Solar Electricity Handbook 2011: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems - Michael Boxwell (2011)

Appendix A. Crystalline Solar Panels and Shading

As mentioned a few times in this book, shading is a big issue with crystalline photovoltaic panels. A small amount of shading has a big impact on the amount of electrical power generated by your solar system.

Unlike solar thermal (hot water) systems, the loss of power through shading is much greater than the amount of the panel that is in shade. With solar thermal systems, if only 5% of the panel is in shade, you lose less than 5% of the power production. With amorphous (thin-film) PV panels, you would usually lose 15-20% of the power production in a similar scenario.

With crystalline solar panels, the difference is significantly more and in some instances can bring power generation down to zero, even if the amount of the panel in shade is small.

The reason for this is in the construction of the solar panel itself. A crystalline solar panel is made up of a number of individual solar cells. Typically, each of these cells generates ½ volt of potential energy. These cells are connected together in series to increase the voltage to a more useful level inside the solar panel. Each individual solar panel has one or more strings of solar cells.

Because these strings of cells are connected in series, a solar panel is only as good as its weakest cell. If one cell produces a weak output, all the other cells within the string are compromised as well. This means that if you have a ‘soft shade’ such as a distant tree branch, creating a soft dappled shade across just one or two cells on a solar panel, the effect can be to reduce the output of the whole string to a similar level to a dull, overcast day.

Worse happens when you have a more direct shadow, creating a bigger contrast between light and shade. When even one cell is in complete shade and the remainder are in bright sunlight, the shaded cell short-circuits as the flow of electrons within the cell goes into reverse.

However, because the cell is connected in series with the other cells in the panel, current is forced through the reversed cell. In this scenario, the reversed cell absorbs the power produced by the other cells in the panel, generating heat and creating a hotspot within the solar panel.

The amount of power that is absorbed by a single reversed cell is disproportionate to the amount of power a single cell can generate. A single cell may only produce ½ volt of potential energy, but can absorb 6-8 volts when it has gone into reversal.

If unchecked, a solar cell left in this state can easily be destroyed. The hot spot generated by the blocked cell can very quickly reach dangerous temperature levels too, and amateur home-built solar panels have been known to burst into flames precisely for this reason.

Thankfully, solar panel manufacturers have a solution to this problem in order to avoid the panel itself becoming damaged. All professionally manufactured crystalline solar panels have in- built protection to route power around a string of cells where one or more of the cells are in reversal.

If your solar panel has only a single string of solar cells within it, this effectively means the entire solar panel is bypassed and produces no power at all. If your solar panel has two strings of solar cells, it means the power output of the panel is halved.

Because a solar array is typically put together by installing multiple solar panels in series, the effects of shading on just part of one panel impacts the performance of the entire solar array.

Types of obstruction

Shade can be broken down into two categories: soft shade and hard shade.

A soft shade is a distant obstruction where the shadow is dispersed or diffused, which significantly reduces the amount of light reaching the solar cells. A shadow from a tree would be classed as a soft shade.

Hard shade is an obstruction that blocks out light from reaching the solar cell completely. Bird droppings, fallen leaves or a tree branch sitting on top of the glass would be classed as a hard shade.

Where cells are soft shaded, you will see a significant drop off in energy production. Effectively, the production of the entire array will drop down to a similar level to a dull day.

Where cells are hard shaded, the production of the entire array will drop down to the same level as the affected cells: if the cells are covered completely, you may see a complete power shutdown (depending on how shade-tolerant your solar panels are). If the cells are only partially covered, you will see a significant drop in energy production.

Designing shade-tolerant solar systems

If you cannot avoid shade in your solar energy system, the solution is to design a shade-tolerant system. In other words, you have to design your system in such a way that the effect of shade on any one part of your system has as small an effect on the overall array as possible.

Designing shade-tolerant solar arrays is a complex subject and a specialist area even amongst solar design experts. There are entire books written on this subject alone. Consequently, it is not possible to cover the whole subject here. However, it is possible to design a basic solar energy system with a reasonable level of shade tolerance without a huge amount of specialist knowledge.

If you need to design a system that will continue to perform well in partially shaded conditions, there are options available to you:

Track the shade

First, you should never design a system that will have to cope with hard shading. Solar panels should be mounted so that they are not covered in leaves and so they can be inspected and cleaned of any hard obstructions if necessary. If there is a permanent obstruction that will be creating a hard shade, you should not be installing solar panels in that particular location.

If you have a soft shade, for how much of the day does this shade your panels? Remember, even a soft shade over a small area can have a big impact. Typical core power generation times for solar energy during the summer are three hours either side of solar noon (i.e. between 9am and 3pm, if your time zone equates to solar time). If you have shading either before this period or after it, you will lose around 20% of your capability in the summer, or 40% of your capability if you have shading in both the early morning and late afternoon.

During the winter, the difference is not so great. Because the sun is lower in the sky and the intensity of the sunlight is significantly lower, almost all of your power is generated during the core power generation times. If you have shading before 9am or after 3pm during the winter, you will probably be losing only around 5-10% of your generating capabilities.

However, if you are suffering from shade within the core power generation times, you are going to severely compromise the performance of your system. The exact impact on the performance of your system will vary according to your location, the severity of shading and the type of solar panels you are using, but in general terms, this is how much of your solar energy production you can expect to lose from one hour of shading during core power generation times:

This table shows the approximate performance loss from your system if it is shaded for one hour during core production times

As you see, you can lose a significant amount of energy production due to shading during the middle of the day. This is why shading during the middle of the day is the number one reason for solar arrays failing to live up to expectations.

However, because you can now quantify the impact of the shading, it is possible to do something to counter the effects.

Increasing the number of solar panels

The first option is the most obvious: if you have the space, you can increase the number of solar panels you install in order to counter the periods of shade.

This is not always possible, either because of space or cost restrictions. Also, it is not always the most efficient way of getting around the problem.

Panel orientation

If shade affects you at a particular time of the day, consider angling your solar panels away from the obstruction. This will increase their effectiveness during the unobstructed parts of the day and reduce or remove the impact of the obstruction in the first place. The reduction in power generation from angling the panels away from due south is often less than the impact on shading if you can eliminate the shade problem altogether.

Choice of solar panel

Another option is to choose amorphous (thin-film) solar panels. Amorphous solar panels do not suffer from cell reversal in the same way that crystalline solar panels do, and consequently provide far better shade tolerance.

Because of their lower efficiency levels, you will need to take into account that you will require around twice as much physical space to install amorphous solar panels. If space is not an issue, using amorphous solar panels is likely to be the simplest and most cost-effective solution for stand-alone solar installations.

Sharp, Mitsubishi, Uni-Solar, Solar Frontier and Sanyo now manufacture high-quality amorphous solar panels that offer excellent performance and reliability.

Use micro-inverters

If you are building a grid-tie system, possibly the best solution is to use micro-inverters, where each panel has its own power inverter and is a full solar energy system in its own right.

With a micro-inverter system, each solar panel runs entirely independently of every other solar panel. If shade affects one panel, none of the other panels is affected in any way.

Using micro-inverters also means that you can have solar panels facing in different directions from each other. For instance, if you are installing solar panels on a roof and your roof has multiple pitches and angles, you can choose to install your solar array on more than one part of the roof using a micro-inverter system.

This approach gives you greater flexibility as to where you mount solar panels: it may even be possible to avoid your shading issues altogether.

Design a parallel solar array

By connecting solar panels in parallel rather than in series, you can reduce the overall effect of shading on just one or two parts of the array. In a parallel system, when one solar panel is in shade, the power outputs of other panels in the array are not affected.

If you are designing a grid-tie solar energy system and want a high-voltage system, it is possible to achieve this with a parallel array by using high-voltage grid-tie specific solar panels. A number of manufacturers now offer these panels, often producing over 100 volts from a single panel. Other panels are available that provide either 24-volt or 48-volt output, and these can be suitable for larger stand-alone solar energy systems that require more than 12 volts.

Design a multi-string solar array

Instead of designing your system to have just one set of solar panels connected in series, you can design your system to have multiple series of solar panels. Controllers and inverters are available that allow you to have multiple strings of solar panels, or you can have a controller or inverter for each individual string.

In effect, this means you end up with two smaller solar power systems, rather than one. This may mean you can face your two different sets of solar arrays at different angles, giving you the opportunity to mount them in entirely different locations if you so wish.

In this scenario, you can design your system so that partial shading will only affect one of your strings rather than your entire array. As with micro-inverters, such an approach may even allow you to avoid the shading issue altogether.

Other options

There are other options, but the designs can become extremely complicated and are really in the domain of highly specialist solar designers. If you are designing a shade-tolerant solar energy system and the above options will not work for you, it is time to call in a solar shading specialist.

Unless you have significant shading issues, you can usually design around the problems using one of the options I have suggested here, or by using a combination of options to come up with a workable solution. Over the past few years, I have designed a number of shade-tolerant systems, including systems that are designed to work in an entirely shaded environment all year round. With careful planning, it is often possible to overcome the issues.

If all else fails...

Sometimes it is not possible to design around shading problems. In this scenario, it requires a rethink of what you can achieve using solar.

This is an issue that I have with my own home. My house is on the edge of ancient woodland. For most of the year, my home is almost entirely shaded by the tall trees that surround it. Even in the height of summer, the sun does not reach my garden until mid-afternoon.

This shading means I could never run my own home completely from solar power. However, I have designed a smaller solar energy system that provides me with enough energy to run my lighting and provide backup power in case of a power cut.

I have achieved this by installing amorphous solar panels onto the back of my home, facing south-west to catch the afternoon and evening sun. Despite receiving very little sunlight, this system provides enough electricity to provide me with my lighting and backup power requirements throughout the year, even in the depths of winter.

In conclusion

· Shade is a big problem for solar energy - especially if using crystalline solar panels

· Even a very small amount of shade can have a big impact on your system

· Amorphous solar panels also suffer in shade, but not to the same extent as crystalline panels

· If you have shade during core power generating periods of the day - typically the three hours either side of solar noon - you will lose a very significant amount of your potential power generation

· There are things you can do to reduce the impact of shading, either through your choice of materials or your system design

· In some cases where you have very significant shading issues, you may need to reconsider what you can realistically achieve with solar