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Selective surface

Selective surface is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Selective surface rather than just read about it. In short: In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratio of solar radiation absorption (αsol) to thermal infrared radiation emission (εtherm).

Selective surface — main illustration
Selective surface — illustration

Key takeaways

  • Selective surface belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Selective surface to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Selective surface from memory before moving on to harder problems.

Reference excerpt

In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratio of solar radiation absorption (αsol) to thermal infrared radiation emission (εtherm). Selective surfaces take advantage of the differing wavelengths of incident solar radiation and the emissive radiation from the absorbing surface:

Solar radiation covers approximately the wavelengths 350 nm to 4000 nm; UV-A, visible and near infrared (NIR, or IR-A plus IR-B). Thermal infrared radiation, from materials with temperatures approximately in the interval -40 to 100 °C, covers approximately the wavelengths 4000 nm to 40,000 nm = 4 um to 40 um; The thermal infrared radiation interval being named or covered by: MIR, LWIR or IR-C.

Materials Normally, a combination of materials is used. One of the first selective surfaces investigated was a semiconductor-metal tandem – simply copper with a layer of black cupric oxide. Silicon on metal is also another example. A different design has ceramic–metal composites (cermets) on metal substrates. Black chromium ("black chrome") and nickel-plated anodized aluminum is another selective surface that is very durable, highly resistant to humidity or oxidizing atmospheres and extreme temperatures, while being able to retain its selective properties, but expensive. One of the more popular designs – a multi-layer broadband solar absorber – consists of a metal substrate coated with multiple layers of metal and dielectric materials. While those have to be vacuum-deposited, they have been widely adopted due to their suitability for vacuum tubes. Although ordinary black paint has high solar absorption, it also has high thermal emissivity, and thus it is not a selective surface. Typical values for a selective surface might be 0.90 solar absorption and 0.10 thermal emissivity, but can range from 0.8/0.3 for paints on metal to 0.96/0.05 for commercial surfaces. Thermal emissivities as low as 0.02 have been obtained in laboratories.

Other applications Selective surfaces are used for other applications than solar thermal collectors, such as low emissivity surfaces used in window glasses, which reflect thermal radiation and have high transmittance factors (being transparent) for visible sunlight.

See also Insulated glazing Solar cooker Solar power Trombe wall

References

External links Selective Surface for efficient Solar Thermal Conversion Table of absorption and emissivity for various materials

Illustrations

Selective surface: Solar thermal collector system based on evacuated glass tubes. Sunlight is absorbed by a special material at the center of each tube that has a selective surface. The surface absorbs sunlight nearly completely, and emits very little of the solar heat as thermal radiation. Ordinary black surfaces are also efficient absorbers, but they emit thermal radiation copiously.
Solar thermal collector system based on evacuated glass tubes. Sunlight is absorbed by a special material at the center of each tube that has a selective surface. The surface absorbs sunlight nearly completely, and emits very little of the solar heat as thermal radiation. Ordinary black surfaces are also efficient absorbers, but they emit thermal radiation copiously.

Worked examples

Example 1 — a first encounter with Selective surface

Start with the simplest possible case. Write down what Selective surface claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Selective surface before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Selective surface ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Selective surface

In research
Selective surface appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Selective surface in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Selective surface is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical filters, Solar thermal energy, so understanding it makes those chapters shorter.
In everyday life
Look for Selective surface outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Selective surface in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Selective surface means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Selective surface out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Selective surface in simple terms?

In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratio of solar radiation absorption (αsol) to thermal infrared radiation emission (εtherm).

Why does Selective surface matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Selective surface?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Selective surface.

Tags

  • Optical filters
  • Solar thermal energy

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