Solar Hot Water Kettle From Plastic Bottles (and Glass)




About: Dad and hubby, good food enthusiast, solar energy, boating, making stuff, melting stuff, and raising chickens.

Intro: 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.



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    74 Discussions


    6 years ago on Introduction

    I like your cautious switch to glass but would strongly advise against aluminum.

    Aluminum 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, no matter how much of a hippie this makes you look. ^^

    5 replies

    Reply 5 years ago on Introduction

    The aluminum-Alzheimer's correlation is dubious; the supposed high quantity of aluminum found in brains of Alzheimer's patients was later proven to be due to use of aluminum-containing water to wash the brain samples before examining them. When later brain-tissue samples were processed with purified, aluminum-free water, no excessive aluminum was found.


    Reply 5 years ago on Introduction, really? They are not as bad as Penn & Teller but they, too, are just another opinion maker and, under the guise of promotin reason, live off discrediting and dispelling. For obvious reasons they don't look too closely sometimes - just like their visitors.
    I suppose you can spend weeks working your way through related on the internet alone. And while it is true that there is some fearmongering going on in the modern day it is also true that, somewhere, there is a rational basis for that.

    The link between Alzheimer's and aluminum is heavily disputed and that dispute is not just based on a screwed up study like the one you mentioned.
    And while the link to Alzheimer's can be debated it cannot be argued that aluminum is both a vital but also a toxic element, overabundant in the civilized world. Which might also be a reason for the heavy dispute.

    Here's a variety of links, from scientific to pubmedia


    Reply 2 years ago

    Right on both counts. Snopes is highly political, and only pretends objectivity. As you noted, the Aluminum-Alzheimer's link is still debated. Before we learned how to process it from Bauxite, to my understanding we were not exposed to it at all in our normal environments. There is no bodily requirement for even a trace amount of Aluminum.


    9 years ago on Introduction

    Let me see if I understand this...

    • all of the energy comes from the electro-magnetic rays of the sun, (hence, this will not work unless it is a sunny day), yes?
    • the absorption of the heat is augmented by the black paint, yes?
    • the retention of the heat is effected by the greenhouse effect where sunlight passes freely through glass or clear plastic, but hot gasses do not effectively pass their heat back out readily - so the heat is largely retained, yes?
    • you have augmented the amount of heat collected by the addition of a somewhat parabolic reflector made of tape, yes?
    • the net effect is that the temperature of the tank (the black-painted bottle) is 95 Fahrenheit after 2 hours.

    If I understand the principles involved and the numbers, this could scale to provide 55 gallons of 95 F water on demand in 35 degree sunny weather simply by doing the same thing with a 55 gallon tank and surrounding air-trapping plastic, no?

    5 replies

    Reply 6 years ago on Introduction

    From what I gather, sun rays go in, heat up medium. Medium gives off heat, which can be trapped using standard insulation methods.

    Additional bottles (aka layers) act as insulation and help prevent energy loss to the outside.


    Reply 7 years ago on Introduction

    On a cloudy day, the maximum possible temperature of the water will be the temperature of the clouds in the sky. The tops of the clouds are of course heated by the sun, thus the temperature of the clouds can potentially be quite a bit higher than the air temperature on the ground.

    As for scaling up -- a 55 gallon tank doesn't have enough surface area to heat up in a reasonable amount of time. You'd be better off insulating it without bothering to limit yourself to transparent insulation, and then use a separate solar thermal collector.

    For most people in developed countries, a modest goal is to offer pre-heated water on the input side of your water heater. That way the water coming into your water heater is either already pre-heated (yeah!) or cold (so the water heater has to fire up). To pre-heat your water, solar power will give you good results on cold and warm days, as long as there is sunlight (electro-magnetic rays). On rainy days, your heater will fire up. It appears to me that ANY VESSEL THAT IS BLACK and SAFE (glass or metal, food-grade plastic) that is BLACK will passively heat your water on MOST days of the year (unless you are in the UK or Seattle). To enhance the impact of the sun, create a greenhouse effect... place a clear glass or plastic container around your black container that will forbid the heated gasses to escape.

    doo da do

    6 years ago on Introduction

    What if you used food grade black container cut to length instead on the inside the two liter bottle. That may encourage more heat inside, hot water faster.

    1 reply

    I did use a glass bottle in one version of this which keeps it a little more chemical free; I'm leery of food grade plastics as well, especially when heated and/or exposed to UV light.
    Separate but related rant: I hate the plastic taste in take-out coffee cup lids, who knows what is leaching into that hot drink. Why don't they make paper lids like the paper cups?


    8 years ago on Introduction

    I would not recommend using anything but HDPE plastic for the bottle that contains the water. The soda bottle plastics are made of PET and they can be toxic when heated.


    8 years ago on Introduction

    Since the "middle bottle" is apparently used only as insulation, I wonder whether a better solution would be to substitute a layer of bubble-wrap, which usually just goes into the trash.

    Maybe even a couple of wraps of the bubble-wrap material, unless it would interfere with the heating of the inside bottle.

    Seems like it would help, actually ... but I'm not sure of the optics involved here.

    1 reply

     That might work nicely. The added insulation might make up for any loss of light to the inner bottle.


    9 years ago on Step 3

    Rubber heater hose is not approved for drinking water. If you want to have potable water you should do the glass bottle upgrade mentioned earlier AND use drinking line approved hose. I am so going to try this with the kids next door!