Generating Electricity and the 'National Grid' power supply

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

  • Energy resources:

    • Non-renewable energy sources

      • These energy resources are finite and will run out eventually, there are major associated environmental issues BUT at the moment, most of our useful energy is derived from them. These are historically, and to the present day, the major energy sources for large power stations.

      • e.g. Coal (mainly carbon), crude oil (certain hydrocarbon fractions), natural gas (mainly methane) and nuclear fuels (based on the metals uranium and plutonium).

    • Renewable energy sources

      • e.g. biofuels, geothermal, hydroelectric, solar radiation, tidal changes, wave action, wind turbines

      • Theoretically these energy resources will never run out and generally speaking their environmental impact is not as great as non-renewables, but they only provide a small % of our energy needs at the moment and can be unreliable like the wind.

      • Most of these energy sources, apart from hydroelectric schemes and a few tidal barrage sites, are deployed on an experimental or small commercial scale.

      • Renewable energy resource technology should be the cheapest to run with no primary fuel costs.

 

Methods we use to generate electricity

chemical/nuclear energy (fuel) => heat energy (steam) => kinetic energy (turbine blades) => electrical energy (generator)

  • Appreciate that various energy sources can be used to generate the electricity we need.

  • Appreciate the need to carefully consider the advantages and disadvantages of using each energy source before the decision to decide which energy source(s) would be best to use in any particular situation.

  • Know that electricity is distributed via the National Grid.

  • You are expected to use your skills, knowledge and understanding to:

    • evaluate different methods of generating electricity,

      • you should be able to evaluate different methods of generating electricity given data including start-up times, costs of electricity generation and the total cost of generating electricity when factors such as building and decommissioning (removing everything of an old power station) are taken into account.

        • You must also be able to consider the reliability of different methods.

        • There are other general issues such as environmental impact - pros and cons for the local community (eg jobs versus environmental damage, visual impact), how long will it take to build?, at what cost versus eventual power output?, planning delays etc.

        • Ideally you would want to site a large fossil fuel/nuclear power station as near as possible to the major/bulk users AND in the case of coal, near a coal mine, since power line transmission involves wasted energy (see National Grid section).

        • Large scale tidal and river/lake hydroelectric schemes and geothermal power plants all need very specific geographical locations.

        • For safety reasons, nuclear power plants are sited in remote locations, often near the coast.

        • Small scale power generation with solar cells and wind turbines can be sited anywhere, but larger wind farms need to be in a windy area eg on low hills or out at sea.

      • Knowledge of the actual values of start-up times and why they are different is not needed, but you must appreciate that the implications of such differences in start-up times are important.

    • evaluate ways of matching supply with demand, either by increasing supply or decreasing demand,

      • you should be aware of the fact that, of the fossil fuel power stations, gas-fired have the shortest start-up time.

        • Power station generator start up times: Nuclear >> coal fired > gas-fired

        • By coincidence (or maybe not?), this order is also paralled by the capital costs, decommissioning costs,

      • you should also be aware of the advantages of pumped storage systems in order to meet peak demand, and as a means of storing energy for later use. See the section on hydroelectricity.

    • compare the advantages and disadvantages of overhead power lines and underground cables.

  •  Know and understand that the flow of water and wind can be used to drive turbines directly.

    • Know that renewable energy sources used in this way include, but are not limited to, wind, waves, tides and the falling of water in hydroelectric schemes and all involve converting FREE kinetic energy into electrical energy using a generator. None of these schemes needs a fuel, or produces any kind of chemical pollution on the site, and all are 'green' in terms of not consuming fossil fuels ie carbon dioxide, but they may have quite an environmental impact. All these methods can contribute to National Grid of electricity supply.

  • Know and understand that small-scale production of electricity may be useful in some areas and for some uses, eg hydroelectricity in remote areas, solar cells for roadside signs, remote telephone kiosks.

    • You should understand that while small-scale production can be locally useful because it is sometimes uneconomic to connect such generation to the National Grid.

  • You should know and appreciate that using different energy resources has different effects on the environment and these effects include:

    • the release of substances into the atmosphere,

    • the production of waste materials,

    • noise and visual pollution,

    • the destruction of wildlife habitats.

    • Also, you should know and understand that carbon capture and storage is a rapidly evolving technology.

      • To prevent carbon dioxide building up in the atmosphere we can catch and store it.

      • Know that some of the best natural containers are old oil and gas fields, such as those under the North Sea.

      • The idea is to capture the carbon dioxide from fossil fuel burning before it is released into the atmosphere and pump it to some suitable storage location.

      • Is it possible to feed the carbon dioxide to algae from which to derive a biofuel?

        • I do know that carbon dioxide from a fermentation process is fed into greenhouses to promote growth of crops like tomatoes! Can we do it on a bigger scale?

    • There are other ways to reduce carbon dioxide, principally by reducing electricity demand, so less fossil fuel is burned. You can reduce electricity demand in the home by insulation, better designed and more energy efficient appliances like washing machines, low energy light bulbs, turning off all devices/appliances not in use.

  • Know that electricity is distributed from power stations to consumers along the National Grid.

    • You should be able to identify and label the essential parts of the National Grid.

    • The National Grid consists of a vast network of pylons and cables (power lines) insulated and suspended from these pylons.

      • All major power stations feed into the National Grid irrespective of their geographical location and many are needed to service millions of users in homes, transport and industry right across the country.

    • You see them stretching for miles and miles across the landscape to supply you, the consumer, very conveniently with a constant (well nearly!) supply of electricity to your city, town or village across the vast majority of the country.

    • power station ==> step-up transformer ==> grid system ==> step-down transformer ==> user/consumer

  • For a given power increasing the voltage reduces the current required and this reduces the energy losses in the cables.

    • You should know why transformers are an essential part of the National Grid.

    • So that you can transmit (transfer) the very large quantities of electrical energy needed you need to use, either, a very high current or a very high voltage or both.

      • the four possible choices are (i) low current/low voltage, (ii) high current/low voltage, (iii) low current/high voltage and (iv)  high current/high voltage.

      • (i) couldn't deliver what is needed, but (iii) is the actual choice.

      • So why is 'low current/high voltage' the desired choice for electrical power line transmission.

      • The greater the current flowing through a wire, the greater the heat generated, which in the context of power lines means waste heat energy, which is why (ii) and (iv) are not employed.

      • However, power = current x voltage, so to deliver a particular power rating, you can increase one of the two variables and decrease the other.

      • Therefore by using a very high voltage (eg 400 000 V, 400 kV) and relatively low current you maximise power transmission for the minimal heat loss of wasted energy.

    • Use of these extremely high voltages (1667 x your domestic voltage of 240 V), means health and safety issues arise and you need lots of big ceramic insulators on pylons and transformers and lots of barbed wire to deter people from climbing up pylons!

  • You should know and understand the uses of step-up and step-down transformers in the National Grid.

    • You do not need to know the details of the structure of a transformer and how a transformer works.

    • Now, (i) since the national power transmission uses 400 kV, you can hardly use this in the home, and (ii) generators themselves cannot deliver 400 kV, you need a way of increasing (for efficiency), and then decreasing (for safety), the voltage in power lines.

    • A transformer is a means of changing an input voltage in one circuit, into another output voltage in a separate circuit.

      • At the power station end is a step-up transformer to increase the voltage for power line transmission.

      • At the user end is a step-down transformer, to reduce the voltage that is a safe level for factories, domestic homes, street lighting etc.

  • Comparing the advantages and disadvantages of overhead power lines and underground cables.

    • The comparison table below addresses the issues.

COMPARISON Installation

- cost

Installation

- ease of

Environmental impact

- visual impact

Environmental impact

- the land itself

Maintenance and reliability
Overhead power lines lower, simple erection of pylons and linking cables carrying the high voltage current relatively easy, simple erection job considerable, miles of pylons and cables stretching across the countryside - controversial in designated areas of outstanding natural beauty slight - foundations of pylons much more needed and much less reliable - weather damage eg from frost, snow, corrosion of structure
Underground cables much higher, costly trenches and insulated cables much more work, all that digging and filling! minimal, not really seen at all considerable, but temporary - digging trenches is disturbing the land, but no lasting damage much less and more reliable, not affected by weather BUT not as easy to trace and access if fault develops

 


Energy and power supplies revision notes index

Energy resources and their uses - a general survey

Renewable energy (1) Wind power and solar power

Renewable energy (2) Hydroelectric power and geothermal power

Renewable energy (3) Wave power and tidal power

Biofuels, renewables and non-renewables

The 'National Grid' power supply


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