Soiling losses : Introduction and effects on solar module
28th May 2018
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.
Figure 1 : Soil accumulation on solar module (Source: Google images)
The factors affecting soiling and the power loss are as follows:
Figure 2 : Dust accumulation at sides of solar module (Source: Google images)
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:
Figure 4 : Energy loss with different frequency of cleaning (Source: DNV GL)
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.
RAHE ROSHAN HAMARA NATION