Solar Energy transmission through polycarbonate and Mylar--two layers

If my memory serves me well, sunlight is a composite of thermal and optical radiant energies; and apparently, you are interested in capturing the thermal component of the energy.

Now question #3 is a consequence of question #2, however it seems as if you meant to say "Conduction" instead of "convection" - if the Mylar does not transmit the thermal radiant energy and merely absorbs it, then the heat transfer to the water is by conduction through the Mylar.

Yet question #4 gives the impression that the optical radiant energy is also of interest to you and that confuses me somewhat.

Then again your question #5 sort of convolutes the object of your experiment it seems: You have Mylar to prevent the condensation of water vapor on the polycarbonate, so why then is this question addressing the effect of water condensates on the polycarbonate? Could it be that you are considering removing the Mylar, in the event that it does not tramsit the thermal radiant energy? Interestingly, here you also give the impression that you seem not to care about the optical radiant energy after all. A little confusing.

Rather then mirroring his questions and making it sound as if you have superiority in this area and giving some ridicule to his questions, You might actually try answering the questions, if you can.

Ridiculing him was the farthest from my mind. Perhaps, it has appeared so, then my regrets.

My intention was to better understand the objects of the questions and the interrelatedness. Now as you can see he has already explained it in the off-topic response which you are welcomed to read as well.

I think it's great that you're experimenting with this, and I think I can help clarify some of the concepts, which might help on the implementation end.

First, I think your premise about the wavelength changes in the PC is not correct - if you look at global warming diagrams, I think what you'll find is that they show the major wavelength change at the earth's surface where the light goes from the visible range to the infrared range (there is also a wavelength change coming from vacuum into atmosphere, but it's for a different reason - one is due to absorption and emission, one is due to a change in the medium the light travels through. Light of all parts of the spectrum (infrared "heat" energy and visible) will be trasmitted, reflected, or absorbed in the PC (and mylar) according to the properties of those polymers and according to the angle of incidence with the surface. Reflection and absorption are also dependant upon wavelength, so the polymer could act like a filter to an extent. There is a wavelength change within the material, but when it exits into the air on the other side, it's back to the original wavelength. Basically, in both cases, the bulk of the light for these two materials should be trasmitted at angles close to 90 degrees. Reducing the thickness might help transmit more light, but will have to be balanced with the desire to keep heat in your panel.

Where the light energy change takes place is the black surface - black surfaces are black because they do not reflect much light and because they do not emit light in the visible spectrum (and for the purposes of your application, you hope, also not much in other parts of the spectrum). So light we shine on them is converted into heat in the felt material (and transferred to your water), and to a far lesser extent, is emitted as infrared or some other wavelength.

My thought is that condensation would decrease your efficiency because of light reflection.

There will be convection inside your panel, but it won't be adding heat to your water or felt, but rather carrying it away. Insulate your panel to the extent you can and it'll help.

Great feedback! I missed the point that it is the medium that controls the wavelength and the wavelength changes back as it leaves the PC. When I read your reply, I went to Wikipedia and typed in "greenhouse effect" which explained that the effect through the atmosphere is very different from the effect in "Real Greenhouses". This was the basis of my confusion. It is the warming of the ground that warms up the greenhouse; it is not heated because the wavelengths were changed going thru the greenhouse glazing!

That being said, I no longer am concerned about the materials reducing the energy tranmission through to the Felt matt. I will concentrate my efforts on reducing the condensation effect on the Felt matt side of the Mylar which is probably blocking so much energy. The fogging is pronounced as the Mylar does not lay as flat to the Felt matt as I would like. I am considering whether something that reduces surficial tension would prevent/reduce this?

All the positive thermal energy coming into your system is in the light. Getting the light to the absorber with minimal losses (reflection instead of refraction) is a high priority.

The black body absorber will collect the thermal energy in the light and the non-thermal radiant energy it reflects may exit the system or be trapped by the polycarbonate. The perfect material for this application would transmit 100% of the visible light and reflect 100% of the thermal energy.

Most of the thermal energy lost by your system will not be lost by reflection, it will be lost by conduction. The polycarbonate will lose heat to the outside air.

Double layers will cut this loss more than in half but will double your material costs. Given the economics I would say single layers will have the best positive return on investment. Glass would be cheaper and glass under Polycarbonate should prove durable and functional with slightly lower costs than double poly but higher weight and likely sealing and fogging problems like you describe for the mylar. Double mylar would be most cost effective in terms of material costs but it would be perhaps limited in being able to restrain the migration of the water.

If the water flow can be controlled such that the first mylar is continuously drawn to the black mat material by the weight of the water trying to exit the panel then panel sealing, mylar leaking, fogging problems may be reduced, but in double pain windows the entire pane must have a tight seal and with the non glass materials you are working with water migration through the materials will likely be a ongoing issue.

If you can stop the fogging with a wetting agent layer on the polycarbonate such as acetic acid from white vinegar, one of the commercial car window sprays or similar. It seems as though hydrophilic coatings of this type perform better than hydrophobic ones like paraffin.

Best wishes,

Mr. Gee

Hello bobdriller:

I find this subject very interesting. There are so many designs for solar water heater. I am a member of a society which sells and advertises 'green' living ways. I am trying to find the link and or the Magazine which is four times a year I think. You may well have the details as you certainly have a more modern version of water heater than the one I cobbled together; just a black painted thin-walled radiator between on the back, black painted glass, and on the from a sheet of plate glass.

If you can, are you able to replace the polycarbonate with glass? And see if there is any difference in water Temp'? I found glass with silver lines painted on the inside seemed more efficient. But this was my own idea and I have not seen in anywhere else.

Will get back to you...........

Hello bobdriller:

I found this piece in this site which looks to have other items which may interest you.........

Comparative evaluatihttp://www.sciencedirect.com/science?

Comparative evaluation of the infrared transmission of polymer films

References and further reading may be available for this article. To view references and further reading you must purchase this article.

P. T. Tsilingiris

Department of Energy Engineering, Technological Education Institution (TEI) of Athens, A. Spyridonos str., Egaleo, Athens GR 122 10, Greece

Received 2 November 2002; accepted 17 February 2003. ; Available online 8 April 2003.
Abstract

The total infrared (IR) transmission of polymer films is a very important property in engineering, which determines their suitability for a specific application at a given temperature level. Aiming to investigate the total IR transmission of various polymer material films of a particular thickness, an analysis is developed for the comparative evaluation of this very important physical property. Excitation of the radical bonds in randomly oriented, long chain polymer molecules by absorption of IR radiation usually causes a large number of sharp resonances at various wavelengths and is responsible for the characteristic shape of the spectral transmission plot for the particular polymer material. The comparative presentation of results shows that at least as far as the accuracy of the available data is concerned, the total transmission of a polymer film is determined by the specific spectral transmission characteristics of the material and may vary remarkably as a function of the radiant source temperature. According to the derived results, polymer films made of materials like plexiglass, mylar, kapton and fiberglass appear to be remarkably less transparent than other film materials like polypropylene or, even more, polyethylene, which are remarkably transparent, almost uniformly, within the wavelength region under consideration. This certainly determines the potential of the particular films for use in a specific application like the design of solar collector glazing systems or convection suppression windscreens in radiative cooling panels.