Effects of the Shading Rate on the Electrical Parameters of CIGS-Based Solar Modules

The aim of this article is to study the effects of the shading rate on the electrical performance parameters of CIGS PV modules. The study concerns a new flexible CIGS type photovoltaic module with a power of 90 W, manufac-tured by the company Shenzhen Shine Solar Co., Ltd. This module, reference SN-CIGS90, is tested under the initial conditions to ensure its correct operation and to determine the initial values of the electrical parameters before shading. After this characterization test, the module is exposed under the actual operating conditions of the Renewable Energies Study and Research Center (CERER), located in Dakar, then 4 types of shading are performed with the same mask: partial shading 25% partial shading, 50% partial shading, 75% partial shading, and 100% full shading. The variation rates obtained on the experimental values of the 4 types of shading carried out determine that the shading phenomenon constitutes a factor that influences negatively on the electrical parameters of a CIGS-based PV module. Indeed, for 25% of the surface of the shaded module, there is a reduction of 58.139% of the maximum power and of 60.507% of the efficiency and for shading of 100%, the module loses 84.436% of its maximum power and 84.135% of its performance.


Introduction
When new, a PV module is characterized by its electrical parameters such as power it delivers. These data vary overtime when the module is exposed to sunlight with the actual operating conditions of the installation site [1].
This variation can be due to several factors, among which we can quote the shading. Studies have shown that the output power of photovoltaic panels is sometimes lowered due to the appearance of shadow on them [2] [3]. In one of their studies, Zaihidee et al. [4] showed that on only 2% of the shaded area of the module, the performance loss was 70%.
Shadows can be caused by trees, buildings, but also by mounting structures of some modules on others, droppings falling on the modules as well as leaves from trees and others… When a module is shaded in a series-connected module system, bypass diodes can prevent the shaded module from being reverse biased. If the bypass diodes open, the shaded module is indeed shorted. And if the bypass diodes fail or are ineffective, the shaded module may experience reverse bias stress, which is very dangerous for the module [5].
It has been observed that CIGS modules, subjected to the stresses of reverse polarizations, develop white characteristics similar to worms visible under the glass which have been associated with hot spots [6]. The presence of hot spots in CIGS modules is a key factor in the failure of these modules due to shading [7] [8].
The use of protective diodes to protect against power loss and reverse bias is difficult in thin-film modules [9].
In our study, we showed the impact of shading on the electrical parameters of CIGS-type photovoltaic modules installed in a Sahelian environment. The analysis of the various rates obtained on the experimental values of 4 types of shading is the object of this study.

1) The flexible CIGS module
To carry out this experimental study of the effect of the shading rate on the electrical parameters of CIGS-based solar modules, we bought a new flexible CIGS type module. Table 1 shows the construction data of the module. Figure 1 shows the module in a new condition.
2) The measurement platform

Experimental study
After this test, the module is exposed on the CERER site and underwent 4 types of shading:  Partial shading at 25%, corresponding to a quarter of the surface of the module.  Partial shading at 50%, corresponding to half of the surface of the module.  Partial shade at 75%, corresponding to three quarters of the surface of the module.  Total shading at 100%, corresponding to the total surface of the module.

Results and Discussions
Using an "ESL-SOLAR-500" analyzer, we sought to determine the effect of the shading rate on the electrical performance parameters of the CIGS module. The study is carried out at the Renewable Energies Study and Research Center (CERER), located in Dakar. The results obtained are presented in Table 2.
In our experimental studies below, we need to determine the rates of absolute and relative variations between the initial parameters and those obtained after the study. For this, we used the following equations: VAT, the absolute rate of change, TVR, the relative rate of change; V F , the final value of the parameter and V I , the initial value of the parameter.    The results obtained on the variation of the electrical parameters for each type of mask are shown in Table 4.
The analysis of these experimental results above allows us to say that the decrease in electrical parameters depends on the nature but also on the thickness of the mask used. Indeed, the loss of power is estimated at 39.910% for the total shading with a blue plastic tarpaulin as a mask, a little transparent, 84.436%, for the total shading with a simple cotton fabric mask (75% cotton) and at the end, 87.905% for total shading with a lab coat (100% cotton) used as a mask. Figure 5 shows the characteristic curves of the module as the function of shading rate.
The general shape of I (V) and P (V) curves shows that the shading causes significant deformations in I -V and P -V characteristics of CIGS-based photovoltaic modules. These deformations are more visible in the case of partial shading at 25% and 50% and we note in these two cases a non-uniformity of the curves.
We also observed a sharp decrease in the short-circuit current caused by shading. This reduction is obvious, insofar as the mask blocks the arrival of solar radiation on the surface of the module, which leads to a reduction in the charge carriers generated by the module and consequently a reduction in the short-circuit current depending on the module.
Studies have shown that for a cell with a high shunt resistance, the effect on the distortion of I -V characteristic will be less severe [10]. Another effect of shading is that of shifting the point of maximum power to lower voltage values.
Indeed, the smallest shading rate (25%) performed on the module, generates a power reduction of around 58.139% which is dissipated in the form of heat. In short, the shading effect can cause poor operation of the photovoltaic system, which is why the solar panels must be protected by bypass diodes and the system must be connected to an inverter following the maximum power point (MPP) in order to be able to carry out permanent monitoring of the system.