Avoid Solar Panel Shading: How To Minimize Its Impact
Sharon Lee • Published December 16, 2022 • Updated on February 20, 2023 • [rt_reading_time postfix=”minute”] read
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Commercial solar system design is a complex endeavor. There are dozens of data points and factors to consider when creating a solar array that meets each customer’s specific energy goals and budget.
If you’re searching for a solar energy partner, one of the best ways to separate the pros from the wannabes is to understand how they deal with PV module shading issues. Solar shading is a fact of life for most installations and if it’s not handled properly, your entire system will suffer.
Table of Contents
How Solar Panels Work
Solar panels are made up of individual solar photovoltaic (PV) cells, which are thin slices of silicon that typically measure 6 square inches. Each solar cell converts sunlight into electricity in what’s called the photovoltaic effect. Multiple solar cells are assembled to form a single rectangular shaped panel or module. Multiple panels are then assembled into an array to provide the solar power your business needs.
Understanding Solar Panel Efficiency
A solar panel’s efficiency rating is the amount of sunlight (solar irradiance) that falls on the solar panel that can be converted into usable electricity. Solar panel efficiencies range between 16 and 22%, with an average of just over 20%.
What that means is that for a panel with a 20% efficiency rating, 20% of the sun’s energy that’s absorbed by the panel will be converted into usable energy. This is also known as the power output or power rating of the panel, which is measured in watts. The higher the efficiency, the more electricity (more watts of power) generated. Most 72-cell commercial solar panels on the market today produce between 350 and 600 watts of power each.
Factors That Impact The Efficiency Of Solar Cells
While a panel’s individual PV cells are the primary driver of the panel’s overall efficiency, there are a number of factors at play when considering the system’s ultimate power output.
Things like the type of cells, how many cells are in the panel, how they’re laid out, how they’re wired together, and the type of material used on the back of the panel (called the back sheet) can all impact a panel’s efficiency.
So can shading, and that’s what we’re going to discuss here. There are two types of shading: dynamic and static. Dynamic shading consists of the shadows from passing clouds, snow or rain, and even dust. Most of these issues can be managed at the equipment level through the use of the power electronics that are built into many inverters, or with the addition of module level equipment like optimizers.
Our focus in this article is on static shading caused by buildings, trees, and other obstructions.
Solar Panels Shade Loss is Not Proportional
You might think that the maximum efficiency of your solar panels is reduced proportionally to the amount of static shading – if one percent of the cells are shaded, you’d lose one percent of the panel’s efficiency, right? While intuitively that makes sense, it’s not actually what happens.
A static shaded module produces electricity at a significantly lower rate than unshaded modules. In fact, studies have shown that shading just one cell in a panel can reduce the solar power output of the entire panel by a whopping 50-80%.
Here’s why: all the cells in the panel work together as a single system; each cell is a link in the chain. If one cell is shaded, its power output is reduced and the entire panel’s efficiency drops.
Think of your solar array as a single lane racetrack. The pace of the slowest car dictates the speed of every other car on the track. It doesn’t matter where you sit in the pack, nobody’s going any faster than that one car.
In the same way, it doesn’t matter where in the panel the static shaded cell is located – it’s the weakest link whether it’s first or last in the chain. Because all the cells are interconnected and work together, the maximum current the whole module can achieve will be no greater than that single shaded cell.
Mitigating the Effects of Shading
With the potential of a 50-80% efficiency loss, clearly solar shading is a big deal and should be avoided if possible. If you’re working with a reputable solar provider like Velo Solar, they’ll consider potential shading issues when designing your solar energy system.
First, they’ll conduct a site survey to identify any potential obstructions like trees, tall buildings, chimneys, poles or even antennas that could cast a shadow on your solar panel array.
They’ll also consider shading when determining the optimal panel orientation. Everything from the surrounding terrain to the location of other panels can cause shading issues.
Finally, a commercial solar pro will consider how the shading on your PV panels might change based on the time of the year and the sun’s position in the sky.
Maximizing the Power Output of Shaded Panels
Sometimes, despite the best efforts of your solar partner, shading simply can’t be avoided. Fortunately, there are solutions that can help reduce any losses.
Stringing Arrangements
Solar panels are connected in series to form strings. While a shaded module in a string will reduce the power output of that entire string, the output power of the strings around it aren’t impacted. For example, if each row of solar panels in your rooftop array is its own string, a shaded panel will only reduce the efficiency of the other panels in that row, not the panels in neighboring rows. In other words, each string is its own racetrack.
Smart solar designers will put all the shaded solar panels in your system on the same string, thus maximizing the overall efficiency of your array. This is called running your panels in parallel.
Inverters and Power Optimizers
As discussed earlier, power electronics help to reduce the impact of shading to your system’s overall performance. Newer model quality inverters offer load shifting, voltage and VAR control that can help to re-balance a system’s output power and other panel output mismatches. This allows the system to create the most usable energy available as shading changes across the day and year.
The next solution is to incorporate power optimizers that have maximum power point tracking (MPPT) capability to rebalance load from both static and dynamic shading. Other benefits from optimizers include module level monitoring, single inverter diagnostics and servicing ensuring that the impact of shaded cells is minimal at all times.
For smaller systems, your solar provider may also employ micro inverters to mitigate the losses that stem from solar panel shading. Inverters convert direct electrical current into alternating current. Micro inverters, as the name implies, are small inverters that are connected to each solar panel.
The War Of The Currents
Before we get into how power optimizers and micro inverters help to reduce shading losses, let’s pause a moment to talk about the two types of electrical currents – direct current (DC) and alternating current (AC). Direct current flows in a single direction while alternating current switches directions (60 times per second in the US). Prior to the late 1800s, Thomas Edison’s DC was the standard in the US; but it isn’t easily converted into higher or lower voltages. Nikola Tesla believed alternating current was the solution to that problem and AC soon became the norm.
As fascinating as Messers Edison and Tesla’s War of the Currents might be, we don’t want to get bogged down in the details. Let’s suffice it to say that your solar panels generate DC electricity, but just about everything you plug into a wall outlet wants AC. The inverter included in your solar system handles that DC-to-AC conversion, making your solar energy useable.
How Micro Inverters Reduce Shading Loss
So now that we know what inverters do, how can micro inverters reduce the impact of solar shading? It’s simple. They provide another way to maximize the energy production of your entire system.
Under perfect conditions, your solar provider might run each string of PV modules to a single inverter (though a large PV system may require more than one). If there are shading issues to be managed, and there likely will be, they may recommend connecting the strings of shaded panels to one inverter and the unshaded panels to another. As noted above, the max power produced by that shaded string will be equal to the power output of the most shaded PV module.
An even better solution is the use of micro inverters. These small inverters are connected to each panel, allowing the module to operate independently at its maximum power output. This means partial shading on one panel won’t reduce the energy production of the panels that get full and direct sunlight. To carry on with our analogy, a micro inverter turns each panel into its own racetrack.
DC Optimizers
Micro inverters are a type of module level power electronics (MLPE), as are DC optimizers. A DC optimizer adjusts the output voltage and current to maintain maximum power without inhibiting the production of the entire string. Simply put, a DC optimizer stops a partially shaded panel from impeding the output of the other PV modules in the string.
Bypass Diodes
Your solar partner may also recommend using bypass diodes. A bypass diode allows you to isolate specific cells within a panel by rerouting the current around those that are shaded. These smart devices allow the higher current from unshaded cells to flow unabated – they’re basically passing lanes on the racetrack.
A typical panel has three bypass diodes allowing your provider to control sections of the panel, not individual cells.
Work With a Pro
Even minimal shading can significantly impact the amount of useable energy generated by your solar system. Contact the pros at Velo Solar to learn more about how their design-build approach can minimize output losses and maximize your solar investment.
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