The deposition density increased, light transmittance reduced at

two factors solar irradiance and temperature play the vital role in determining
the power output from the PV module. Dust degrades the performance of the PV
module by influencing the intensity of solar irradiance, surface temperature and
producing partial shading.

are several studies related to transmittance loss caused by dust deposition in
from of scattering, absorption and reflection of sun light which depends on
wavelength of solar radiation. Hasan and Sayigh 1992 performed the experiment
to find the transmission loss due to the dust deposition with two wavelengths 540nm
and 720 nm. They observed that as dust deposition density increased, light
transmittance reduced at all visible wavelengths.

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and Hussein1993 performed the experiment to investigate the effect of the
physical properties of dust particles deposited on the surface of PV module.
They used cement, carbon and three types of limestone particles having average
diameter of 10, 5, 50, 60, 80 micro m.  The four parameters short circuit current,
power output, solar radiation and fill factor fig.. The result shown that for
carbon particles deposition with 28g/m2, the short circuit current more reduce
compared to cement with 73g/m2 for 10 micro m, and limestone with 125g/m2 for
50 micro m, 168g/m2 for 60 micro m and 250g/m2 for 80 micro m. They found that
carbon particles absorb more solar radiation compare to cement and limestone
particles. Thus carbon particles reduce the solar light entering to the solar module
and also increase the surface temperature of the PV module.  They reported that small dust particles are
uniformly distributed compare to coarser dust particles, resulting reduce the spaces
between the dusts particles through which sun light can pass. With small amount
of coarse dust deposition, there are sufficient spaces to enter sun light to
solar module. This way small dust particles have more scattering losses
compared to that of the larger dust particles. The degradation in performance
PV module depend not only dust deposition but also on the dust properties.

Al-Hasan(1998) investigated the effect
of dust deposition on light transmittance at PV module with the wavelength
range190 to 900nm. The transmittance of clean and unclean samples at different
dust deposition densities up to 15.6g/m2 was tested with spectrophotometer. The
test data showing the variation of reflectance as a function of wavelength can
also be deduced in form of transmission losses. They conclude that the
transmittance reduced as the dust deposition densities on the glass sample increased
with no wavelength dependence.. They also found that the light transmittance reduced
as the incidence angle increased.


Qasem et al. (2012) reported that
transmission losses are higher at smaller wavelengths (300nm-570nm) compare to that
at larger wavelengths. When dust particle size is equal to the wavelength of
light, scattering losses increase. In the case when the surface mass density of
dust particles is small, each particle behaves as a single scattering object.
When the mass density increases to form dense layer, behave as multiple
scattering objects. In this case, optical modeling can be used for analyzed the
transmission losses.

Maghami 2016 stated the shading due to dust particles
is divided in two types; soft shading refers as atmospheric concentrated dust,
and hard shading refers as dust deposited on surface of PV module. Soft shading
occurs when atmospheric dust concentration is high or some dust particles
deposited on the PV module. The effect of soft shading is reducing the current
of PV module and keeping voltage remains the same. Hard shading occurs when heavy
dust deposited and blocks the light to enter into the PV module. The effect of
hard shading depends on the numbers of shaded cells of PV module. If few cells
are shaded and other cells receive solar light, there will be some output power
with decreased output voltage of PV module. Due to some dust particles such as
bird dropping, leaves and dirt patches block the few cells of PV module. In
this condition blocked cells behave as a resistance. Due to this reasons, blocked
cells heat up and create to hot spot that can harm the PV module.