Solar Hot Water Kettle From Plastic Bottles (and Glass)

I love solar energy and making things out of trash. This project combines the two loves into a solar water kettle made from old plastic bottles.

This kettle will heat 16 oz of water in full sun to a nice hot/warm temperature for some sun tea or washing hands, but you'll have to wait a few hours or less depending on outside temperatures. I've only recently made this during the winter and have achieved 95 degree water temps during 30 degree outside temps. Someone in a warm area of the world will have to make one and tell me how hot it can get during summer-like temperatures.
(Instructables member Kopomeroy built a few versions of this; they achieve 140-160 F during 80 F outside temps. See comments below.)

This takes three plastic bottles to build, one 2-liter, one 16 oz, and one 20 oz. Also needed are flat black spray paint, 5/8" (16mm) rubber heater hose, aluminum foil tape. The outer 2-liter bottle should preferably be clear.

After making this I realized a major problem when using a plastic bottle core; the breakdown of the plastic from heat and UV rays which could leach plastic chemicals into the water. This hot water might be useful just to wash your face or something but not to drink. An alternative to this design is to use a glass or aluminum bottle for the center bottle.

Step 1: Clean, Prep, and Paint

If your bottles held contents other than water, you'll probably want to wash out all the sticky drink residue. Mmmmmm....sticky drink!

Peal off all the labels.

Give the 16-oz bottle a light spray with the flat black so it drys fast, a second coat may or may not be necessary.

Step 2: Cut Insulating Bottles

The 2-liter and 20-oz bottles are insulating bottles to keep the solar heat in the center 16-oz bottle. Cut the bottoms off where the curve of the bottom stops (see pic.) More clear bottles of increasing sizes could be added for more insulation.

Step 3: Install Connection Hose

Cut about 1.5 to 2 inches of the 5/8" (16mm) rubber heater hose. Wet the hose and wrestle it into the 16-oz bottle lip about 1/2". Next wedge the hose with 16-oz bottle up into the 2-liter bottle; you may need a screwdriver to help pull the hose through the neck. This holds the two bottles together into one bottle and lets you fill from the 2-liter spout into the 16-oz bottle.

Step 4: Add Insulation

Slide the 20-oz cut bottle over the 16-oz inner bottle; this is the secondary insulation. You may need to make a cut lengthwise on the 20-oz so it fits better. * Next carefully force the 2-liter bottom back onto the bottle.

*Add about 2 tablespoons of dry rice inside as a the 2-liter before assembling. This will increase efficiency of the kettle by absorbing the reflective condensation that can form on the walls of the bottle.

Step 5: Add Reflector Foil Tape

Attach foil tape to the outside of the 2-liter, just go half way around the bottle. This becomes somewhat of a parabolic reflector and lets the sun shine a little brighter on the backside of the center bottle.

Step 6: Performance Notes

Fill it with water, leave some air for expansion, put on the lid slightly loose and lean it up against something while facing the sun. The foil side is the side away from the sun. Now wait a while; the time depends on outside air temps and the position of the sun. It can take several hours to get hot in winter weather.

I tried the plastic bottle version, the glass bottle version, and just a plastic bottle painted black. I laid them all out in the afternoon winter sun (outside temps were in the lower 30s Fahrenheit with full blue clear sky) and checked them after a two hours. I don't have a good digital thermometer currently, just a kitchen candy thermometer which is slow and not precise.

The glass and plastic kettles were about the same temperature, although the water from the glass kettle felt slightly warmer than from the plastic one. They measured about 95 degrees F if the candy thermometer worked right.

The black plastic bottle was just cold, just barely warmer than outside air temps.