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 (as shown in Figure 1 ) 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. 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 important, one cannot afford to lose money. Now with scheduling and forecasting regulations in place, the effect on plant owner would be two pronged i.e. first they would lose money due to reduced energy generation and secondly they would have to pay deviation charges for the reduced generation (compared to what was forecasted/scheduled). Hence it is important to understand the factors effecting soiling, the factors that necessitate cleaning cycle and key takeaways. This article hence aims to educate its readers on the above matter.


Solar Panel
Figure 1 : Soil accumulation on solar module (Source: Google images)

The factors affecting soiling and the power loss are as follows:

  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 (Figure 2).This cemented dust particles are (mostly) irremovable which causes a fixed reduction of power output from the module and further damaging the module permanently.

    Solar energy

    Figure 2 : Dust accumulation at sides of solar module (Source: Google images)

  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. However 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 lesser known fact that such lower tilt angles (as low as 5% in few cases) causes an increased deposition of dust. A factor of energy loss due to soiling (generally between 3 to 5%) is considered while designing the power plant. However power loss as high as 10-12% may be observed and are reported due to decrease of such tilt angle.
  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 Figure 3) 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.

    Solar module
    Figure 3 : Effect of on decrease in transmittance (Source: Appels. R, et. al. “Effect of soiling on photovoltaic modules”)

While the above mentioned parameters 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 cleaning cycle. These factors are as follows:

  1. 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 should be the cycle time as cleaning cycle is usually associated with a cost. The outcome of the cleaning cycle is that the energy output of the cleaned array (and/or the power plant)increases which would lead to increase in revenue. However 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 no. of cleaning cycle is shown in Figure 4, it is clear that there would be more than 25% of energy lost if no cleaning cycle is undertaken for a month. For one biweekly, one weekly and twice weekly cycle the energy output increases accordingly. For a country like Abu Dhabi where the local climatic conditions are dry and dusty, it makes sense to clean the solar panel once or twice in a week however for country like India, once in a week or two week cleaning cycle is fine.
  2. Solar panel installation
    Figure 4 : Energy loss with different frequency of cleaning (Source: DNV GL)

  3. 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. 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 module array.

To conclude, we can safely say that soiling has adequate impacts both at plant and module level. Thus it is important to keep the plant cleaned. However for number of cycles per week/month, one may keep it after a thorough evaluation of performance, cost and availability of resources. It is also suggested that the cleaning of power plants are carried out only by distilled water and/or suggested liquid by the module manufacturer/EPC provider. Additionally, we have seen many cases where performance of plant is gauged by its Performance Ratio (PR) (a ratio of how efficiently the plant is performing to the expected value). In few of those plants, we have seen that there is dust settlement on the irradiance meter (pyranometer) as well. Many a cases, these meter are located at such places where cleaning them may either be difficult or forgotten as it is unnoticeable. This decrease offsets the decrease of energy output of plant and the PR of the plants almost remains constant. Thus it is suggested that proper cleaning cycle is undertaken of such meters as well.

The underestimation of soiling losses is 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.

Let us all pledge to make solar energy the primary source of energy in the near future.


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