Soiling (solar energy)

Soiling is the accumulation of material on light-collecting surfaces in

There are several ways to reduce the effect of soiling. The antisoiling coating[2] is most important solution for solar power projects. But water cleaning is the most widely used technique so far due to absence of antisoiling coatings in past. Soiling losses vary largely from region to region, and within regions. Average soiling-induced power losses can be below one percent in regions with frequent rain.^[3] As of 2018, the estimated global average annual power loss due to soiling is 5% to 10% percent. The estimated soiling-induced revenue loss is 3 – 5 billion euros.^[1]

Physics of soiling

Soiling is typically caused by the

Soiling losses vary greatly from region to region, and within regions.[3]^[6][7][8]

The rate at which soiling deposits depends on geographical factors such as proximity to deserts, agriculture, industry, and roads, as these are likely to be sources of

The soiling rate (see definition below) varies from season to season and from location to location, but is typically between 0%/day and 1%/day.[1] However, average deposition rates as high as 2.5%/day have been observed for conventional photovoltaics in China.^[1] For concentrated solar power, soiling rates as high 5%/day have been observed.^[1] In regions with high soiling rates, soiling can become a significant contributor to power losses. As an extreme example, the total losses due to soiling of a photovoltaic system in the city of Helwan (Egypt) were observed to reach 66% at one point.^[10] The soiling in Helwan was attributed to dust from a nearby desert and local industry pollution. Several initiatives to map out the soiling risk of different regions of the world exist.^[3][11][12]

Soiling losses also depend on

Quantifying soiling losses

The level of soiling in a photovoltaic system can be expressed with the soiling ratio (SR), defined in the technical standard IEC 61724-1^[16] as:

$${\displaystyle SR={\frac {\text{Actual power output}}{\text{Expected power output if clean}}}.}$$

Hence, if ${\displaystyle SR=1}$ there is no soiling, and if ${\displaystyle SR=0}$, there is so much soiling that there is no production in the photovoltaic system. An alternative metric is the soiling loss (SL), which is defined as ${\displaystyle SL=1-SR}$. The soiling loss represents the fraction of energy lost due to soiling.

The soiling deposition rate (or soiling rate) is the rate of change of the soiling loss, typically given in %/day. Note that most sources define the soiling rate to be positive in the case of increasing soiling losses'^[1][17][18] but some sources use the opposite sign[NREL].^[3]

A procedure for measuring the soiling ratio at

• Site selection and system design: The effect of soiling can be mitigated by careful planning during
  mirrors, in the case of concentrated solar power) should be stowed at the maximum inclination angle (or upside down, if possible) during the night.^[1] In summary, soiling is a concern for the system designers, not only the system operators.^[1]
• Solar panel design:
  diodes that allow current to pass the soiled parts of the panel).^[1] In the future, the fraction of solar panels with half-cells and without frames are expected to increase.^{[citation needed}
  ]

This means one can expect solar panels to be more resistant to soiling losses in the future.

Wet-chemically etched nanowires and a hydrophobic coating on the surface water droplets was shown to be able to remove 98% of dust particles.^[23][24]

• Cleaning: The most used approach to
  solar panels/mirrors. Cleaning can be manual, semi-automatic, or fully automatic. Manual cleaning involves people using brushes or mops. This requires a low capital investment, but it has a high cost of labor. Semi-automatic cleaning involves people using machines to aid the cleaning, typically a tractor equipped with a rotating brush.^[25]

This approach requires a higher capital investment, but involves lower cost of labor than manual cleaning. Fully automatic cleaning involves the use of robots that clean the solar panels at night.[26]

This approach requires the highest capital cost, but involves no manual labor except for maintenance of the robots. All three methods may or may not use water. Typically, water makes the cleaning more efficient. However, if water is a scarce or expensive resource at the given site, dry cleaning may be preferred.^[4] See Economic consequences for typical costs of cleaning.

• Anti-soiling coatings: Anti-soiling
  solar panels or mirrors in order to reduce the adhesion of dust and dirt. Some anti-soiling coatings are meant to enhance the self-cleaning properties, i.e. the probability that the surface will be cleaned by rain.^[27]

The coating can be applied to the panels/mirrors during production or retrofitted after they have been installed. As of 2019, no particular anti-soiling technology had been widely adopted, mostly due to a lack of durability.[1]

• Electrodynamic screens:
  solar panels or mirrors. Time-varying electromagnetic fields are set up by applying alternating voltages to the grid. The field interacts with the deposited particles, moving them off the surface. This technology is viable if the energy needed to remove the dust is smaller than the energy gained by lowering the soiling loss. As of 2019, this technology has been demonstrated in the lab, but it still remains to be proven in the field.^[1]
• Electrostatic dust removal[28]^[29]

Economic consequences

The cost of cleaning depends on what cleaning technique is used and the cost of labor at the given location. Furthermore, there is a difference between large-scale

Soiling leads to reduced power production in the affected solar power equipment. Whether or not money is spent on mitigating soiling losses, soiling leads to a reduced revenue for the owners of the system. The magnitude of the revenue loss depends mostly on the cost of soiling mitigation, the soiling deposition rate, and the frequency of rain at the given location. Ilse et al. estimated the global average annual soiling loss to be between 3% and 4% in 2018.^[1] This estimate was made under the assumption that all solar power systems are cleaned with an optimal fixed frequency. Based on this estimate, the total cost of soiling (including power losses and mitigation costs) in 2018 was estimated to between 3 and 5 billion euros.^[1] This could grow to between 4 and 7 billion euros by 2023.^[1] A method to obtain the power loss, energy loss and economic loss due to soiling, directly from PV remote monitoring system time-series data has been discussed in ^[30] which can help the PV asset owners to timely clean the panels.

See also

• Solar Energy
• Solar power
• Photovoltaics
• Photovoltaic system
• Photovoltaic power station
• Photovoltaic system performance
• Solar tracker
• Solar panel
• Solar cell
• Concentrator photovoltaics
• Concentrated solar power
• Heliostat

References