Chapter 4

Introduction

Over the past three chapters, we've discovered what electricity is, how engineers describe it and why resistance affects its flow. But electricity has another important characteristic that we haven't yet explored.

Sometimes electrons travel steadily in one direction. At other times, they repeatedly reverse direction many times every second. These two forms of electricity are known as direct current (DC) and alternating current (AC).

Although both transport electrical energy, they behave very differently and are used for different purposes. Understanding the difference is essential because solar panels naturally generate one type of electricity, while our homes and the electricity grid rely on the other.

Direct Current - One Direction

Direct current, usually shortened to DC, is electricity that flows continuously in a single direction. Rather than constantly changing course, the electrons move steadily around the circuit, carrying energy from the power source to the electrical load.

A useful analogy is a river. The water continually flows downstream in one direction, carrying everything with it. Electricity behaves in much the same way. Once the flow has started, the electrons continue travelling in the same direction for as long as the circuit remains complete.

Solar panels naturally generate direct current because the electric field inside each solar cell continually pushes electrons in one direction. Batteries also produce direct current, making them ideal for storing the electricity generated during the day. Many electronic devices, including laptops, mobile phones and LED lighting, also operate internally using direct current.

Alternating Current - Constantly Changing Direction

Alternating current, or AC, behaves very differently. Instead of flowing continuously in one direction, the electrons repeatedly reverse direction many times every second.

In Australia, this happens 50 times every second, giving the electricity a frequency of 50 hertz (Hz).

At first, this might seem strange. If the electrons keep changing direction, how can energy ever reach our homes?

A useful analogy is shaking one end of a rope. The rope itself doesn't travel across the room. Instead, a wave moves along its length, carrying energy from one end to the other.

Alternating current behaves in a similar way. Although the electrons move backwards and forwards over very short distances, the electrical energy continues to travel efficiently through the circuit.

Why Does the Grid Use AC?

If solar panels naturally produce direct current, why doesn't the electricity grid use it as well? 

The answer lies in efficiency.

Whenever electricity travels through a cable, a small amount of energy is lost as heat because of resistance, as we discovered in the previous chapter. The longer the cable and the higher the current flowing through it, the greater these losses become. If power stations simply generated electricity at low voltages, enormous amounts of energy would be wasted before it ever reached our homes.

One solution is to transmit electricity at a much higher voltage. For the same amount of power, increasing the voltage means the current can be greatly reduced. Since heat losses increase rapidly with current, transmitting electricity at high voltage makes long-distance power transmission far more efficient.

This is where alternating current has a major advantage: its voltage can be easily stepped up or down using transformers, allowing electricity to be transmitted efficiently at very high voltages.

Alternating current is constantly changing direction, causing the magnetic field around a wire to continually grow, collapse and reverse. This changing magnetic field is exactly what a transformer needs to operate.

A transformer consists of two coils of wire wrapped around a shared iron core. When alternating current flows through the first coil, it creates a changing magnetic field inside the iron core. That changing magnetic field then induces a new voltage in the second coil. By changing the number of turns in each coil, engineers can either increase the voltage (a step-up transformer) or decrease it (a step-down transformer).

Direct current behaves differently. Because it flows steadily in one direction, it produces a magnetic field that quickly becomes constant. Once the magnetic field stops changing, the transformer can no longer induce a voltage in the second coil. In other words, after the electricity is first switched on, a transformer powered by direct current effectively stops working.

This ability to easily increase and decrease voltage made alternating current the perfect choice for electricity transmission. Power stations can generate electricity, step the voltage up to hundreds of thousands of volts for efficient transmission across the country, and then step it back down several times before it finally reaches our homes at around 230 volts.

For more than a century, this simple but powerful idea has allowed electrical grids to deliver vast amounts of energy over hundreds of kilometres with remarkably small losses. Although modern high-voltage direct current (HVDC) systems are now used for some specialised applications, alternating current remains the worldwide standard for distributing electricity to homes and businesses because transformers make it practical, efficient and economical.

Why Solar Panels Produce DC

Unlike a power station, a solar panel has no need to generate alternating current.

Inside every solar cell, the P–N junction creates a permanent electric field. When sunlight strikes the cell, photons transfer their energy to electrons, allowing them to break free from their atoms. The electric field immediately pushes these electrons in a single direction, creating a continuous flow of electricity.

As long as sunlight continues to shine on the panel, this process repeats millions of times every second. Because the electric field always points in the same direction, the electrons are continually pushed the same way, naturally producing direct current (DC).

This makes solar panels perfectly suited for charging batteries, which also operate using direct current. Many modern electronic devices also run internally on DC, even if they are plugged into a standard household socket.

The challenge comes when we want to use solar electricity throughout our homes. Almost every household appliance is designed to operate using alternating current (AC) because that is the form of electricity supplied by the national grid. Lights, kettles, washing machines and air conditioners all expect an AC power supply.

Somewhere between the roof and the power socket, the direct current produced by the solar panels must therefore be converted into alternating current before it can safely power our homes or be exported back to the electricity grid.

Looking Ahead

We now understand that electricity comes in two forms.

Solar panels and batteries naturally produce direct current, while our homes and the electricity grid rely on alternating current. Both transport electrical energy, but each is suited to different applications.

This raises an obvious question. How does the electricity generated on a rooftop become the alternating current needed to power our homes?

The answer lies inside one of the most important components of every modern solar installation: the solar inverter.