more on the energy absorber

to absorb all of the energy in sunlight, almost all of which are colours at wavelengths between 0.2 micron to 2.5 microns, the absorber must be all absorbing, dark. good

as we’ve seen, the absorber re-radiates (usually equally in all directions) much of the captured sunlight after transforming it into invisible near-infra-red water heating colours with wavelengths between 2.5 microns to 30 microns – sunlight does not carry much energy at these wavelengths

a blackbody absorbs as well as it radiates. i e if we were to heat up our absorber to the temperature of the sun (~ 5500 deg C), it would turn into a mini-sun, radiating the same colours, in the same energy proportions, as the sun. we can also say the absorber has high emissivity at sunlight wavelengths (emissivity = 1 for the ideal blackbody)

more interesting is the absorber’s behavior as a radiator at the temperature of 100 deg C (or thereabouts), and the games we may play therein, to our benefit. let’s take a closer look at the happenings around the hot (100 deg C or so) absorber

as we just said, the hot absorber radiates energy in the 2.5 – 30 micron wavelength band, usually evenly, i e the absorber’s surface throws as much heat towards the side where it is needed – to heat water – and, indiscriminately, a more or less equal amount of heat towards the side facing the sun – to waste away into the atmosphere. not good

once the absorber has looked at the sun for a while and reached its operating temperature (how quickly? that is defined by the emissivity at sun wavelengths – higher the emissivity , quicker the sunlight absorption), how quickly or slowly the absorber gives off this heat depends on the emissivity at these different-from-sunlight wavelengths – a different infra-red wavelength emissivity

a ‘natural’ material, such as an absorber painted matte black (why matte or non-shiny? so that the paint does not wastefully reflect away any energy carrying sunlight) has high emissivity at infra-red wavelengths. it radiates infra-red heat quickly. while quick heat transfer into water is good to make steam, quick heat transfer back to the atmosphere is not good – it is wasted heat

we want an absorber that minimizes losing heat back to the sky. we can do this by choosing (or making) a material that has low emissivity at long wavelengths

and not to worry, as water will heat up once the absorber reaches operating temperatures

as an aside – a low emissivity object, because it retains the heat it captures without radiating it away, can get quite hot. a shiny metal car seat belt buckle, provided it is exposed to sunlight long enough, can get very very hot, for, although the shiny metal absorbs less sunlight (it  has low emissivity at sunlight wavelengths – it is reflective-shiny), since it radiates less energy overall (at its preferred infra-red wavelengths), it retains more of the absorbed energy

to summarize, the absorber must:

have high emissivity at sunlight wavelengths, to absorb most sunlight

have low emissivity at infra-red wavelengths, to minimize radiative energy loss

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About Vishwa Narayan

who can live without water? drinking water for all is my goal, through Theertham, a not-for-profit organization i founded and registered in Texas. you can reach me at drvishwanarayan@yahoo.com
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