ELEC 5564 Electric Power Generation by Renewable Sources - Shading Losses and Mitigation

Shading from trees, buildings, clouds, other panels, or dust can significantly reduce the output of a solar cell.
Partial shading is a major cause of energy generation losses in photovoltaic (PV) systems.
Even a small shaded section can significantly reduce the performance of the entire solar photovoltaic panel.
Past studies have used a shading factor, assuming a proportional decrease in power production to shaded area and solar irradiance; while true for a single cell, this is not always accurate at the module or array level.

Figure A shows the P-V characteristics of a PV module.
Figure B shows the P-V characteristics of a PV module under partial shading.
The highest point in Figure A represents the Maximum Power Point (MPP).
For maximum power utilization, solar PV panels should operate at MPP.
Under uniform irradiance, the P-V characteristics exhibit one peak, representing a global and local MPP.
Multiple peaks can result from partial shading.

Figure A illustrates single-cell partial shading in an array.
Figure B shows the effect of one shaded cell in a PV module, which reduces the current through healthy cells, causing them to produce higher voltages that can reverse bias the shaded/faulty cell.
Hot spot heating occurs when the operating current exceeds the reduced $I_{SC}$ (short-circuit current) of a shadowed or faulty cell.
The affected cell or group of cells is forced into reverse bias, and the entire generating capacity will be dissipated in the shaded/faulty cell.
This power dissipation in a small area results in local overheating, or "hot-spots", leading to destructive effects like cell or glass cracking, melting of solder, or degradation of the solar cell.

The effects of hot-spot heating can be mitigated using a bypass diode.
A bypass diode is connected in parallel, but with opposite polarity, to a solar cell.
Under normal operation, the bypass diode is reverse biased and acts as an open circuit.
If a solar cell is reverse biased due to a mismatch in short-circuit current between series-connected cells, the bypass diode conducts.
This allows current from the good solar cells to flow in the external circuit, rather than forward biasing each good cell.
The maximum reverse bias across the poor cell is reduced to about a single diode drop, limiting the current and preventing hot-spot heating.

A blocking diode prevents battery discharging through the PV modules.
When the battery voltage exceeds the generator voltage, the diode becomes reversely biased and blocks the discharging path.
It prevents current from flowing from one parallel string into a lower-current string, minimizing mismatch losses in parallel-connected arrays.
In larger PV arrays, individual PV modules are connected in both series and parallel.
An open-circuit in one of the series strings can cause a problem: current from parallel-connected modules will be lower than the remaining blocks (similar to the shading effect).
In parallel arrays with series-connected modules, the bypass diodes of the series-connected modules become connected in parallel. If the bypass diodes are not rated to handle the current of the entire parallel-connected array, this presents a problem.
To minimize mismatch losses, an additional diode, called a blocking diode, is used.

Example: 36 cells connected in series, 35 are irradiated identically, but one is shaded by 75%.
The current through all cells is the same; the terminal voltage is determined by $V = VS(I) + 35VF(I)$
The module characteristic can be obtained by choosing a range of current.
$V = V1 + V2 + … = \Sigma V_I$
$I = I1 = I2 = I3 = …. = I{36}$

Module characteristic when stopping at the short circuit current of the partially shaded cell.
Power reduces from 20.3 W to 6.3 W.
When current is higher, voltage of shaded cell becomes negative.
$V = V1 + V2 + … = \Sigma V_I$
$I = I1 = I2 = I3 = …. = I{36}$

Cell shading reduces the module output power drastically. In the example, module power reduces from 20.3 W to 6.3 W, although only 2% of the module surface is shaded.
The partially shaded cell operates as a load.
At higher irradiance, the power dissipated in the shaded cell further increases, heating the cell and causing hot spots, potentially destroying the cell.
To protect single cells from hot spot-related thermal damage, bypass diodes are integrated in parallel into the module.

To protect single cells from hot spot related thermal damage, by- pass diodes are integrated in parallel into the module.
They are not active unless shading occurs.
1 diode per 18-24 cells.

Depending on the degree of shading, the MPP shifts, and high losses occur despite integrated bypass diodes.
A 36-cell module with two bypass diodes.