Issues with shading and solar panel arrays

Partial shading and solar PV arrays: The 2 don't mix

by James Martin II on September 14, 2012

Shading is not good for solar PV system output. A shadow cast on even just part of one solar panel in your solar array can potentially impede the output of the whole solar PV system significantly. What are some ways to avoid the problem of shading on solar arrays?

Why do solar panels hate 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 convert sunlight into DC (direct current) electricity, and the inverter then transforms the DC electricity AC (alternating current) electricity, which can be used by appliances and is compatible with the power grid.

The majority of small-scale solar systems, such as a 3 kilowatt (kW) system, for homes and small businesses will include a minimum of 6 panels. However, systems often are designed with more panels in order to meet the electricity demand of the occupants of the building they are connected to.

For reasons related to the voltage requirements of a system’s inverter, solar arrays are usually separated into ‘strings’ of solar panels. Smaller systems may only have 1 string, while large systems may have a larger number. One string could theoretically be as small as a single panel, but there are usually more than this.

You can think of a string of panels as something like a length of pipe, with the power running through it like water. In conventional solar panel strings, shade is something that impedes that flow. If, for example, shade from a tree or a chimney is cast on even one of the modules, the output of the entire string will fall to almost zero for as long as the shadow remains there. If there is a separate, unshaded string, however, this string will continue to generate power as normally.

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

Dealing with shading on solar panels: Strategies and technologies

Although the power output and return on investment (ROI) from a solar PV system can be significantly hampered by shading–especially shading that occurs often due to an object that casts a shadow at the same time every day or during a certain as the sun passes through the sky–there are a number of ways to safeguard against this.

Make sure your solar array is installed where there is no regular shading

This is the first and most obvious step to making sure your system does not suffer the effects of partial shading. It is of utmost importance 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 may be able to check this yourself. Alternatively your Solar Selection broker can check to ensure there your roof is shade-free year round using a program called Nearmap.

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.

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 ensure that there are no nearby trees which might grow tall enough to eventually block the sun. Solar system lifespans are typically expected to be 25+ years, during which time a tree would have plenty of time to get taller.

Clouds passing through the sky during the day may also result in fluctuations in system output, but there’s little that can be done about this. Amorphous silicon solar cells are said to be better at handling shading than crystalline silicon solar panels, but generally speaking the comparatively low overall efficiency of amorphous panels means that crystalline modules tend to be a better choice.

There are some other technologies under development that may offer reasonable efficiency even in overcast conditions. These include ‘super black’ solar cells and a few other technologies, but most products like this are still either expensive or not yet commercially available.

Make sure your inverter is MPP tracking-capable

Maximum Power Point Tracking (MPP Tracking or MPPT) is a technology that is now 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 inverters, so that even if the output from one is compromised by shading, the system is still able to take advantage of whatever power (however little) being produced and add it to the more powerful string, producing a consistent, usable amount of electricity. Inverters without MPPT capability simply lose the output from the weaker string once it sinks below the usability threshold.

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

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) which offer module-level MPP Tracking technology. These inverters offer MPPT for each individual solar panel in an array, averaging the output of all panels more ‘intelligently’ than a conventional, centralised inverter, which can make adjustments only at the string level.

Microinverters are another another technology that deal with the issue of power loss and strings. More and more brands of these are becoming available on the market, and although their price point tends to be higher, they promise significant advantage in terms of overall system output.

© 2012 Solar Selections Ltd

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