Correlating photovoltaic soiling losses to waveband and single-value transmittance measurements

Abstract

This paper presents the results of an investigation on the spectral losses of photovoltaic (PV) soiling. The transmittance of a glass coupon exposed to natural soiling outdoors in Jaén, southern Spain, has been measured weekly and used to estimate the soiling losses that various types of photovoltaic materials would experience if installed in the same location. The results suggest that measuring the hemispherical transmittance of the soiling accumulated on a PV glass coupon can give enough information to quantify the impact of soiling on energy production. Each PV technology is found to have a preferred spectral region, or a specific single wavelength, for which the transmittance through a PV glass coupon could be used for the best estimation of soiling losses. Overall, considering the average spectral transmittance between the extreme wavelengths of the material-specific absorption band, or the transmittance of soiling at a single wavelength between 500 and 600 nm yields the best estimations for different PV technologies. The results of this work can lead to innovative approaches to detect soiling in the field and to estimate the impact of spectral changes induced by soiling on PV energy production.

Selection of optimal wavelengths for optical soiling modelling and detection in photovoltaic modules

“…Optical sensors take a single measurement and thus neglect the wavelength dependent effects of soiling, together with the different spectral responses possible for the various types of PV absorber materials. In reality, the transmission of light due to soiling has a gradually increasing spectral profile [3,4], with larger losses in the blue region, as is shown in Fig. 1.…”

“…1. In our previous study [3], we demonstrated that a transmittance measurement at a single wavelength or the use of an average transmittance can be successfully used to identify the soiling profile trends over time, with extreme accuracy (R 2 >0.99). Each PV absorber material (PV technology) had a preferred wavelength at which the measurement returned the lowest error.…”

“…The spectral behaviour of a PV module is established, in part, by its bandgap and is described by its spectral response, which expresses the ratio between the current produced by the PV absorber material and the incident power density at a given wavelength. This means that the same amount of soiling deposited on the PV module's cover glass, with the same transmittance profile, can lead to different losses for each PV material [3,4]. Additionally, sunlight does not have a constant irradiance spectrum.…”

“…In addition, the impact of soiling on the transmittance of the PV cover glass is not uniform across the spectrum of sunlight, but varies with the wavelength. More losses are registered in the blue region of the spectrum at shorter wavelengths [3,4]. This means that soiling has a double impact on the incoming solar irradiance: (i) it reduces the broadband intensity and, at the same time, (ii) changes the spectral distribution of the irradiance reaching the PV material [5].…”

“…The aim of this work, which builds on our previously published studies [2,3], is to analyse the correlations between transmittance loss and electrical losses due to soiling, based on the characteristics of the solar irradiance spectrum and different PV semiconductor materials. These include: monocrystalline silicon (m-Si); poly-crystalline (or multi-crystalline) silicon (p-Si); amorphous silicon (a-Si); Cadmium Telluride (CdTe); Copper Indium Gallium (di)Selenide (CIGS) and a representative perovskite.…”