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The advantage of using DC electricity

Home » The advantage of using DC electricity

We make electricity in two forms: alternating current (AC) and direct current (DC). Each form has its disadvantages and advantages in the way it is made and how it can be used.  The important advantage of AC electricity is that it is easier to transport over long distances.

Mains electricity is AC because it comes to us over long distances from the large centralised generating stations spread throughout Australia.  For independent power plants this advantage is not important because the electricity doesn’t move very far to get to where it is used; for independent power plants the advantages of DC electricity are more important.

Storing Electricity

The biggest advantage of DC electricity is that it is easier to store than AC electricity, especially on a small scale. Storing electricity when it is made, for use later when it is needed, is a critical concept for a hybrid independent power plant. When electricity is stored we can use it when we need it, and storing it increases its density, so we can take as much power as we want to operate high-powered equipment.

When we store electricity, we can turn the generator set off most of the time which saves fuel, and we can use electricity when our renewable sources such as solar cells are not providing electricity. The best way to store electrical energy for relatively small-scale applications is to use rechargeable storage batteries.

We can retrieve electricity from a storage battery bank silently, efficiently, and immediately when we need it. Storage batteries require DC electricity, so to get the advantages of storing electricity in a storage battery bank, we must generate DC electricity.

Providing AC Mains Electricity

In practice, we ultimately need to provide mains-style AC electricity because most of the equipment that we want to use is designed to run on mains-style electricity. An important advantage of using DC electricity for an independent power plant is that AC mains electricity can be generated twice as efficiently by using a diesel DC generator and converting the DC electricity to mains-style AC electricity, than by generating mains style AC electricity directly using a petrol AC synchronous generator.

Variable Engine Speed

An AC synchronous generator must turn at a single specific speed to control the characteristics of the mains-style electricity that it generates; if more power is needed, the engine cannot be sped up; if only a little power is needed the engine cannot be slowed down.  

For a DC generator, the characteristics of the mains-style electricity are determined by the DC to AC conversion process, so the turning speed of a DC generator doesn’t matter, and the driving engine can turn at a range of speeds.  This means that if a lot of power is needed the maximum potential of the engine can be used by running it at its maximum speed; if only a little power is needed the engine can be slowed down, saving fuel and wear, and making less noise.

Providing the alternator’s magnetic field

Because the turning speed of an AC synchronous generator can’t be changed to match a changing electrical load, the generator’s output must be controlled by changing the strength of the magnetic field in its alternator.

To make the magnetic field controllable it must be produced using an electromagnet, called a field winding, which uses some of the electricity made by the generator.  Because a DC generator can be controlled by varying its speed, its alternator can use permanent magnets to create its magnetic field, and so doesn’t lose any of the electricity that it makes to the field winding. Variable speed and no field winding make a DC generator much more efficient than a synchronous AC generator.