Macrogeneration

Question 1

Generation of electricity in a fossil fuel power plant stages

Answer 1

Stage 1; Combustion of the fossil fuel

Stage 2; Evaporation of water

Stage 3; Steam used to turn a turbine

Stage 4; Condensing in the cooling towers

Question 2

Combustion of fossil fuel

Answer 2

The fossil fuel is combusted in a furnace to release the chemical energy that it contains. This combustion specifically releases thermal energy (sometimes incorrectly termed heat)

Question 3

Evaporation of water

Answer 3

The thermal energy produced by the combustion of the fossil fuel is then used to increase the temperature of the water in the furnace. Water enters the furnace in liquid form. The temperature in the furnace is very high and increases the temperature of the much cooler water as energy is transferred to the water. The temperature of the water increases considerably, and it evaporates at 100 °C into steam. This steam is ejected from the furnace and directed into the steam turbine

Question 4

Steam used to turn a turbine

Answer 4

This steam turbine turns when the high-pressure steam is forced through it. Steam, which is extremely high velocity water molecules, collides with the fins of the steam turbine causing the turbine to turn. But the steam molecules have less energy after each collision. Hence the steam which is entering the steam turbine at its hottest temperature, leaves at a lower temperature, but is still hot. The hot steam is now sent to the cooling towers.

Question 5

Condensing in the cooling towers

Answer 5

The hot steam from the steam turbine enters the cooling towers. Here the steam loses substantial energy so that the steam condenses back into water (liquid). Then the liquid which is cold is recirculated back to the furnace and the process is repeated

Question 6

Direct renewable energy sources

Answer 6

These renewable energies take energy from a renewable source such as the wind or waves in the ocean and can convert this into electrical energy directly

Examples include wind, hydroelectric, wave and tidal

Question 7

Indirect renewable energy sources

Answer 7

These take energy from a renewable source such as the Sun using a physical characteristic of the material being used, and convert this energy into electrical energy

Examples include solar PV and Biomass.

Question 8

Electromagnetic induction in the generation

Answer 8

The generator has a metallic conductor (typically copper) that can rotate when the steam turbine rotates. This is due to the metallic conductor and steam turbine being connected to a common shaft. Placed around this metallic conductor, inside the generator, are strong magnets. When the metallic conductor rotates, the magnetic field lines from the magnets cut through the metallic conductor. This induces a current to flow in the metallic conductor. This electromagnetic (EM) induction generates electricity in the generator.

Question 9

Wind

Answer 9

The process of electricity generation by wind is covered in detail throughout chapter seven. Only the energy transfers are indicated below in Figure 5.3. For this solution and all others that follow, kinetic energy will be shortened to KE

Question 10

Solar

Answer 10

The process of electricity generation by solar PV is covered in detail throughout chapter eight.

The solar system must be the solar PV system as the solar thermal system simply heats water.

Question 11

Operation of hydroelectric

Answer 11

When the Sun strikes water on the surface of the Earth some of the water will be evaporated. This water vapour then rises upwards and eventually as the water vapour loses energy to the cooler air, it condenses and forms clouds. When water falls, in the form of rain, the water can be collected in high mountainous areas. Rivers can be blocked off to create a large depth of water behind a dam.

The water at high level trapped behind the dam has a lot of gravitational potential energy that can be utilised to produce electricity. A door can be opened inside the dam’s turbine’s shaft allowing water to flow from the high-water level to the low water level. As the water travels through the turbine shaft it turns a generator to produce electricity.

Question 12

Operation of tidal barrage

Answer 12

The gravitational attraction by the moon, and to a lesser extent the Sun, causes the oceans’ water levels to rise and fall every day. When the tide comes in through a barrage, the gates along the barrage can be shut at high tide. This maintains the high-water level in the estuary. As the tide returns to its low level again the gates may be opened and the water flows through turbine shafts much in the same way as the hydroelectric plant. This flowing water turns a generator which produces electricity.

Question 13

Operation of a wave generator

Answer 13

The Sun heats the Earth’s surface. However, this heating does not produce the same increase in temperature of the land and water. If equal quantities of energy are given to the same masses of land and water the temperature of the land will increase more rapidly. Less energy is needed to make 1 kg of land to rise by 1 °C than 1 kg of water rise by 1 °C. As the land and sea receive energy from the Sun the temperature of the land becomes larger than that of the water. This different rise in temperature causes the air above land to gain more energy than the air above the oceans.

This is because there is a larger difference in temperature between the land and the air above it, than between the sea and the air above it. This larger difference in temperature causes more energy to be given to the air from the land than from the sea to the air. The hot air above the ground rises as it is less dense. The air at sea rushes into this vacant space left and wind therefore moves from the ocean to land. Between the wind and the surface of the oceans there exists a form of friction.

The particles in the air rub against the surface of the ocean as they rush towards land. This causes waves on the surface of the ocean to move towards the land. These waves possess vast amounts of kinetic energy. They cause tubes of a wave generator to oscillate up and down along the water surface. This causes a fluid to flow at the joints between tubes which causes a generator to turn. This produces electricity.

Question 14

Biomass

Answer 14

A biomass generator operates in a similar fashion to the fossil fuel power plants discussed in LO2 of this chapter. The only difference is that instead of combusting coal, natural gas or oil, biomass is combusted. This of course can be considered a renewable form of energy. This is only the case provided that the trees utilised are regrown at a rate similar to, or greater than, the rate at which they are cut down.

Question 15

Transmission of electrical energy across the grid

Answer 15

Stage 1; Producing electricity at the power station

Stage 2; Step up transformer

Stage 3; Transition lines

Stage 4; Step down transformer

Question 16

Producing electricity

Answer 16

The initial part of the process is of course generating the electricity. The various parts of the process are discussed in detail in LO2 of this chapter. The electricity produced in the power station is at 22 kV which is 22,000 V.

Question 17

Step up transformers

Answer 17

The electricity produced in the power plant is at 22,000 V. If this electricity is sent across the National Grid there would be substantial power losses as the cables would be heated considerably. This is due to the current being excessively high. It is critical to reduce the current to the minimum value before sending electricity across the National Grid. This is achieved through the use of a transformer.

This is a device which allows the size of the voltage and current to be changed. A step-up transformer is used to step-up the voltage of the electricity from 22,000 V to somewhere in the range of 275 kV to 400 kV (275,000 V to 400,000 V). The huge increase in voltage results in a huge decrease in current. This minimises the heat losses occurring in the cables along the National Grid, which makes the energy distribution network much more efficient.

Question 18

Transmission lines

Answer 18

When the voltage has been raised by the step-up transformer to somewhere in the range of 275 kV to 400 kV, the electricity can now be sent across the National Grid through transmission lines. Materials are selected with low resistances to again minimise energy losses. The cost of the transmission lines materials must also be considered.

Question 19

Step down transformer

Answer 19

When the electricity reaches the consumer, the voltage is much too high and very dangerous. As a result, a step-down transformer is used to reduce the voltage down to levels required by the consumer. This also leads to an increase in the current. The voltage is stepped down to 33,000 V for industry, and 11,000 V for distribution to towns, villages and direct to small industrial customers. It is reduced to 240 V for use in homes, schools, shops and businesses.

The step-up transformer does increase the voltage, but this is completed to reduce the current to a minimum. Minimising the current to reduce energy losses before transferring electricity across the grid is the goal of the step-up transformer.

Question 20

The conventional electricity distribution method

Answer 20

Over the past number of decades electricity was distributed from a small number of producers, power plants run from various different fuels, to the end consumer of which there are many (every member of the population who uses electricity, which in an industrialised nation is everyone apart from a very few living on isolated islands). This involved the flow of electricity in one direction; this was from the power plant to the end user.

Question 21

The incorporation of renewables

Answer 21

Since 2008 there has been a surge in the percentage of electricity generated from renewables. As outlined in chapter three, NI in 2008 provided only 8% of its electricity from renewables. However, renewables contributed to 44% of electricity generation as of June 2019 (Macauley, 2019). The problem with this increased contribution from renewables, is the intermittent nature of these renewable generators. On days when there is little or no wind the output from wind turbines will be minimal. Similarly, on days of low solar radiation values due to cloudy conditions, the output from solar PV systems will be minimal.

Question 22

Challenges of the national grid

Answer 22

1. Incorporating intermittent renewable electricity onto the distribution network. 2. Facilitate power production from all consumers who own properties and business premises. This will require a new dynamic: two-way flow of electricity on the grid. 3. Accurate real time data is required to establish, at any time, the renewable electricity production and overall demand for electricity from all consumers.

Question 23

The SMART grid

Answer 23

A SMART grid is needed to incorporate renewable sources of electricity and plan and manage the intermittent nature of these energy sources.

Although not explicitly stated in the LO, a student must be aware of the definition of a SMART grid, which is as follows: ‘A computer-controlled electricity network that switches input from one energy source to another as demand for energy varies on the grid’.

Question 24

Requirements of a SMART grid

Answer 24

1. Predict demand. The SMART grid will have a network of intelligent meters located throughout the National Grid to accurately predict demand. Only when accurate demand levels are established can accurate power production levels be achieved.
2. Measuring output from renewables. The SMART grid will also need intelligent meters to detect the output from renewables across the National Grid. All of the power from renewables will be incorporated for final consumer usage.
3. Adapting non-renewable power outputs. The electricity provider must establish the demand for electricity throughout the day and night. The output from renewables must be monitored constantly, so that the additional contributions from fossil fuel power plants can be calculated. Information can then be sent to natural gas fired power stations to increase or decrease output as required to meet the demand at that time. Natural gas power stations are used here as they can quickly adapt output to the required values. Some natural gas fired power stations (dependent on design and thermodynamic cycle implemented) can be fully operational from shutdown in as little as a few minutes. This cannot be achieved by coal or nuclear power stations.
4. Exporting or importing to other nations. There exists the possibility of producing more electricity than required at a given time by a nation. In the absence of effective energy storage solutions (see chapter ten), a nation can export energy to other nations as they are becoming increasingly interconnected. Similarly, if a nation is faced with the possibility of having to use a fossil fuel power plant to provide the required electricity to the National Grid, importing renewable energy from other nations can be more cost effective.

Question 25

Benefits of a SMART grid

Answer 25

Maximises renewables; A SMART grid maximises the proportion of renewables providing electricity for a nation. This increases a nation’s fuel security as they are less reliant on fossil fuel imports.

Minimises fossil fuels; A SMART grid minimises the proportion of electricity from fossil fuel power plants resulting in lower greenhouse gas emissions. This is hugely beneficial to the environment but also helps in achieving national carbon reduction targets.

Control electricity demand; Electricity providers can offer different electricity prices during the day. This can move consumers away from peak times of power demand such as morning and evening. This results in the National Grid itself not having to be upgraded with new pylons and cables to meet increased electricity demand in the future. Such increase in infrastructure would be bad for the environment in terms of habitat destruction. These upgrade costs would result in increased prices of electricity.