Supply and demand
4 marks · NormalA power company is deciding how to best prepare for regular changes in electricity demand throughout each day.
Describe one predictable change in electricity demand that occurs on most days.
Suggest one way the company can plan to supply extra electricity during these predictable times.
Explain why fossil fuel (coal, oil, or gas) power stations are often run continuously, even at night when demand is lower.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Morning or evening rise in demand (e.g. when people wake up or return home). · must show predictable pattern
(b) Use power stations that can be started and stopped relatively quickly, such as gas turbines, or keep some power stations running at low output to increase supply as needed. · must show one practical planning method
(c) Because fossil fuel stations, especially coal and oil, cannot be switched on and off quickly; it is safer and more efficient to keep them running rather than shutting down and restarting every day. This ensures supply and avoids delays in meeting demand. · must show explanation related to start-up time/efficiency
Common mistakes: Not linking answer to predictable demand; Incorrect statements about shutting down power stations
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Gives a correct example of a predictable change (e.g. evening or morning demand spike)
B1 (b) · 1 mark · States a method for planning extra supply during predictable increases
B1 (c) · 1 mark · Explains that fossil fuel stations cannot start and stop quickly
B1 (c) · 1 mark · Mentions reliability/efficiency or security of supply
Worked solution
a) There is a predictable rise in demand in the evening when people return home and use appliances.
b) The company can use quickly-responding power stations or run some plants at reduced output, increasing output when needed.
c) Fossil fuel power stations take a long time to restart, so it is more efficient to keep them running at night, ensuring continuous supply.