In this instructable, I will show you how to make a heat-powered fan. Set it on top of a wood stove and it blows warm air around the room. There are no power cords or batteries to fuss with and it doesn’t cost anything to operate. This is especially useful if you want to use it in an off grid cabin or live in the developing world. The fan self regulates its speed, meaning the hotter the fire gets, the faster it spins. It helps distribute heat more evenly throughout a room and allow it to warm up faster than it would without a fan. And it’s whisper quiet.
I have seen this type of fan for sale in the Lehman's catalog for $160, but I was able to make this one for about $5, mostly because I had a lot of stuff laying around. I am not claiming that my fan is as well made or reliable as the $160 version, but it was certainly fun to build and works pretty well. Even if you had to go and buy all the parts, you could probably still build it for under $30. However, many of the parts can be obtained free.
If you are looking for inspiration, you can check out YouTube. One video that inspired me in my design was this one:
If you stay within certain guidelines, there is a lot of wiggle room as far as design and materials are concerned. I will try to lay out the guidelines for you.
How it works
The magical part about this fan is that it converts the heat of the stove top into electricity, which it then uses to spin the fan blade. This conversion from heat to electricity is performed by a peltier plate (the white square thing with the wires sticking out of it, see picture above), which can be purchased on eBay for about $2. Sometimes, peltier plates are called thermoelectric coolers, and that is because when peltier plates are connected to a power source, they transfer heat from one side of the plate to the other. When used in this way, a peltier acts as a solid state heat pump. Mostly they are used for cooling things like computer processors.
Simply put, peltiers normally work like this:
power in = heat transfer from one side to the other
For this project, we will use the peltier plate in reverse:
heat transfer from one side to the other = power out
To achieve this, we have to hold one side of the peltier plate at a high temperature, and the other side at a lower temperature. That way, As heat makes its way from the hot side to the cooler side, it passes through the peltier plate, generating a small current. Yup, the peltier will act as a thermoelectric generator!
As we discussed, one side of the peltier plate has to be held at a high temperature. It isn't a good idea to just lay it on the stove however, so for us it means attaching the peltier to a chunk of metal and that sits on top of the stove. As for the cooler side, we will attach a chunk of metal to that as well. This one, however, will be a heat sink or other radiator type design that will shed heat efficiently into the surrounding air. Any common metal could be used for these parts, but aluminum, copper, and brass are much better choices than steel. Steel is comparatively poor at transferring heat and is very hard to machine, so it isn't recommended for this project.
The parts that make up the fan:
- chunk of aluminum I-beam
- chainsaw engine block (robbed from dead chainsaw)
- aluminum heat sinks (robbed from dead computers)
- peltier plate ($2 eBay)
- small low-voltage DC motor (the ones from CD players work well)
- sheet metal
- small (#4 diameter) bolts
- assorted brass fittings
Tools and equipment you will want:
- soldering iron w/ solder
- drill w/ bits
- flat metal file
- ruler and pencil
- tin snips or nibblers
- assorted flat and Phillips head screwdrivers
- vise for holding things while you cut them
- tap sized for tapping #4 NC threads
- RTV gasket maker or electrical tape
I didn't have this but you might want it:
- thermal paste
Step 1: Obtain Materials
Aluminum heat sinks like the ones used to cool computer processors work really well for this project, and they are easy to obtain. For one thing, many of us have an antiquated/not-working desktop computer or two lurking in a closet somewhere. Also, try going to computer repair places and asking about any heat sinks they might be willing to part with. Plus, don't forget that recycling collection/drop-off/processing locations are great places to look for dead 'puters. Worst case scenario is, you have to buy a heat sink off of eBay.
The motor for spinning the fan blade has to be a low-voltage DC motor, and CD player motors (the one that actually spins the CD) work very well. All you have to do is get your hands on a CD player, or even a computer CD drive, and tear it apart. As a last resort, you could probably buy a motor off eBay.
The engine blocks of small air-cooled engines, while not as effective (at shedding heat) as heat sinks from computers, make interesting pieces. I chose the engine from a chainsaw, an old Stihl 02 WoodBoss.
I got my hands on a chunk of aluminum I-beam somehow, but I am not sure where it came from. It works real well as a base but just about any chunk of metal will work.
One thing we will have to do is sandwich the peltier between the two chunks of metal. For me the easiest way to do it was to bolt the two halves to each other. The drawback is that heat will be conducted through the bolts, and this will hurt efficiency. My compromise was to use small bolts, diameter #4.
I threaded some brass plumbing fittings together to make the part that holds the fan motor. It seems that a ¼ inch NPT (national pipe thread) fitting fits well into the 14mm spark plug hole. For you, a simpler way to make the brass part may be to buy a dishwasher elbow or something like that. Go to the hardware store and start playing around, seeing what you can make. I had some brass pieces laying around, so I used those, though I did have to modify one of them considerably, as I will get into in a minute.
I made my own fan blade by cutting it out of a sheet of aluminum. You could use other metals but they won't be as lightweight. You could also try to buy a fan blade off of eBay, or rob one from an existing fan.
Step 2: Cut a Slice Off Your I-Beam
Okay, so maybe you don't have access to a free piece of an aluminum I-beam, but if you do just put it in the vise between two pieces of two-by-four and saw off a chunk with the hacksaw!
Step 3: Cut Out a Fan Blade
I have included the PDF of my fan blade design if you want to replicate it, and also the image file that you could scale to the size of your liking. Choose one, then simply print it out, cut it out, and lay it on another larger piece of paper (my full size pattern won't fit on an 8.5 by 11 inch sheet) or the sheet metal that your fan blade will be made out of. As the pictures show, trace the blade pattern once, the flip it over and trace it again to make the rest of the blade pattern. Then, simply cut it out with a metal shears, tin snips, or nibblers.
Step 4: Bend Your Fan Blade and Figure Out How to Mount It
Now you have to put a couple of bends in the fan blade. The first pair of bends will be made towards you, the other will be away from you. The pencil lines in the pictures show where the bends have to be made. It is hard to describe in words but the pictures should help you understand. If you are new to instructables, you should know that you can click on pictures to make them larger. Also, some of the pictures have little boxes over them, and placing your cursor over them will display additional details.
I simply held my fan blade down on a board and pushed down on the unsupported part (the part hanging off the edge of the board) to make the desired bend.
To mount the fan blade on the motor, I first tried punching a hole in the middle and forcing it onto the end of the motor shaft. It worked... sort of. The fan blade could still wobble on the shaft. One of the CD player motors I found had a small gear on the end, and I was able to drill a larger hole in the fan blade and force part of the gear into the hole. Now I could just push the gear/fan blade assembly onto the end of the motor shaft. I don't love how this part ended up, as plastic isn't a good choice around so much heat, but it is working for now. I may add some epoxy glue to help it hold up.
Step 5: Brass Parts to Hold the Motor
The pictures above show three of the four brass fittings I used, and how I put them together.
Step 6: Find Something to Thread Into the Spark Plug Hole
The brass parts in the previous step accomplished a lot, namely making a 90 degree turn and adapting up to the size of the motor. However, they don't adapt small enough to thread into the spark plug hole on the engine block. To do that, we have to adapt down to 1/4 inch NPT (national pipe thread). I did have a fitting with the correct size threads, but it was an elbow, so I cut the part off that I didn't want (see the first picture). Then, I had to grind/file the remainder down until I could drive it into the elbow fitting as seen in the second picture. I also had to drill all the way through it, so that the wires to the motor could pass through. I don't have a lathe, so to turn down the part I fed a small headless bolt (I cut the head off with a hacksaw) through the fitting and put a nut on the threaded end. It then chucked the bolt shaft into a drill and as it spun the brass fitting I held a flat metal file against it. Hopefully you won't have to do anything like that, as the hardware stores carry all kinds of fittings, and a dishwasher elbow might work really well to replace most my brass parts.
Step 7: Assemble Your Fan, Part One
Okay, so lets jump to assembly. I will fill you in as we go along. First, we start with the base, which for me is the I-beam. I cleaned up the top part nicely, then set the chainsaw engine block on top of it. I fed a Phillips head screwdriver through the block and down through its mounting holes, then tapped the screwdriver to make a mark. I did this for each of the holes to show me where to drill. Then I drilled the holes in the I-beam.
Step 8: Assembly, Part Two
First, feed the wires of the peltier plate through the engine block. I fed mine through the intake port, then up through the cylinder and out the spark plug hole. I had the brass fittings installed up to the elbow, but you will have to determine the best way to feed the wires. Next, lay the peltier on the I-beam, followed by another piece of metal to bridge the hole in the bottom of the engine block (I cut mine from a scrap part of the I-beam I cut off). The peltier supposedly has a "hot side" and a "cool side". The hot side is the one that should go against the I-beam. If the hot side isn't marked, you can touch the two wires of the peltier onto the posts of a car battery very briefly. The side that gets hot is the hot side.
This would be the point where you would want to spread heat conductive paste between the mating surfaces of peltier and the different heat sinks, to help with heat transfer. I don’t have any, but you could either try to scrape some off of old CPUs or you could just buy some off eBay.
Step 9: Assembly, Part 3
Now, feed the little bolts through the holes and tighten them down.
Step 10: Assembly, Part 4
Now, finish assembling the brass parts if you haven't already. I should mention that I drilled three holes through the brass and tapped them with threads so that I could thread little #4 bolts through them to tighten down on the motor. You can see it in the last picture. You don't have to do this, as you could use only one hole w/bolt, or you could use epoxy glue. Or, better you don't use glue, as you might want to take the thing apart. Solder the wires of the motor to the ones of the peltier. Insulate one of the connections with a little RTV gasket maker or electrical tape. Put the motor in its place and tighten it down.
Step 11: Finish!
You can finish by attaching any heat sinks you want to the engine block. I attached one using the existing holes and bolts that held the muffler on. The very last thing to do is attach the fan blade and place the whole thing on a hot stove and see if it spins. If the motor spins the wrong direction, you will have to switch the wires going to it.
To be honest, my fan still needs some work. The heat sinks I used looked good, but don’t shed heat very well. Also, I used plastic to hold the fan blade on, which might not hold up under extreme conditions. The wires of the peltier get hot, and the insulation gets soft. It could probably be better protected. Anyway, some things I need to work on. Thanks for reading, and if you have the time and are so inclined don’t forget to comment or vote on this instructable! It means a lot. Happy making, Hulkbuild out.