Reflective Material

I would look for some kind of roller that can form aluminum foil to fit the parabolic dish; the foil itself will not be costly.

At more expense, it might be possible to use metal spinning or stamping to form specular stainless steel into a parabolic reflector.

Get some metallized polyester (Mylar) film. Cut it into pie shaped sections and apply to the dish.

Hi Lyn

I have looked at Mylar however although it will work, I'm looking for something more reflective.

If I can find a mirror finish similar to the finish found in a torch lens reflector or a car headlight....

How is that reflective coating/process achieved?

The metal is vacuum deposited in a continuous reel to reel process.

http://www.google.co.za/#hl=en&source=hp&q=reflective+mylar&oq=reflective+mylar&aq=f&aqi=g10&aql=&gs_sm=e&gs_upl=12108l18389l0l29357l16l16l0l5l5l0l328l2377l0.5.5.1l11l0&fp=50604dd709f6caad&biw=1312&bih=820

You could use acrylic mirrors which are somewhat flexible. If you need a really steep curve you could bend them using a heat gun or an oven.

Another option is the reflective (mirror) film made by 3M. It's very flexible.

The MOST reflective cheap material is supposed to be brushed (not anodised) aluminium. (even if it's not mirror look) You can use that and occasionaly clean it with polishing cream. S.M.

The cheapest material that you can find for your manufacturing process can only be found by you. The thermal reflective quality of the foil, paint, sheet metal, glass, tape, or other reflective material will certainly be a pivotal attribute in your solar cooker design. But as the Florida Solar Energy Center found out, some preferred materials may be no longer available. Many different approaches have tackled this problem of a low cost solar cooker by using a variety of different reflector materials. All of these designs have advantages and limitations that each respective designer chooses to utilize or work around. Depending on which attributes (portable, tracking, multi-purpose, temperature control, etc.) the final design is supposed to have, will guide you to which approach you should use.

Since your goal here is a solar cooker, I do not understand your desire for a mirror finish. The light does not have to be focused to image quality on the cooking utensil. Actually you'll be making a new problem with a sharp image by having uneven heating of the utensil.

He didn't actually say he was going to place the heating vessel at the exact focal point.

A well-polished mirror-like surface will direct nearly all of the sun's energy toward the focal point. He can place the heating vessel at, or just inside, the focal point to capture the energy. An irregular, brushed or matte finish will scatter a lot of the light and be less efficient.

I agree that our OP did not claim that he will be placing the heating vessel at the exact focal point. However, by having a well defined focal point the operator of this solar cooker must now keep track of heating vessel placement. IMHO I believe the loss in efficiency (slight by my guess) from using a smooth but matte finish reflector will be more than made up from not having to critically readjust the geometry of sun, reflector and heating vessel during the cooking process. But this is not my design. I may just be projecting my concerns from a 40°N latitude perspective onto a 33°S latitude situation.

The dish is the correct shape. Get some glass mirror stock, make sure the back is painted with an epoxy paint and cut them into one inch squares. clean the parabolic surface and place a small dab of silicone sealant on the back of each square and press them into place in as close a pattern as you can manage, making sure the glass edges meet the metal (small dabs). This will give you a mosaic that covers about 90% of the surface (cutting the glass into rounded sections will increase the %, but is harder to do)

Glass is about 95% reflective = over all about 85-88% +/-, when new. remove dust and point down in the rain to avoid weathering.

This will give you a good cooker, with a reasonable cost - about $20 for the mirror squares and $5 for the silicone.

Do not use acrylic, or siliconized acrylic = frauds that trade on silicone. GE and Dupont make is. With proper dabs one tube should do it.

A spun aluminium parabola will cost lots more and be about 5-7% better. Best is a perfect glass parabola, 97-98% , front surface coated with silver, but is loses % fast from sulfur in the air. Front surface aluminium is about 96-98%

a 1.6 meter parabola = 2 meters in area and in full sun will collect about 2800 watts that shines onto your black parabolic pot = twice the heat of a 1500 watt frying pan - if you can deal with the geometry of the pot suspension, etc. Any shadows cost heat, save for the pot. Good luck,

That would be my approach as well. You may be able to find 1" mirror tiles on a nylon mesh backing for quick and easy handling. You may be able to find a cheaper, easier to use glue than epoxy. Given the different rates of expansion, something like silicon seal or construction adhesive might give better results, as long as you get the tiles aimed right, not floating on too much glue.

do not use epoxy glue - it will fail on glass. Water proof epoxy pain for the back. Use silicone to stick it

You might try this:

http://www.youtube.com/watch?v=SjT1dPeXX9o

For real cheap it would be hard to beat used compact discs and DVDs. If you already have a dish, like a satellite aerial you could just wire or glue them on. What it lacks in efficiency can be made up for by increased area.

It would also be possible to cut and fit the pieces of disc together like a mosaic, but that is a lot of work.

If you have a satellite aerial you could extend it by attaching lengths of wire to continue the curve and cover these with CDs.

BAB

I think the reflectivity of CDs is too low ~~25% or even less,due to a high ratio of off-axis reflectivity plus low reflectivity of the good areas and void areas.

I agree, but if you double the diameter you make up for the poor performance so long as there is no size restriction.

BAB

If you would have viewed the video link I posted you would have seen a fella lighting a piece of newspaper from a triangular parabola using CD's for a reflective surface. Regardless of what their reflectivity is they are capable of doing the job. I have seen one video of a guy using a six foot round dish, and he made the statement that with having as many CD's as he could pack on there, he was getting 850 degrees F.; so, he had to take some off for his purposes to lower the temperature.

Yes, I did see the video, and he did light paper:- but the whole apparatus was woefully inefficient, since the CDs were so low in reflectivity. A high efficiency unit would make the paper burst into flames instantly - have done this. It looked like an attempt to find some use for old CDs.

searches on the topic

Images

Videos

Look through these searches and see what hints you can find.

He would have done better by using sticky aluminium shelf paper (which would also conform better and cover 100% of the area. There are also sheets of plastic coated with aluminium - mylar type - but make sure you buy the zero transmittance type - some are so thinly coated that a large % shines through.

Been there done that. Even on the underside of the satellite dish it lasted about 6 months and all just disintegrated. I used it as a garden light with a spot light from below.

I think I would vacuum seal an elastic foil over the satellite dish (We called it astronaut foil back then). OP should suck it onto the pre-glued (epoxy) surface and after curing unplug the vacuum pump. Perfect parabola.

I've never done it but I would give it a try. Maybe even heat the foil?

It's my birdbath nowadays

Hope all goes well, Ky.

Space Blanket?

so far this is the cheapest mylar, I've found

That's the stuff. Mylar, aih. We stretched it over Styrofoam for film set lighting. It was quiet durable and cheap and not as heavy as some.

Hi Dali,

I would suggest trying to by some flat sheet reflective plastic of some form, and then using your parabolic dish have it vacuum formed to that shape.

I have no idea what kind of plastic you would need to get the finish that you require, so I suggest you talk to a plastic vacuum forming expert to give you the advice needed, I am only making the suggestion, I dont know the result.

Best Regards

Joe

I have looked into it a bit. Parabolic dishes need pretty constant very accurate tracking. A coupe of years ago I tried to design a dish that only needs crappy tracking of the sun. (I used laser pointers and I called the design thing the solar design t-square. My version is clam shaped and aligned with the path of the sun. The cooking pot is surprisingly high near the wall of the clam. The only problem with the clam is "aligned with the path of the sun". So you basically need an equatorial platform to mount it on because it is difficult to know exactly where the sun will be in 2 hours time. You can find an album about it (and other designs from different people) at http://solarcookers.ning.com/

I believe it is currently way too hard to provide the accurate tracking that full dish parabolics need and that you should try a different approach. Either a half parabolic dish on equatorial platform with timed tracking or something like the horizontal troughs in the following link http://www.eps.hw.ac.uk/~tm74/thesis/chap1.pdf

(Nobody tries these curves because nobody knows about them). I found this pdf about 6 months ago and it was a breath of fresh air to me.

Thank you all contributors.

Think I will go either the heavy foil or 1" mirrors route

Now I need ideas on sun tracking.

The pot stand will not be attached to the dish and will be approx 1.5m high and basically is just a 20mm iron stake driven into the ground, onto which I will weld a pot stand/rack which will swivel on the stake (so if the focal point is too hot, I will be able to swivel the potstand so that the other side of the pot gets the heat. I will only use heavy black cast iron pots

My ideas on the tracking of the sun are:

a) to cast a concrete base approx. 3m x 3m

b) embed a circular steel pipe track in the concrete which would have a diam of approx 2.5m. The focal point for my dish is approx 520mm

c) attach the dish base stand onto the rail with a tracking wheel not unlike a gate guide wheel sliding mechanism

d) add two rubber wheels on the inside of the base stand which will run on the concrete

This will allow the complete dish and it's stand to travel in a circle around the pot stand, with the dish remaining at the optimum angle. This would allow for precise sun tracking however I need your ideas for how to mechanise this rig with a timer so that this will all happen automatically.

I think I may also need a vertical correctional travel machanism as well which would allow the dish to tilt vertically

For tracking the Sun East to West:

Why not just stick a small photovoltaic panel in the shade behind the reflector so that when the sun shines on the panel it drives a motor which is then geared down to move the reflector. OK it will not work when there is cloud cover but it will catch up pretty quick when the sun comes out:-

__________________

Tracking is much easier if your concrete base is an equatorial platform. So you have to tilt it to the north at 90 minus your latitude. (Because you are in South Africa) That means that once your dish is set up in the morning, you rotate it at 15 degrees per hour round your pot and it keeps it at the focus all day.

If your concrete base is flat on the ground you can check out http://solarcooking.wikia.com/wiki/Solaria for 2 designs. Either one might work for you