The Magnato Water Bottle Heater was developed by Magnar, creator of the SolarCookers blog, written from Brazzaville in the Republic of Congo in Africa. He came up with this idea of reusing discarded ten liter transparent plastic bottles, and lining them loosely with a black plastic bag to heat water bathing and household tasks.
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.
To make the heater, stuff the bag, which needs to be slightly oversize for the bottle, down into the bottle, keeping the top of the bag out for filling. Once it is mostly filled, it is not so important to keep the bag out of the bottle. It will help in heating if the bag has some folds inside. This will help collect more of the sun's rays.
Then tip the filled bottle back with it facing the sun. The round shape of the bottle helps gather the sun's rays, and it should not require repositioning for the approximately six hours it is left in the sun. In full sun, the water temperature will rise to 50 - 60 °C (122 - 140 °F). If you dont need the hot water right away you can store it in a freezer bag. Magnar found this to be a very cheap and easy way to get all the hot or warm water he needed for himself and his family. This method is especially 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 materials needed can be found in almost any African villages, and that is a advantage, and a great start to free hot water.
It is important to remember that for water pasteurization, water needs to be heated to at least 66 °C (150.8 °F) to be considered safe for drinking. Any temperature below boiling must be verified with a WAPI or thermometer. Chemicals from the bag may also be a problem. This technique looks to be a great way to heat water for other household uses.