This Instructable describes how I built a low power, electric kettle-thermos to heat up water. It is made of reused/repurposed components and is suitable for use with an off-the-grid PV panel. I have a single panel connected to a small inverter that puts out 120V AC.
Electric kettles that you typically see use 1000 or more watts, because people want their water heated ASAP. However, if you’re not in a rush, then you can get by with a lot less power. Mine uses only about 15 Watts, and, on sunny days, while I’m gone at work, it heats up 1.25 liters of water to boiling. Even on days when it has been mostly cloudy, the heater was on enough of the time that the water temperature was 160 F when I got home from work. In the evening, I can use the water to wash dishes, and since the kettle is also a thermos, the water is still hot enough for dishes the following morning. I don't use the water for drinking because I'm not sure that the heating element is completely free of materials that I would want to ingest.
Pros and Cons
I know that electricity is an inefficient way to heat water, and that a solar thermal system could easily to as well or better. However, if one has the available PV power, then efficiency isn't really an issue. Furthermore, it would be quite challenging and complex to use solar thermal to heat water in a thermos. This device is really compact, and the water will stay plenty hot for hours after the Sun goes down. By using the water to wash dishes, I save a little bit of natural gas. I dilute this water with tap water, and I get enough hot water to wash a load of dinner dishes for 2-3 people.
Step 1: Materials and Tools
Materials List - These are the materials I used. You may substitute anything that works
An old, discarded thermos, 1.25 liters, the kind made for hot liquids
An old 15 watt soldering element that was discarded from my work
A small piece of rigid foam insulation, 1-inch thick
A discarded computer power cord
Small piece of 1/8 inch copper tubing
Shrink Tubing Heat Gun
Step 2: Electric Heating Element
I used a 15W 120V AC soldering element. These can be purchased, but there are also other ways to do this if you don’t have one. You basically need to use Ohm’s law to figure out what resistance you want, and then you need an electrical resistor of the right value and power rating.
In my case: V=120
R = ~1000 Ohms
Power = V^2/R = ~15 Watts
Say you are using 12V DC:
V = 12
P = 15W
R = V^2/P = 144/15 ~10 ohms
So, you would want to get a 10 ohm resistor that is rated at, say, 25 Watts (for some added safety). With a resistor, you wouldn’t want to immerse it directly in the water. I haven’t thought about this a lot, but you would want to seal it in something that is waterproof, not electrically conductive, and very good at conducting heat.
Step 3: Joining the Heater to the Power Cable
I cut off the end of the power cord that normally plugs into a computer, then cut a two-inch slit down the outer insulation of the cable, revealing the white, green and black wires. The soldering element has two little electrodes that are made to be inserted in a connector (See Photo in Step 2). Since I don’t have the connector, I had to improvise. For each connection, I used a small piece of copper tubing, with an inner diameter of about 1/16 inch,cut with a hacksaw to about twice as long as the pin on the solder element. I placed the tubing over the pin and then inserted the wire in the other end of the pin. I then flooded the interior of the tube with solder and that made a pretty secure joint. After checking each connection with a multimeter, I sealed the entire joint with hot glue and then added a layer of shrink tubing on top.
Step 4: The Insulated Top
This thermos was missing the top when I got it (That's probably why someone else got rid of it :). I cut the foam into a circle that will just fit the top of the thermos, then covered the sides with aluminum tape. With my hand, I used a drill bit to make a hole on the foam and then pushed the heating element in so that it will be thoroughly immersed in the water when the top of the thermos is on. By trial and error, I found that I had to push the heater in so that about one inch sticks up from the top. I then used aluminum tape on both sides of the foam to fix the heater in place. On the opposite side of the top, I inserted the meat thermometer in such a way that it won’t touch or be too near the heating element. In my case, I had to push the thermometer in at an angle, as shown in the photo.
I also made a second top identical to the first one, but without any holes poked in it. I use this top when I take the thermos inside to use the water.
Step 5: Plug It In!
A fews words of safety
This device is not intended to be used, from a wall socket and I wouldn't consider it safe for that. With a small PV system, even if you forget and leave it on, it will either run the battery or turn off at night. For mine, I plug the kettle-thermos in outdoors and have it sitting on concrete in my backyard. I wasn't able to use the ground wire on the power cord as a safety feature. As long as the connections on top are intact, there is little chance of getting shocked, I always connect the electric kettle-thermos to a power strip and turn the power off before handling the power cord or any other part. If there were some kind of short circuit in my absence, the inverter would shut itself off due to the voltage dropping. Also, the PV panel is not capable of supplying much current, should there be some type of short. Of course, handle the thermos carefully, as you would with any other container of boiling water.
Plug it in and enjoy "green" hot water at the end of the day!
Step 6: Future Upgrades
I'd like to add a small LED on the top, so I would know for sure when the kettle is turned on, but not so easy to do when running off of 120V AC. In the future, I would like to expand this concept to maybe 50 watts of power and heat a larger quantity of water in a manner that would be safe for drinking, as well as washing dishes.