SolarCookers

Solar cooking for everyone


In the making of the Voukana Solar Cooker, I was using the same team that made the Deep Double Funnel Cooker in 2009. They started out with paper models and already after second try; the result seemed to be quite satisfactory. The only thing I still am waiting for is some good reflection materials that have some durability in tropical climate. Her smaller brother, the patented Kolamba Solar Cooker is meant for a family of two – six people, but the Voukana Solar Cooker can cook for up to 20 people.  

Voukana also means community in the African language Lari.

The Voukana Solar Cooker is an efficient cooker, even compared to wood-stove and coal stoves. We are often used to wait for hours to have our solar cooked meal. This is fine when everything is well planned and cooking can start in the morning. But often african woman does not have food in the fridge, and have to go to the market to buy food each day.

Finding and cutting wood can also be time-consuming in many places. That means preparation can often not start before noon. With the Voukana Solar Cooker this is not a problem.

The african girl on the picture, Sara, has boiling water within 8 minutes, without even starting to search for wood or coal. Before her African sister has even started to cook, she is almost finished with her meal, thanks to instant solar heat and no time spent finding wood or arranging the fire place.

If cooking in a big pot for a community of ten people or more, it may still take hours to prepare the food, but the food can be left unattended.

Because it is inclined towards the sun it can cook from almost 0800 o'clock in the morning to 4 pm in the afternoon.

I tried to sterilize water in a normal 10l PET bottle and did not have to wait long before the thermometer showed 75 C.

As you can see from the picture, there are no other moving parts (other than the casserole), it only has to be adjusted azimuth with the sun to be effective. Max effect is calculated to be1200 W directly on the cooking pot. 

The size of the Voukana Solar Cooker is not meant for mobile transport, at least not yet. It is meant to be installed outside in specific place and is intended for families in villages with plenty of space. The model can for now only be produced locally, thus also giving work-places and important capacity to the local community. It is made from 6m2 of metal plates and its weight is about 10 kg. 

See also the video, where we have been using a preassure cooker with Voukana Solar Cooker !

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Comment by Magnar on October 29, 2012 at 11:47pm

Hello Edgar!

Theory is good as a guide, but we have to do tests to know for sure.  Would be very interesting to see your results if you will share them with us here!

My guess is that it will be an advantage with the accumulator if you are heating the stove well above the intended operating temperature before you put the food inside. If operating temperature is 140 degrees C, and you can heat the oven to 200 degrees C, you might have 140 degrees C right away with the right accumulator.

This is possible to calculate. Let’s say starting temperature for food is 20 degrees C. The oven is heated to 200 degrees C. Let’s also say "the heat capacity"*, C of oven is two times that of the food.

We should begin to say that the internal thermal energy, U of an isolated system is conserved.  In a solar cooker this is not quite true, sunlight is coming in and heat is escaping as well. But for now, to simplify, we will just say it is a completed isolated system.

The thermal energy U of a system is given by its heat capacity C, at any given absolute temperature T => U=C*T  (http://en.wikipedia.org/wiki/Thermal_energy)

Just after you put the food in the oven, U = c1 * t1 + c2 * t2,
where:  C of food=c1, T of food= t1, C of oven= c2, and T of oven= t2

After a few minutes, the heat would transfer from the oven to the food, and the temperature would be equal everywhere.   We want to find this temperature, t3 and now U = t3(c1+c2)

This gives: c1*t1+c2*t2=t3(c1+c2) =>  t3 = (c1*t1 + c2*t2)/(c1+c2)  which is a general formula to calculate the middle temperature.

Remember that we said that the heat capacity of the oven is two times that of the food, then c2=2c1

  t3 = (c1*20C + 2c1*200C)/(c1+2c1) = c1(20C + 2*200C)/ 3c1 = 20 + 2*200C/3 = 420/3= 140C

Then without even using the sun, you have with a good accumulator been able to heat the food at a temperature where you can start baking. That is a great timesaver! Now, the only thing you need to do is to keep the temperature at this level.

Please keep us posted, would be interesting to see what you find out!

 

* C= Heat Capacity is the amount of energy required to change the temperature of a unit with one degree. C=Q/deltaT.  (http://en.wikipedia.org/wiki/Heat_capacity)  

 

Magnar

Comment by J. G. on October 29, 2012 at 10:20am

Interesting work. Please keep up these excellent efforts!

Comment by EDGAR on October 29, 2012 at 10:16am

Hello Manger.

Thanks a lot for all explanations. You are right.

Observing my first "square-box" SC I've noticed that during partly cloudy and windy days the temperature inside my cooking chamber "jumped" up and down when coulds cover the Sun.

The chamber itselve is very well insulated, also double glass cover was installed.

My concern - in that situation - is not to increase tepmerature inside the cooking chamber, but to keep it more stable. The "accumulator" will be mounted inside the CCh with some separation from surrounding construstion - i.e. walls of CCh. and mounted in such the way so it will create air circulation inside the CCh. I expect that - even there will be always a balance between the energy added by the sun and its loss throuhgout the insulation - the accumulator could stable temperature jumping providing the food will remain in more or less same temterature during cooking period.

I will have to accept the extra time necessary to heat all up to cooking temperature, but then I suppose the temperature should stabilize and remain. Clouds appearance make big difference so far. Concluding - my new experiment will give me an answer for my doubts and will clarify my knowledge. Let us see :-))

I'm also trying to build a boiling water device - made of black metal cylinder with cover closed inside glass cylinder with a cover and with mirrors behind them all. All comes from my fascination with Solar Energy, to which I didn't pay so much attention so far. My fault ! Many thanks for Your valuable comments. RGDS - Ed

Comment by Magnar on October 28, 2012 at 11:53pm

I think 51N is not too far north, but right now it is probably difficult to cook with the sun. 
Myself I have spent 12 years in Oslo which is close to 60N. The sun is hot even there, but mostly in the summer, and yes there is quite a difference to 4S in Brazzaville! :)


You are again asking a question that is very interesting. Generally I think the accumulator can be effective, if the oven is heated to a much higher temperature, than what you really need when cooking your food. I will try to explain this, but I have not done any tests on this matter. 


We know that if preheating the oven first, there will be a fall in temperature when you put the food inside. How much depends on the heat capacity of the food compared to that of the oven. If the heat capacity in the oven is very low compared to the heat capacity of the food you want to cook, the temperature will fall a lot. If the temperature in the oven in the beginning falls twice as much as the increase in temperature in your food, then the heat capacity of the oven is only half of that of the food. With an accumulator it could be the opposite. Instead of starting to heat the food from 40C, you could now start to heat the food from 60C, theoretically speaking.


Now, if you only preheat the oven to temperature close to what you consider as operational temperature to cook your food, let’s say 80 C, I don’t think you should use an accumulator. The energy you need to heat let’s say 0,5 liter of water from 20 degrees C to 80C equals to 0,5kg*4200J/kgK*60K = 126 000 Joules. In that case the food increase from 20 to 60 degrees only represents the heat transfer inside the system, and you can theoretically start heating from 60 C instead of 40 C.


But there is a problem. The amount of energy the sun needs to add into the oven is still the same right? It is still 126 000 Joules, but as you now are operating at a higher temperature it also mean higher loss of energy and heat. Wich means there is less energy left to increase the temperature. Therefore it will probably take much more time to increase the temperature from 60C to 80C, than if would to increase from 40C to 80C in a unit with proportional lower heat capacity!


When the food is at cooking temperature, you need to consider the oven, the cooking pot and the food as one unit when it comes to heat capacity. But the fall in temperature is not only depending on the heat capacity and the heat storage, but also on the heat insulation. Basically the temperature in the oven will rise as long as the added energy from the sun is higher than the energy lost by heat flow. When the oven has reaches its max operating temperature, it means that the energy the sun adds to the oven is equal to the energy lost by heat flow. With this in mind, it is worth noting that the accumulator cannot reduce the loss of energy from the oven; it can only reduce temperature loss.


As I argued for above, it will take more time to restore the loss of energy with an accumulator inside due to higher heat capacity and higher temperature. Now, even if the temperature does not fall so much, the system has even lost more energy with the accumulator than without. This is because, the rate of heat loss from a unit is proportional to the temperature difference to its surroundings, and with a higher temperature more energy has been lost to the surroundings. It would therefore take much more time to restore the right operating temperature with an accumulator than without. An aluminum accumulator is therefore questionable in my view, unless the loss of heat is within an acceptable operating temperature. Another way to increase heat capacity is to build a bigger stove. Likewise I think reducing the heat flow by better insulation, is also a good idea.

Magnar

Comment by EDGAR on October 28, 2012 at 6:07pm

Hello Manger,

I know You are mostly concerned on SC for areas - let say - with intense Sun operation, where water is dirty and the fire wood is difficult to achieve. OK. That is very good and very helpful that people invented devices in order to help those people.

My lat is 51 N, and I'm looking for a solution to capture and to store as much solar energy as possible. Therefore I'm trying to use :

a. reasonably big reflectors "pushing" sunlight INTO the cooker

b. dark cookin pot, where food is closed into baking-bag with very little water included

c. additional black-alluminium heat accummulator, because here mostly we have partly cloudy sky

I know, I will have to "cook" both an accumulator and a cooking pot, but I can expect I will be able to keep temperature more or less stable. And that is my basic idea - for my particular area . 

Thanks in advance for every comments to abowe.

RGDS

ed

Comment by EDGAR on October 28, 2012 at 5:44pm

Hello Manger,

I do agree with everything You said, since that is very important base knowledge ref to SolarCookers.

My comment, however, reffers only to heating/boilng water, and not to cooking food itselve. 

I've seen a very simple (another) test where they exposed to the sunlights two glasses of clear water.

But te one of them had something dark inside on the bottom. After a while - there were noticable "bubles" comming from dark object - means the heat from the object was transmitted to the water, while the other glas didn't changed at all. Sunlight comes and goes away and no hit appeared.

Ref. to cooking food - everything You said is 100% right and every new SC constructor should take it under her/his considerationas the Base Knowledge.

Thanks, RGDS - Ed

Comment by Magnar on October 28, 2012 at 4:28pm

Thanks for your comment Edgar! - and very interesting question! 

Many people are asking if they should use black, or if they should use reflective materials in their solar cooker  - and there is a huge difference between reflective and black. Good reflective materials, can reflect up to 95% of the sunlight and will have a temperature very close to air temperature even when exposed to direct sunlight. With black materials it is quite different, and my experience is that it will approach up to 60 C without any reflector. 

So the question is what do we want to heat?  The answer depends somewhat to if you are cooking or baking, but the essentials is that we want to heat the food. And to transfer the heat into the food we can use different means; direct sunlight, air, water or materials that leads heat well like aluminum.  And also if we wants to heat the food as quickly as possible we should not use a lot of water or big chunk of metal, but reduce the dead weight as much as possible. 

Typically the bottom plate will not touch the food, but the casserole will.  If you put the casserole on the bottom plate it might transfer heat to the casserole, but we don’t want to include the bottom plate in our effort to heat food. We just need the sunlight to hit the casserole directly to reduce loss of energy on the way. Earlier with a single funnel reflector we found there was in fact a huge difference in placing the casserole on the bottom plate or on a support. We have not tested the double reflector in that regard and the result could be different. But we think the best position is to place the casserole on a 7 cm high support and let the sunlight be reflected directly from the bottom plate onto the casserole. 

When heating a bottle of 10 liters it has to rest directly on the bottom plate. To find out what is best between black and reflective, we would have to do some testing, but my guess is that reflective is best outside the bottle and black right under the bottle.

If we want to use an oven, it would make sense to have a black bottom plate inside the oven to absorb the sunlight and then heat to the air inside the oven.  Would be very interesting to try to have some glazing and see how effective that could be! 

 

 

best, 

Magnar

Comment by EDGAR on October 28, 2012 at 3:38pm

Hello Magnar

Your Voukana SC looks and works impressive, congratulations.

I just wish to add one small note to all above.

1. Once I've seen an experiment made for home-pool. A guy complained its water to bee "too cool", so he exchanged every second square ceramix which were in linght-blue colour into black...

The effect was impressive. Black seramix absorbed solar heat and increased temperature in the pool.

Conclusion: What will happen if You enter a black steel plate or just black flate stone or ceramix to the bottom of Voukana SC ? Most probably the water will boil even faster. What do You mean ?

RGDS - Ed

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