Solar cooking for everyone

Water heating with a plastic bottle.


10 liters transparent plastic bottles can be fond everywhere in Africa and elsewhere as well. Lately I have found a way to heat water with these transparent plastic bottles. If you follow the pictures, I will show you how you can try it out for yourself.


The only thing you have to do is to buy a black plastic bag and find an empty transparent plastic bottle. If the plastic bag has textures on the outside you can turn it inside out.








Put the plastic bag into into the transparent (PET) plastic bottle and fill up a bucket of water.

Then fill up the plastic bag with water. You might also fill some water in the transparent bottle.

Then tilt it backwards towards the sun and let it rest in a optimal position for about six hours. You will find that the temperature of the water will rise slowly up to 50-60 degrees Celsius.  Find out how hot you can get the water and let us know how well it works in your location.

It is worth mentioning that the bottle is round (of course) and this helps the bottle to stay in focus of the sun for a long time. Just make sure the bottle is placed in a position where it will receive the the rays from the sun in an 90 degree angle all day long. You can do this if you find what track the sun is following. If you imagine  an axes from the top to the bottom of the bottle, the angle between this axe and the plane of the sunpath should be 90 degrees. If you are able to pose the bottle in its optimal position you don’t have to worry about changing the position throughout the day. Just make sure there are no shadows that blocks the sun. If you dont need the hot water right away you can store it in a freezer bag.

I found this to be a very cheap and easy way to get all the hot or warm water I need for myself and my family. Especially is this helpful if the only alternative is to heat water with gas or wood fire. The cost of charcoal to heat so much water can easily be a couple of dollars wich is a lot of money for many poor people. The material needed can be found in almost any African villages and that is a great advantage.

If you also add a small reflector you can get water even hotter. In a 5 liter plastic bottle I once got the temperature to the boiling point. The plastic bottle got a little bit deformed, but it did not melt and there was no leak.

To prevent the bottle from deforming you should normally not heat water in a plastic bottle to more than 80 degrees Celsius and the focus area should be evenly distributed around the bottle.


Have a nice sunny day! :)




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Tags: cooking, solar, warming, water


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Comment by Magnar on April 13, 2011 at 10:48pm

Hi, Magnar, Have you tried having the bag as a loose fit on the bottle and spread out wide and hanging down as far as possible ? That would mean more plastic surface to collect heat and hopefully the heat will rise and heat your bottles even more. In that case, at the tropics, you might need the bottle on a stand and maybe tilted back with the bag put down over the top of the bottle and then tied at the bottom.

Would be an interesting comparison test. One with a tight bag and one with a loose fit black garbage bag.




Hello Brian

The advantage of having the black plastic bags spread out inside a plastic bottle is to make sure that most of the heat is transferred from the bags and into water. If you have a black plastic bag hanging in the sun, some of the heat will transfer into the water on the inside, but some of the heat will also heat up the air around the bag.

I have tried to fill the bags with different amount of water to find what is most efficient. The bottle to the right have about 2 liters in the bag and 8 liters outside between the bag and the bottle. In the bottle in the middle the black bag has about 5 liters of water and 5 liters on the outside, and in the bottle to the right there is about 8 liter of water in the black bag and 2 liters on the outside. The bag with 8 liters of water gave the best result. Then there are also other alternatives you can try out. You can use a bigger black garbage bag and fill all the water inside of it and nothing outside the black bag. I have not tried this yet, but will soon do it. It is also possible to leave some airspace between the bottle and the black plastic bag. As air isolates better than water, temperature is likely to rise. A loose bag might also work well, but I have not tried it yet. It is likely that loose fit bag will work out better than at tight bag because sunlight will bounce back and forth between the folds in the bag and this will decrease reflections and loss of energy. In my example the plastic bag is big enough to fill the whole plastic bottle making it all black. But when there are folds they seems even darker than where the bag is tight. The reason for this might be that less light escape through reflections. But only tests can give us the full answer.



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