Their findings are presented in the paper An analytical indoor experimental study on the effect of soiling on PV, focusing on dust properties and PV surface material, published in Solar Energy and on the ScienceDirect website.
The Exeter group used a solar simulator, spectrometer and energy-dispersive X-ray spectroscopy to analyze the effect of the accumulation of 13 soiling agents: ash; bird droppings; carpet dust; cement; charcoal; clay; coarse sand; reddish, clayey ‘laterite’; loam and sandy soils; salt; stone dust; and wood dust. The team behind the study claim it was more extensive and rigorous than previous research in the same field.
Samples were deposited through a dispenser developed by the group and examined in wet and dry conditions on low-iron glass and acrylic plastic encapsulation materials.
“Each piece of low-iron glass and acrylic plastic had a dimension of 13 by 13 by 0.4cm,” stated the group. “A mini-module with an active area of 120.84 cm2 was developed using four monocrystalline cells with dimensions [of] 5.2cm by 5.2cm.”
Wet deposition was used to simulate dew and dust deposition during wet weather. An image characterization was used to analyze the morphology and chemical composition of each dust sample. The performance of the mini module was then tested using a Wacom continuous solar simulator at a controlled temperature of 25 degrees Celsius.
The experiments showed the encapsulant materials had high light transmittance, of around 92% for the glass and 91% for the acrylic, and the Exeter group found soiling agents could almost negate panel performance.
“Our result revealed that larger particles create wider gaps between them and could allow light to pass through the gaps but small and uniformly spread particles would not have enough spaces that light can penetrate,” stated the researchers, who added, energy yield was further depleted by heavier weights of accumulated dust particles.
Charcoal powder was found to be the worst soiling agent, with a degradation of PV yield performance of around 98%. Salt was the least inhibitive, and reduced performance only 7%.
The scientists determined panels with an acrylic plastic encapsulant accumulated more dust than glass covered modules. Wet conditions were found to reduce performance more than dry accumulation as capillary force can create bridges between particles and surface, according to the researchers.
“The asperity [harshness] of degradation observed in this study cannot be overlooked and, therefore, proper mitigation techniques must be provided to prevent soiling of PV surfaces,” the authors of the paper concluded.
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