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Thermodynamics and engines (A-level only)

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Question 6

The National Grid is supplied mainly from power stations which have overall efficiencies of up to about 35%.

Table 1 shows the average power requirements of a large chemical processing plant.

Table 1

RequirementAverage power / MW
for driving electric motors, compressors, and pumps32
for heating, steam creation, and distillation in the chemical process95
for running electrical control systems, office equipment, and lighting8

The chemical plant can either:

  • take all of its energy from the National Grid, or
  • install an electrical generator of output 45 MW driven by a gas turbine of efficiency 32% as part of a combined heat and power (CHP) scheme. The hot exhaust gases from the turbine are used to produce steam for heating.

The directors of the chemical plant are considering the CHP option.

Explain, with reference to the data above and any other factors, the advice you would give them.

In your answer you should:

  • explain why the maximum theoretical efficiency of a heat engine is much less than 100%
  • use the information above, including the numerical data, to compare the two options.
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Markscheme

Thermodynamics and engines (A-level only) Questions

  1. A Level
  2. /Physics
  3. /Thermodynamics and engines (A-level only)

29 exam-style questions on AQA A Level Physics Thermodynamics and engines (A-level only), covering First law of thermodynamics (A-level only), Non-flow processes (A-level only), The $p$ - $V$ diagram (A-level only), Engine cycles (A-level only), Second Law and engines (A-level only), and Reversed heat engines (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.

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