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.
SEE THE COMPLETED INSTRUCTABLE ON HOW TO BUILD A VERY EFFICIENT ONE BY GOING TO THIS INSTRUCTABLE LINK: www.instructables.com/id/Solar-Water-Heater-for-Backpacking-Using-Water-Bot/
Solar Water Heater for Backpacking