Solar Cooking: Pot Volume vs. Pot Surface Area

Hello gaiatechnician:

This sounds like an interesting idea.

And, no offence intended OK?...............Look at the pots already designed and what they are used for.

The very shallow and wide pan for a wide heat source, is made to heat food, usually with the use of fat or oil of some kind, quickly, with little steam. The steam is created but allowed to evaporate almost instantaneously.

The other extreme is a stock-pot, which is taller than it is wide, which is designed to prevent loss of liquid as steam and is designed for use on a narrow (less powerful) heat source.

I am no expert OK? With your 'theory' I think it would be more beneficial and heat saving if, when you talk of doubling the volume, you did this with a pan in the same width range.

Once the pan and contents have reached optimum temperature, it seems to me that a tall narrow pot will need the same amount of heat to continue cooking, even if it was twice as high.

This has nothing to do with the heat source, but if you are talking of solar heating the pot, it would be a very simple experiment to try a pan which is 20 cms wide and 10 cms high. Compared to a pan of 20 cms wide but double the volume (approximately) and, 20 cms high? Or am I way of here?

Pots in a modern kitchen were designed over a long period of several hundred years at least, and shapes and sizes have evolved for specific ways of cooking. I cannot see why this fact can be carried forward into your idea.

Thank you for your replys. I will not have much chance to continue experiments due to the cloudy season (It is like a fall monsoon here sometimes!) I have a new site with details that is easier than the utube site on bandwidth http://www.instructables.com/id/Make_a_quotcompoundquot_parabolic_solar_cooker/ and it has the pdf file for download on the last step. My results compare not too badly with those of a sk14 cooker in india which can be viewed at www.barli.org/documents-PDFs/solar-cookers2.pdf Their reflector was about twice as big as mine and their pot was bigger too. I had cloudy periods in both my 7 liter water heating test runs and I put in a graph to allow people to take account of that. I think the thermal absorber thing is a good idea. I think aluminium is as good as or better than copper for heat movement through the metal and its oxide layer is probably harder too. I like the idea of heating iron, or limestone (I have seen black limestone) or soapstone to a higher temp and cooking on it later. My points in the question was about the heat getting into and out of the pot too. Basically with twice the volume of pot, subjected to twice the amount of solar energy, the surface area is not double. So will it be able to transfer the heat into the liquid fast enough or will the heat just re-radiate immediately off the hotter surface and be lost? For cooling down, retaining the heat a long time, I think the bigger pot will definitely be better. (But for heating up, that is my question). With my design of solar cooker the heat can be mostly on the bottom and the back of the solar cooker. (You can say evenly all around if you like for doing the calculations). Brian Thanks for the comments

Hello gaiatechnician:

I thank you for getting back to me. I have not had time to look at your site. But will as soon as I can.

In the UK a 'Solar cooker' rather like a sky TV antenna, can be bought from camping shops.

I think as has been mentioned in other posts on this thread, a sealed pressure pot is more efficient because any heat gained can be used for cooking.

I think at least one post has mentioned a doubling in pot size in both the height and width. Doubling the size in height will double the volume. I think doubling the size of the pot in width and height I think increases the volume by four? I know you did not say this but, it has been said.

As long as there is costant.........or should that be, enough sun to power your idea, there could be a situation where the solar cooker is not in use for some time. Then if it will be generating heat in anything, it can be aimed at lumps of metal (rather than stone) as the stone can easily explode if it is not heated relatively gradually. The solar heater could however heat metal, or some safe stone type substance which is not likely to break, and these 'heaters' could be used for simple water heating for drinks and the like?..............Just a thought..............

You may have done it, but, have you checked for the efficiency curves of different metals, copper, Aluminium etc as to how quick they heat up? And, or keep the heat?

You will see further down I give ideas on a composite pot.

Is it 'that' critical that the pot should not radiate any more than any usual cooking pot, if, you have what is in effect a 'free' heat source?

Also, when talking about your cooker.........are you talking of the element used for heating which is a solar heater, or the type and design of the cooking vessel?

Because cooking vessels with a composite fabrication could possibly hold and retain more heat without hardly any escaping by radiation............And I am thinking of maybe a metal pot, lined on the outside but, not where the solar heater will be aimed, with a very highly reflective outer surface on the pot, and (perhaps) a porcelain liner into which this is placed, which has a highly reflective inner surface. The actual cooking pot can be removed for cleaning?

Nothing gets to over 100C

You can easily get to over 100°C if you use a pressure vessel, and, this is a good way of conserving energy on a conventional cooker. As babybear says most of the lessons already learned should still apply.

Excellent idea as pressure cookers use very little energy to keep cooking once the working pressure has been achieved.

I would also like to add for the original poster that doubling the "SIZE" of the post is an excellent idea, "PROVIDED" he still only places in it the same volume of food to be cooked as he had before. The larger surface for heat collection, Will help him to cook faster and hotter.

Furthermore, there are a plethora of relatively cheap aluminium cooking/pressure saucepans around with a black external finish, that reflects very little, which should take on the heat much better than a normal pot from a Solar source......

In this case "Bigger is better" provided you do not increase the amount of food to be cooked. I have not done the Math (too lazy!), but the larger post will take a lot more food, which will probably have the same or similar problems that the smaller pot exhibited, if not worse, unless the reflector is also increased in size and given a better focus.......

Are you mandated to hit the pot itself with the solar flux?

Couldn't you make a target with high specific heat, conductivity and mass to hold the heat that the pot would sit on and increase the thermal stability of the system?

I'm thinking like a copper plate. First catch the heat in the copper, then transfer it to the pot. The copper is likely to be a better capturer than the pot and any not water contents. (because the non water solids will not circulate they tend to scorch rather than convect heat away.)

my 2sense...

milo

Twice the volume would be 10" wide and 17" high (change it the other way, it becomes more like a huge skillet for frying) and should be amenable to similar experimentation.

I have also done a lot of stuff that is not pertinent to the question. As I am increasing the size of the pot, the surface area is increasing slower than the volume. We are not changing shapes of pots. It is aluminium filled with water. All the dimensions are linked. What happens as you increase the size of the reflector as you increase the size of the pot at the same rate? Aluminium is one of the very best conductors of heat. What happens? Will the solar energy just go in more than twice as fast or will it go in the same speed or will more just radiate off the hotter surface before it goes through it? Thats the question I would like answered. I cannot do the experiment because I no longer have decent sun. Someone who knows his materials physics probably can easily answer it.

I apoligize. I now realize that the question is more complex than I thought. The solar fluc is increasing at one rate as the size of the apparatus increases, the area of the pots surface is increasing at another rate and the volume that the pot contains is increasing at yet another rate. And none of these rates are linear. If you double lengths, you square area (the dish) and you cube volume, (the pot) and I am not yet sure what happens to surface area. I was interesested in finding out how my compound parabolic solar cooker might scale up and I just thought linear. So sorry for being snippy about the segways. brian

First of all, I remeasured the pot and it is 9 inches wide. (I had several candidate pots and I ended up using a different one). I asked about scaling it up and I had the wrong idea. Basically when you scale everything up equally, the surface area scales up at the same rate as the collector area. So your solar flux on the surface remains the same as you scale up. The volume however scales up at a faster rate. About 2.8 times for every 2 times the surface area increases in my case. I guess this means that there is an ideal size depending on the use If you need to bring water to the boil, you would chose a smaller system with a smaller pot. If you just need 70 degrees for pasturisation, a larger system with a larger pot and slower heating (but probably more efficient heat transfer. because you get the same solar flux but a steeper temperature gradient down into the pot). If a larger system is required to heat to a higher heat, I guess you need to drop back the acceptance angle. Mine is 45 degrees. If you have a 30 degree acceptance angle, you can get more energy into a smaller space (but have to move the solar cooker more often) or track more accurately. Having looked into it a little bit, it is quite the problem. 100C is still the intended top temperature for this project. It is just a simple solar cooker for use in poor countrys. Brian

re : 9& 10.

Now you're thinking!

Do you have a copy of direct use of the suns energy by farrington daniels?

http://www.amazon.com/Direct-Suns-Energy-Trends-Science/dp/0300003994

A great resource.

And you'll love the cover photo.

milo

Brian

I have not seen your design but would like to. One thing I might mention to you sinse I am guessing that when you are talking about acceptance angle you are refering to the relationship to the sun. It might be something you might want to research by checking out www.solarpanelsplus to use for your idea. It uses solor tubes instead of flat panels and is not dependant as much on the relationship to the sun.

Dale

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