Effects of Shade on PV Output
Since PV systems generate electricity based on the amount of sunlight they receive, it makes sense that when a shadow is cast on a panel, for example by a nearby tree, its power output decreases. However, the decrease in power could be a lot worse than it initially seems.
Intuition suggests that power output of the panel will be reduced proportionally to the area that is shaded. However, this is not the case. In his book Renewable Energy and Efficient Electric Power Systems Stanford University’s Gil Masters demonstrates how shading just one out of 36 cells in a small solar module can reduce power output by over 75%.
To conceptualize why shading results in such severe losses, it is helpful to use the analogy of water flowing in pipes. The flow rate of water through the pipe is constant, much like the current through a cell string is constant for a given irradiance level.
Shading a solar cell is similar to introducing a clog in a pipe of water. The clog in the pipe restricts the flow of water through the entire pipe. Similarly, when a solar cell is shaded, the current through the entire string is reduced.
This is significant because every cell in the cell string has to operate at the current set by the shaded cell. This prevents the unshaded cells from operating at maximum power. Therefore, only a small amount of shading can have a dramatic effect on the power output of a solar panel.
Similar principles apply to PV modules connected together. The current flowing through an entire string of modules can be heavily reduced if even just a single module is shaded, leading to potentially significant loss of power output.
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).
Modules connected in series form strings, and strings can be connected in parallel to an inverter. The current through all the modules of a string has to be the same, and the voltage of parallel strings has to be the same. As we saw in the last section, a shaded module in a string can bring down the power output of the string significantly. However, a shaded module in one string does not reduce the power output of a parallel string. Therefore, by grouping shaded modules into separate strings, the overall power output of the array can be maximized.
For example, in a commercial system with parapet walls, it can be beneficial to group modules that receive shade from the parapets into strings, and keep modules that do not receive shade from the parapets in separate, parallel strings. This way the unshaded strings can maintain a higher current and power output.
Bypass diodes are devices within a module that allow the current to “skip over” shaded regions of the module. By utilizing bypass diodes, the higher current of the unshaded cell strings can flow around the shaded cell string. However, this comes at the expense of losing the output of the cells that are skipped over.
Although it would be theoretically ideal to have a bypass diode for each solar cell, for cost reasons a typical solar module will have three bypass diodes, effectively grouping the cells into three series cell strings (Figure 5). For instance, a 60-cell module will typically have one bypass diode for every 20 cells.
Module Level Power Electronics (MLPEs)
MLPEs are devices that are attached to individual modules in order to increase performance under shaded conditions (though there are other benefits, such as mismatch mitigation and module-level monitoring). This is done by performing maximum power point tracking at the module level. MLPEs include DC optimizers and microinverters.
A DC optimizer adjusts a its output voltage and current to maintain maximum power 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; to compensate, the optimizer 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 to convert electricity from DC to AC.
As opposed to having a single inverter servicing all of the panels, each panel can have a small inverter attached to it to convert its output from direct current (DC) to alternating current (AC). Since each microinverter has an MPPT, and their outputs are connected in parallel, each panel will operate at its maximum power point, without impacting other panels.
Effects of MLPEs on PV System Performance
Using Aurora’s simulation engine, we compared the performance of three different PV systems subject to significant shading. As shown in Figure 7, we placed a 3.12 kW system near the edge of a roof, which has tall trees next to it. Note that while this design effectively showcases the performance difference of these system topologies in shaded conditions, it is not an optimal — or even a practical — design. Our findings are summarized in Table 1.
|System Topology||Annual Yield||Improvement with MLPEs|
|String Inverter||2,585 kWh/year||N/A||Microinverters||3,033 kWh/year||+17.3%|
|DC Optimizers||3,035 kWh/year||+17.3%|
Our results show that using MLPEs under these conditions increases system output by 17.3% annually, showing the benefit of using these components for shade mitigation. Additionally, the effective yield of a system using a microinverter or a DC optimizer is approximately the same, although there could be small differences (on the order of 1%) in some cases due to differences in efficiency curves.
For the same reason that they can mitigate shade losses by decoupling module output, MLPEs can eliminate module-to-module mismatch losses. These losses are typically caused by manufacturing variations that lead to slight differences in the electrical characteristics of two modules of the same type. Since MLPEs allow the modules to operate independently from one another, these variations will not impact the system’s overall performance.
About Solar PV Education 101
How a Photovoltaic System Produces Electricity is part of Solar PV Education 101, a five-article series introductory primer to the fundamentals of solar PV for beginners.
Article 1: The Beginner's Guide to Solar Energy
Article 2: How a Photovoltaic System Produces Electricity
Article 3: How to Size a PV System from an Electricity Bill
Article 4: Shading Losses for PV Systems, and Techniques to Mitigate Them
Article 5: The Basic Principles that Guide PV System Costs