Chapter 4

Introduction

By now, we've found the best roof, chosen the ideal orientation and selected an appropriate tilt angle. The design appears perfect.

Then you notice a small chimney casting a shadow across the corner of one solar panel for an hour each afternoon. Surely such a small shadow can't make much difference. Surprisingly, it can.

Unlike many electrical systems, solar panels don't simply lose power in proportion to the amount of shade they receive. Because the solar cells inside a panel are electrically connected together, shading just a few cells can reduce the output of a much larger part of the panel. In some situations, shading can even cause individual cells to overheat and suffer permanent damage.

Understanding how shading affects solar panels is therefore one of the most important aspects of solar system design. In this chapter, we'll discover why seemingly insignificant shadows matter so much and explore the engineering solutions that help minimise their impact.

Why Shade Is Different From Dirt

Not every reduction in sunlight affects a solar panel in the same way. A cloudy day reduces the amount of sunlight reaching every solar cell fairly evenly. Likewise, a thin layer of dust or pollen usually decreases the output of the entire panel by a relatively small amount. Shading is different.

Instead of reducing the sunlight across the whole panel, a shadow often covers only a small group of cells. This creates an imbalance between shaded and unshaded cells, and because those cells are electrically connected together, the effect spreads beyond the shadow itself.

Engineers often distinguish between soft shading and hard shading.

Soft shading occurs when clouds, haze or atmospheric conditions reduce the intensity of sunlight across the entire panel. Every cell receives less light, but they continue behaving in much the same way.

Hard shading occurs when an object such as a chimney, tree branch, antenna or neighbouring building blocks sunlight from only part of the panel. It is this type of shading that causes the greatest electrical problems.

Shading can also be either permanent or temporary. A nearby building may cast the same shadow every day, while a tree may only shade the roof during certain seasons or times of day. Understanding how these shadows move is one of the reasons site surveys consider the Sun's position throughout the entire year rather than at a single moment.

Why One Shaded Cell Can Affect Many Others

At first glance, it seems reasonable to assume that shading 10% of a solar panel should reduce its output by roughly 10%.

Unfortunately, solar panels don't work that way.

As we discovered in Part 3, the individual solar cells inside a panel are connected in series. In a series circuit, the same electrical current must flow through every cell.

Imagine a line of people passing buckets of water from one person to the next. If one person slows down, everyone behind them must also slow down because the buckets cannot move faster than the slowest person in the chain. A series-connected solar panel behaves in much the same way. When one cell becomes shaded, it produces less current than the surrounding cells. Because every cell in the series string must carry the same current, the shaded cell effectively limits the performance of all the other cells connected with it.

This is why a shadow covering only a small part of the panel can reduce the electrical output of a much larger section. The electrical connections inside the panel, rather than the size of the shadow itself, determine the extent of the power loss.

When Shade Becomes Heat

Reduced power isn't the only problem caused by shading. Under certain conditions, a heavily shaded solar cell can begin consuming electrical energy instead of producing it. The surrounding illuminated cells continue generating current and force that current through the shaded cell. Rather than acting like a tiny generator, the shaded cell begins behaving more like an electrical resistor.

Instead of converting sunlight into electricity, it converts electrical energy into heat. These highly localised regions of excessive heating are known as hot spots. Hot spots can become surprisingly hot, placing the silicon cell and surrounding encapsulation materials under considerable thermal stress. Repeated heating and cooling can accelerate ageing, reduce performance and, in severe cases, permanently damage the panel.

Protecting against shading is therefore about more than maintaining energy production. It is also about protecting the long-term reliability of the solar installation.

How Bypass Diodes Protect the Panel

Fortunately, solar engineers have developed an elegant solution. Hidden inside the junction box on the back of most solar panels are small electronic components known as bypass diodes. Their purpose is exactly what their name suggests. If part of the panel becomes heavily shaded, the bypass diode provides an alternative route for the electrical current. Instead of forcing current through the shaded cells, some of the electricity flows around that section of the panel.

The shaded group of cells contributes little or no power, but the remaining sections continue operating much more effectively.

Most modern solar panels contain several bypass diodes, with each diode protecting a different group of cells.

Although bypass diodes cannot eliminate power losses caused by shading, they dramatically reduce their severity while also helping prevent damaging hot spots from developing.

It is a remarkably simple solution to what would otherwise be a serious reliability problem.

Designing Around Shade

The best solution to shading is usually to avoid it altogether. Before installation, designers carefully assess the site to identify anything that could cast shadows across the array throughout the year.

This includes nearby buildings, chimneys, roof structures, television antennas, trees and even neighbouring properties that may influence sunlight during winter when the Sun remains lower in the sky. Where possible, the panel layout is adjusted to avoid these obstacles. Sometimes moving the array only a short distance across the roof can significantly improve its long-term performance.

On more complex roofs, designers may also divide panels into separate electrical strings or use technologies such as power optimisers and microinverters. Unlike a conventional string inverter, these devices allow individual panels to operate more independently, reducing the impact that shading on one panel has on the rest of the system.

These technologies do not create additional sunlight. They simply allow the available sunlight to be used more effectively.

Small Shadows, Big Consequences

One of the biggest surprises in solar engineering is that the smallest details often have the greatest consequences.

A shadow cast by a television antenna.

A chimney covering just a handful of cells. A nearby tree that shades the roof for only part of the afternoon. Each may appear insignificant. Yet because of the way solar cells are electrically connected, these small shadows can reduce energy production far more than most people expect.

This is why experienced solar designers spend so much time analysing shade before a single mounting rail is installed.

Good solar design is not simply about finding the sunniest roof.

It is about finding the roof that remains sunny for the greatest proportion of the day, throughout every season of the year.

Sometimes, removing a small amount of shade can increase annual energy production more than upgrading to a more efficient solar panel.

Understanding shading isn't just an interesting piece of photovoltaic physics. It is one of the keys to designing a reliable, high-performing solar power system.

Looking Ahead

We've now chosen the best location for the solar array and learned how to minimise one of the biggest causes of lost performance.

The next step is selecting the equipment itself.

Modern solar panels, inverters and batteries may look similar, but they differ significantly in efficiency, performance, durability and functionality.

In the next chapter, we'll explore how engineers choose the right combination of equipment to match the customer's needs, the site conditions and the overall system design.