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# Sizing a Parabolic Reflector

03/31/2009 9:26 PM

Can anybody advice me how to calculate the size of a parabolic reflector needed to focus a certain amount of heat energy onto a spot? I'm thinking of creating a low cost solar-thermal device using recycled material for use in impoverished Third World nations.

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#1

### Re: Focusing solar energy

03/31/2009 11:13 PM

Does this work, or were you looking for more details such as the type of glass and reflector material (in which case you will need to get another input)?

Divide the power you need by the power density of insolation (incoming solar radiation) and that will give you the capture area of the dish:

Area (m^2) = Power needed (W) / Power density (W/m^2).

Wikipedia cites 250 W/m^2 as an average insolation number.

Another factor is that the reflector won't be 100% efficient - it will absorb some energy itself.

Therefore you will need to correct the theoretical area calculated above for the inefficiency of the reflector:

Area actual (m^2) = Area theoretical (m^2)/η,

where η is the efficiency of the reflector:

η = reflected power density/incident power density.

Of course, this all assumes that average level of insolation being available. If you need it to work on sub-average days, the dish has to grow in area proportion to the lower insolation value.

And if the amount of heat delivered must be controlled within an upper as well as a lower bound, then you will need some sort of reflective iris near the target or similar device to reduce any insolation above the target level.

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#2

### Re: Focusing solar energy

03/31/2009 11:20 PM

Thanks. Looks like what I'm looking for.

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#3

### Re: Focusing solar energy

04/01/2009 4:05 AM

The values for power density go (according to data published for the solar centrals in USA) up to 1kW/m^2.

The suggestion with an iris is excellent.

There is also the problem of orientation during the day to be analyzed if needed or not.

For cost reasons small mirrors have been made with several plane "mirrors" fastened on a frame and with a cylindrical collector.

One of the problems requiring a good solution is the absorbtion capability of the collector.

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#4

### Re: Focusing solar energy

04/01/2009 4:12 AM

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#5

### Re: Sizing a Parabolic Reflector

04/01/2009 11:41 PM

full sun on a clear day from 10 AM to 2 PM with the sun overhead at noon will give you about 1000 watts. Your reflector will be about 85-95% efficient, with a bright first surface mirror reflector of chrome or silver being best Aluminium a few % less. Of course a bad parabola will reduce it as well, so a safe bet may be 85%. Sun angle earlier or later will also reduce the insolation, as will dust or cloud cover. Higher latitudes will have more hours of high insolation in the summer and shorter hours in the winter.

Your cooking container will need to be insulated to reduce losses.

Now a 1500 watt electric heater runs a hot water kettle, so you will need about 1.8 square meters to boil the same amount of water in the same time on a hot day. This will also work for frying, but I would suggest you arrange things so your focal point is directed at a mirror you can steer to bring the focussed energy into a vertical plane and arrange a height adjustment to shape the hot spot to the bottom of your cooking container. Use an adjustable shade in front of the parabola to reduce heat to avoid burning stuff.

All doable and being done all over

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#6

### Re: Sizing a Parabolic Reflector

04/01/2009 11:46 PM

Okay, thanks. That really helps.

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#7

### Re: Sizing a Parabolic Reflector

04/02/2009 12:53 AM

1) Also make the Parabola to cover 180 degrees and in rotation to track the sun from sun rise to sun set.

2) Let the pipe line run along the focus point.

3) I believe you know how to make the parabola with focus point as per mathematical calculations. Y2 = 4a x.

4) Try to concentrate on material reflectivity as well with shining surface. Worka manship has lot to contribute here. Highest relective index possible- May be aluminimum or better material.

5) Insulating the pipe line will get the better results.

6) Any aditives in water to aborb heat for better heeat transfer.Any other fluid in place of water can be thought of.

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#8

### Re: Sizing a Parabolic Reflector

04/02/2009 1:26 AM

I have been experimenting with solar cookers for some time with conical cooker and parabolic.

The parabolic cooker with about 30" diameter can cook rice (150gm) in about an hour, but burns at focal point after that.

I found conical cooker made by a friend more suitable. The diameter is 28" and can cook in same time(1 hour). There is no need to focus again once the food is kept for cooking and is focussed with the help of shadow of washer mounted on a bolt. The weight being hardly 2kg, this is a very good practical design

The cost of the cooker is about \$35(Rs. 1600) but there are no takers. The awareness needs to be increased. In fact, last year, we distributed 10 such cookers in tribal area, but there is no feedback. I have given below a picture of the cooker with a friend of mine.

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#10

### Re: Sizing a Parabolic Reflector

04/02/2009 2:47 AM

I believe this needs good marketing and visibility. Some channels could be

4) Taking a video of rice cooking and poutting on some social site.

This is a good cause and need to be taken as social and sustainability initiative.

shivaram

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#13

### Re: Sizing a Parabolic Reflector

04/02/2009 5:37 AM

In fact, in our country, this would be a very useful thing. I am trying to reduce the cost of the cooker to less than Rs. 1000(Material cost, no profit), so that people in villages can afford it. Here, in tribal area, the school children spend 2 to 3 hours every day just to gather firewood. But they do not have money to buy solar cooker. hardly 30 cookers have been made in last 4 years. I was president of Rotary club last year and We distributed ten in a tribal village.

Thanks for your suggestions. I will try these things.

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#16

### Re: Sizing a Parabolic Reflector

04/02/2009 11:57 AM

Cool! Where can I get one?

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#26

### Re: Sizing a Parabolic Reflector

04/05/2009 1:20 PM

Great idea for Nomads living in the desert. But you're looking at a cost of 10 days labor in the third world, or 1 year in the outback. Not to mention the covetiveness inspired keeping up with the Jonses'. 20 or so years ago I heard of a mud/brick oven that ran on a few sticks of wood with about the same cooking time, with enough residual heat to make congee from rice, or yougert from boiled milk. I think it showed up in Mother Earth news as a solution for areas where deforestation for cooking was wrecking havoc.

Got Mud?

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#9

### Re: Sizing a Parabolic Reflector

04/02/2009 2:23 AM

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#11

### Re: Sizing a Parabolic Reflector

04/02/2009 3:41 AM
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#12

### Re: Sizing a Parabolic Reflector

04/02/2009 3:46 AM

Great, thanks.

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#14

### Re: Sizing a Parabolic Reflector

04/02/2009 9:12 AM

It may not be quite what you are looking for but I found an online calculator for microwave dishes that might interest you. You can leave the frequency as it is, but give in your dish size and depth and it will give you the focus angle......will work for sunlight as well!!

http://my.athenet.net/~multiplx/cgi-bin/parabolic.main.cgi

By the way, do not focus your sunlight too much as this will give a too concentrated heat in one place, this will cause burning of your food. You need a loose focus that heats the whole of the base of the cooking pot at the same time.....

An "Offset" reflector, upside down, will allow you to place the cooking pot above the dish. I hope my simple diagram will help:-

The pot stand and the reflector need to be mounted together so that the pot is always horizontal, no matter at what angle the reflector is......

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#15

### Re: Sizing a Parabolic Reflector

04/02/2009 10:41 AM

I guess it's too bad those pesky rebels blew up the death star.

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#17

### Re: Sizing a Parabolic Reflector

04/02/2009 1:13 PM

If it is non tracking, it should not be parabolic because parabolic goes quickly out of focus. So you have the poor cook standing in the 35 degree heat under the sun moving the dish every 15 minutes!

I have a project where I try to use "art of illusion" free modeling software to compare different solar cooker reflector curves as the sun moves overhead. Art of illusion is the easiest modeling software I have ever found.

http://www.instructables.com/id/Use_software_to_design_a_better_solar_cooker/

Anyway, a hemisphere, a compound parabolic dish and a w curved dish (taking one of winstons designs out of context) ALL beat a parabolic dish as a nontracking dish. The software also allows you to model cookit's and other panel cookers. (I am not proficient at it, so it anyone else wants to draw them up in art of illusion I will put them into the scene and we can see how good they do.

The scene file is available for download and it is a joint production between me and others.

More help is welcome.

In the calculations for heat absorbtion for a solar cooker, nobody mentioned that much of the heat never makes it throught the pot surface. much light and heat gets re-radiated almost immediately.

I would guess that the pot surface re-radiates almost half of the heat before it ever reaches the food.

Brian White

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#18

### Re: Sizing a Parabolic Reflector

04/02/2009 1:33 PM

well, your cooking pot needs to be black and what it emits is a function of it's temperature. With most metals it will be the water temperature.

The sun is a radiator at 4000 to 5000 kelvin, so the equilibrium will strongly favour the heat entering the pot. A portion will radiate, and portion will evaporate as well with the boiling of water. A frying pan will be somewhat hotter, so will radiate more.

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#29

### Re: Sizing a Parabolic Reflector

04/05/2009 10:41 PM

Does it radiate straight back to the sun? I think it radiates in all directions and conducts and convects a lot too. With solar cooking I have not yet approached 4000 kelvin. What am I doing wrong?

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#20

### Re: Sizing a Parabolic Reflector

04/02/2009 5:26 PM

With regard to re-radiation, I have a few (unproved) ideas for you:-

1) Buy black pots, there are plenty of thick sided aluminium pots with a dull black finish all over. Use lampblack to make any shiny surface black.....or stove blacking or car exhaust paint that handles upto 600Â°C.....

2) Insulate the sides and the top with fiber glass or some other material in some way....the top insulation should easily removable and the pot should have a glass lid only....

3) There are long metal pots for the oven that might be better, the suns focus could be held longer without adjusting anything for example, also they are often made in black....

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#19

### Re: Sizing a Parabolic Reflector

04/02/2009 5:13 PM

The cheapest solar reflector I have seen was made from flat sheet aluminum,with a spiral cut starting at the center and progressing outward. To achieve a parbolic shape, spacers were located at strategic points to hold the spiral layers to conform to the parabolic shape.It was very light,easy to transport (it layed flat) and set up and very inexpensive.

Good luck --------------SSB--------

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#21

### Re: Sizing a Parabolic Reflector

04/02/2009 8:37 PM

Okay, thanks guys, your suggestions were most helpful. As for the Death Star, well, I did cut off my son's right hand as payback for it.....

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#22

### Re: Sizing a Parabolic Reflector

04/03/2009 7:19 AM

Now I know that you are not "Darth Vader", you are "Daft Ada"!!!

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#27

### Re: Sizing a Parabolic Reflector

04/05/2009 8:41 PM

Yeah, dumb of me, isn't it.

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#31

### Re: Sizing a Parabolic Reflector

04/06/2009 3:16 AM

I was joking Sir Darth, honest!!

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#23

### Re: Sizing a Parabolic Reflector

04/03/2009 9:15 AM

I couldn't resist with that comment. When I saw the topic, the question "Can anybody advise me how to calculate the size of a parabolic reflector needed to focus a certain amount of heat energy onto a spot?", and that DVader posted it, I instanly thought it was some sort of April fools thread in which the "spot" that would need the heat energy concentrated on would be a planet...Alderan maybe.

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#24

### Re: Sizing a Parabolic Reflector

04/03/2009 9:39 AM

Humour is also a requirement for the CR4 blogs!!!

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#30

### Re: Sizing a Parabolic Reflector

04/05/2009 10:55 PM

Aw, you caught me out. GA for you from me for it. Anyway, it was Grand Moff Tarkin who made the decision to blow up Alderaan, not me. I was too busy torturing my daughter at the time, except that I didn't know she was my daughter back then.

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#32

### Re: Sizing a Parabolic Reflector

04/06/2009 3:19 AM

How damned awkward!!!!

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#33

### Re: Sizing a Parabolic Reflector

04/06/2009 3:25 AM

And all this time I've always wondered why I never get anything from either of them for Father's Day.

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#34

### Re: Sizing a Parabolic Reflector

04/06/2009 3:29 AM

Well at least you have cleared up that important point!!

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#35

### Re: Sizing a Parabolic Reflector

04/06/2009 3:36 AM

You were right about me being daft. I mean, my son looks like me, my daughter looks like her mother, and I couldn't recognize either one of them?! No, wait, I did recognize my son. So why couldn't I recognize my daughter then I wonder.....

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#25

### Re: Sizing a Parabolic Reflector

04/03/2009 11:24 AM

People will not stop offering you help on this subject, no matter how many times you say "thank you" as a polite way of terminating the continual offers of help. Remember the expression: Be careful what you wish for [because] you just get it?

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#28

### Re: Sizing a Parabolic Reflector

04/05/2009 8:43 PM

Thanks for the consideration, but my mother raised me to always show appreciation to anyone who helps me.

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#36

### Re: Sizing a Parabolic Reflector

11/01/2010 10:12 AM

While viewing this thread, an idea occured to me:

I have a solid fiberglass 10 ft.Diameter satelite dish that has been obsoleted by the digital age.What would be a good coating to apply to the fiberglass to reflect maximun solar energy to the focal point? How about aluminized self adhesive tape?

Also, is there an efficient thermal powered alternator that would be able to utilize this heat efficiently,using possible NH3 or Freon as boiler/turbine fluid?

What would be a reasonable expectation of the electrical output of this system at a lattitude of N36 degrees?

The system was originally designed to track an equatorial arc, (polar mount), but I can build a tracking system for tracking the sun, I have some old actuator positioners and controllers, so horizontal and vertical programming of the dish is possible, but will require some minor modifications of the polar mount.

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#37

### Re: Sizing a Parabolic Reflector

11/01/2010 1:20 PM

an excellent recycling idea... and fortunately, an abundance of previous attempts.

If nothing else, just line them with old CD's

Chris

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