Tuesday, December 27, 2016

Pieces of art to bring rain

See how to build a solar air heater at
Your piece of art can be built in the shape of a solar air heater and the dark painting inside can absorb solar energy.
To heat up air well requires hot surfaces for the air to come into contact with. Black objects absorb solar energy well and so if you have black gauze or a black wire mesh that has a large surface area that air can come into contact with and the surface is hot, you will heat the air well. When a black object heats up it radiates heat and this heat will generally be lost to the surroundings. The glass (or other glazing) on the solar air heater keeps heat in, but even without it you can heat up the air with a piece of art made of black material, such as a wire mesh, that stands in the sun.
For those interested: The book Heat Transfer by JP Holman tells us that ordinary window glass transmits radiation up to about 2.5 microns (the energy goes through the glass if its wavelength is less than about 2.5 microns).
Now the question is: How much solar energy has wavelength of less than 2.5 microns?
The answer is 'about 97% of solar energy.' So about 97% of the solar energy passes through the glass and is absorbed by the black surface of the solar air heater (assuming a perfect absorber).
Now how much of this energy escapes? Well if the absorber heats up to 50 deg C, then radiation from it that is above 2.5 microns will not escape. The answer is that far less than 1% of this radiation can escape through the glass. The reason is that the energy of wavelengths greater than 2.5 microns radiated from a black body at 50 deg C is more than 99% of the total.
If there is no glass and your piece of art is at 20 deg C, it will radiate 419 W per square metre. If your piece of art is at 60 deg C it will radiate nearly 700 W per square metre. You do not want heat loss by radiation. Instead you want the heat to heat the air so there can be convection - so it is better to have your dark piece of art in the solar air heater. "Cool roofs" can reduce rainfall and "dark art in greenhouses" can increase rain - see https://www.scientificamerican.com/article/cool-roofs-may-have-side-effects-on-regional-rainfall/

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