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.
Step 1: 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
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.
Step 3: Preparing the Holding Bottle
The holding bottle is the inner bottle that will be used to absorb the heat generated by the reflector. The outer bottle actually just acts as an insulator. The bottle itself offers little insulation. It is the air between the two bottles that insulates. In a warm climate, the outer bottle may not be necessary.
I used a 500 ml (16 oz.) large-mouth Nalgene bottle as the Holding Bottle. I coated it with several coats of black spray paint. This will aid in the absorption of the heat by the water in the bottle. Be sure to paint the bottom also. You could also scale this down with a smaller sized-bottle, but 500 ml of hot water is about perfect for two hikers.
Step 4: Preparation of the Reflector
Find a very shiny window screen and cut out a rectangle about 20" X 24". Cut down each diagonal of the rectangle about 6". Reinforce the areas at the end of the cuts with metal ducting tape (on the shiny side and regular duct tape on the non-shiny side..
Velcro strips should be applied onto four corners as indicated below on the shiny side, and four on the dull side.
Step 5: Prepare the Outer Insulating Bottle
The outer bottle's major purpose is to act as an insulating thermos for the inner bottle to prevent it from cooling down too rapidly after heating. I found a bottle that was actually two bottle screwed together that was ideal once I cut out the bottom of the top bottle so that the inner bottle could fit inside. A large jerky container may also work, and I have used a 1-liter soda bottle. (All of these are made of PET.) Make sure this outer bottle is relatively clear so as to not impede light entry.
To use the soda bottle, just cut off the bottom just above where the bottle starts to curve into the bottom. You will then make a hinge out of duct tape and the other side opposite from the hinge will insert itself nicely into the top of the bottle. Place a crumbled napkin on the bottom (or cut some neoprene, to insulate the bottom of the bottle.) The inner bottle is then inserted through the bottom of the 1-liter bottle and the two halves are fit together.
I have also used a 1/2 gallon juice or water bottle as the outer bottle.
Step 6: Final Construction
Now just fill the black inner bottle with water, place into the outer insulating bottle, place them in the center of the reflector and point the device at the sun. I have also placed small slits into the reflector reinforced with the metal ducting tape so that it will accept straps. The device can then be strapped on a backpack and the reflector and the bottle can be secured while hiking - heating your food as you walk. (I am getting ready to test this latter device.)
Step 7: Testing Results
Yes, this can produce very hot water. One of my early prototypes using a PET bottle (a 16 oz. Dr Pepper bottle) became so hot that it melted the plastic, generated steam and deformed the top of the inner bottle.
I ran some tests on just a black-painted inner bottle, a black-painted inner bottle with a reflector and the entire system of reflector, outer bottle and inner bottle.
The ambient temperature was 83 F. Temperature measurements were taken every 10 minutes. The final temperature after 100 minutes was 115 F for the black bottle, 153 F for the black bottle with the reflector and 181 F for the complete solar heater. The outer bottle is probably more important when the ambient is cool to keep the inner bottle insulated from the ambient.
The system has heated water to 196 F in just two hours.
Thanks to my son Hunter for helping me with this project!