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

[FieldOpsGirl]

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?

 

[Danger]

Why not just fill a black box with water, measure the before-and-after temperatures, and calculate the caloric increase?

 

[Bob S]

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.

 

[FieldOpsGirl]

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.

 

[FieldOpsGirl]

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?

 

[Danger]

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.

 

[negitron]

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

 

[FieldOpsGirl]

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.

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

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?

 

[negitron]

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:

If you have cash to spare, there are special "ultrablack" coatings (such as certain nickel-phosphorus alloys) with reflectivities in the .1-.2% range.

 

[FieldOpsGirl]

Great. Thank you all very much.