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Generally, solar panels are installed in an open environmental condition where they face numerous types of obstructions by various soiled particles. These soiled particles harsh the solar output at a great level. Moreover, PV sites are highly affected by surrounding such as industrial activities, transportation, and local climatic conditions. After a long time, small dust particles and pollutants are deposited over the flat glass cover of the panels and block the incidence amount of solar radiation significantly. A degradation in solar PV output is directly concerned with the area of shadow which affects the performance of the solar cell. The term soiling losses refer to the loss in PV performance resulting from snow, dust/dirt (minute solid particles less than 500 µm in diameter) accumulation, soot, ash, bird droppings, and other small particles that cover the surface of the PV module facing partial and complete shadow effect [1-3].

Small solid particles affect the performance of the PV system at a great level. It measured dust deposition densities ranging from 25.8 to 277mg/m² for an exposure period of a week [4]. Moreover, a 6.9% power loss was found in deposition due to sandy soil and 1.1% caused by compact soil [5]. A comprehensive review is assessed on a loss in PV performance caused by the surrounding environmental effect [6] as presented in Fig.1. In another study, a maximum power loss of ≈23.8% has been observed due to bird dropping deposition on the front glass cover of the PV module [7]. Similarly, a great cut in PV module efficiency was measured due to the deposition of industrial pollutants i.e., 64%, 42%, 30%, and 29% for coal, aggregate, gypsum, and organic fertilizer dust, respectively [8].

Fig.1 Vulnerability of soiling on the front glass cover of PV module in a real open environmental condition

Experimental Study

Fig. 2. Sample collected from particular locations: (a) transport activities; (b) industrial activities; and (c) open field
In a real-time experimental study, A power loss of the PV system caused by soiling has been assessed. In this regard, we have considered ~2mm thick transparent glass plates for the output electrical power assessment. These three glass plates were exposed to soiling in outside natural environmental conditions at different locations as presented in Figure 3 below.

Fig. 3. Glass plate samples after one month of  exposure at the particular locations: (a) clean; (b) open field; (c) industrial activities and (d) transport activities 

Electrical experimental description

The experimental study is performed by attaching the exposed soiled (dirt) glass to a reference solar PV module as shown in Fig.4. The test bench PV system includes a standard silicon solar module of area (7cm×7cm) and the professional solar wattmeter (Solar Module Analyzer PROVA 210) is used for assessing the PV output [11, 12]. Output electrical data is collected every four days a week, frequently measured under natural sunlight for the evaluation of PV output. The data were recorded manually with a clean module glass cover and after that with exposed soiled glass plates.

Fig.4. Experimental setup for the electrical measurement

As result, the outcome of soiling can be measured in terms of power losses (%) by comparing maximum output power P_max (i.e., P_mpp) before (clean) and after (soiled) [12] deposition by using Solar Module Analyzer as shown in Equation. 1 below.

Fig.5. the effect of the soiled glass coupons on the average power losses of the PV system collected from the different locations

It is promptly seen that every PV module has a different amount of soiling deposition attributes that the soiling level greatly depends on the surrounding activities in particular locations. In a study, it is found that the most critical site is Loc#1 i.e., critical power loss of 30.05% obtained as presented in Table.1. In everyday life, frequent movement of a large number of heavy vehicles promotes the settlement of high-level of small particles (airborne) over the front glazed surface of PV module that blocks (i.e., transmittance loss of radiation) the more solar radiation falling on it. As result,  a critical loss of power generation has been obtained. Especially in the winter season, moist air ables to more accumulation of small pollutants over the front cover of a PV system.

Table.1. Average power loss at the different PV site locations

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