A recent research paper called “Cleaning of Floating Photovoltaic Systems: A Critical Review on Approaches from Technical and Economic Perspectives” investigates different techniques of FPV systems cleaning and categorises them into water-based and water-free approaches. In addition, their cleaning frequencies, as well as economic aspects, are presented and discussed to determine their merits and demerits. The paper has been authored by Rafi Zahedi and Gevork B. Gharehpetian, Electrical Engineering Department, Amirkabir University of Technology, Tehran; Parisa Ranjbaran, Department of Renewable Energies and Environment, Faculty of New Sciences and Technologies, University of Tehran; Fazel Mohammadi, Department of Electrical and Computer Engineering, University of Windsor, Canada; and Roya Ahmadiahangar, Department of Electrical Power Engineering and Mechatronics, Tallinn University of Technology, Estonia. REGlobal presents an extract from this paper.

Dust and soils have negative impacts on the efficiency of PV panels. Therefore, the importance of cleaning PV panels is considerable from both economic and performance points of view. A key factor to maximizing the economic advantage is the determination of cleaning times. There is no specified cleaning cycle for PV panels, and the soiling rate of the region mostly determines the cleaning frequency. The optimal cleaning frequency mainly depends on the environmental conditions of the installation place, such as precipitation and humidity, wind velocity, particle type, source of particles, and soiling rate. In this regard, it has been suggested in that PV panels should be cleaned weekly in moderately dusty places. Furthermore, it has strongly been recommended that all equipment should immediately be cleaned after a dust storm to maintain nominal operating efficiency. 

Economic Evaluation

Another determinative issue for optimized operation and maintenance of PV systems is the cleaning cost. Soiling is one of the influential parameters, which is effective in operation and maintenance costs. Hence, it should carefully be taken into account, especially in desert regions. The cleaning cost of PV systems is mainly dependent on the cleaning frequency during a specific period (by year or month). In 2006, the results of a study have indicated that the increase of the total revenue of a 100 kWp PV system, installed in Los Angeles (CA, USA) should be $1500 under the California Solar Initiative incentive program and $3000 under the lucrative European feed-in tariffs in case of cleaning twice during the dry summer period. In another evaluation, it has been shown that cleaning the soiling and snow from PV panels has not been economic in Stockholm (Sweden). In two locations of the Sahara Desert of Algeria, it has been illustrated that cleaning can be profitable under circumstances that PV panels have been cleaned twice a year for an estimated cost of $15843/MW, and soiling is superior to 7%.

Critical Analysis of Techniques

The cleaning techniques are categorized into water-based and water-free approaches throughout this paper. In addition, four and six subdomain techniques are considered for water-based and water-free approaches, respectively. Each individual technique has its own merits and demerits, which can come in handy in cleaning FPV systems, but the lack of comprehensive analysis is obvious for the selection of the best solution. Therefore, this section provides a critical analysis for applying the techniques to FPV systems. Some of the described techniques use chemical solutions to reach more effective cleaning, and accordingly, the techniques should be analyzed from an environmental point of view. The water quality of the reservoir and water shortage in the region lead to different approaches. Therefore, the availability of techniques for each site must be taken into consideration. Two main factors considered for comparison of techniques are cost and cleaning efficiency.

Rainfall: Rainfall can help the cleaning process, but it is not accurately predictable and does not follow a regular pattern. Moreover, the rainfall cannot completely remove the accumulated soil, and usually, a complimentary washing step is needed. It must be noted that a cloudy and rainy environment is usually not suitable for a PV system.  Furthermore, many PV and FPV systems are located in arid regions with low or inadequate rainfall. Therefore, in such regions, other cleaning techniques should be applied to provide higher output power. From an economic point of view, considering the fact that this technique does not need any apparatus for cleaning, it may seem to be cost-effective. In some countries, such as Japan, where the application of FPV systems is widespread, the precipitation is more than in arid regions and it may be enough for cleaning.

Manual Cleaning: The most crucial reason for using manual cleaning is the simplicity of its application, which is desirable for many utility operators. The human laborers employed for this job need to be professionals because of the risks and the need for selecting the right materials. Constant use of some chemicals for cleaning may decrease the performance of PV panels. Also, the leakage of such materials into the water reservoirs has a harmful impact on the environment.  In addition, in this technique, the amount of wastewater is relatively high, which makes it costly for arid regions with non-freshwater reservoirs or in the case of installing FPV systems with the aim of water evaporation reduction.  Nevertheless, this technique may have other limitations for FPV systems, including the difficulties in accessing panels and the need for weight-bearing floating structures.

Self-Cleaning: As mentioned before, both water and air can be used in self-cleaning techniques. Sprinklers are a water-based approach. Although this system is suitable for arid regions because of its cooling effect, it cannot spray the whole surface of PV panels.  As a result, it cannot infiltrate all crevices of PV panel surfaces.   It has the same cleaning effect as rainfall and can clean PV panels at a relatively low cost. However, there is a significant water wastage during the operation of sprinkler cleaning systems for land-based PV panels because the nozzles spray the water a few meters outside the panel perimeter.  For FPV systems located on the surface of the freshwater reservoir, most of the sprayed water returns to the reservoir and can be reused.

The method of forced airflow using air-conditioner return air has a low energy consumption, but it is only applicable in the regions, whose penetration factor of air- conditioners is high, and it is not appropriate for large-scale FPV systems. Besides, transferring the returned air to FPV systems through the water reservoir increases the initial cost.

Robotic: This technique can also be categorized as water-based or water-free. One downside of utilizing robotic techniques is their high total cost consisting of high maintenance cost for repairing, operating, and monitoring and controlling the robots. Nevertheless, considering the true cost of water, labor, and frequency of cleaning, it is found that the installation of robotic systems can be cost-effective. In addition, this system can effectively decrease the wastage of water.

Deliberation of movement of FPV systems on the surface of the water reveals a key issue for utilizing robots. These movements are unpredictable because they are highly dependent on the buoyancy force, as well as on continuous regular and irregular oncoming waves, unlike existing land-based systems. Considering the impossibility of installing fixed rails on PV panels, due to the independent movement of each floating structure, any sudden movement of the FPV system can detach the robot from the PV panel surface and drown it into water or take it into a position, which is not planned. Hence, it reduces the reliability of the system.  In addition, using robots, vehicles, or mechanically integrated mechanisms for cleaning purposes increases the possibility of damage to the PV panel surface that has been cleaned.

Airflow: Airflow improves dust removal of PV panels, mainly in the regions with water short- age. A low-speed airflow is desirable for FPV systems due to the low dust density of the air on water reservoirs. However, high-speed winds hit the surface of the PV panel with sand particles that may scratch the surface. Long-term exposure to such a wind creates problems of random scratches on the PV panel surface, which results in a reduction of irradiation transmission and reflection. Meanwhile, wind can create small cracks on PV panels via differential pressure, which in turn results in lower efficiency. This problem is intensified for FPV systems due to their offshore installation and higher repair costs.

Coating: Although coating prevents soil from sticking on the PV panel surface, it requires water for soiling removal. By using this technique in arid regions, the volume of water utilized for washing is decreased, while regular washing is required.  Nevertheless, because of the humidity upon the water reservoir’s surface, this technique is adequately efficient. Moreover, the coating surfaces can provide other features, such as anti-icing, stability due to heavy rainfall, anti-reflecting, photocatalysis reaction (this process can chemically break down the organic dirt through the reaction to UV light), and anti-fogging. It is noted that coatings accumulate more soiling when the coating deteriorates due to UV light.  Considering the initial cost for the coating of all PV panels and the recoating cost after several years, it can be said that concerns should increase the cost of this technique. On the other hand, their released chemicals can be a threat to water reservoirs and damage the environment. In the end, it must be noticed that this solution has not been developed for the industrial level yet.

EDS: The EDS technique is distinctively faster than other techniques. This technique has shown proper efficiency in arid regions, but it has been shown that it is not effective for wet or cemented dust. Besides, its efficiency is low for fine particles. Due to the proximity of water with PV panels in some applications and high humidity, the ESD is not a proper option for these cases. Moreover, as illustrated before, the traveling wave method is not cost-effective for both large-scale PV and FPV systems.

Surface Vibration: The vibration method is used periodically, and accordingly, this technique consumes negligible amounts of power in comparison with other active cleaning techniques.  The advantages of the piezoelectric actuator are its lightweight and compact structure, which makes it feasible for utilization in FPV systems. It is noteworthy that as time passes, the vibration can increase the risk of creating major cracks on the PV panel surface.  These cracks may lead to the disconnection of cells and a total loss of generated power. Also, employing a vibration system for each panel can affect the initial and operation and maintenance cost of FPV systems.

Analysis Remarks

Considering the merits and demerits of different techniques, as well as their approaches, it can be said that manual cleaning (without using chemical materials and before sunshine) can be fine for FPV systems that are located on the surface of freshwater reservoirs as it requires no additional water source, produces no wastewater, and consumes no electrical power. The effect of coating techniques on the environment should further be studied, and it has not been developed for industrial applications yet.  Considering the high stability of the coated layer, the combination of the coating technique and manual cleaning is a noteworthy solution for FPV systems located on freshwater reservoirs. Many FPV systems have been located on the surface of non-freshwater reservoirs, which their water cannot be employed for the cleaning procedure. For such an FPV system, which has been installed for water evaporation reduction, using the airflow technique as well as high stability coating layers is preferred.  In the regions that do not have water scarcity problems and FPV systems have been developed for energy generation, using additional manual or self-cleaning water-based techniques leads to cooling and higher efficiency.

Conclusions

This paper has reviewed a variety of techniques for cleaning FPV systems from both technical and economic perspectives. It has been explained that considering the purpose of their development, FPV systems are classified into two groups, namely energy generation and evaporation reduction.

Plenty of techniques have been developed for cleaning PV systems. However, many of them are not applicable due to the particular characteristics of FPV systems. Furthermore, water-based techniques are typically considered the best solution for cleaning. The application of these techniques for FPV systems depends on water availability and reservoir water quality, which are always feasible. Therefore, this paper has categorized such solutions as water-based and water-free approaches and analyzed each technique individually. Furthermore, the following conclusions are drawn:

• There is no specified cleaning cycle for all FPV systems, and the environmental conditions determine the frequency of the cleaning.
• For freshwater reservoirs: The manual cleaning (before sunshine and without using chemical materials) can be fine, as it requires no additional water or electrical source. Assuming high stability of the coated layer, the combination of the coating technique and manual cleaning is an ideal solution for FPV systems installed on the freshwater reservoirs.
• For non-freshwater reservoirs: For FPV systems that are developed for water evaporation reduction, using the airflow technique in conjunction with a high stability coating layer is preferred. For FPV systems that are developed for energy generation, in addition to the above- mentioned solution, using manual or self-cleaning water-based techniques leads to cooling and higher efficiency.