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Shedding light on PV system shading

By Ilan Yoscovich | May 06, 2010

Installing a PV solar system is an exciting opportunity to get energy from a free and natural energy source - our sun. But are we really getting the maximum power out of our PV installation?


Standard PV systems are built with the naive approach of connecting power sources in serial strings, like we do with batteries for consumer electronics devices. Just as with batteries, you usually don't mix types. If the solar panels are not matched then the total energy potential drops rapidly. Actually, as discussed in many articles and measured in numerous installations, standard PV systems suffer from many mismatch problems.

 

The most interesting (and natural) mismatch problem is “partial shading”- mismatch between the shading (and therefore illumination) of each panel.

 

Shading may occur due to clouds, environmental obstructions such as trees and buildings, self-shading between panels in parallel rows (see Figure 1), dirt and dust, bird droppings etc. These shading effects may be static, i.e. slow due to sun-light angle during the day or may be very dynamic (e.g. moving clouds).

Self-shading in PV systems

Figure 1 - self-shading by parallel rows of

panels in a rooftop PV installation

 

 

electrical characteristic curves of shaded and unshaded PV panels

Figure 2 - Electrical characteristics graph of fully

illuminated and partially shaded PV panels

If shading was uniform (not “partial”) then everything would be simple - the electrical current of all PV panels would drop by the same proportion and lost power would be proportional to the actual shading (we cannot do better than that, aside from removing trees).

Let’s dig into the effect of partial shading where different panels have different illumination. A PV panel electrical characteristics curve is shown in Figure 2 for two illuminations (full / 25% shaded). In this example of a 300W PV panel, a fully illuminated panel has a maximum current of 10A but achieves highest power at a lower current of 8A.


Let’s assume that we have a string of such panels, which has one shaded panel with 75% illumination (limited to 7.5A maximum current and achieving maximum power at 6A). This shaded panel achieves maximum power at 75% of the current that the other panels need for maximum power. Since the panels are connected in series, their current is the same. Does this mean that all the panels will operate at 75% of their maximum power, resulting in a 25% energy loss just because of one shaded panel?

 

For a traditional PV system, the answer is that the inverter will search for the optimal voltage/current for the string and therefore will “prefer” to work at the maximum current. The shaded panel will therefore be bypassed by a parallel diode so that it won't ruin the energy yield of its string-mates and allow the high current flow through the string. So in this case, 75% of the energy of a single shaded panel is lost. If there are additional shaded panels, then their energy will be lost as well, up to the point where the traditional inverter will “prefer” to lower the string current below 8A thereby reducing the power from each fully-illuminated panel but starting to gain power from the shaded panels.

 

 

Partial shading of PV panel substrings

Figure 3 - Partial shading distribution

on substrings of PV panels

Another effect of partial shading is related to the internal structure of the panels which is based on several substrings connected in series. If all sub-strings have the same illumination/shading, then the total current/power will be degraded according to the shading ratio. But if the substrings have different shading, then like partial shading in a serial string, the total power performance will be degraded by more than the shading ratio. For example, see Figure 3 cases where shading can occur either on several sub-strings or on a particular sub-string. It is possible to determine the orientation of the PV panel during system design such that during daylight hours, the overall performance will be optimal. However, in many cases the analysis is not simple.

Yet another interesting case relates to mismatch between panels that is not caused by shading, but behaves the same. The electrical characteristics of PV panels have some manufacturing variance (e.g. 5%) at manufacturing and it gets worse with temperature changes and over time (and PV systems are usually installed for many years). This mismatch results in a variance in the optimal current work-point for achieving maximum power from the panel. Since the string has the same current for all panels, traditional inverters try to find the optimal current that will fit most of the panels. However, there will be some energy penalty that may reach 3-5% overall and even more over time.

 

So what can be done in order to mitigate partial shading and other mismatch effects? Fighting shading itself by designing a better installation can help to some extent but the main problem remains (see for example this site movie where daily partial shading effects are caused during the day by rooftop chimneys in a standard installation).

The correct solution is straightforward: each PV panel should be treated separately even when connected in a serial string. This individual treatment is achieved by adding a power optimizer to each and every PV panel.

 

Partial shading across panels

 

Using SolarEdge power optimizers, an energy gain of 5-20% is automatically achieved in many standard installations with partial shading and other mismatches, relative to traditional installations. I believe no one can overlook a 20% energy loss in their PV system.

 

Our power optimizers compensate for partial shading effects even in very dynamic environments, such as tree branches moving due to wind, and therefore provide the PV system with as much energy as the sun provides - this is where we started this discussion, isn’t it?

 

Let me show you a glimpse of our power optimizer system in action. Our PV monitoring system shows the energy yield during the day when shaded panels yield less energy but produce as much as they can. More importantly, the non-shaded panels keep producing maximum power with no penalty due to their string-mates. The next monitoring screenshot movie of our site physical layout map also demonstrates this: shaded panels are marked by a flag and the cumulative energy generation is shown for all panels.

 

Pinpointing underperforming panels on a physical site map

 

As seen in the next screenshot movie, a power graph of each panel in the string is shown in our monitoring web system. It is clear from the graphs that each panel builds its energy at a different rate depending on its specific illumination.
Another benefit of our monitoring system, which is shown above, is the ability to pinpoint energy losses of each panel, including effects of partial shading. In many cases, it can lead to improvement of the installation and the total energy yield.

 

Per-panel power curves provide high accuracy remote maintenance

 

Ilan is a Senior System Engineer at SolarEdge, in charge of system design and advanced features development

 

 

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Categories: Technology PV systems
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