A DIY solar kettle for free boiling water
This post is about a cheap but effective DIY solar kettle that can make plentiful boiling water for hot beverages using just the power of the sun.
Note: This post is n two parts. The first is my established simple direct heating solar kettle and the second part will be about my development of a hopefully safer, more convenient and powerful kettle that will use indirect water heating. The second part is a work in progress.
DIY solar kettle- Part 1 direct water heating
Introduction to a DIY solar kettle
This DIY solar kettle was inspired by the installation of both PV electricity panels and a solar HWS on my roof. The DIY solar kettle uses the sun to make and store boiling water for household use. I have used a couple of these devices successfully at my home and on my and on my yacht.
Evacuated tube source
The DIY solar kettle is made from replacement evacuated tubes for solar water heater such as in the following link Apricus. The cost of a single tube, as a replacement part, was only ~$20.
These ‘value-for-money’ pieces of high-tech equipment are beautifully crafted from borosilicate glass and are evacuated. The outside of the inner tube is coated with a dark glistening thin mysterious heat-absorbing coating. The tubes are such strong heat collectors that they will easily produce steam!
Caution 1: Work in the shade when making the kettle. Otherwise, the device will quickly reach hazardously high temperatures that could cause serious burns.
“I watch professionals install my solar HWS. After a nasty encounter with the first tube that was lying in the sun, they put the tubes in the shade and also wrapped each tube in a towel as they plugged them in one-by-one into the heat exchanger manifold”.
Me
Caution 2: Also when using the kettle, be careful to avoid steam/boiling water burns when handling the kettle as the reduced pressure of tilting the kettle WILL cause water/steam to blurt out of the opening. “More on this later”.
Making the DIY solar kettle
Making the DIY solar kettle is very simple as it just requires the careful removal of the silicone rubber seal at the opening of the tube. Keep this seal as it will be used as part of the kettle.
Next, remove the copper heat transfer tube. Keep this lovely device just in case you find a use for it. “I have not yet, but they seem too nice to cut up to make a copper tube or scrap copper”. They contain a liquid that is sealed under low pressure to make the liquid boil at low temperatures. Next, remove the thin aluminium heat tube support if it did not already come out with the heat transfer tube. This is also sure to have some future use in some project.
Tie a short length of durable thin cord to the silicone rubber seal. Then tie the cord around the evacuated tube near to the opening. This makes a keeper for the seal so that it will be easy to find when inevitably a ‘steam-blow’ will blasts the seal out. The small hole in the seal; forms a steam vent, acts as a closure to retain heat and reduce evaporative cooling of the hot water. “Just like a lid on a saucepan does. It also forms a resonator for the steam hisser described later.”
Mounting the solar kettle
The solar kettle needs to be mounted upright and, if possible, tilted back at an angle so that as far as is practical it is at a right angle to the suns rays at midday. A 30-degree angle from perpendicular is my choice as it is the optimum angle for winter heat collection. Efficiency doesn’t matter in summer. I am also lucky as my house has A-frame poles facing North with this ideal slope. On my yacht, I use the side stays on the mast as a cradle for setting the collection angle.
[Photo of kettle mounted on my yacht]
Decanting the DIY solar kettle
When it is time to use the water, I like to replace the water immediately to protect the tube from overheating. To do this I empty the solar kettle into my electric kettle which has a wide opening. This means that it is easier to transfer the hot water that will ‘blurt out’ as the tube is tilted and reduces the hydrostatic pressure in the water and allows it to boil.
“This is a good demonstration of lowering boiling point with lowered pressure (boiling point Wikipedia) or conversely raising it with the higher pressure of the water column. It might sound a bit tricky, but it is quite easy to do safely with understanding and due care.”
I also use the electric kettle so that it can be used to quickly bring the water temperature to full boiling point for making a good pot of tea.
[Add a video of a steam/water blow out while decanting boiling water into an electric kettle]
Caution 3: I only remove the seal by pulling on the restraining cord. This protects my fingers from steam burns. Also, I keep the tube perpendicular while removing the seal so that I do not trigger a steam ‘blow-off’ through pressure reduction (as described above) during the seal removal.
The tube should be kept filled with water when exposed to the sun. When not in use I store it in the shade with some water in it. Consequently, when I wish to get water from the solar kettle, I take the replacement water with me. I use a 3L juice bottle to fill the kettle. If I require a quick recovery time for the next load of water I use hot water for my solar HWS. I refill the tube as soon as possible and leave an expansion volume of about 100-200 cc. This leaves ~2.5 L water load.
The inside of the empty tube can get very hot in just a few seconds and a hissing sound can often be heard when starting to pour in a new load of water. Always refill the tube without delay to prevent overheating. The video in the link below shows what can happen with excessive delay.
Video of overheated tube imploding with the addition of water
Solar kettle operational observations
When I first started to use my solar kettle I can remember these unexplained harsh hissing sounds from my garden. Living in a
Eventually, I heard this hissing sound and my eyes caught a glimpse of a little puff of steam condensing above the solar kettle and the mystery sound was solved. However, I am still startled each time it ‘blows-off’ as it is such a harsh aggressive sound as the steam fires out through the small vent in the silicone rubber seal.
Under particularly bright sunny conditions the ‘blow-off’ will eject the seal and the requirement for the restraining cord is evident.
The solar kettle has a nice feature in that it keeps the water hot even when the sun goes down and the day cools. This is because the vacuum between the inner and outer tubes acts in a similar way to an evacuated drink flask.
Tim
