the digital hygrometer, digital infrared gun thermometer and digital solar power meter respectively.

Data Processing and Measurements
This study was carried out in real-time outdoor conditions with varying atmospheric parameters.
With the aid of the intelligent MPP tracker, the maximum power at maximum power point P mp , the open circuit voltage V oc , the instantaneous current I mp and voltage V mp at maximum power under a particular real-time atmospheric condition were measured. The efficiency of a solar panel is significantly influenced by the current and voltage it can generate which is affected by maintenance and atmospheric parameters. The V mp and the V oc are immensely influenced by temperature (T) which can be ascertained by (1) as revealed by [39]. Also the I mp and the short circuit current I sc are immensely influenced by irradiance (H) and can be ascertained via (2) also revealed by [39]. In contrast, from the data obtained, the normalized power output efficiency was computed with (3) while the solar panel efficiency at STC was confirmed with (4) as shown by [40][41].
Open circuit voltage: (1) Short circuit current: (2) Normalized power output efficiency: Module Efficiency: Where K and Q are Boltzmann constant and electronic charge respectively. i 0 is the saturation current while b is a constant which is influenced by the semiconductor junction properties. P mea and P max are measured power and power at STC respectively.

Study Area
Calabar lies on Latitude 4°57'06''N and longitude 8°19'19''E. It has an elevation of 32m above sea level and is the capital of Cross River State located in southern Nigeria. The dominant climate is the tropical monsoon climate and it experiences precipitations almost throughout the entire year excluding the core months of the dry season which occurs in two short periods of January to March and October to December [42]. Rainfall is substantial in most months with the short dry season having little effect. In Calabar, it is hot and oppressive throughout the year, the rainy season is overcast, while the dry season is mostly cloudy.  [43].
Calabar experiences significant seasonal variation in its average hourly wind speed. From May 23 to October 15 (4.7 months) is the windier part of the year with an average wind speed of more than 5.8 miles per hour, with August as the windiest month having an average hourly speed of 7.5 miles per hour. The calmer part of the year begins from October 15 to May 23 (7.3 months) with an average wind speed of less than 5.8 miles per hour, with December as the calmest month having an average hourly wind speed of 4.2 miles per hour [43]. C) which were recorded in the months of January and April respectively [44].
The least humid month is January with a mean relative humidity of 69%, while July through September are the most humid months with a mean relative humidity of 87%. In regard to rainfall, December and July are the months having the least (13.5 days) and most rainfall (29.9 days) respectively [45]. In regard to sunshine hours, December (9.5 hours) is the month with the most sunshine, while August (4 hours) is the month with the least sunshine [45]. The location for this study is on Latitude 4 0 57'38.6161" N and Longitude 8 0 18'58.482" E which is less than 400 metre away from the Calabar river as displayed in Fig. 4.

RESULTS AND DISCUSSION
This section presents the results acquired from experimental measurement and analysis, it is divided into four parts. The first part discusses the variations in the atmospheric condition during data acquisition in the study area. The second part presents the analysis of the performance of the solar panels with respect to ambient temperature. In the third and fourth part, analysis is given on how both solar panels responds to varying levels of relative humidity and irradiance respectively. It should be noted that the voltage, current and power used in the analysis of the results are the maximum voltage, current and power respectively that the modules produces instantly under a particular atmospheric condition. Table 3 displays the summary of the atmospheric data obtained at the site, while Table 4 displays the summary of the output electrical parameters of both solar panels under study.

Variation of Atmospheric Parameters at the Site
As displayed in Fig. 5a, there was constant fluctuation in the atmospheric parameters (temperature, relative humidity and irradiance) at the location of study. A constant rise in irradiance was observed in the morning which peaked at 11:00. Between 11:00 to 13:00 a decrease was observed, between 13:00 to 14:00 another increase was observed, beyond 14:00 there was constant decrease in irradiance. As for the ambient temperature of the location, it increased gradually from morning to 11:30, then attained fair stability until 16:00, then begin to decrease gradually. Also, the relative humidity was high in the morning, an increase in relative humidity was observed between 6:00 to 7:00, then begin to decrease beyond noon time to 14:30, beyond 14:30 an increase in relative humidity was experienced again; this shows that as irradiance increased, relative humidity decreased. Transient variations of atmospheric parameters are triggered by turbulences within the atmosphere.
The impact of dust on solar panel temperature is revealed in Fig. 5b. the figure shows that dust accumulation on solar panels enhances its operating temperatures which is undesired. This observation was made in the process of acquiring data from both panels which revealed an average panel temperature difference between the clean and dusty panel to be 5.30 0 C as evident in Table 3. Furthermore, the rise and fall in panel temperature also depends on the level of solar irradiance at the surface of the solar panel which is clearly evident in Fig. 5a. Our observation about the impact of dust on the solar panel operating temperature corresponds with earlier studies by [38] which reported that the operating temperature of the clean module was observed to be lower than that of the dusty one.

Analysis of Electrical Performance of both Solar Panel with Varying Temperature
The impact of dust on the output electrical parameters of the solar panels with respect to ambient temperature is displayed in Fig. 6. Fig.  6a and Fig. 6b reveals a higher voltage and current performance from the clean solar panel over the dusty one respectively. The low voltage and current performance from the dusty panel is linked to its higher panel temperature (as revealed in Fig. 5) which is as a result of the dust on its surface absorbing heat and also obstructing air from reaching its surface to enhance cooling. The poor voltage and current performance from the dusty panel is translated to poor output power and efficiency which is evident in Fig. 6c and Fig. 6d respectively. These figures reveals that temperature rise in solar panel which is triggered by dust deposition hiders the performance efficiency of solar panels. Our results in Fig. 6 is in agreement with earlier researches by [13][14][15] which revealed that temperature, dust and shade affect the performance efficiency of solar panel.

Analysis of Electrical Performance of both Solar Panel with Varying Levels of Relative Humidity
The effect of relative humidity on the clean and dusty solar panel performances is shown in Fig.  7. From Fig. 7a and Fig. 7b respectively, it is revealed that high levels of relative humidity adversely affect the voltage and current performance of both panels. An increase in voltage was observed for both panels as the relative humidity decrease to 80%, below 80% stability in voltage was achieved while a linear High level of humidity hinders solar radiation from reaching the surface of the panels, while the dust on the surface of the dusty panel further hinders the available solar radiation at it surface from going through to reach the cells, which further causes a degradation in its voltage and current performance. Fig. 7c and Fig. 7d which displays the power output and efficiency of both panel respectively is as a result of the voltage and current performances from both panels. It shows that high levels of relative humidity negatively affect the power output and efficiency of solar panel regardless of the mechanism employed for dust wiping. It depicts that for the same level of relative humidity around both panels, the lost in power by the dusty panel was up to 43.40% which is a massive lost. Our observation about the impact of dust on the solar panel output power corresponds with earlier studies by [31] which reported that dust accumulation on solar panels up to 6.0986 g/m 2 triggered a 21.47% decrease in the output power.

Analysis of Electrical Performance of both Solar Panel with Varying Levels of Irradiance
The impact of dust on the output electrical parameters of the solar panels under the same level of irradiance is displayed in Fig. 8 [4]. Furthermore, the figure shows low performance in voltage, current, power and efficiency from the dusty solar panel as displayed in Fig. 8a to Fig. 8d respectively which is as a result of the obstruction towards solar radiation caused by dust deposition and accumulation on the panel surface. Solar radiation is one of the most important parameter necessary for the effective operation and functionality of a solar panel. Once it is hindered from reaching the cells of a solar panel, the efficiency of the panel is drastically reduced as evident from Fig. 8d. The difference between the   average maximum efficiency attained by the clean panel and the dusty one is 29.26% which is more than 50% efficiency lost as displayed in Table 3, and this shows that for solar panel to keep functioning at its best it must be free from dust and shade at all time. Our observations about the impact of dust on solar panel output electrical parameters corresponds with earlier studies by [38] which reported they observed maximum panel efficiency loss to be 64%, 42%, 30%, and 29% for coal, aggregate, gypsum, and organic fertilizer dust, respectively.

CONCLUSION
A photovoltaic system with an automatic dust wiping/cleaning mechanism was designed and its impact was evaluated. Loss in performance efficiency of a polycrystalline solar panel due to dust accumulation was investigated in real-time outdoor conditions in an environment where human activity takes place. The impact of dust deposition was ascertained by subjecting both panels to the same environmental and atmospheric conditions. One panel was installed with the automatic dust-wiping mechanism while the other panel was left unattended for the dust to accumulate on it. It was observed that the photovoltaic system with the automatic dustwiping mechanism performed better in voltage and current performance and efficiency over the dusty panel. The average panel temperature of the photovoltaic system with the automatic dustwiping mechanism was 5.30 0 C lower than the panel without the mechanism. This lower panel temperature led to an increase of 16%, 32.5%, 43.40% and 43.37% in average voltage, average current, average power and average efficiency respectively over the dusty panel. Plummet in voltage and current output due to dust accumulation leads to a loss in power output and efficiency and consequently, huge economic loss to photovoltaic power with concern towards large-scale photovoltaic power generating plants.
Solar radiation is one of the most essential parameters necessary for the effective operation and functionality of a solar panel. Once it is hindered from reaching the cells of a solar panel, the efficiency of the panel will immensely decrease. From this study, it is advisable not to mount photovoltaic systems just one metre above the ground in a dusty environment. The study reveals that just 16 weeks after the mounting of any photovoltaic system more than 40% efficiency can be lost due to dust. If a photovoltaic system must be mounted one metre above the ground it must be given considerable care and attention or an automatic dust-wiping mechanism must be installed with it, especially for large-scale photovoltaic power generating plants.