P3SUSTAINABLE

ENERGYLesson 10: The National Grid

OBJECTIVES MUST understand that electricity

is transmitted through the National Grid

SHOULD understand why electricity is transmitted at high voltages

COULD interpret a Sankey diagram showing how efficiently energy is transferred and understand the difficulties in assessing risks

KEY WORDS You need to be able to define the following:

National Grid

TEXTBOOK ANSWERS1) Carries electricity from power stations to users.2) Does not pollute at the point of use; can be transmitted

long distances; can be used in many ways.3) By transmitting electricity at low current and high

voltage.4) 230 V is high enough to provide enough power for most

of our needs in the home, without providing an unacceptably high risk of electrocution; 120 V has a lower risk of electrocution, but it does not provide as much power unless the current in the equipment is higher and this increases the fire risk.

5) Some energy is wasted when transmitting the electricity; Sankey diagrams show the average proportions of energy transferred in these types of power station; different types of power station input electricity to the Grid.

6) Useful output is 80 kJ; efficiency is 80% (0.8).

WORKSHEET ANSWERSActivity 1Power station – generates electricitySubstation 1 – increases voltages to

thousands of voltsPower cables – carries electricity

from power stations to homesPylons – supports power cablesSubstation 2 – reduces voltages to

safe levels

WORKSHEET ANSWERSActivity 2Substation 1 – Making the voltage

even higher, about 400,000V reduces the current, so reducing the energy loss

Substation 2 – The voltage is reduced for the use in the homes to the very much lower level of 240V

Power station – Electricity is generated at about 25000V

WORKSHEET ANSWERSActivity 3 (High demand)1) It shows that the power station uses 40 kJ from fuels to

produce 15 kJ of electrical energy, so 25 kJ is transferred to the surroundings; the National Grid delivers 13.5 kJ while 1.5 kJ is transferred to the surroundings from the transmission lines; the delivered energy is split between industrial and non-industrial use, 6 kJ and 7.5 kJ respectively.

2) Efficiency of power station = 15/40 = 0.375 or 37.5%3) Efficiency of transmission lines = 13.5/15 = 0.9 or 90%4) Burn the fuel more efficiently, e.g. use gas power

stations that use the heat from burning gases as well as steam; allow sensible suggestions such as only generating the electricity that is needed as surplus electricity cannot be stored; in the transmission lines, transmit electricity at even higher voltage to reduce current further.