Solar energy as a source of renewable and a clean energy has known a significant growth over the last few years. In fact, the great potential in solar energy especially in the MENA (Middle East and North Africa) region needs to be massively exploited to cover the electricity demand which is increasing over the years. Morocco is an example of countries having a considerable solar potential with 3000 hours/year of sunshine and an average irradiation of more than 5 kWh/m² per day (Othieno and Awange 2016). The Moroccan Kingdom has therefore launched special projects for clean energy based on solar energy (Aarich et al. 2018; Cantoni and Rignall 2019). However, soiling is a challenge that faces solar technologies (Dahlioui et al. 2022a). It is manifested by the accumulation of dirt on the surface of solar panels and can cause significant power losses. A daily and monthly power reduction in some areas can reach respectively more than 1% and 80% (Kazem et al. 2020). This makes maintaining or enhancing the performance of the solar power plants through low cost and ecological soiling mitigation techniques the trend today (Costa et al. 2017; Gupta et al. 2019a).
Indeed, water-based cleaning methods are nowadays the most commonly used in cleaning of solar collectors (Fernández-García et al. 2014; Kazem and Chaichan 2019), but they are considered as less sustainable since they require an important amount of water for large reflective areas especially in a region suffering from water scarcity (Bouaddi et al. 2018). As detailed by (Jamil et al. 2017), different cleaning techniques were the result of the awareness to keep the solar panels constantly clean and improve their performance. Figure 1 presents a classification of the cleaning techniques that can be divided into three categories. Each category will be detailed in the following section.
In this paper, a recent review on the cleaning and soiling mitigation techniques has been done. A focus has been given to the investigation of the adaptability of the mechanical cleaning solutions to solar trackers. Based on this rigorous analysis, an innovative cleaning technique has been designed, realized and tested. Moreover, in this work, the soiling losses for dual-axis tracker have been evaluated for the site of Rabat under real environmental conditions. The second section of this paper presents the evaluation of the manufactured cleaning system based on an artificial soiling.
1.1. Natural cleaning
The first category concerns the natural cleaning which can be done by rainfall, wind and dew depending on the climate conditions. No cost is required by natural cleaning but it has been reported as being not effective for small dust particles (Gupta et al. 2019b). The efficiency of cleaning by natural events depends on the amount of rainfall (rain or dew) and the wind direction. It has been reported in Qatar that an amount of 3 mm of rainfall can ensure an efficient cleaning and restore the initial state of solar panels (Elminir et al. 2006; Javed et al. 2020). Less than this amount can lead to a partial cleaning. The effect of rainfall can be affected positively or negatively by wind velocity (Hee et al. 2012). Indeed, high wind speed with presence of rain enhance the cleaning of the solar panels by blowing rain to remove the dust out of the panel. The effect can be reversed in absence of rain and, depending on wind direction, the dust can be blown on the surface and lead to dust accumulation.
In the work of (Jiang et al. 2018), the cleaning of PV panels by wind has been analyzed and it has been found that large particles with diameter larger than 1 µm were effectively removed by wind due to the low required resuspension velocity compared with small particles. In the same work, the wind velocity that can lead to a natural cleaning is ranged between 0.82 to 2219.8 m/s. Regarding the cleaning by dew, an important amount of dew water is observed on the solar panels which leads to their self-cleaning especially in early mornings (Dahlioui et al. 2019). The effect of this latter as a cleaning agent has not been well investigated in literature. A recent review has been recently published aiming to exploit dew in soiling mitigation (Dahlioui et al. 2022a).
1.2. Corrective cleaning
Manual cleaning
Manual cleaning requires human resources (labor), material such as soft brushes for dry cleaning as well as water in the case of wet cleaning. Karcher (kaercher.com/us/) which is a company specialized in the cleaning products, has developed cleaning brushes made by natural and nylon bristles that avoid micro-scratches to be created on the solar glass surface. These brushes can be fitted with air pressure cleaners.
As benefits, the manual cleaning is considered the most efficient method to recover PV performance and it can be performed whenever required (Gupta et al. 2019b). However, it requires high cost and water which is limited as well as scratches may be produced. According to the study of (Fernández-García et al. 2014), the use of demineralized water and a brush was the most effective compared with a jet of air or water with high pressure. The use of detergent is not required since it does not increase the effectiveness of cleaning.
Mechanical systems
The corrective cleaning category includes as well mechanical techniques that can be fully automatic or semi-automatic. Mechanical cleaning systems denote any cleaning technique with motorization able to replace the physical effort provided by the operator. They are characterized by a large dust removal force, fast operation, good environmental adaptability and control performance (Lu et al. 2013). According to the system dimensions (motorization axes), the mechanical solutions are divided into three categories; single dimension, two dimensions and autonomous robots.
According to literature, the cleaning combined with tracking may increase the efficiency of the solar panel by 50% (Abhilash and Panchal 2016). However, the most of these cleaning solutions are dedicated to fixed panels and they showed less degree of adaptability to the trackers as shown clearly in Table 1. In order to make these solutions appropriate to trackers, it will be necessary to rely on their adaptation, which will involve a high investment cost and makes the cleaning solution economically less attractive. Few systems that show more adaptability to tracker systems as the case for Hector and Ecoppia T4 (Hardt et al. 2011; 2019a). However, the need for operators is required to perform the distribution tasks for the fleet of solar power plants. Several solutions are in the research stage, furthermore, they show more adaptability to trackers (Tejwani and Solanki 2010; Singh and Ravi 2014) since they perform a rotation of 360° during the day, which results in sliding of cleaning system twice over the PV modules. For the autonomous cleaning robots, they are still the most appropriate for the tracking system as they move without restrictions on a given surface and, with a single robot, a large area can be scanned. However, the passage of a slightly heavy robot on hard soils may not ensure effective cleaning (Juzaili et al. 2017). According to what has been cited earlier, a real gap in cleaning systems dedicated to solar tracker is highlighted. Hence the need to take the obtained improvements points into consideration and propose a cleaning technique integrated into solar trackers.
Table 1 Commercial and near commercial mechanical cleaning systems of solar panels, [V]: Verified; [NV]: Not Verified; [A]: Adaptive; [VA]: Very Adaptive
Cleaning system*
|
Autonomy
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Simplicity of design
|
Adaptable for trackers
|
Adaptability
|
Economic feasibility for trackers
|
Comments
|
Nomadd (Services 2019)
|
V
|
V
|
NV
|
NV
|
NV
|
The need of twin mounted rails. To be used, this system requires that the PV modules should form a row
|
SolaRobot (2019b)
|
V
|
NV
|
NV
|
NV
|
NV
|
The cleaning system should be guided by rails
|
SunPower
|
NV
|
NV
|
NV
|
A
|
NV
|
Effective for one-axis tracking solar PV panels
|
Greenbotics
GB1 (Dailygreen.it 2013)
|
NV
|
NV
|
NV
|
NV
|
NV
|
Complicated structure which is not compatible with trackers in general
|
Geva-Bot (Geva-Bot 2019)
|
NV
|
V
|
NV
|
A
|
NV
|
Not compatible with a tracker composed of many modules mounted in wide rows
|
Washpanel (Verdi 2019)
|
V
|
V
|
V
|
A
|
NV
|
This cleaning system does not require rails and it accepts wide rows. But if the tracker is composed of multiple rows, the use of this system is not considered low cost.
|
Hector (Hardt et al. 2011)
|
V
|
V
|
V
|
VA
|
V
|
This cleaning system is developed for the cleaning of Heliostats in CSP plants
|
SunBrush
(SunBrush 2019)
|
V
|
V
|
V
|
A
|
NV
|
Adaptable for a wide row of PV modules. Multiple units are needed for a tracker composed of many rows.
|
Solmaks (Solmaks 2019)
|
V
|
NV
|
NV
|
NV
|
NV
|
The use of this system requires rails
|
Miraikikai
(2019c)
|
NV
|
V
|
V
|
VA
|
NV
|
The autonomy of this cleaning system is relative since it needs one operator to be placed on the PV modules surface
|
Sinfonia Resola (Sinfonia 2019)
|
NV
|
V
|
V
|
VA
|
NV
|
As a cleaning robot, it needs to be placed on the surface which is subject to cleaning
|
Aerial Power (Aerialpower 2019)
|
VV
|
NV
|
V
|
VA
|
V
|
One drone can be used for many trackers. However, it will involve a lot of time in cleaning since it uses a small brush.
|
hyCLEANER (2019d)
|
V
|
NV
|
V
|
VA
|
V
|
The need for an operator to place the system on the PV panel surface as well as a long time to clean a considerable surface
|
PSE-BOSON(Solrenen 2019)
|
V
|
V
|
NV
|
A
|
V
|
This system presents a smart correction of moving position in real time as well the advantage of turning back after failing to pass an obstacle to 3 times (Security)
|
Ecoppia T4
(2019a)
|
V
|
V
|
V
|
A
|
V
|
As a cleaning robot, it needs to be placed on the surface which is subject to cleaning
|
* The names of the cleaning techniques mentioned in the table correspond in most cases to the names of the companies. Note that the use of these names is only to compare the compatibility of these techniques with the tracker presented in this work and it does not present any publicity interest. |
Electrodynamic screen (EDS)
As a corrective cleaning method, the electrodynamic screen has been proposed and tested in different environments and adopted as the main dust removal by the National Aeronautics and Space Administration (NASA) on Mars and Moon missions (Horenstein et al. 2013). The EDS uses traveling-wave effects to deter dust out from the panel surface. The principle of EDS is based on the manufacture of electrodes on a substrate (Fig. 2). The electrodes are either transparent or very thin in order to minimize the shading effects. In order to insulate the electrodes from the air, a transparent dielectric cover is placed over the electrodes. This dielectric layer becomes the outermost layer thus requiring protection from dirt or even its mitigation (Guo et al. 2018). During field operations, dust deposition occurs on the air side of the dielectric cover, so that the activation of the EDS can repel the deposited dust, taking advantage of the electrostatic charges carried by the dust particles (Kawamoto and Guo 2018). The dielectric blanket is a thin sheet which is bonded to the electrodes / substrate through an adhesive, or applied as a coating (Kawamoto and Shibata 2015). The activation of the EDS consists in applying a high alternative voltage to the electrodes, which leads to an alternative electric field. The electrically charged dust that has settled on the air side of the dielectric cover can then be pushed out of the EDS. This latter can be either a self-contained thin structure that covers the front surface of a solar panel or a component integrated into the solar module (Guo et al. 2018).
The EDS is considered faster compared to other methods cleaning. However, there is a risk of screen degradation due to ultraviolet (UV) rays (Deb and Brahmbhatt 2018b). Also, the system involves high voltage supply to generate electric field, thus reducing the generation efficiency by 15%. It has been also reported that the EDS is not effective for wet or cemented dust particulates (in presence of dew), and so it is less efficient for small sized particles (Guo et al. 2018).
1.3. Preventive soiling mitigation
The preventive methods include different approaches aiming to repel the dust out from the panels surface based on the treatment of the surface properties (He et al. 2011). This first category of soiling mitigation focuses on special coatings that can be super-hydrophilic (Son et al. 2012) or super-hydrophobic (Nguyen-Tri et al. 2019). The second part is dedicated to tilt angle and tracking effect as a soiling mitigation approach.
Super-hydrophilic coating
The common super-hydrophilic coating is based on Titanium dioxide TiO2, which has hydrophilicity and photocatalytic activity (He et al. 2011). This preventive approach has two phases. The first one is a photocatalytic process which TiO2 film reacts under the UV radiation leading to splitting the dust particles. Then, because of the hydrophilicity, the rainfall will diffuse to the whole surface instead of get together and rinse the dust. Several works have been developed related to the preparation, doping and amendment of this material. This self-cleaning method cannot be used in solar cell array because they worked mostly in desert region where the occurrence of rain is very limited.
The performance of the hydrophilic coating has been improved by using an automated mechanical vibrator as demonstrated by (Al-Badra et al. 2020). Indeed, the results showed only a decrease of 12.94% in PV panels efficiency while the one of coated PV only has reduced by 24.46%.
Super-hydrophobic coating
Inspired by the Lotus leaf (Przybylak and Maciejewski 2016) with a hydrophobic effect and less wettability, has raised a great interest among the research community because of its capability to be reproduced as coatings for self-cleaning by developing Nano-structures and micro-structures (Xiao et al. 2017). The hydrophobic coating consists of forming a layer which is considered as a barrier so that water accumulates on the substrate in a spherical shape without being adhered to the substrate surface. These spherical water droplets could roll off easily on the treated tilted surface thus leading to its self-cleaning. Indeed, it has been reported in literature (Park et al. 2011) that the contact angle (CA) can be enhanced by reaching more than 150° (Fig. 3).
Many works are ongoing with a great focus to improve the non-wettability property of the hydrophobic coating as well as the lifetime and durability concerns especially for real environmental conditions (Sarver et al., 2013b). In fact, the lifetime of the coating is considered very limited. A durability study conducted in Denmark on candidate coatings has shown that they started to degrade after only two weeks outdoors, and this degradation was manifested by the contact angle decrease (Oehler et al., 2020).
Tilt angle and tracking effect
Soiling is highly affected by a surface’s tilt angle. Indeed soiling rates significantly decrease at steep tilt angles (Sarver et al. 2013b; Anana et al. 2017). In Portugal, a model based on irradiance and soiling data has been developed to set tilt angle configurations to maximize the energy production (Conceição et al. 2018). For large solar field, the different tilt angle configurations could be concretized by movable tilt angle frames which present better costs optimization rather than single or dual-axis trackers. Since soiling can reach higher rates during the night (Ilse et al. 2018), stowing the PV modules equipped with solar tracker vertically or upside at night can significantly reduce soiling (Figgis and Ilse 2019) as well. In case of trackers, many stowage positions have been proposed to mitigate soiling (Ilse et al. 2019). It has been found that facedown position is obviously more opportune (Sayyah et al. 2014).
1.4. Synthesis
In order to restore the losses in optical performance due to soiling phenomenon, several cleaning techniques can be adopted in solar power plants as presented in Table 2.
Table 2
Summary of the cleaning and soiling mitigation techniques with their advantages and disadvantages
Soiling mitigation techniques
|
Advantages
|
Disadvantages
|
Natural
|
Rain
|
- Free of charge
|
- Availability
|
Wind
|
- Climate site dependent
|
Dew
|
- Less efficient for birds dropping and cemented dust
|
Manual
|
Labor
|
- Efficient
|
- High costs
- Micro-scratches
|
Corrective
|
Electrodynamic Screen (EDS)
|
- Faster compared to other methods
|
- Screen degradation due to UV
|
Mechanical systems
|
- Fast operation, good environmental adaptability and control performance
|
- Less adaptability to solar trackers
- Maintenance costs
- Less efficient for fine dust particles
|
Preventive
|
Trackers/Tilt angle
|
- Efficient in soiling mitigation compared to fixed structures
|
- Economic applicability for large solar field
|
Anti-soiling coatings
|
- Passive (no energy requirements)
- Efficient
|
- Durability concerns
|