Solar Water Heater for Backpacking Using Water Bottles and a Car Shade

Picture of Solar Water Heater for Backpacking Using Water Bottles and a Car Shade
After trying several methods, this is the best method I found for heating up a small quantity of water for rehydrating freeze-dried backpacking food using just the sun as an energy source. My goals were to make a simple but light solar still. This project is inexpensive (<$20), efficient and light. Several options for its construction are offered.

I have heated water to 196 F in the matter of just a few hours.

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Step 1: Materials Needed

Picture of Materials Needed
1 HDPE (Nalgene) Bottle 500 ml size
1 Flexible reflector auto shade (the shinier, the better)
1 Large plastic bottle with a screwable mid-section (This is hard to find. I got mine at a dollar store and it contained a lot of small bungees. This is convenient, but not necessary. You can use a 1 liter soda bottle, a half gallon square water bottle or a large jerky container.)
Metal duct tape.
Regular Ducting Tape.
Self-stick Velcro strips.

Step 2: Notes on Bottle Types and Hazards

Picture of Notes on Bottle Types and Hazards
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.

The photo shows the alternate outer bottle - a 1 liter PET soda bottle with the bottom cut off about 2 inches from the bottom and attached on just one side with duct tape. This allows you to hinge the bottom to load the black-painted HDPE bottle.

epitts22 months ago

Aren't you worried about carcinogens from the plastic?

Actually I am working on a product that can heat water for paseteurization. I want to introduce the product in developing countries for save drinking water. Thanks for sharing this great idea!

Did I get it right: The bigger the outer bottle, the hotter the water gets inside?

Maybe people in developing countries could colour the bottle black by using soot...

fady4y3 years ago
a more portable solution:

- put your water bottle in a clear nylon or plastic bag
- inflate the bag as much as you can
- tie it with a rubber band or else
- put the bag on a reflector, the tied part down!
- the hot air won't go out as it goes up (like in a hot air balloon)

... and voilà!
kopomeroy (author)  fady4y1 year ago
Great idea! As long as it is quite clear it should work.
Thanks, this is very interesting. A glass inner bottle would be best, have you ruled it out entirely? Maybe a tempered glass flask would work.
kopomeroy (author)  mandolinible1 year ago
It probably would work nice, but I chose plastic to reduce the weight.
What about one of those metal water bottles? Metals have very low heat capacity, so wouldn't it transfer heat to the water faster?
kopomeroy (author)  gnomecatcher1 year ago
They could melt the outer bottle though by conducting heat from the metal to the outer plastic bottle. (Metal is a great conductor of heat.)
m2753zz63 years ago
Clear polyesters are a great choice too as they have much higher heat resistance because they are a thermoset material so they don't break down until very high heats are attained. They are also a lot more durable from a strength standpoint and less susceptible to UV degradation unlike polyolefins (PP, PE).
bondel3 years ago
sir iam working on the plastics recycling and trying to adopt this methods in our campus and then aware the people about its usage so, kindly help me sir by sending regarding data to my mail ahmedshareef.k@gmail.com which would be helpful to me alot
vontzy4 years ago
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.

gtoal vontzy3 years ago
That should work if you fit the valve in the lid of a jar type heater, but for the ones where the outer bottle is a coke bottle etc - sealing the bottle back up to withstand vacuum after cutting off the bottom in order to insert the internal bottle just may not be practical.
RobinHPS4 years ago
Hi man,

Realy like this instructable, it's a super-easy and cheap way to positively use solar energy - thanks very much for sharing your handywork!

I really liked this instructable as a potential for preheating water for kettle use. (that's how tight I am about energy consumption!), and am just generally interested in solar water heating.

I also think this would be a good candidate for emergency a kind of water purification through distillation device. If you had another, empty bottle, placed in a colder location (shaded, or maybe buried a little) and a conduit put between the two, I'm pretty sure you would be able to purify a decent amount of water through distillation. Very good for emergency situations, or for use in the tropics, etc. I think there are several NGO's / charities which are looking produce similar devices en masse.

zcshiner4 years ago
This is a really good idea. I may do this next time I go backpacking with scouts (which is very soon).

Have you thought about suspending the bottle above the reflector so the rays can also hit the back of the bottle?
kopomeroy (author)  zcshiner4 years ago
Yes, this might increase the efficiency but I don't think it would really give you much in return. Maybe instead of waiting 1 hour for hot water it would take 50 minutes. When you're hiking, what's another 10 minutes?
I Like The Idea. Think You Could Use A Bottle That Tennis Balls Come In Instead Of A Mid-Section Screw In Bottle?
kopomeroy (author)  kingsmanname4 years ago
Great idea. It would work assuming the size of the inner bottle was suitable for your use.