Water Walls

You may copy whatever you want from these web pages for personal use or for academic purposes. You may not copy information from this web site for monetary gain.

Most objects only absorb energy in very specific wavelengths. The peak absorption spectrum for water is around 3000 nm (3 micrometers) but absorption increases significantly at around 1,450 nm (1.45 micrometers). The peak of the absorption spectrum is neither in the range of direct sunlight or in the range of long wave IR radiation emitted from common materials that have heated due to direct solar gain.

A very small percentage of the incoming solar radiation is near the 1.45 um mark. Water used as a thermal mass which is not experiencing direct solar gain must rely on conduction heat transfer from the air or another object that it is in contact with. Most objects right around room temperature are emitting radiation in the 8 - 25 micrometer band.

The following image shows the sunlight wavelengths that water absorbs energy at. It does not display how strong the absorptance is at any particular wavelength. The text describes that.

http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water

Water absorbs energy best in the UltraViolet range. Its absorption of energy is poor in the visible portion of the spectrum. The ability of water vapor, not liquid water, to absorb energy varies over many frequencies rising to about 2500 nanometers (2.5 micrometers) where it has a peak. This is right on the line between Near and mid IR. After about 1 micrometer, its ability to absorb energy falls off again.

For water vapor, strong absorbance occurs at wavelengths of 2500, 1950 and 1450 nm. Weaker absorption takes placea at 1200 and 970 nm. Some absorption will also occur at 930, 820 and 730 nm, which are all in the IR spectrum. For liquid water, shift each number by 60 nm in the longer wavelength direction.

Adding pigment to water may help gather energy from visible light upon the water. In this case, it is not the water that is effected rather the pigment. Colors that absorb visible light well such as black, may be reflective in the IR portion of the spectrum. White paint, either latex or oil based, is an excellent absorber of IR energy although it is an excellent reflector of energy in the visible light range.

Most of the sun's energy in the IR portion of the spectrum is between 750 and 1500 nm.

Given the poor absorption of water in the wavelengths of direct sunlight, it may make more sense to use a metal like chrome, in contact with the water thermal mass. The chrome will heat up and impart energy to the water mass via conduction, just like a burner on a stove.

Return to main page on modeling basic solar home design Click Here