Evaluation of the impact of partial shading on the performance of photovoltaic panels

This experimental work presents an investigation on the impact of partial shading of a photovoltaic (PV) solar cell on the performance of different types of PV solar panels. A mono-crystalline panel and a polycrystalline panel were used for the experiments as well as three types of covering materials with different transmittance levels (white leaf, tree leaf and transparent paper). The shaded cell area varies by 20, 50 and 100% and the electrical parameters of the modules were measured in each case. The transmittance level of each material used was characterized. The study shows that the performance degradation depends principally on the type of material used and the shading rate applied. The performance degradation also depends on the type of panel used. The position of the shaded cell has no impact on the results obtained.


INTRODUCTION
The production of electricity by solar energy has become one of the most widely used technologies in the world. This production depends essentially on the sun, and is made through photovoltaic (PV) cells that absorb this energy and transform it into electrical energy. PV cells are wafers a few centimeters in thickness that are usually composed of silicon (Si) semiconductors. These cells must be combined together in series to form a PV panel (Wang and Hsu, 2011). In order to exploit the energy better, the PV panels are installed in open sky, which exposes them directly to several factors that have a harmful impact on their performance. One of these factors is shading. The PV cells must be identical and benefit from the same operating conditions, however, if any of these cells is shaded, the current through the cell will be very high and voltage across its terminals will be inverted causing a high thermal dissipation on its surface called hotspot (Skomedal et al., 2020;Zhang et al., 2021). Niazi et al., 2019) Partial shading mostly caused by obstacle shadows arising at specific times, and by clouds that move temporarily (Lee et al., 2021), it can be one of the main causes of mismatch losses which are Responsible up to 10% of the total energy generated power (Roy, 2015). Partial shading is not predictable not measurable and unavoidable (Srinivasan et al., 2020). type, duration and pattern of the shade can influence the performance, lifetime, reliability and energy yield of PV modules (Hanifi et al., 2019;Sun et al., 2014;Teo et al., 2018).
Several studies done on partial shading around the world Rajput et al. (2016) developed a mathematical model to calculate solar cell temperature, hot spot temperature and module efficiency in opaque and semitransparent mono crystalline silicon PV module; calculations of the model estimate the power efficiency of PV models for hot spots (opaque 10.41%, semitransparent 10.62%). Vargas et al. (2015) found that when number of shaded modules increase small reverse current flows, but when the number of shorted PV modules increases the reverse current increases significantly. Satpathy et al. (2018) studied the effect of partial shading in different grid and sub grid structures; they found that total-cross-tield (TCT) and series-paralell (SP) interconnections have better energy production with reduction in redundancy. In addition, according to Mehedi et al. (2021) the net gain in energy of the micro inverter and power optimizer is significant. when the installation is heavily shaded, but the use of these devices can be counterproductive Ndiaye et al. (2014) showed that the impact of shadow can cause hot spots on the PV module and lead to a degradation of the latter.
In addition, partial shadows can create multiple peaks on the P−V curve, thus making it difficult to identify the optimum operating point (Roy, 2015). Faye et al. (2017) investigated the impact of partial shadow on the performance of the PV module installed in a sub-Guinean climate. The different types of shadow transmittance showed that the power loss of the module varies according to the nature of the shadow, the illuminated surface, the study medium and the technology as a function of the transmittance. Gupta et al. (2021) developed an electromagnetic strategy to improve the performance of photovoltaic panels under the effect of partial shading, this strategy consists of replacing the bypass diodes by an electromagnetic relay, which prevents the formation of hot-spot and open-circuit defects.
The aim of our work is to study the effect of partial shading on the performance of two types of solar panels; mono-crystalline and polycrystalline. The idea is to shade a part of PV cell (100, 50 and 20%) with different materials (white paper, transparent paper and tree leaf) of different transmittance level and evaluate the performance of the panels for each shading case. Thus, study the impact of changing the location of the shaded cell on the results obtained.

METHODOLOGY
Exposing PV panels to open sky makes them vulnerable to all kinds of dirt such as dust, bird droppings, and tree leaves. These pollutants deposit on their surface arbitrarily in terms of concentration and location. As a result, the PV cells can be shaded, causing a malfunction of the panel and even a degradation of its performance.
To illustrate the impact of partial shading on the performance of PV panels, we performed an experimental study on a monocrystalline PV module with 72 cells of 16.63 cm×9.97 cm, and a polycrystalline module with 36 cells of 14.80 cm×6.70 cm. The mono-crystalline panel is recently installed, while the polycrystalline panel has been used for 2 years. Both modules contain bypass diodes in their junction boxes ( Figure 1 and Table 1).
First, we characterized the different materials used in the experiment by evaluating the light absorption rate for the materials used in the experiment by using a Luxmeter. The experiment is performed in a closed room in the laboratory with constant lighting, the instrument is placed on a work table and the initial value of the light illuminance is measured and noted. Then, one of the materials used for the study is placed on top of the luxmeter while respecting a distance of 15 cm. The new value of the received illumination is measured and noted again.
This experiment is done in the same way for each type of leaf to determine their rate of light absorption. This rate is calculated for each material, using Equation 1.
where  the rate of light absorption by a leaf, Id is the initial luminous intensity measured by the luxmeter, and If is the final luminous intensity measured by putting one of the material used for partial shading. Second, a partial shading experiment of the panels is performed by moving the position of the shaded cell. Thus, for each panel, a part of the cell surface is shaded according to the following proportions: 100, 50 and 20% of the cell with the three types of materials (white sheet, transparent sheet and tree leaf). Before the start of the experiments, the panels were well cleaned and characterized with an IV tracer (IV400).
The study is done on the roof of the building which shelters a part of the laboratory LE3PI of the Polytechnic School of Dakar. Figure 2 shows the flowchart of the experimental procedure. To estimate the loss rate for each electrical parameter of the panels, Equations 2, 3 and 4 are used:

RESULTS AND DISCUSSION
The shadowing of buildings or objects and the deposition of pollutants on the surface of solar panels can cause the phenomenon of partial shading; this phenomenon depends on several factors, including the transmittance of the deposited objects. The rate of light absorption for each type of leaf used is as shown in Figure 3. The light  We find that the degradation rate varies with the shading rate and the leaf used. It increases when the shading rate increases and decreases when the shading rate decreases. Also, the polycristallin module has a more remarkable degradation rate compared to the monocrystalline. Figures 7 and 8 show, respectively a comparison of the I-V and P-V characteristics of the studied modules operating without partial shading and with partial shading of 100% of the cell using the 3 materials. Figure 7 shows that the short-circuit current registers a minimum value of 7.2 A when shading by white leaf and a maximum value of 8.3 A for shading by transparent leaf, while the P-V characterization, undergoes the same variation as I-V. The power decreases when the shaded area increases (Figure 8). Moreover, we also notice that   The results of the 50 and 20% partial shading of the cell show the same tendency of variation for each material and for each type of panel technology, shading with transparent sheet gives the best results followed by tree leaf and then white leaf. Table 2 shows the maximum power and short-circuit current results for each type of partial shading. When the illuminated panel area is 99.31%, the power

Conclusion
The effect of partial shading on two types of panels, mono-crystalline and polycrystalline PV panels, was studied using three materials and varying the surface area and the shaded cell.
The experiment showed that the power loss of the module depends on the technology of the panel, the nature of shading and the shaded surface, moreover, we found that the polycrystalline module undergoes a higher rate of performance degradation than the mono-crystalline module and the change of the place of the shaded cell does not have an impact on the obtained results. From the results obtained, before implementing a photovoltaic installation, it is necessary to carefully study the site, considering any element causing partial shading.

CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.