Solar Water Bottle Heater




After reading a great Instructable on building a solar water bottle heater by robbtoberfest, I decided to try to design a system for backpacking and heating up water for use in dehydrated food. It appears the key to the temperature of the water is not so much the outside temperature, but the amount of sunlight. The original method used a 3 bottle system. I compared various alternatives including a 1, 2 and 3 bottle system.

With robbtoberfest's original 3 bottle system I could get water to about 135 F in 3 hours. If a larger outside bottle is used (a juice bottle), the temperature could reach about 150 F. If I used a 2 bottle system with a cut out half of a car window shade reflector, I could go as high as 196 F. Unfortunately, I think the water may have boiled and it melted and deformed the bottle.

I also have changed the outer bottle to type of PET with a wide mouth so that you don't have to cut it open and tape it after each use. You just screw the two halves together.

Although I would like to use just PET (like soda bottles) bottles due to their low weight and abundance, I now use HDPE (like the cloudy Nalgene bottles) for the inner bottle. (Lexan bottles may release estrogen like substance so I avoid those.) I also will investigate using a lighter reflector - mylar with a collapsible plastic frame.

A 3 bottle system may insulate the bottle better when the weather is cold. I will test this later.


Solar Water Heater for Backpacking

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    48 Discussions


    4 years ago

    use glass jam/pickle bottles they are non toxic

    My complete instructable talks in detail about this concern. I have included it here for you: There are five major types of bottles that can be used for this project, but the inner bottle that actually touches the water should always be HDPE (Nalgene), which is cloudy semi- hard plastic. Soda bottles are made of PET. When PET gets hot, it may leach DEHP, a potential carcinogen. Lexan containers, the clear plastic that is often brightly colored, may contain traces of BPA which may interfere with your hormones (It is an estrogen-like compound. Doesn't sound good to me.) Then there are PVC (polyvinyl chloride) and polystyrene bottles, which can also produce carcinogens. Polypropylene or LDPE may be safe choices.

    And don't forget the dangers of using the sun to heat this, as sunlight is a known carcinogen. Water is also a very dangerous solvent, and can cause poisoning per the MSDS.
    The entire website has become a contest of who can poo poo the instructables as too dangerous. It's really pathetic.
    Good job on the instructable. I will try it and if I get cancer, too bad for me.

    consider the health risks of plastic!!!!

    why doesn't anybody use metal, plus some types are better conductors of heat anyways, but i get what you guys saying about the inner bottle being plastic to insulate a bit better.

    but plastic poisoning!!!!!!    =(   =( =(
    See full size image

    Plastics are used for several reasons, the least of which is lower weight. In addition, if I used a metal container in my system the heat generated would be hot enough to melt or burn anything it touched - plastic is a good insulator. In addition, not all plastics are toxic. BPA is a concern when polycarbonate (a hard clear plastic often dyed bright colors; same as Lexan) is used and estrogen like compounds are a concern with PET (thin clear flexible soda bottles). I have used glass, but the risk of breakage, weight, etc are too high.

    or you could paint one of those reusable metal bottles that you get at the store black and then when your done heating it, cover it with a good insulator, or stitch a sleeve for it out of one of those neck covers for skiing.


    6 years ago on Introduction

    Awesome idea. Wondering if I can strap one of these to the rear rack of my bike for touring/camping. Someone mentioned they were going to try to come up with a system to attach it to the top of a backpack -- any news? I would be interested in that, too!


    7 years ago on Introduction

    As much as I regret spoiling the whole lightweight trekking/camping idea of this
    I would strongly advise against aluminum. It has become a major suspect for Alzheimers and I suppose if not so much of our dishes were made out of it this would have been made popular in a much broader sense already.

    As rule of thumb, it might be best to not use anything that doesn't occur naturally on the surface of the earth. Clay, Glas, Wood, Stone, you get the idea.

    1 reply

    Actually, the impact of aluminum on the formation of Amyloid-Beta protein structures is very poorly understood. Other metals have also been implicated, including iron, copper and zinc, but the research is very far from unequivocal. The most that can be said so far is that there may be some impact from aluminum and other metals on the development of Alzheimer’s. I think calling it a major suspect is unwarranted at this time.

    I would also point out that fired clay and glass are not naturally occurring, and many glazes for ceramics are far more toxic than aluminum. I believe that diversification and moderation are key. Don't use only one type of container; try to thoughtful about what you use; discard (recycle) items that are showing signs of wear.


    Reply 10 years ago on Introduction

    Actually, I think that would just reflect the sunlight. The inner bottle is painted black because black coloring absorbs all three colors of primary color (red, blue, green), thus absorbing the highest amount of energy, thus becoming VERY, VERY HOT. This is the reason why you heat up more when you wear black clothing during a sunny day than when you wear white clothing (white reflects most of the visible spectrum of light).


    Reply 8 years ago on Introduction

    I think that putting silver paper on *half* of the outer bottle to reflect back the light on the side away from the sun is probably a good idea. the bottle shape should also fortuitously focus the light inwards. That is, if an external reflector covering more area isn't practical in your setup.

    I know that all those grade school lessons have confused the crap out of many people and it's hard to switch modes, but with light it's called additive color and the primaries are Red Green and Blue. Red and Green make Yellow. What you are thinking of is pigment mixing which is mixed to reflect not project light, so the pigments subtract (fail to reflect) portions of the light, so it works differently between additive mixing and subtraction. You can look it up there are a million pages, like the one I already linked to for you.