Introduction: Solar Water Bottle Heater

Picture of 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


Mayank_j (author)2015-06-21

use glass jam/pickle bottles they are non toxic

evergreensolar (author)2010-03-25

My concern is heating plastic which releases toxins in water intended for consumption.

kopomeroy (author)evergreensolar2010-03-25

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.

Mugsy Knuckles (author)kopomeroy2013-09-24

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.

kopomeroy (author)Mugsy Knuckles2013-09-24

I love your comment!!

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.

worldstrad (author)2013-01-05

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!

antioch (author)2012-06-19

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.

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.

stephenniall (author)2009-04-15

Hmm could i add some tin foil around the Bottle to make it attract sun too!!

robotguy4 (author)stephenniall2009-05-15

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).

gtoal (author)robotguy42011-08-23

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.

dezertdude (author)robotguy42009-07-27

uhh its red blue and yellow.

No yellow is for reflected light. RGB is "additive" color.

red blue and yellow are the primary colors! yellow and blue combine to make green!green is NOT a primary color!

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.

Note to readers:
Reflected light as in "pigments".

EARTHFORSOLAR (author)2011-02-16

man that babby gets hot 196 F VERY HOT!!

gtoal (author)2011-01-04

I tried something like this myself before I realised other people had done it already; for the inner bottle I found a beer bottle that is actually a metal can shaped like a bottle (I forget which as it is now painted black; it may have been a Bud Light?)

Using a metal bottle solves the issues of contaminants leeching from the plastic.



SdaMaster (author)2009-06-11

hey what if you used a brown reclosable or screwtop glass bottle(beer or rootbeer) inside no risk of harmful chemcial transfer or bootle expanding

wolfe.chris (author)SdaMaster2010-11-19

That would work, no doubt, but the purpose of this particular heater is to be portable for hiking. Glass is fragile and heavy compared to most plastics. Neither of these qualities is desirable while hiking.

harlyquin (author)wolfe.chris2010-12-09

A beer bottle is not fragile: a whiskey bottle is fragile. A beer bottle is built to put up with being dropped while holding back built up beer pressure. if anything is likely to fail on a beer bottle, it's the plastic seal on the underside of the cap. I bet you one of those little jobbers won't put up with repeated loosening, tightening and heat and pressure.

it's true that glass would be heavier, but the increased mass, and the insulative properties might mean hot, hot, heat.

wolfe.chris (author)harlyquin2010-12-09

I was talking about fragile (in regards to impact) in comparison to a PET bottle. If we're comparing things that don't matter though, how about this. A whiskey bottle isn't fragile: a light bulb is fragile. It doesn't matter, the point is, glass is not a good material to have to haul around while backpacking.
My experience with re-sealable bottles is as follows. The most likely point of failure on a screw top glass bottle is actually the inside-threads on the metal lid. They tend to stop holding tightly after a short period of usage or from a single over-tightening. Beer bottles with the traditional pry-off crown cap are more likely to fail than anything in the capping mechanism. Yes, it takes a tremendous amount of pressure to burst a beer bottle due to over-pressurization, but that's not terribly difficult to produce if you were to make a miscalculation in the amount of priming sugar to add to a batch of beer. When they do rupture it can be a dangerous situation; they call them bottle grenades for a reason!

harlyquin (author)wolfe.chris2010-12-09

Haha, touche', WC.
PET it is then. At least, until DuPont comes up with the next big thing in plastic bottles that is...

vontzy (author)2010-12-02

I believe one way to improve the efficiency of this type of Solar Heater would be to remove the AIR between the Bottles for the best Insulation. If the Outer bottles are of stiff Plastic, or Glass, then they wouldn't collapse. There is a very inexpensive hand operated Vaccum pump and "Cork" at the URL below. They are sold for Wine Storage. They are highly portable and you can purchase additional "Corks" separately.

trebligumbo (author)2010-06-15

What type of plastic are those cups made from that you get hot drinks in out of vending machines? Are there any bottles out there made of similar stuff? That type of plastic should be safe.

joanofarc (author)2009-11-22

Do you think I can easily adapt this system to an outdoor waterer for my chickens during the winter so their water won't freeze? 

suezq (author)joanofarc2010-02-13

Hi. I used to be frustrated constantly by my chicken waterer freezing until I came up with this idea.
I had the large size galvanized automatic waterer. I took a rubbery type heat tape, the kind you keep your pipes from freezing with, wrapped it around the outside part of  the waterer and duct taped the whole thing down. I wrapped it so the end with the plug was at the top and left enough to attach an extension cord to.
Then I suspended the waterer and ran the extension cord up the cable holding the can. The heat tape thing has an automatic sensor that only turns on if the temperature drops below a certain amount. I never had to chip water and the hens had drinkable water even down to 0 degrees. It kept the metal conatiner just warm enough to keep the water flowing.
Hope this helps.

kopomeroy (author)joanofarc2009-11-23

That would be a great idea! A lot would depend on the quantity of water you need and whether you could place this in a sunny (south facing) area. I would just make a big reflector (s) from sun shades. You may even be able to do this and get it hot enough by just painting the reservoir black!

Pader (author)2009-06-11

I found that one bottle works pretty well when left on a pallet in Helmand, Afghanistan lol

ladybanksia99 (author)Pader2010-01-24

I'll bet it does!  Somewhat similar to being left on a bench in Arizona in July! :^)

I cook rice in my solar oven in a canning jar painted black.  What principles could feasibly be adapted (non-emission plastics, lightweight materials, portability, etc.) for a hiking situation from this?

kopomeroy (author)ladybanksia992010-01-25

See my complete Instructable at I will be field testing my reflector and two bottle backpacking design soon.

tesla coil (author)2010-01-19

Could you use electrical tape for a light/heat absorber or will it produce toxins?

Eye Poker (author)2009-11-30

My concern is heating plastic which releases toxins in water intended for consumption.

kopomeroy (author)Eye Poker2009-11-30

You have a valid concern. That is why you should not use PET (soda bottle plastic) nor Lexan (the pretty, often bright colored translucent plastics). I used HDPE which should be fine.

mickey_shimitz (author)2009-06-03

uhh 1 problem, who wants to drink something hot on a day when its hot enough to heat up whatevers in that bottle?

The obejective is to get the water hot enough to use it on dehydrated foods or foods or drinks that require hot water.. Ramen noodles, Tea, coffee, etc..

Africorn (author)mickey_shimitz2009-06-25

LOL, yeah... :o)

kopomeroy (author)mickey_shimitz2009-06-04

This system depends on radiant heat and so wil work well as long as there is sunlight. A high outside temperature is not necessary. The air space between the bottles acts as an insulator. Some loss will occur to convection especially after the water is heated, so I have made a neoprene sleeve to keep the inner bottle warm. You can then stick it in your backpack if the outside temp is cold and open it for a nice hot cup of cocoa when you get into camp.

prcsolar (author)2009-07-01

we usually need cold drink no hot drink.

gmjhowe (author)2009-02-18

Nice work, I look forward to your 3 bottle idea if you do it.

kopomeroy (author)gmjhowe2009-06-04

I did try the three bottle design, but the two bottle design with a reflector heated the water more quickly and hotter. I will take it out soon for field testing and then I hope to make something lighter using mylar and a carbon frame.

lukeyj15 (author)2009-04-02

What happened to putting some black pipe on a tin roof in the summer?

static (author)2009-02-20

I have read putting a length of black plastic tubing in a clear bottle works also.

blakeredfield (author)2009-02-18

Very very nice. Thank you for posting these images so quickly. I applaud your knowledge of plastics. I had not considered the harmful effects from the release of chemicals during heating.

About This Instructable




More by kopomeroy:Easy Boiling Breakfast Burrito in a Bag - great activity for kidsSolar Water Heater for Backpacking Using Water Bottles and a Car ShadeSolar Water Bottle Heater
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