THE DIFFERENCES BETWEEN ALTERNATING CURRENT AND DIRECT CURRENT

Alternating current and direct current are the two types of electric current that we encounter in particular for the transport and distribution of electricity, up to the consumption of electricity in our daily lives. However, alternating current and direct current do not have the same characteristics or the same applications. The focus of this article is on the specificities of these two electrical currents, the type of current used in daily life (heating, lighting, domestic hot water, etc.), and the possible impact on the price of an electricity offer.

Operation of alternating and direct currents

Alternating current and direct current come from two different modes of production. But before detailing these points, it is useful to remember the definition of electric current.

Principle of electric current

What we call electricity or electric current is the energy produced by the movement of electrons (carriers of mostly negative electric charges) within a conductive material. As a reminder of our (sometimes distant) physics lessons, all matter is made up of atoms, themselves made up of electrons and a nucleus. The nucleus is made up of protons (positively charged) and neutrons (neutral charges); the movement of electrons, caused under the influence of an energetic voltage, produces electric current.

The speed and direction of movement of electrons determine two types of electric current: direct current and alternating current.

What is direct current?

Direct current (DC) consists of an electric current where the movement of electrons is continuously in one direction – from negative to positive. The speed of movement of electrons is several meters per hour.

Inside a closed electrical circuit, direct current is produced from the chemical activity of a cell battery, or even photovoltaic panels. The typical example is the case of an electric lamp:

the electric lamp is connected to a switch, via which the electrical circuit is closed;

by flipping the switch to “on,” the battery produces a chemical reaction, a reaction that produces electrons. These circulate out of the negative terminal of the battery, and then move via a conductor (metal rod);

the movement of electrons on the filaments of the bulb creates heating of said filaments, which results in the creation of light;

the electrons finish their movement by returning to the battery, via the positive terminal.

Direct current, which was promoted by Thomas Edison at the end of the 19th century as the first use of electricity in the majority of everyday activities (lighting, heating, transport), gave way over time and alternating current experiments.

What is alternating current?

Alternating current (AC) is an electric current in which the movement of electrons inside a conductive material occurs in one direction, then in the other, alternately and periodically.

Alternating current is produced from a turbine and an alternator driven by primary energy.

Knowing that this primary energy can come from  :

either from sources of mechanical energy from renewable energies such as wind energy from the force of the wind, hydraulic energy produced by the movement of water, biomass energy from the production of biogas from organic waste, etc. ;

either thermal energy from water vapor produced during the combustion of fossil fuels (coal, natural gas, or oil) in thermal power plants;

or nuclear energy from the fission of the atom in nuclear power plants.

Regardless of the type of primary energy used, the alternating current production process is used in the same way, in a power plant:

the primary energy used turns a turbine, which drives in its movement the axis on which the alternator rotor is fixed;

the rotor of the alternator, composed of a series of electromagnets, rotates on itself creating a sinusoidal movement of the electrons;

the movement created causes the electromagnets of the rotor and the stator (made up of coils of copper wires) of the alternator to interact;

this interaction creates the alternating electric current;

the electricity produced is consumed immediately because it cannot be stored.

The speed of the alternator rotor will determine the frequency of the alternating current expressed in hertz (Hz); or the change in direction of the electric current in one second. In France in particular, alternating electric current is called periodic and sinusoidal current. It is 50 Hz, which means that it has made 50 alternations per second (i.e. 100 times/second, 50 times positive, and 50 times negative). The movement of electrons, unlike direct current, is extremely long, a few millimeters per second.

Direct current VS alternating current: for what uses?

Direct current and alternating current have distinct characteristics which give them unique uses:

direct electric current is reserved today for less universal uses than in its beginnings. It is used for the transport of electricity on very high voltage lines (between 225,000 and 400,000 V) over long distances, or when transporting electricity via submarine cabling. However, direct current is not completely excluded from our daily lives, since it is found in the majority of battery-powered electrical devices such as electric lamps, telephones, and laptop computers, as well as electric cars which are powered by different batteries. In addition, the development of renewable energies allows useful and practical production of direct current: solar energy makes it possible to produce photovoltaic energy in the form of direct current. This direct current is then transformed into alternating current, via an inverter, to be injected into the electricity distribution network;

alternating electric current is much more “manipulatable” than direct current, its voltage as well as its intensity can be lowered via electric transformers, without too much energy loss (expressed in Joules); which makes it the preferred electric current in the transport and distribution of electricity via high voltage lines and low voltage lines (230 V to 400 V). Alternating current is, generally speaking, the type of electric current suitable for our domestic uses since it is commonly used for heating, lighting, cooking, and all types of use of electrical appliances.

Alternating current: single-phase or three-phase?

Now that we have made the distinction between direct current and alternating current; Let's take a closer look at alternating current, since it is the type of electric current used mainly in our homes.

Principles of phases and neutral

Alternating current circulates in your home, via two power supply modes: single-phase current and three-phase current. These two alternating currents are defined by the principle of phase and neutral, unlike direct current which is determined by the positive terminals and the negative terminals.

But what exactly do we call phase and neutral?

The phase is the electrical cable used to transport electricity to the point of consumption. It provides the voltage necessary for the operation of an electrical device.

The neutral is the electrical cable which allows its good distribution. As its name suggests, its tension is zero. The neutral is mainly used to send alternating current back to the electrical circuit and the general network.

Differences between single-phase current and three-phase current

Single-phase current uses a phase and a neutral, or two wires inside a cable. It allows the entire home to be supplied with electrical energy via a single electrical circuit connected to a single-phase meter. Single-phase current is defined by a voltage difference of 230 V between phase and neutral. This voltage difference is particularly suitable for powering most electrical appliances for domestic use. In single-phase current, electrical outlets are of the classic type with two holes and a ground pin. This type of alternating current is the most used/installed in France, via the single-phase meter.

Three-phase current uses 3 phases and a neutral. Its particularity is to be able to absorb significant electrical loads, as well as distribute the electric current via these 3 phases throughout the home. Three-phase current is defined by a voltage difference of 230 V between each phase and neutral, but also by a voltage difference of 400 V between 2 phases. It is used in high voltage lines managed by Enedis in the distribution of electricity, but also in individuals via the three-phase meter, for very specific needs (use of energy-consuming devices such as heat pumps, need for a connection power equal to or greater than 18 kVA, consumption point too far from the electricity meter). In three-phase current, electrical outlets are of the type with three holes (the three phases) and an earth rod.

Choose your connection power and meter power

At this stage of the article, the question legitimately arises as to how the principle of single-phase current or three-phase current is decisive about its electricity consumption? Quite simply because the choice between single-phase or three-phase will determine a connection power and a metered power. Explanations:

the connection power of your home defines the maximum power that your electrical installation can support. This connection power is generally defined by Enedis during the construction of the home and its electrical connection to the distribution network. Two power levels are available: 12 kVA in single-phase and 36 kVA in three-phase; knowing that for classic domestic use, the power of 12 kVA is more than sufficient and that the power of 36 kVA is rather reserved for significant energy needs ( professional oven type, installation of a heated swimming pool or a heat pump) ;

the power meter which defines the maximum possible power, when using all the electrical appliances in the home simultaneously. Included in a power range between 3 kVA and 36 kVA for individuals and small professionals, the meter power must be lower than the connection power. The choice of meter power is determined by the energy supplier when taking out an electricity subscription, based on your energy needs. Please note, that the greater the power of the meter chosen, the higher the amount of the subscription will be; Conversely, if you subscribe to a meter power lower than your needs, you expose yourself to frequent micro-power outages or risks of tripping.

What are the consequences of switching from single-phase to three-phase?

In certain circumstances (such as high energy needs), it may be useful to switch from single-phase to three-phase. However, it must be understood that this transition from one current to another is not as easy as changing the tariff option with your energy supplier, where a simple request by telephone is generally sufficient.

This modification requires several mandatory interventions  :

to change the private installation of the home's domestic network, and for this, to call on an electrician at his own expense. Because, if the three-phase current allows a good distribution of the electrical load when the electrical installation is in order; otherwise, it exposes you to numerous inconveniences. A simple imbalance between phases can cause untimely disjunctions. To avoid them, it is essential to ensure that this distribution is equal, for identical electricity consumption between each phase;

to contact your energy supplier, who will dispatch Enedis, the network manager, to modify the connection power and any work to replace the circuit breaker. The average cost of the intervention: is €160.18 including tax (Source: Enedis 2020 catalog ).

Choosing a three-phase or single-phase electricity supplier

The supply of electricity, whether connected to single-phase or three-phase current, is provided by identical energy suppliers. No special three-phase meter or single-phase meter energy offer exists at this level and does not develop specific prices.

You can choose your electricity subscription from all the existing electricity offers on the energy market. The only difference between a single-phase meter and a three-phase meter is in the power subscribed, and therefore inevitably the cost of your subscription. And this will impact the amount of your electricity bills.

As a reminder, the subscription price corresponds to the fixed part invoiced by each energy supplier. However, you can act on the variable factor – the price of your electricity consumption (price per kWh excluding VAT) – by choosing the cheapest energy offer on the market.

To do this, you can use an electricity comparator such as that of Choisir.com. Free and easy to use, the comparator allows you to establish a list of electricity offers corresponding to your consumption profile.

The prospects for direct current

Although reserved today for certain uses, direct current has not said its last word, particularly for the years to come in areas such as renewable energies, energy transition, and digital technology  :

in fact, direct current which allows the transport of electricity over large distances via very high voltage networks can be a solution of choice for the development of renewable energies. This is not done by alternating current, whose energy losses during transport can be significant. Take for example the exploitation of wind and solar sectors currently produced and consumed locally. Direct current could be a key ally in the development of these sectors, by ensuring the transport of the electricity produced from one end of a territory to another. The construction of very high-voltage lines has proliferated almost everywhere in Europe for around ten years;

the all-digital trend, where most equipment developed (more than 75%) and placed on the market (mobile phones, laptops, tablets TVs, etc.) uses direct current. For them to be usable, it is necessary to convert alternating current into direct current. Which leads to unnecessary energy consumption; consumption which could be avoided if the current was continuous from production;

the development of LED lighting (particularly for public lighting), using direct current (DC 48 V) is also a formidable lever for its presence throughout the world. The advantage of using LEDs is to do without transformers installed for current lighting. Very energy-intensive and expensive to purchase transformers;

the development of direct current in buildings can allow energy savings of around 25% (less wiring for the electrical installation, particularly for the riser, less electrical equipment).

Will direct current replace alternating current in the years to come? Nothing is less certain, as the costs of replacing infrastructure (wiring, networks, and private electrical installation) would be significant. However, it is more possible and consistent to have a mix of use between the two types of current.