Design challenges for solar photovoltaic arrays operating in partially shaded environments Power Electronic Tips

A partially shaded environment on a photovoltaic (PV) panel refers to a situation where the irradiance on the PV panel is reduced due to factors such as passing clouds or a falling shadow on the PV panel. This reduces PV output power and creates complexity in the maximum power point tracking (MPPT) algorithm. This FAQ will discuss three challenges that power electronics engineers face when extracting maximum power in partially shaded environments.

Figure 1. Three challenges while designing a PV array under partial shading conditions. (Image: Rakesh Kumar, Ph.D.)

Output power reductions and fluctuation create challenges

Partial shading can cause severe power losses disproportionate to the shaded area. Even minimal shading on a single cell can drastically reduce the output of an entire module or string. For example, shading just 1/36 of the cells in a module can reduce power output by as much as 75%.

Under partial shading conditions, multiple peaks emerge in a PV system’s power-voltage (P-V) curve. These peaks represent both local and global maximum power points, complicating the task of MPPT.

Figure 2(a) shows a nine-module PV array with three modules shaded. The shading reduces irradiation on these three panels, significantly lowering the array’s output power, as evident in the P-V graph.

Figure 2. Effect of partial shading on the P-V graph creating multiple peaks and leading to complications in the MPPT algorithm. (Image: ResearchGate)

Figure 2(b) shows the P-V graph with no partial shading on any PV panel. In this case, the P-V graph has only one peak when the PV output voltage and current are maximum, resulting in maximum power. However, shading creates multiple peaks on the graph in the form of local maximum power point (LMPP) and global maximum power point (GMPP), as shown in Figure 2(c). Such a phenomenon of multiple peaks confuses the MPPT algorithm and complicates the control system.

Effect of bypass diodes in system complexity and thermal management

Bypass diodes are connected in parallel, but with opposite polarity, to PV cells or groups of cells. Their primary function is to prevent hotspot heating (Figure 3), which can occur when shaded cells become reverse-biased. By providing an alternative current path, bypass diodes limit the reverse bias voltage across affected cells, preventing damage and fire hazards.

Figure 3. Thermal imaging on a PV panel shows a hotspot, a localized area of high temperature, that can negatively affect power output. (Image: Maysun Solar)

Figure 4 shows how a bypass diode is connected to PV cells and how they work during partial shading. The bypass diode has an on-state resistance, so even a minimal current flow through it leads to power loss in the form of heat. The problem is aggravated during a hotspot when the bypass diode becomes the sole medium of current flow, resulting in a higher power loss.

Figure 4. Connection of bypass diode with PV panels and how it helps bypass the shaded panels to keep the continuity in current flow. (Image: PVEducation)

Therefore, it is necessary to consider the proper design of the bypass diode, which must have the lowest possible on-state resistance and the least numbers used while ensuring high reliability. The non-linear nature of bypass diodes also demands sophisticated electrical solvers and simulation tools for accurate modeling and optimizing system performance.

Mismatch in PV array output power causing array reconfiguration challenges

The problems that the bypass diode creates on a single PV panel can quickly escalate into power mismatches in the PV array. This is especially true when multiple PV panels face issues with their bypass diodes. This often results in each bypass diode behaving differently according to the shaded conditions that their respective PV panels face.

In Figure 5, nine PV panels form a PV array. With no shading on the PV panels, the P-V curve is smooth and ideal, as expected. In the case of non-uniform shading on PV panels, the output power is unpredictable, resulting in a high mismatch loss. This phenomenon is an extended version of the power mismatch we discussed in Figure 2, where the shading was considered uniform on all the affected PV panels.

Figure 5. Illustration of irregular power vs voltage curve in PV array caused by variations in power output of individual PV panels. (Image: ScienceDirect)

The challenge that mismatch loss creates is that it forces PV designers to look for array reconfigurations that can minimize this mismatch loss. The P-V curve is expected to improve when the difference between the maximum and minimum power-generating rows is brought to the minimum. Therefore, the number of multiple peaks will reduce power fluctuations, resulting in smoother MPPT control.

Summary

From the three challenges we discussed, it is observed that smooth control of the MPPT controller, proper design of the bypass diode, and finding the right combination of PV panels in a PV array are central to optimizing solar PV arrays for operation in partially shaded environments. All three challenges are linked; resolving even one will positively impact the other two.