The physics of electricity

QUESTIONS FOR ENERGY DEMOCRACY: What are the characteristics of electricity systems which every energy system will need to take into account? 

Electricity is particularly important for a sustainable energy system, in fact sometimes when people say energy they are actually talking only about electricity. Electricity is very versatile, and renewable energy resources typically produce electricity rather than heat or fuel. So many sustainable energy scenarios involve a shift from fossil fuels to electricity for heat and transport.

Note again that electricity is an energy vector or carrier, rather than a primary form of energy, as discussed in the physics of energy page.

Additionally, electricity has some specific technical challenges. It can only be stored by converting it – for example to chemical energy in batteries, or gravitational potential energy in pumped storage. Both involve energy losses and expensive infrastructure. The instantaneous supply of electricity either by new generation or conversion, and the instantaneous demand must be balanced at every moment. The entire national grid is electrically connected, and changes in one location are almost instantaneously felt everywhere.  When supply matches demand voltage and frequency in the network stay the same. One of the key jobs of the electricity network operator is to keep voltage and frequency constant.

Some approaches to energy democracy involve decentralisation of our electricity systems. Technical innovations that should allow this include advances in battery and other storage technologies, smart monitoring and control systems such as active network management and smart meters.  Understanding the pros and cons and challenges of decentralisation is important, starting with basic electrical concepts.

Some of the description below involves equations. For some people, this is really helpful for understanding. If you find equations intimidating, don’t worry, it’s not essential to understand it all. Hopefully the key points and implications for the energy system will come across even if you skip through the equations.

Basic electrical concepts

Electricity is the movement of charged particles called electrons. Electrons will flow from a point of higher electrical potential to a point of lower electrical potential, a bit like water flowing down a hill.  In a battery powered circuit, the two terminals of the battery have different electrical potentials, and electricity flows from the higher to the lower potential.

Electricity is generated by moving a conducting mass (using mechanical energy) close to a magnet, via electromagnetic induction. This usually involves a spinning coil of wire – which produces AC (alternating current) in a smooth sine wave (see below). The only other major source of electricity generation is solar photovoltaics, which converts sunlight into electrical energy directly with no moving parts.

In large electrical power systems, such as the UK electricity grid, there are several terms that are important to understand:

Charge: When talking about electricity, we are interested in the charge carried by electrons, which causes the physical effects of electricity, rather than the number of electrons in itself. Charge is measured in Coulombs. Each electron carries 1.6×10-19 C of charge. So 1 Coulomb is a lot of electrons!  Charge is usually represented in electrical equations by the letter Q.

Voltage: this is the ‘electrical potential’, or the amount of energy that a unit of charge would release in moving from its current position to the ground – analogous to height, or a pressure difference.   If you dropped one coulomb of charge (6.25×1018 electrons) from a ‘height’ of one Volt, this would release one Joule of energy. Voltage is usually represented in electrical equations by the letter V. You may have seen that the electricity supplied to the appliances in your home is at 230V.  Battery-operated off-grid systems are typically at 12V.

Current: this is the flow of charge, analogous to the speed of water flowing in a pipe. We measure current using the Ampere (A), usually abbreviated to Amp. If 1 Coulomb of charge flows per second that is equal to 1 Amp of current. Current is usually represented in electrical equations by the letter I. A very high current can be dangerous. That is why fuses cut off the flow of electricity  when the current goes above a safe level. The plugs of household appliances usually have fuses rated between 3 and 13 Amps, depending on the appliance. The service cable and fuse box where electricity comes into your house typically cuts the house off if the current drawn is above 60-100A.

Power: as discussed in relation to energy generally, power is the rate of flow of energy, measured in Watts.  When charge moves across a voltage difference, this is a flow of energy. Power is usually represented by the letter P. The rate of flow of charge is current, so the rate of flow of energy is the current times the voltage.

P= V x I

At the mains voltage, 230V, the power that would make a 13A fuse blow is:

230V x 13A = 2,990W, or nearly 3kW

(see ‘physics of energy‘ section for explanation of W and kW)

The power that would trip the fuse for the whole house is:

230V x 80A= 18,400W or about 18kW

Resistance: this is a measure of the difficulty of moving electricity through a material.  It depends on both the type of material used, and the width of the wire. Resistance is measured in Ohms (Ω), and is usually represented in electrical equations by the letter R. The resistance causes energy to be lost as heat – not usually what we want.

Losses from transmission of electricity in wires

One of the reasons often given for distributed (local) generation is to avoid the loss of energy from transporting electricity over long distances in cables. The amount of energy lost (converted to heat) every second, or power lost, depends on the resistance of the cables. It is given by the following equation:

P(loss)=I²x R

where I is the current through the cable and R is the resistance of the cable.

To minimise power loss, we can reduce the resistance, by:

  • choosing a material with low electrical resistance, such as copper wire
  • making the wire or cable thicker (but it gets heavy and uses a lot of material)
  • reducing the distance – length of cable

And we can minimise the current, which will have a bigger impact, as the power loss is related to the square of the current. This means that small changes in current have a surprisingly big impact on the amount of power lost.

Remembering the equation for power:

P= V x I

If we want to transmit a given amount of power across the cables and to minimise the current, this can be achieved by increasing the voltage.

For those who like equations, I = P/V , so P(loss) =I ² x R = P²/V² x R – so the loss in power is proportionate to the square of the voltage. This means that maximising voltage makes a big difference to the efficiency of the transmission of electricity. However, high voltages are dangerous, and so voltages in homes and appliances are much lower than those used in the transmission of electricity.

Direct current and alternating current

The discussion above presents a simple view of electricity flowing in one direction like water in a pipe. This is Direct Current (DC). But the electricity system, including what comes from the socket, uses Alternating Current (AC). Electrons vibrate back and forth instead of just going in one direction down a wire. The number of times they change direction in a second is the frequency measured in Hertz (Hz).

DC systems have advantages, but AC is used in the mains electricity system because the voltage of AC electricity can easily be changed using transformers, allowing efficient high voltage transmission and safe low voltage consumption. There was intense debate about whether AC or DC was better during the early days of electricity – the war of the currents, between Thomas Edison and Nikola Tesla in the late 1880s.

Although the electricity in the mains electricity system is AC, many electrical devices, particularly computers and other electronic devices, use DC power. Electricity is converted from AC to DC in the power pack in your laptop or mobile phone charger.  Any device with a charger that is more than a simple plug with a fuse, and that heats up when it is plugged in, is probably DC. People who live in boats, vehicles or off-grid houses, usually use a 12 Volt battery. There are plenty of guides available online for people who want to build DIY 12 Volt systems. Batteries are DC electrical systems. When we convert between AC and DC, there is always a loss of energy.

Additionally, solar photovoltaic panels (solar PV) generate DC electricity. To connect them to the grid, the electricity must be converted to AC using an ‘inverter’.

Key concepts for alternating current, or AC systems:

There are a number of concepts which become relevant with alternating current, which do not apply with direct current.

Frequency: This has already been mentioned – it is the number of times that electrons change direction in a second, measured in Hertz (Hz). In the UK the electricity grid is operated at 50Hz. Large deviations can cause failures in different parts of the electricity system including blackouts sogrid operators must ensure that the frequency is maintained as close to this figure as possible. Deviations can be caused by an imbalance in supply and demand.  Small changes in frequency can easily be detected, and this is used in an AC electricity system to enable automatic correction of imbalances between generation and demand, through a mechanism called ‘frequency response’.

Harmonics and power quality: In an ideal AC electrical system, the change of direction of the electrons takes place in the shape of a sinusoidal wave, or sine wave. This means that they get faster and faster, then gradually slow down, then go back in the other direction, speed up, gradually slow down, and go back in the other direction in a smooth repeating pattern. 

sine wave.jpeg

A sine wave is a very common pattern in nature, and is seen in the rate of flow of water in an estuary as the tides ebb and flow, and in the rate at which the length of days changes faster around the equinox, and slower around the solstice.

However, when DC devices, including computers, mobile phones, batteries and solar PV panels are connected to the AC system, the process of converting from AC to DC or from DC to AC disrupts this smooth sinusoidal curve, by adding jerky vibrations into the system.  These jerky vibrations can lead to a phenomenon called ‘harmonics’, which can damage equipment in the electricity grid.

sine and harmonic

Network operators have to minimise the impact of these harmonics, and so the increase in use of DC devices on the electricity grid causes a technical challenge.

Reactive power: In an ideal AC electrical system, the ebb and flow of voltage and of current are in time with each other, or ‘in phase’. This means that they peak and trough at the same moment in any given location. However, some electrical devices, in particular electrical motors, cause the current to lag behind the voltage – they become out of phase.  

real and reactive power

This lag is called reactive power, and is independent of the consumption of ‘real’ power. The law of conservation of energy tells us that real power in must equal real power out (including all losses), but with AC electricity it becomes more complicated, because reactive power must be kept in balance too. The concept of reactive power is quite difficult to get your head around, so don’t worry if you don’t understand it exactly – the main thing is to know that this is something electricity system operators need to consider.

AC power is currently used for long distance high voltage transmission with lower losses, and safe low voltage electricity in buildings. New technology means that high voltage DC (HVDC) is being proposed for long distance transmission. DC power in buildings is also being trialled (e.g. the SoLA Bristol project) because many electrical devices such as computers, mobile phones and LED lights use DC power, and batteries and solar PV are also DC.  Using local DC based systems for these devices avoids conversion losses going from DC to AC and back again.

LEARNING POINTS FOR ENERGY DEMOCRACY:   

  • Electricity is a particularly important form of energy for a zero carbon system, because many renewable energy technologies produce electricity, and it doesn’t pollute the air or emit GHGs at point of use.
  • There are losses involved with the storage of electricity, so it is important to balance generation and consumption as much as possible, to minimise the need for storage to plug the gaps.
  • AC power has been an effective way of enabling high voltage transmission (reduced heat losses) with safe voltages at point of use. However, this wasn’t the only way forward envisioned in the early days of electricity – and HVDC (High Voltage Direct Current) is now being considered as an option. 
  • Special technical issues e.g. harmonics and reactive power also need to be taken care of by the electricity system operator – so new forms of generation may lead to a need for new management strategies.

Next: Physical Infrastructure