AQA · GCSE · Physics · Higher

Overall Efficiency of a CAES System

Q06.1 3 marks

Energy can be stored using a Compressed Air Energy Storage (CAES) system. When more electricity is generated than is needed, a CAES system uses the surplus electricity to compress air in a storage tank. When there is a greater demand for electricity, the CAES system releases the compressed air to generate electricity. Figure 7 shows a diagram of the CAES system.

Calculate the efficiency of the CAES system.

Give your answer to 2 significant figures.

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Marking points

1 Multiplies the efficiencies 0.72 and 0.86.
2 Obtains the unrounded value 0.6192.
3 Gives 0.62 to 2 significant figures (or the equivalent correctly marked percentage).

Full-mark answer

Overall efficiency $= 0.72 \times 0.86 = 0.6192$ , which to 2 significant figures is $0.62$ .

Why this answer loses marks

Adds the two stage efficiencies.

The stages act successively, so multiply them to find the fraction retained through both processes.

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