Why even partial shading is bad for solar power systems

by James Martin II on May 10, 2012

in Installation advice,Positioning solar PV panels

Although it probably goes without saying, shading is not good for solar panels. A shadow cast on even just part of one solar panel in your solar array can potentially compromise the output of the whole system. What are some strategies for dealing with potential shading of solar arrays?

Why are solar panels averse to shade?

In most instances, solar photovoltaic (PV) systems for homes and businesses consist of solar panels (the collection of which is referred to as the ‘array’) and an inverter. The solar panels catch sunlight and convert it into DC (direct current) electricity, and the inverter in turn converts the DC electricity into grid- and appliance-compatible AC (alternating current) electricity.

Most small-scale solar systems, such as a 1.5 kilowatt (kW) system, for homes and small businesses will include at least 6 panels. However, systems can easily be composed of more panels where necessary, depending on the output of the panels in question and the electricity demands of the occupants of the building they supply power to.

For technical reasons related to the voltage requirements of the system’s inverter, solar arrays are usually divided into ‘strings’ of solar panels. Small systems may only have 1 string, while large systems could have many more. One string could consist of a single panel, but usually they have more.

You can think of a string of panels as something like a piece of pipe, and the solar power is like water flowing through that pipe. In conventional solar panel strings, shade is something that blocks that flow. If, for example, shade from a tree or a chimney is cast on even one of the panels in the string, the output of the entire string will be reduced to virtually zero for as long as the shadow sits there. If there is a separate, unshaded string, however, this string will continue to produce power as per usual.

In extreme cases, a shadow does not necessarily need to fall on an entire panel–depending on the technology used in the solar panel in question, shading of even just one cell could flatten the output of the panel and in turn the entire string. Many modern panels, however, come equipped with devices called bypass diodes which minimise the effects of partial shading by essentially enabling electricity to ‘flow around’ the shaded cell or cells.

Strategies and technologies for dealing with shaded solar panels

Although the performance and therefore the return on investment (ROI) from a solar power system can be severely affected by shading–especially shading that occurs regularly due to an object that casts a shadow at the same time every day as the sun passes through the sky–there are a number of ways to avoid or mitigate these effects.

Site your solar panel array where there will be no regular shading

This is the first and most obvious step to making sure your system does not suffer the consequences of being partially shaded. It is extremely important to consider all times of day for all seasons of the year when working out whether some nearby object might cast a shadow onto your roof. You can check this yourself or alternatively your Solar Choice broker will check to ensure there is no shading on your roof using a program called Nearmap.

shaded solar array

An example of a tree which has grown tall enough to cast a significant amount of shade on the solar array of a nearby roof.

Solar system owners should also be vigilant in making sure that there are no nearby trees which might grow tall enough to eventually cause shading issues. Solar system lifespans are typically expected to be 25+ years, during which time trees have plenty of time to grow.

Clouds are another source of potential shading. Clouds passing through the sky during the day may also result in fluctuations in system output, but these are basically unavoidable. Amorphous silicon solar cells are said to be better at handling shading than crystalline silicon solar panels, but generally speaking the relatively low overall efficiency of amorphous panels means that crystalline modules are a better choice.

There are some other technologies under development that may offer high efficiencies even in inclement weather, such as ‘super black’ solar cells, but most of these are still either expensive or not yet commercially available.

Use an inverter that has MPP Tracking capability

Maximum Power Point Tracking (MPP Tracking or MPPT) is a technology that now comes standard in most quality inverters. An inverter equipped with an MPP Tracker is able to ‘average out’ the difference between the current and voltage of 2 or more strings of panels, so that even if the output from one is sub-optimal, the system is still able to take advantage of whatever juice it is creating and add it to the more powerful string, producing a consistent, usable volume of power. Inverters without MPPT capability simply lose the output from the weaker string once it passes below the required output threshold.

MPPT and shading in a multi-string solar power system

An example of partial shading in a conventional multi-string inverter system (Image credit: pvsolarchina.com)

There are also a number of companies (such as SolarEdge) who offer module-level MPP Tracking technology. These inverters offer MPPT for each individual solar panel, averaging the output of all panels more ‘intelligently’ than a central inverter, which can make adjustments only at the string level.

Enecsys is another company that produces a type of mirco-inverter. Enecsys micro-inverters sit on individual modules and converter all power from each panel directly into AC electricity, avoiding power losses from shaded strings. Tindo Solar panels are an Australian-made solar panel brand that come equipped with Enecsys inverters.

© 2012 Solar Choice Pty Ltd

James Martin II

James Martin II

James has been working as analyst and online development manager for Solar Choice since 2011. He holds a master's degree in Environmental Management from UNSW, and a bachelor's degree in Philosophy from Bridgewater State University in his native Massachusetts.
James Martin II

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