How much energy can a black metal surface absorb from direct sunlight?

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Discussion Overview

The discussion revolves around estimating the energy absorption of a black metal surface exposed to direct sunlight, particularly in the context of heating water. Participants explore methods for measurement, the efficiency of energy absorption, and the characteristics of different coatings.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests estimating energy absorption by heating water in a black box and measuring temperature changes.
  • Another participant provides a calculation of total incident sunlight, estimating about 8 kilowatt-hours per square meter per day, while noting challenges in preventing heat loss.
  • A question is raised about the realistic absorption rates for a black-painted metallic surface, with a reference to conventional black paints reflecting 5% to 10% of incident light.
  • Discussion includes the spectral reflectance of black paints, indicating that reflectivity is consistent across visible light and into the infrared spectrum.
  • Special coatings, such as "ultrablack" coatings, are mentioned as having significantly lower reflectivity, potentially enhancing energy absorption.

Areas of Agreement / Disagreement

Participants express various viewpoints on the efficiency of energy absorption and the characteristics of different coatings, indicating that multiple competing views remain without a clear consensus.

Contextual Notes

There are limitations regarding the assumptions about the effectiveness of different coatings and the conditions under which energy absorption is measured, which remain unresolved.

Who May Find This Useful

This discussion may be useful for individuals interested in solar energy applications, materials science, and thermal dynamics, particularly in the context of energy absorption and heat transfer.

FieldOpsGirl
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I'm trying to figure out a simple way to estimate how much energy (in the form of heating water) can be collected by a black metal surface of a given area exposed to direct sunlight during the day. Any thoughts?
 
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Why not just fill a black box with water, measure the before-and-after temperatures, and calculate the caloric increase?
 
The total incident sunlight is about 1000 watts per square meter when the surface is normal to the sunlight at noon, so assume that the daily incident energy is about 8 kilowatt-hours per square meter. Preventing this from being reflected back or convected away is the real problem.
 
Danger said:
Why not just fill a black box with water, measure the before-and-after temperatures, and calculate the caloric increase?
Ideally, I would do that very thing. However, if I could get some idea of power/area I could much more directly optimize the size of my container to maximize the amount of warm water I could get. i.e. If I have a small amount of water and a large area, I'm sure the water will be quite warm, but the amount of heat transfer will have decreased over time since the water will have become a closer temperature to the heating metal. At least that's my initial conception of this little device.
 
Last edited:
Bob S said:
The total incident sunlight is about 1000 watts per square meter when the surface is normal to the sunlight at noon, so assume that the daily incident energy is about 8 kilowatt-hours per square meter. Preventing this from being reflected back or convected away is the real problem.

Ah, very good. Thanks.

How much absorption is realistic for a metallic (say, steel) surface that is painted black? Is it even close to that 1kw per square meter?
 
FieldOpsGirl, I hope that you didn't think my reply flippant. It was just the best that I could think of being a non-scientist. I knew that someone like Bob or Astro would eventually give you a technical answer. :smile:
 
Conventional black paints reflect between 5% and 10% of the incident light, so figure .9 kW/m2 worst case.
 
Danger said:
FieldOpsGirl, I hope that you didn't think my reply flippant. It was just the best that I could think of being a non-scientist. I knew that someone like Bob or Astro would eventually give you a technical answer. :smile:

No problem. It was a good thought, just not quite what I was looking for this time. Thanks, though.

negitron said:
Conventional black paints reflect between 5% and 10% of the incident light, so figure .9 kW/m2 worst case.

Excellent. Is that just visible light, or also the spectral neighbors?
 
The spectral reflectance spectrum appears pretty flat across the band for standard black paints/coatings, well into the IR. The following chart compares these to a special heat-reflective black coating:

black_reflection.jpg


If you have cash to spare, there are special "ultrablack" coatings (such as certain nickel-phosphorus alloys) with reflectivities in the .1-.2% range.
 
  • #10
Great. Thank you all very much. :smile:
 

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