Shading Effects on PV Output

THIS ARTICLE WAS ORIGINALLY PUBLISHED ON AURORA SOLAR’S BLOG

Since PV systems generate electricity based on the amount of sunlight they are receiving, it makes sense that when a shadow is cast on a panel, from either a passing cloud or nearby tree, the power output decreases. However, the decrease in power is a lot worse than it seems.Solar panels with shadingFigure 1. Solar panels experiencing shading. Source: lowcarbonlivingblog.wordpress.com

Intuition suggests that the proportion of the panel shaded will reduce the power output by a proportional amount. This is not the case. For instance, in a lecture given by Gil Masters, professor at Stanford University and author of the textbook Renewable and Efficient Electric Power Systems, a single cell shaded in a module made up of 60 cells has the potential to reduce power output by over three quarters.

How Electricity Delivered by PV is like Water Flow

To conceptualize why shading results in such severe losses, it is helpful to use the analogy of water flowing in pipes. A PV module is like a water pipe. Each solar cell in a PV module is like an inlet that lets in some more water. Throughout the length of the pipe, water flows at the same rate.

Analogy between string of PV modules and water pipe with inletsFigure 2. Analogy between string of PV modules and water pipe with inlets.

Shading is the not the equivalent of simply closing some of the inlets off. In addition to tightening the valve and cutting off most of that water source, shading is like putting an obstruction in the pipe itself. The obstruction blocks a lot of the incoming water, and only a little bit may trickle through, effectively reducing the water flow of the entire system dramatically.

Water pipe analogy with shadingFigure 3. Water pipe analogy when shading occurs. Shading of a solar cell is like introducing an obstruction in a water pipe.

In a PV system, complete shading of a solar cell is that obstruction. Electrical current has difficulty going through a shaded cell, effectively blocking the majority of electrons trying to flow through the length of the string. Less electrical current means less electricity delivered, and vastly reduced power output.

Similar principles apply to PV modules in an array. Modules that are on the same string, that is modules that lie on a single path for electrons to travel along, will be subject to severe losses when even a single module is experiencing some shading.

Approaches to Reduce Shading Losses

Fortunately, there are a number of different mechanisms that can be used in a PV system to reduce shading losses. They include stringing arrangements, bypass diodes, and module level power electronics (MLPEs).

Stringing Arrangements

Shaded modules reduce the electrical current in the entire string, thereby reducing the power output. To help combat this, modules can be arranged in either series or in parallel.

Modules that lie on the same string are said to be in series, while modules that lie on different strings are said to be in parallel. Due to Kirchhoff’s Current Law, all components along the same string must have the same current going through them, while they can be different on parallel strings.

Since modules on parallel strings can have different currents, shaded modules will not reduce the power output of modules that are on parallel strings, therefore mitigating the effect that a shaded module has on a PV system.

PV arrays with modules in series and in parallelFigure 4. (Right) A PV array with modules in series (left) and modules in parallel (right)

Bypass Diodes

Bypass diodes are devices within a module that allow the current to “skip over” shaded regions of the string. By utilizing bypass diodes, the current is able to flow at the same rate without being lowered by shaded cells at the expense of losing output on all the cells it skips over.

According to the results presented in Renewable and Efficient Electric Power Systems by Gil Masters, skipping over cells to avoid lowering the string current can oftentimes be favorable. Although it would be most efficient to have a bypass diode for each solar cell, a typical solar module will have bypass diodes grouping the module into three segments. For instance, a 60-cell module will have a bypass diode for every 20 cells.

Bypass diodesFigure 5. (Left) PV module with three bypass diodes to segment the solar cells in series. Source: Gil Masters, Stanford University

Module Level Power Electronics (MLPEs)

MLPEs are devices that are attached to the individual modules in a PV system to increase performance under shading conditions. MLPEs include microinverters and DC optimizers.

Microinverters:

As opposed to having a single inverter servicing all of the panels, each panel has a small inverter attached to it to convert the produced electricity from DC to AC. This effectively makes each panel independent from one another, and poor performance from shading of one module will not affect the output of the other modules.

DC Optimizers:

A DC optimizer adjusts a PV module’s delivered voltage and current to maintain power output without compromising the performance of other modules.

For instance, when a shaded module produces electricity with a lower current, the DC optimizer will boost the current at its output to match the current flowing through the unshaded modules; because the DC optimizer cannot produce extra energy, it reduces its output voltage by the same amount it boosts the current. This allows the shaded module to produce the same amount of electrical power without impeding the output of other modules.

A system utilizing DC optimizers still needs an inverter at the end of the string to convert electricity from DC to AC.

Simple schematics of PV systems utilizing MLPEsFigure 6. Simplified schematic of a PV system utilizing microinverters (top) and a PV system utilizing DC optimizers (bottom)

Effects on Performance for PV Systems Equipped with MLPEs

Using the same Palo Alto home referenced in The Beginner’s Guide to Solar Energy and How to Size a PV System from an Electricity Bill, performance simulations were run to determine how much more energy is produced annually by a system utilizing MLPEs against a system that does not.

This was done by running three separate simulations on identical rooftop and panel configuration. Each simulation either utilized a standard string inverter, microinverters, or DC optimizers with a string inverter, with the panels arranged in series.

The Palo Alto home used for MLPE performance simulationsFigure 7. The Palo Alto home used for the three different performance simulations to test the relative output of MLPE systems to a non-MLPE system. Source: Aurora Solar

Compared to the standard string inverter case, the system utilizing microinverters lead to a 1% increase in annual energy output, while the system with DC optimizers lead to a 2.8% increase in annual energy output. The differences in output for the microinverter and DC optimizer system can be attributed to the 2% difference in the efficiencies of their respective inverters. It is important to note that this home did not feature severe shading. In the presence of more severe shading, the benefit from using MLPEs would be larger.

MLPE or NO MLPE Annual Energy Output % Increase
No MLPE 6,060 kWh/year N/A
Microinverters 6,120 kWh/year +1%
DC Optimizers 6,227 kWh/year +2.8%
Table 1. Results from performance simulations of PV system on a Palo Alto home utilizing different MLPE components. The difference between the two MLPE outputs is attributed to the differences in their inverters’ efficiencies. Source: Aurora SolarAdditionally, MLPEs are able to eliminate module-to-module mismatching losses. They are able to do this by adjusting the output of the each module independently such that all of them are the most compatible with one another. This effectively eliminates the slight variances the components may have from one another due to the consequences of the manufacturing process.
READ THE ORIGINAL ARTICLE HERE.