A electric storage heater uses electrical energy to heat up special bricks overnight, when electricity is cheaper. During the day, the stored thermal energy is released slowly to warm a room. Explain why the efficiency of the storage heater can never reach 100%, and describe TWO ways in which the efficiency of the storage heater could be increased.

Edexcel A-Level Physics (9PH0) — 3.2 EMF, resistance and circuits · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

An electric storage heater is rated at 3 kW. Over one night it stores energy in its internal bricks. During the day, some of this energy is released as useful heat into the room, but some energy is lost to the surroundings through the outer casing of the heater.

Model answer (5 marks)

Energy cannot be 100 % because, by conservation of energy, the total energy supplied to the heater must equal the sum of useful heat released into the room and energy lost to the surroundings. Some of the stored thermal energy inevitably escapes through the outer casing as waste heat, so the useful output is always less than the input.

1. Improve the insulation of the outer casing. Better insulation reduces heat loss to the surroundings, so a larger fraction of the stored energy is released into the room.

2. Recover or recycle the waste heat that escapes through the casing and redirect it into the room. By capturing this heat and using it as additional heating, the overall efficiency of the system increases.

Examiner tips

  • Use the conservation‑of‑energy principle to justify why 100 % is impossible; mention waste heat loss to the casing; give two specific, realistic improvements; keep language concise and use terms like ‘insulation’ and ‘recovery’.”]
  • common_mistakes
  • :
  • Claiming 100 % is possible by ignoring the casing heat loss; confusing electrical efficiency with thermal efficiency; not specifying how insulation or heat recovery actually improves efficiency.”

Mark scheme (5 marks)

  1. Energy cannot be created or destroyed (conservation of energy), so waste energy transfers mean useful output is always less than total input
  2. Some energy is transferred as waste heat to the surroundings through the outer casing (waste energy transfer), reducing useful output
  3. Thermal insulation applied to the outer casing of the heater would reduce heat loss to the surroundings
  4. Correct explanation of how thermal insulation increases efficiency — less energy is wasted / a greater proportion of the stored energy is released as useful heat into the room
  5. Recycling / recovering the waste heat that escapes through the casing and redirecting it into the room as useful energy input, increasing efficiency

Key terms in this question

efficiency

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