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Thermal Energy Transfer: Question 7

Syllabus 2.3.3

Multiple choice Core 1 mark

An engineer is choosing a surface finish for the metal absorber plate of a rooftop solar water heater. The plate is mounted in direct, unobstructed sunlight for most of the day, and its job is to absorb as much of the Sun's infrared radiation as possible so that water flowing through pipes attached to it warms up quickly.

Which surface finish should the engineer choose for the absorber plate?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall how surface finish affects absorption

Surfaces differ in how much infrared radiation they absorb compared with how much they reflect. Dull, matt black surfaces absorb infrared radiation very effectively, while shiny, polished or light-coloured surfaces reflect most of the radiation that reaches them instead of absorbing it.

Step 2: Apply this to the absorber plate

The absorber plate’s job is to absorb as much of the Sun’s infrared radiation as possible, so its surface must be a strong absorber, not a strong reflector.

Step 3: Compare the four finishes

  • Polished aluminium and chrome-plated steel (A and B) are both shiny, reflective finishes, they reflect most incoming radiation away rather than absorbing it, so very little energy would be captured.
  • Glossy white paint (D) also reflects most of the radiation striking it, which is why white surfaces are chosen when the goal is to stay cool, not to absorb heat.
  • Dull, matt black paint (C) absorbs infrared radiation far more effectively than any of the other finishes, so it captures the most energy from the sunlight.

Step 4: Match to the correct option

The absorber plate should be finished with dull, matt black paint, option C, so that it absorbs as much of the Sun’s radiation as possible and warms the water in the pipes quickly.

Why the other options are wrong

OptionFinishWhy it doesn’t fit
APolished aluminiumA shiny surface, so it reflects most incoming radiation instead of absorbing it.
BChrome-plated steelAlso shiny, so it behaves like a mirror to infrared radiation and absorbs very little.
DGlossy white paintA light, reflective finish that minimises absorbed radiation. Useful for staying cool, but the opposite of what the absorber plate needs.

Final answers

  • The engineer should choose dull, matt black paint, option C, since dull black surfaces are the best absorbers of infrared radiation.