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

In summary, conventional black paints reflect between 5% and 10% of the incident light, so a .9 kW/m2 worst case can be estimated. Special heat-reflective black coatings can have reflectivities as high as .1-.2%, if you have the cash to spare.
  • #1
FieldOpsGirl
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0
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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  • #2
Why not just fill a black box with water, measure the before-and-after temperatures, and calculate the caloric increase?
 
  • #3
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.
 
  • #4
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.
 
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  • #5
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?
 
  • #6
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:
 
  • #7
Conventional black paints reflect between 5% and 10% of the incident light, so figure .9 kW/m2 worst case.
 
  • #8
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?
 
  • #9
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:
 

What is energy absorbed from sunlight?

Energy absorbed from sunlight refers to the process by which light from the sun is converted into usable energy. This energy can be harnessed through various technologies such as solar panels and used to power homes and businesses.

How is energy absorbed from sunlight?

Energy from sunlight is absorbed through a process called the photovoltaic effect. This involves solar cells made of semiconductor materials, such as silicon, which convert the sun's light into electricity.

What are the benefits of energy absorbed from sunlight?

The benefits of energy absorbed from sunlight include reducing our reliance on fossil fuels, which contribute to air pollution and climate change. It is also a renewable energy source, meaning it will never run out, and it can help reduce electricity costs for individuals and businesses.

What are the challenges of using energy absorbed from sunlight?

One challenge of using energy absorbed from sunlight is the initial cost of installing solar panels. These can be expensive upfront, although the long-term savings in electricity costs can offset this. Additionally, the efficiency of solar panels can be affected by weather conditions and the angle and location of the panels.

How can we increase the use of energy absorbed from sunlight?

To increase the use of energy absorbed from sunlight, we can continue to invest in research and development to improve the efficiency and affordability of solar technologies. Additionally, governments can provide incentives such as tax credits to encourage individuals and businesses to switch to solar energy. Educating the public on the benefits of solar energy can also help increase its use.

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