Here at engineering timelines, we wanted to know more about the mechanics of electricity generation and how power reaches us at home. We also wanted to know things like whether electric cars are 'green' or not. So, we decided to ask questions.
The conversation below expands on the content of the diagrams that appear on the supplying electricity
conversation with John Biscoe
, Director, GDM
First, John Biscoe summarises fuel in relation to power generation ...
In the UK, six different energy sources are used as primary fuel for power generation.
Each has pros and cons.
Environmental impact is calculated by adding together DIRECT and EMBODIED energy costs embodied energy is the energy it takes to extract and transport the fuel.
engineering timelines: People can sometimes be confused about the environmental impact of electricity there's a misperception, for example, that electric cars are more environmentally friendly. But the reality is that electricity is produced in power stations by burning fossil fuels. Can you explain what the environmental impact is of using different fuels in the production of electricity?
: We all know that fossil fuels are a finite resource, that they are being depleted and that they are contributing factors to global warming. I did a calculation that shows how much carbon dioxide is produced by heating up 1.7 litres of water in a kettle using electricity produced from different fuels (see switching on the kettle
). For coal we emit 0.153kg, oil 0.136kg, gas 0.075kg nuclear, hydro, and wind power produce no carbon. Of course, these calculations exclude the embodied costs it takes to source and transport all those fuels. We also looked at the figure for when a kettle is on a gas hob above a direct source of gas. The result was 0.042kg of carbon.
engineering timelines: So it's better to use a kettle placed directly on the gas hob than to use an electric kettle?
John Biscoe: Absolutely. Think about it. When you plug in an electric kettle, you're drawing down energy from the National Grid, which means that fossil fuels were burned to produce that electricity in a power station miles away, and then the electricity travelled through the distribution network to get to you, and there are invariably going to be energy losses in that distribution process. So it's much better to use energy at source if you can.
engineering timelines: What do you think is the best alternative to fossil fuels?
John Biscoe: That's tricky. There's wind, but the disadvantage of wind power is that some days there's no wind. As a designer, if I wanted to use wind power, I'd have to have a dual system for no-wind days. So I'd still have to take electricity from the grid or from a fossil-fuel-burning resource. Then there's nuclear, but there's the issue of where to put spent material, and also the issue of embodied energy in terms of building new nuclear power stations. One could consider biomass oil-powered stations. Biomass is basically vegetable oil, or wood chips, but there again, there are issues about how to produce and transport the material, and what to do with spent material. And those fuels are going to produce carbon as well. So as an engineer I don't think the answer lies entirely in alternative energy measures. There have to be some but we have to look carefully at how our buildings, cars and planes waste electricity. You can't just rely on the industry to come up with a carbon-friendly alternative. It's not viable.
engineering timelines: What choice does the consumer have in all this? Obviously you can't choose what fuel is used to produce the electricity you use when you flick on the switch at home.
John Biscoe: I think the choice you can make as a consumer is looking at the potential for on-site renewables. You can have a wind turbine installed on your house, or consider installing photovoltaics or solar thermals because residences are big users of hot water. If you use an alternative energy source to heat up your water, such as solar thermals, you're avoiding using fossil fuels.
engineering timelines: So the alternative is to get off the National Grid?
John Biscoe: The alternative is to supplement. You'll never get off the National Grid entirely because residential buildings are high users of energy.
Then, John Biscoe summarises power generation issues ...
The fuel is used to heat water, which drives the motion of huge turbines. The ever-moving turbines produce the electricity.
It is produced at a value of 25kV (25,000 volts).
The newly-generated current is fed through a series of transformers that step the voltage up to 400kV it's more efficient to distribute it at high rather than low voltage, since this minimizes energy losses.
Electricity cannot be stored. It is transmitted as soon as it is produced. When consumer demand falls, power generation halts.
engineering timelines: Can you explain why electricity is produced at a low voltage and then it's immediately stepped up to a higher voltage?
: This has to do with something called I²R losses. Basically, whenever you transmit electricity via cables, you always get energy losses along the way. These energy losses are much lower when you transmit at high voltage than when you transmit at low voltage. That's why we see the distribution of 400kV (400,000 volts) from source to throughout the UK. The voltage is then transformed down in various substations in stages until it reaches 230V the voltage you have at the power socket in your home. That's what we're seeing through these different panels (on the supplying electricity
page). The other thing to keep in mind is that it's very difficult to store large quantities of electricity. It's not like water or gas, where you can put it in a container.
: What about batteries
don't they store electricity?
John Biscoe: Batteries don't store electricity. They turn chemical energy into electrical energy. This issue of not being able to store electricity is a real problem for the electricity companies. It's why they're keen to have people buy certain tariffs. So they might ask people to buy electricity at night until 7am on an economy tariff. They want people to buy cheap electricity at night and use that for storage heaters. What they don't want is to have to turn off their power stations when there's low demand because it takes a lot to get a power station up and running again.
engineering timelines: So there's a constant dance between supply and demand, and the point is to balance them at all times.
: Yes, and that's why the National Grid is so important in terms of evening-out those distribution problems across the entire network. The Central Electricity Generating Board, which used to run the power stations and the network (see history of public supply
), actually had trends for electricity use. So, for example, they would know that if there was a very important football match, at half time there would be huge demand for electricity because everyone would be getting up to make a cup of tea. They looked for these trends in order to bring power stations online to cope with demand. In the summer, it's air conditioning. There's huge peak demand for it in London so they'd bring certain power stations on line to meet it. If you could make electricity and store it in a container, you wouldn't have to do that.
Some points about transmission ...
Electricity is transmitted across long distances via aluminium cables suspended on .
Overhead cables are more efficient than underground cables for transmitting at high voltages underground cables must be insulated to prevent short-circuiting.
Transmission is at a higher voltage than generation or distribution, to maximize efficiency across the system.
engineering timelines: How does electricity travel from the power station to the end user?
: There are two types of high voltage distribution. One is through overhead cables, which you can see in the diagram on the supplying electricity
page. This is the more efficient method. The alternative is underground, which is ten times more costly.
engineering timelines: Why?
John Biscoe: Well, you have to dig trenches and you have to insulate the cables in the trenches. Cables overground don't have to be insulated. They're bare aluminum cables and their heat is dissipated into the atmosphere. The way overground cables cope with short-circuiting is by having an air gap between them but underground, you have to insulate the cables to prevent short-circuiting. This, though, interferes with the natural dissipation of heat. To compensate for this, you have to use bigger cables. That's why the regional electricity companies adopt overhead cables where possible. The disadvantage with overground cables is they're unsightly. As you get into inner cities and urban environments, the cables go underground.
Some points about distribution ...
The electricity is sent to bulk supply transformer stations, closer to end users.
These transformers step the voltage down to 132kV or 110kV.
At this voltage, it is distrubuted to individual or small groups of end users.
: So, electricity is generated in a power station, then it goes through a transformer
and is transformed up to a higher voltage for distribution. It's then carried via transmission towers (pylons) to a substation location that is nearer the place where the electricity is needed.
John Biscoe: Yes. The substation consists of a series of transformers that transform the 400kV or 275kV electricity down to either 132kV or 110kV. And it's quite normal for large factories or buildings to take in electricity at 110kV (11,000 volts). It's then transformed down to either 400 volts or 230 volts for use by the end user inside a building.
engineering timelines: How does a transformer work?
John Biscoe: A transformer is a simple piece of equipment. It consists of an iron ring core wrapped in copper coils. It's the number of coils wound in two sets, one each side of the ring that determines the ratio of transforming up or down of the voltage. It's a static piece of kit but one of the major issues is dealing with the heat that's produced by the electrons running round the copper coils. Quite often the coils are immersed in oil to deal with the heat (an oil transformer). Actually, there are three types of transformer oil, gas and air. Mobile phones use small transformers for re-charging (air transformers). Plug your mobile phone in for a few hours to charge it up. When you're done, you'll notice that it's warm. Well, scale that up about a million times and you've got a major transformer in a substation. The key thing about the transformer is its transience. Once you put a current in to one side of the ring, you can only sustain a current in the other side, in the other windings, for a very brief period of time before it returns to a static state.
And finally, the electricity reaches you ...
In the UK, electricity is distributed to consumers at 230 volts (standard UK voltage).
Transformers are used to step the voltage down from distrubution levels to consumer level.
Energy losses increase with this drop in voltage, so larger cables used are used inside buildings to compensate.
engineering timelines: The sense I'm getting is that the energy that's flowing through the National Grid is fluctuating much more than we might imagine.
John Biscoe: Exactly. And as an electrical engineer, you have to be able to address this diversity in your designs.
engineering timelines: What do you mean by diversity?
John Biscoe: If I'm designing a residential development with 100 flats, first I calculate the maximum load for one representative flat, including all the electric cooking, heating, and all the electrical appliances that might be on at any one time. If I multiply that by 100 then I have the building load. Diversity means that we acknowledge that not everybody's going to have their cookers on at once. The worst case might be Christmas maybe 70% use. So I might apply a diversity value of 70% or 75% of the maximum electricity load. If I don't apply a diversity factor, the systems coming into the building (i.e., the cables) would be too big and I'd be paying for an unused supply. On the other hand, I have to be careful to apply a sensible diversity value. Whenever I apply for electricity for a building, whether it's for one house or for a block of 400 apartments, I apply for a high voltage (HV) supply from the electricity company. They will say to me, "What is your maximum demand?" And I have to tell them what I've calculated and what diversity I've applied. They charge my client a contribution charge for bringing this service into the building. So developers are very keen for me to get it right because they don't want to be paying for unused electricity. And we need to build the infrastructure the cables and systems coming into the building to be able to take the demand I've identified. If we're too lean on the diversity calculations, there's an overload problem because are we demanding too much electricity and the protective devices and all the cables may not be strong enough to take that load. So, as engineers we have to get it right.
engineering timelines: What's interesting about your job as an electrical engineer is that you're managing a sort of choreography of energy. It's moving and fluctuating all the time.
John Biscoe: Yes, and the electricity is only one part of it. We also have to take care of the lighting design. We have to include motors and pumps for mechanical systems. I think the best comparison I could use to describe a building system is the human body. If you think of what all the organs in the human body do, that's what mechanical systems within a building do. So you have the lungs corresponding to the air handling system, for example. In terms of your nervous system and your veins with the blood moving around them, that's the electricity. And the plumbing's the plumbing!
The conversation above expands on the content of the diagrams
that appear on the supplying electricity