Why Are The Pipes Of Solar Heaters And Containers Painted Black?
You may notice the exterior of the pipes of solar heaters and containers are primarily black. It’s natural to be curious whether there is some specific reason behind this. There are so many colors. Why do they always choose black? You can see the same thing in cookware, and mostly they are black. Here is also the reason is the same. Let’s discuss that.
Black surfaces absorb more heat than any other color. And shiny white surfaces reflect heat more than any different color. So the pipes of solar water heaters and containers are painted black to increase the heat absorption to some extent.
Does black absorb more heat, or is it a myth?
As you know, black is not a color in the visible light spectrum. Colors are nothing but the reflected part of the incident white light from any object. Technically when no amount of visible white light reflects back, we see the object as black in color. So, black is an absence of color. Hence, the object, which is black in color, absorbs the whole visible spectrum of the incident light.
Black observers are indeed sound absorbers of incident light. But keep in mind that I’m only talking about the visible part of the spectrum. Heat can still be reflected back in the form of infrared waves. The wavelength of visible light ranges from 400nm to 700nm roughly. Infrared wavelength ranges from 700nm to 1 mm (106nm).
You can see how small a part visible spectrum is of the whole electromagnetic spectrum. And if you even don’t consider the entire electromagnetic spectrum. Just look at the contribution of the IR part to the spectrum. So this is partially true that black objects are sound absorbers of light and heat.
They absorb the visible part of the incident light, and the other colors always reflect some part. So, it’s true that black things are better absorbers than any different color. But that doesn’t make a huge difference.
What about other colors?
As I said previously, colors are the reflected part of the visible spectrum. So, when we see some color, the object absorbs the complementary color(e.g., blue and yellow are complementary to each other). Every color has one particular range of wavelength. And wavelength corresponds to the amount of energy.
Greater wavelength means a lesser amount of energy. Anyway, different colored things absorb different wavelengths of the spectrum. But as black means no reflection of visible light, it can absorb every wavelength that belongs to the visible spectrum.
But, the infrared wavelength can be reflected back in the case of black and other colors. So, infrared wavelength can’t make any difference. And you can see how small range is associated with the visible lights. Also, the absorbance of an object is directly proportional to the number of sheets of the colored material (Lambert’s Law).
Darker colors absorb most of the lights that incidents upon it. On the other hand, lighter color means it reflects back most of the light. If some surface absorbs most of the incident light, the surface gets heated up quickly.
A quantitative overview
Well, you cannot measure how much more heat black surfaces do absorb than white surfaces or any other color. Also, the color of the surface is not the only factor that affects the heat absorbance of the object. But still, I am trying to provide a quantitative overview here.
A Black single-ply surface can be heated to 193 degrees Fahrenheit when the ambient temperature is 95 degrees Fahrenheit. The red tile surface gets heated to 172 degrees Fahrenheit in the same ambiance.
The ratio of absorbed heat to incident light is called absorptivity. That is a fraction that shows how good an absorber some material or color is, and it’s a dimensionless quantity that may vary from 0 to 1. You can show the comparison in the chart below.
Heat absorptivity vs. Colors
Black has highest absorptivity, while this data shows how absorptivity varies with different color of the surface.
You can also see solar reflection indexes (SRI) that tell you how much light or heat is reflected from the surface. The more light is reflected, the much cooler the surface will be. Higher SRI objects are cooler than the lower SRI ones as they reflect more light.
Black vs. transparent pipes
Some researchers at the Federal University of Technology experimented to know whether black lines are more efficient than transparent ones. They used similar tubes in each case with a 50-yard length and 1inch diameter.
They tested for 29 hours while the ambient temperature and temperature of the water initially were the same in both cases. They measured the peak water temperature in the black hosepipe to be 47 degrees Centigrade and that of the transparent pipe to be 46 degrees Centigrade.
Hence on significant temperature difference was noted in this case. There was only a 1-degree temperature difference while the sun was up. After sunset, it started to cool down by radiating heat. The lowest temperature was noted to be a 19-degree Centigrade in the case of the black pipe and a 26-degree Centigrade in the case of the transparent one.
So, it can be concluded that despite having no significant difference in the peak temperature, they lose heat differently. The rate of losing heat makes them different. That’s because of the process the water inside them is getting heated. The black pipe uses the standard conduction method for heat transmission.
The transparent one also does the same, but in this case, there is a greenhouse effect. Due to this effect, transmitted heat cannot come out and radiate back to the surrounding when the ambiance is cooler. So it resists the heat from radiating back from the water inside the pipe.
What are the other ways to increase heat absorption?
Heat absorption also depends on the material used, as I said earlier. Using thermal insulation is another popular way to increase the efficiency of solar heaters. Indeed, insulation doesn’t help to increase heat absorption. But it reduces the rate of heat transfer through the walls.
That, too, is necessary to improve the performance as it’s no use having an excellent heat absorbance if it loses heat quickly. Insulating materials are generally put between two metal walls as most are fragile. Some popular insulating materials include fiberglass, mineral wool, cellulose, foam, and urethane.
Conclusion
To conclude, I must say that even if the color black doesn’t make a huge difference, we still count what it does. Black surfaces are proven to absorb a somewhat greater amount of heat than the other colors.
Insulation is also an essential and popular material to increase the efficiency of solar heaters. As these devices solely depend on sunlight, we must provide them with every possible condition so they can receive and absorb the possible solar heat and light.
