World’s energy demand is supposed to be increased by more than 50 per cent by 2030. For last three years, it is seen that carbon dioxide emitting energy source use is decreased abruptly. Different technologies are built for the efficient use of renewable energy. Many governments in the world have committed to implement renewable energy source to cope energy demand in heating, transportation as well as in power sector. Around 9.8 million people got employment on renewable energy sector in 2016 which was an increment of 1.1 per cent over 2015. By technologies most of the jobs is related to solar PV and biofuels. Solar PV being one of the renewable energies is making a lead in global market for past few years. Solar PV energy is harnessed from solar radiation, for a mono crystalline solar PV panels under STC it is seen that only 15-18 per cent of solar radiation is used to produce electricity. This efficiency changes due to several factors like: lower irradiance; higher AM; higher temperature; regardless of this solar radiation is failed to be harnessed due to accumulation of foreign particles like dust, bird excrement, snow, and many other. Solar PV panel cleaning has been a challenge in renewable energy sector to achieve maximum efficiency possible. Different cleaning systems have been designed to cope up reduction of efficiency and studies have been made to show the effectiveness of cleaning system.
The article discusses about how the constraints such as dust, humidity affects the output of the solar PV panels and the automated technologies that are involved in solar PV panel cleaning system that help in maximising the output by minimising the effect the constraints have on solar panels. The article presents the constraints that directly and indirectly influence on the efficiency of the solar panel generation and their impacts and different existing techniques for improving the efficiency. It also discusses the problems occurred in the PV cleaning system worldwide.
Efficiency of Solar Panel
Every system having input and output parameters have their efficiency, which is the measure of how much of input energy is converted into output energy. In solar PV panel, input energy is solar radiance striking on solar panel, and output is electrical current from two terminals of the solar PV panel. Mathematically, efficiency of solar panel is:
There are different factors that effects on the efficiency of solar panel internal and external factors. Internal factors mean from which material solar cell is made, depending upon different materials and manufacturing technologies. Efficiency of the solar PV panel varies, whereas the external factors include temperature, humidity, solar irradiance, panel orientation.
PV cell requires some light absorbing material for conversion of energy carried by a photon into electric current. Different types of solar cell technologies have their own efficiency representing what percentage of solar energy striking on PV cell can be converted into useful electrical energy.
Mostly solar PV panel manufacturers use poly crystalline silicon, mono crystalline silicon and amorphous silicon due to economic factor and their availability. Table 1 illustrates different types of solar PV technologies along with respective efficiencies.
Different environmental factors such as temperature, humidity, solar irradiance affect on efficiency of solar panel. Cell temperature affects the output of solar panel, as the temperature of solar cell rises, band gap decreases following the decrease in solar short circuit voltage (VoC) which finally results in decrease of output power.
Figure 1 illustrates I-V and P-V (Power-Voltage) curve of a solar PV panel, which shows decrease in output power with the increase of temperature. In the figure, output of solar panel is monitored at two different temperatures; 25C and 43.5C, it is clearly seen that IV curve declines at 43.5C comparatively than at 25C. Humidity is another factor where due to humid environment, only 55-60 per cent of solar energy can be utilised with the decrement of 10-15 per cent. Table 2 illustrates the relationship of humidity with solar output power. It can be seen that at higher humidity level, output current tends to decrease which results decrease in output power of PV module.
Short circuit current of solar panel is directly proportional to incident light intensity whereas open circuit voltage increases logarithmically with incident light intensity. Figure 2 shows IV characteristics curve for a solar cell with different illumination. In the figure above IV curve is monitored at different irradiance level (1000W/m2, 800W/m2, 500 W/m2), it is seen that at lesser irradiance current decreases, as a result efficiency of panel decreases.
Tilt of panel also affects the efficiency of the panel as amount of incident light intensity differs with tilt angle for maximum efficiency it is suggested to face the panel at an angle of latitude minus 10 degree. Atmospheric dust and dust deposition on solar PV both reduce efficiency of solar panel. Due to atmospheric dust, most of the light intensity is absorbed and as a result, efficiency is reduced. Figure 3 shows the IV characteristics curve for different dust densities. It illustrates decrease in short circuit current with the increase of dust densities resulting decrease of efficiency. To analyse the effect of dust deposition on efficiency of solar PV panel, experimentally it is seen that efficiency of the solar panel decreases by 30-40 per cent for indoor set up with constant illumination whereas there is loss of 4-5 per cent for outdoor set up with natural lightening condition. Opaque objects like moss tremendously decrease efficiency of solar panel by 80 per cent. Figure 4 and Table 3 illustrate the effect of dust deposition on solar PV panel. Figure 4 represents decrease in short circuit current for different types of dust among which accumulation of sand decreases the efficiency at most.
Techniques to Improve or Maintain Solar PV Efficiency
The loss of illumination plays a vital role in generating solar energy. This can be caused by factors that are a part of the integrated system or a member of the atmosphere. Furthermore, this leads to the lowering of efficiency in the system. Some of the problems that drag the output of the panels require into consideration that a different material needs to be introduced that can be utilised to its maximum potential. However, problems as such including effect of dust, effect of humidity can be minimised and the efficiency of solar panels can be optimised by cleaning of the panels. Various methods have been introduced with their own pros and cons whose sole purpose lies on improving the efficiency through cleaning. Solar panel cleaning can be both automated and manual. Manual cleaning demands extensive labour. Moreover, manual cleaning maybe practical to households with a few solar panels. For large scale projects, the situation favours an unmanned automated cleaning system that can take in the real time data with retrospect to the dust accumulated, power generated and clean henceforth. Many automated systems built as of today are dependent on the landscape of the place where the solar panels are situated. They also rely on the structure of the panels. Depending on these factors, many technologies have come to surface that optimises the performance of solar panels by cleaning them.
Linear Piezoelctric System
A linear piezoelectric system consists two driving feet integrated face to face onto a guide which is operated at the same operating frequency to form two symmetric elliptical trajectories at two driving tips.
Fuzzy Logic Control based Solar Panel Cleaning
Fuzzification of sunlight sensor output (irradiance) and cell output current is done and defuzzification of input variables decides whether to clean solar panel or not. Motor driver for cleaning action is dependent of fuzzy logic. Cleaning action is decided by the fuzzy logic of two input variables irradiance and output current.
Gekko Solar and Gekko Solar Farm
The cleaning is done by a rotating brush and demineralised water. Its movement is based on feet, with vacuum technology, which are rotating on two trapezoid- shaped geared belt drives. This enables the robot to move on every desired direction. The robot is designed for places that are difficult to access. It can easily be transported from one place to another. For larger application, ‘Gekko Solar Farm’ has been used. The GEKKO Solar Farm robot is designed for utility scale solar farms. With an efficiency of up to 2000 m2 per hour the robot is very powerful.
HECTOR is an automated cleaning system, which can be used for solar PV panel cleaning. It is wireless, rechargeable and carries water solution tank with itself. It requires no external power or water supply for its operation; it carries its own batteries and water tank. HECTOR is designed for night and day operation. Its performance is very slow and the weight of HECTOR is over the panel.
Solar Brush is a robotic cleaning system for SPV panels. The robot ‘solar brush’ walks over the solar PV panel. It can function up to an inclination of 350. It is wireless and rechargeable. It is having a cleaning brush which swipes the dust. Solar brush is light weight of 2.5 kg. Also, the manufacturers have been trying to introduce drone-based cleaning in the market.
Greenbotic’s robotic cleaning system
‘GB1’ from Greenbotic’s is a robotic cleaning system for SPV panels. It is wireless and rechargeable. It comprises rotating cleaning brushes perpendicular to the axis of panel and a wiper system, such that not only does it clean the panel, but also clear the dirty water.
PLC based cleaning system
This is an example of cleaning system that boosts the output power performance of PV compromised by the effect of shadowing and shading. The main component of this system is the PLC which controls and powers both the mechanical and electrical parts of the design. The system is successful in removing dust and bird droppings on the PV panels which led the PV to have better output hence, more efficient and more reliable for offshore application.
Automatic Solar Panel Cleaning System
It automatically washes and rinses the solar panels. It attaches nozzles to the solar panels. It comprises a reservoir for soap concentrate. There is also a sediment ﬁlter that contains water softener media. It also has an anti-siphon valve to prevent backwashing into the system. System consists of a controller which automatically provides wash and rinse cycles, the controller programming can be changed as per seasonal requirements.
Electrostatic Charge System
Places with very scarce source of water such as deserts, dry areas like Saudi Arabia or even places with no water like Mars use electrostatic techniques for cleaning. Electrostatic charge concept for lifting and transporting charged particles of insulating materials has been used for providing standing wave-type electric curtain. In this case, we have a standing wave, and at any point, the electric ﬁeld has a deﬁnite direction and amplitude oscillating at the imposed frequency. A single charged particle oscillates along the ﬁeld line. The present context of solar cleaning demands various prospects to be encompassed keeping in mind the optimum performance of the panels. Table 4 lists the advantages and disadvantages of different solar cleaning techniques. Among the available techniques for cleaning solar panels, Gekko Solar and Gekko Solar Farm Robot are the most popular and effective techniques for cleaning.
Various studies have concluded that there lies a negotary effect of dust on the output of solar panels. An average of 1 per cent with a peak of 4.7 per cent in two-month observation period was observed in United States, 40 per cent degradation in 6-month period in Saudi-Arabia, 11 per cent decrease in efficiency in the tropical climate of Thailand and from 33.5 per cent-65.8 per cent reduction in efficiency from a study conducted in Egypt. So, an effective cleaning method for the respective scenarios can bring significantly improvement in the output of the solar panels. However, many drawbacks are also associated with the cleaning systems. Cleaning systems can be both automated and manual. Manual cleaning systems demands manpower and is usually time consuming which may not be of suitable practice at large scale utility solar farms. Moreover, most of the manual cleaning systems that have been made as of present require external power supply. Some automated solar PV cleaning systems do not introduce treated water to wash off the dust accumulated, hence cannot be completely effective to remove bird droppings that remain on the panels. Cleaning systems with electrodynamic screen require dry surface and is not suitable for rainy climates. Moreover, they require high amount of voltage to complete their operation saving less energy. PLC based cleaning system require heavy gears that add load in the surface of the panels. Most of the automated cleaning systems have constraints defining a threshold of upto certain inclinations that they can work effectively and not suitable for all the scenarios.
The article discusses effects of various parameters on the solar PV panel, and various cleaning systems that is developed and being used in present days. The key points are:
- Environmental factors like temperature, humidity, solar irradiance affect efficiency of solar panel.
– It is seen from the studies that for an increase of 18.5 degree Celsius, the power is reduced by 5W.
– Due to humid environment, only 55-60 per cent of solar energy can be utilised.
- Atmospheric dust and dust deposition on solar PV both reduce efficiency of solar panel. Experimentally, it is seen that efficiency of the solar panel decreases by 30-40 per cent for indoor set up with constant illumination whereas there is loss of 4-5 per cent for outdoor set up with natural lightening condition. Opaque objects decrease efficiency of solar panel by 80 per cent.
- Most of the cleaning system require human interface to clean efficiently, and uses high pressure water to spray on surface thus requiring high power to operate.
- Electrostatic Charge System and Electro-Dynamic Screen is used for cleaning in areas where water is not accessible, like for solar panels in Mars.
- Gekko Solar (farm) is recommended to use in big solar farms or plants, where the panels are kept in array. Its efficiency reaches up to 400m2 per hour. It is a good choice for extended roof top installations, where cleaning by hand is hardly feasible. The disadvantage of Gekko Solar is that it puts heavy load on the surface of panel which may damage the panel. Also, this system is limited to 300 in case of solar farm, and 450 in case of Gekko Solar Farm.
- For automated and uninterruptible cleaning PLC based cleaning system is best but requires heavy gears that add load in the surface of the panels.