The problem of partial shading in photovoltaic modules, in addition to causing loss of generation, is often responsible for the appearance of hotspots (heating points) in photovoltaic cells, as illustrated in Figure 1. To avoid the hotspot effect, manufacturers add bypass diodes in parallel with a set of photovoltaic cells. In most commercially available modules there are three bypass diodes.
Typically, shadows cover a certain number of cells in the PV module. However, in extreme situations, localized shadows may occur on a small number of cells – in the worst case, on just one cell – which may prevent the bypass diode from working, leading to the occurrence of a hotspot.
Localized shadows can be caused by obstacles such as chimneys, poles, power distribution cables or bird litter, as illustrated in Figure 2.
Total hotspot prevention can be accomplished by adding a bypass diode to each cell of the PV module.
Although most manufacturers prefer the collective bypass diode solution (for a group of photovoltaic cells), the individual bypass diode solution (as illustrated in Figures 3 and 4) already exists commercially, having been introduced exclusively to the market. by AE Solar, a photovoltaic module manufacturer based in Germany.
Experimental study
A study conducted by the Fraunhofer Institute (Germany) with AE Solar modules validated the individual diode solution based on an experiment with two photovoltaic modules, one traditional (with collective diode) and the other with dedicated diode, of the hotspot-free type. The experiment was carried out by applying partial shadows over the modules by adding opaque screens over cells or groups of cells, as shown in Figure 5.
Learn more: AE Solar develops hot-spot-free and shading-resistant module
In the experiment, three conditions were used, applied to the two types of modules analyzed:
- Test condition 1: Shadowing of only one cell;
- Test condition 2: Shading of a row of cells;
- Test Condition 3: Shadowing of multiple rows of cells.
A row of cells is defined as a group of cells aligned horizontally (considering the module in portrait position), but not belonging to the same string, as shown in Figure 6.
Results of the study
The results of the experimental study carried out are shown below. The results show the influence of the effect of the bypass diodes on the power generated by the photovoltaic module. Although the main function of the bypass diode is to protect against the occurrence of hotspots, there is a side effect (desirable) of the diode on the performance of the photovoltaic module.
In the traditional module, the activation of a bypass diode causes the loss of a complete string of cells. In the hotspot-free module, on the other hand, the loss is gradual, proportional to the number of shaded cells, as shown in Figure 7.
The graph in Figure 7 shows that shading only 60% of a cell causes a loss of 1/3 of the output power of the conventional photovoltaic module. This is explained by the removal of an entire string (20 cells) when one of the bypass diodes is activated. On the other hand, as also shown in Figure 7, in the AE Solar hotspot-free module the power remains around 97% under the same test conditions.
An interesting fact that can be observed in Figure 6 is that there is an almost linear relationship between the shading percentage and the power reduction of the hotspot-free module. In the case shown, only 1 cell (out of 60) of the photovoltaic module is shaded, which would correspond to a shading of 98,3% – considering only the active area of the module, that is, the area corresponding to the surface of the photovoltaic cells.
The results in Figure 7 are explained by the IV curves of the photovoltaic modules, shown in Figure 8, when subjected to the conditions described above. Figures 9 to 12 show the results obtained with the other test conditions, that is, with the shading of 1 row and several rows of photovoltaic modules.
Analysis of results and conclusion
An experiment was conducted to evaluate the performance of an AE Solar hotspot-free PV module compared to a conventional module under partial shading conditions. The experiment conducted by the Fraunhofer Institute involved tests under different shading conditions, according to three standards applied equally to both types of modules evaluated: single-cell shading, single-row shading and multi-row shading.
With the shading of just one cell, the AE Solar hotspot-free module showed losses of only 3%, while the conventional module lost 35% of its power. With the shading of one row, the standard module loses 100% of its power, with its three bypass diodes activated. In the same condition, the AE Solar hotspot-free module loses only 20% of its total power.
With the shading of several rows, the power of the AE Solar hotspot-free module decreases linearly with the increase in the shaded area of the module. On the other hand, the conventional module loses all its power when 50% of the rows are shaded. It is important to note that the result obtained with the test in condition 3 is only valid when the shading occurs from the bottom to the top.
If the shadow advances in a horizontal direction (from right to left or vice versa), the power loss in the conventional module may be even greater. This article focused on the problem of power loss with partial shadows. This problem can be mitigated by using a greater number of bypass diodes in the PV module – ideally, one diode for each cell, as is the case with AE Solar's hotspot-free modules.
References
Electrical Characterization of hotspot-free and standard module. Hamed Hanifi, Frank Wenger, Jens Schneider, Report No. 389/2018, Fraunhofer
https://hsfsolar.com/smart-hot-spot-free-modules/ – accessed on 28/07/2021
Aerial thermography of photovoltaic solar plants https://canalsolar.com.br/termografia-aerea-de-usinas-solares-fotovoltaicas/ – accessed on 28/07/2021
An answer
This means that the use of diode per cell is highly effective, high quality article of information, I hope the next article will be about the use of mppt per cell in panels, to solve shading, as they are already being applied by Jinko Solar.