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Effect of Soiling on Solar Photovoltaic Performance

Soiling on Solar Modules

The light-collecting surfaces of solar power systems cover areas of more than 3,000 km2 worldwide, with solar modules accounting for the majority. An often-neglected problem is the contamination of these surfaces, so-called “soiling,” which leads to significantly reduced energy yields, especially in high-insolation arid and semi-arid climates. Indeed, an inadequate soiling mitigation strategy in high solar-potential and soiling-prone locations can cancel out in a few weeks the impressive progress in solar cell efficiency made in recent decades.

Before Solarud there was no one-solution-fits-all to the problem of soiling due to its site-specific and seasonal variability, differences in local energy costs, and the availability and costs of resources required for cleaning, such as water or labor. Indeed, frequent cleaning can increase the O&M costs and water consumption dramatically, leading to a need for water-less and inexpensive soiling mitigation technologies.

The problem

The solar module, in order to produce power, requires direct irradiance (meaning that this light is directly coming from the sun). However, other than internal factors (such as refractive index of glass, refractive index of EVA, composition of glass, etc.) there are various external factors as well which affect the amount of irradiance entering the solar module. One such factor is soiling, and the loss of power associated with such factor is known as soiling loss. Soiling refers to accumulation of soil, dust particles, etc. on the solar module. This soil accumulation hampers the solar irradiance to pass into the solar module. This primarily leads to reduction of power output from the solar module. This reduced power output may remain till the module is cleaned which may not be soon enough, causing hotspot defects and other malfunctions. The end result of soiling is that it leads to loss of money if not tackled properly. With a market where payback and economics are so important, no one can afford to lose money.

3 factors affecting soiling and the power loss:

  1. Climatic conditions: The local climatic condition in conjunction with the geographical location of the solar power plant can have significant effect on soiling. The local conditions may be extremely dry/humid or a combination of extreme dry and humid weather (in few cases). This in addition with the continuous flowing wind would deliver soil and dust particles on the solar module. In longer run, due to repeated dry & humid weather cycle it may so happen that these particles accumulate along the module frame which is known as cementing. This cemented dust particles are (mostly) irremovable which causes a fixed reduction of power output from the module and further damaging the module permanently.
  2. Tilt angle of modules: The tilt angle of modules is known to affect the production of a PV power plant. It is known that the optimum tilt angle is the latitude of a particular location. Nonetheless, due to increased shadow length at such higher angles and/or space constraints, the tilt angle is usually kept at lower angles. However, it is a less known fact that such lower tilt angles (as low as 5º in few cases) causes an increased deposition of dustSolarud can work from tilt angles as low as 2 degrees. A factor of energy loss due to soiling (generally between 3% to 5%) is considered while designing the power plant and power loss as high as 10%-12% may be observed and are reported due to decrease of such tilt angle.
Picture 1 – Types of soiling in the bottom of the modules.

Beware that 2 Solarud pieces are advised near each corner of the module if the modules are in portrait position, and 3 Solarud pieces are advised if the modules are in Landscape position (the third is placed in the middle of the module width). Looking at the above picture you can surely understand why.

3. Type of liquid used for cleaning: This factor can be directly attributed to the chemical composition of the liquid and its direct effect on the glass surface. Few droplets of the cleaning liquid always tend to stick to the solar glass when the module is cleaned. This liquid while evaporates may leave behind few of its deposits which usually vary in thickness. This would result in decrease in transmittance from the glass (as shown in Graphic 1) which directly leads to loss of power output from module. Additionally, the varying chemical composition primarily reacts with the glass surface (and its ARC) which may either causes the dust to stick and settle on the glass.

Graphic 1 – Effect of decrease in transmittance (Source: Appels. R, et. al. “Effect of soiling on photovoltaic modules”).

While the parameters mentioned above are important as they give information on how soiling loss occur and the factors which affect them. With soiling in place on the module, it is important to clean such modules to regain its power output.

However, there are few crucial factors which affect the cleaning cycle. These factors are as follows:

Power gain vs frequency of cleaning: It is a known fact that cleaning of solar module is important. But it is necessary to understand what exactly the cycle time should be, as cleaning cycle is always associated with a cost. The outcome of the cleaning cycle is that the energy output of the cleaned string (and/or the solar power plant) increases which would lead to increase in revenue. Though, it is important to understand whether such increase in revenue would offset the cost of cleaning. Additionally, it is also important to understand how often the cleaning cycle is required and would such regular cleaning offset its cost. The energy gain vs the number of cleaning cycle is shown below. It is clear that there would be more than 25% of energy loss if no cleaning cycle is undertaken for a month. For one biweekly, one weekly and twice weekly cycle the energy output increases accordingly.

Graphic 2 – Energy loss with different frequency of cleaning (Source: DNV GL).

Effect on module components (primarily Glass, EVA): While the above two points could be physically visualized and easily monitored, effect on module components cannot be seen or measured. This is because once the dust starts settling over the glass, it would decorate the quality of glass. Additionally, with the dust longer settling on the glass, there are chances that it, along with moisture could seep-in to the module. This seep-in addition to module power loss (in short term) leads to deterioration of module quality in long term and finally rendering the module useless. Such module (if not changed immediately) could have a significant effect on power output of the entire string. You can calculate the energy savings using our calculator software.

Another side effect of soiling and most of the time neglected because it needs special equipment (thermographic camera) to be detected is hotspots in an early stage and open substrings in a longer term.

When deposits stay above the glass, they tend to create hotspots. These hotspots create a resistance in the module cells and the performance of the module also decreases, affecting not only that module but sometimes the full string or the whole installation depending on the site layout. If these hotspots tend to stay longer than it is supposed, they could lead to major defects like open substrings or module bypass.

These hotspots can be avoided using Solarud, specially if these result from the accumulation of water and dust near the bottom of the modules. As you can see below a hotspot in the bottom of the module was created from dust deposition. This could have been minimized if Solarud was being used in this solar plant.

Also, these hotspots can sometimes lead to fires where some or all the modules of the solar installation are damaged by the fire, and nearby buildings and persons can be affected. So, if you can minimize or eliminate this by using a simple piece of plastic near the bottom of the modules, why take the risk?

Picture 4 – Burned PV solar system.
Picture 5 – Burned PV solar system.

At last, but not least, the underestimation of soiling losses is also due to a particularly stealthy effect. In most cases, the irradiance sensor suffers from the same amount of dirt that is covering the solar PV panels. Consequently, the measured irradiance level decreases, despite the actual irradiance remaining the same. The decrease in measured irradiance balances out the decrease in electricity generation of the panels, thus the PR does not change, effectively hiding the losses. This final comment will be reported in a future blogpost.

All the best,
Solarud Team.


Characterization of Soiling Bands on the Bottom Edges of PV Modules
PV Magazine USA
Power loss due to soiling on solar panel: A review
Soiling losses : Introduction and effects on solar module
Hotspots Detection in Photovoltaic Modules Using Infrared Thermography

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