energy absorber

the absorber must absorb as much of the sunlight upon it and get hot

a good absorber must:

i) absorb as much sunlight as possible

ii) convert into heat as much of the absorbed sunlight as possible

iii) minimize wasted heat radiated (mostly) to the atmosphere

when does an object appear black? when all the light striking the object is absorbed by it, leaving no light at all for our eyes to detect. if the object instead were to reflect away all of the light upon it, there would be plenty for us to see and it would appear white. and objects of other colours reflect that colour, and that colour only, while absorbing (or allowing to pass through, or scattering away) all other colours. light coloured surfaces reflect more, dark coloured surfaces absorb more. when sunlight strikes a dark object, such as the face of our absorber, (we will design the absorber and choose specific materials later) most of it gets, what else, absorbed, and heats up the absorber

now, all hot objects throw off heat and the absorber is no different. the hotter the absorber the more heat it gives off, usually to the atmosphere, which is usually cooler than the object. heat is like water. water flows from higher up to lower down. heat ‘flows’ from hotter places to colder places. until everything is level. until there is nothing left to flow anymore

ideally, the absorber devours the energy borne by each of the wavelengths of sunlight upon it, converts all of the sunlight into heat of the kind, i e of the wavelengths that water can absorb, without radiating much of it wastefully away to the atmosphere, or otherwise losing any of the heat in any manner

recall the ‘blackbody’ construct, an ideal world concept, a body that is so black that it absorbs every last wink of energy, without reflecting, passing through, or otherwise losing any of the energy. more about blackbodies at http://galileo.phys.virginia.edu/classes/252/black_body_radiation.html

so it is clear the absorber, to work best, has to be ‘black’. how ‘black’? ideally, totally totally totally black. in the regular world, we’ll settle for ‘black as we can get’

here is the energy that 1 square meter of the sun’s surface emits as sunlight at each colour, assuming a sun surface temperature of 5500 deg C (+273 to convert to K, = 5773 deg K)

sun radiates most energy at wavelengths between 0.3 to 3.5 microns

the most energy delivered by any one colour is at a wavelength of 0.5 micron – cyan blue green – slam bang in the middle of the visible wavelength band

50 % of the energy in sunlight is in the 0.7 to 3.5 micron near-infra-red wavelength band

37 % of the energy is in the 0.4 to 0.7 micron visible wavelength band

12 % is in the 0.1 to 0.4 micron ultra-violet wavelength band

the remaining 1 % in other wavelengths

energized by this influx of sunlight, the absorber captures all the energy in sunlight across wavelengths of 0.2 micron to 3.5 microns , and after due time, reaches a surface temperature of 100 deg C (or 373 deg K)

the 100 deg C hot ideal or blackbody absorber, which is also an emitter, in turn, throws off heat evenly every which direction, in the following infra-red colours (our eyes do not detect these colours but infra-red cameras do) – a different set of colours from those of the sun

100 deg C blackbody radiates 93 % of its energy at wavelengths between 3 and 30 microns

the most energy delivered by any one colour is at a wavelength of 7.76 microns – well into the invisible infra-red band of wavelengths

more than 93 % of the absorber’s energy is radiated within a wavelength band of 3 microns to 30 microns

being much much hotter, the sun puts out energetic wavelengths of high frequency. the 100 deg C absorber, being much cooler than the sun, puts out energy as an infra-red heat source, at much longer wavelengths

many thanks to http://www.spectralcalc.com/blackbody_calculator/blackbody.php for their excellent online blackbody plotter and calculator

and from which wavelengths of light (= heat) does water prefer to absorb energy well?

100 deg C blackbody radiates right into water's absorption band

water absorbs well heat or electromagnetic energy at wavelengths from 2 microns to well over 100 microns – comfortably enveloping the 100 deg C absorber’s radiation spectrum of wavelengths 3 to 30 microns

armed with this information,  we look to design a practical absorber

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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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