Peltier cells are flat devices that draw heat from one side to the other through a thermoelectric principle called the Peltier effect. The cells are commonly used to pump heat away from CPUs or graphics cards, and are also found in camping coolers and heaters. The Amazing Seebeck Generator uses one of these devices in reverse, to turn a heat differential into electricity, rather than using electricity to produce a heat differential.
I originally made the project because I wanted something like a steam-powered generator, but without the noise and maintenance issues associated with steam. I was pleasantly surprised when I found that my $5, 37-watt Peltier cell from eBay could capture the heat from a single tea candle or alcohol burner and use it to generate about 5 volts at 1 amp, which made it perfect for powering radios, mobile phones, and LED lights. You can make the Amazing Seebeck Generator in less than an hour using mostly scrap or recycled parts, and it has a distinctly steampunk feel to it.
In the Cooler
The Peltier and Seebeck effects exchange temperature differences and electricity. In a thermoelectric cooler, aka Peltier device, alternating slices of different semi-conductor materials connect in a zigzag pattern between 2 plates. Heating one plate drives electrons away in one material while attracting them in the other. This induces an electric current in 1 direction — the Seebeck effect. Conversely, running a voltage across the junction draws heat toward one side while cooling the other — the Peltier effect.
The multiple junctions in the zigzag work in parallel, which multiplies the effects. Whether the device is used to convert a heat differential into voltage, or vice versa, it performs the conversion with no moving parts.
Unfortunately, thermoelectric devices are typically only 1%–2% efficient, or 5% with the latest advances. This isn’t enough to make large-scale thermoelectric power generation (TEG) practical, although many researchers are trying to raise the efficiency. But thermoelectric generators are useful for other things; they can measure extreme differences in temperature, and are used in heating systems to power convection fans and pumps by using waste heat recovered from stovepipes and boilers.
The principle behind our Amazing Seebeck Generator is simple. We position our Peltier cell horizontally over our tin-can “furnace,” heat the underside with a candle or alcohol burner, and cool the topside with a heat sink and fan.
1. Make the Can Furnace
1a. Using a small knife or Dremel with a cutting bit, make 3 or 4 U-shaped cuts equally spaced around an empty tin can, near the open end (the bottom of the furnace). Bend the resulting metal tabs 90° into the can to form little brackets. These will hold the can lid later, to make a level platform for the candle.
CAUTION: Wear gloves to protect your hands when cutting metal.
1b. Cut another, larger hole in the can between 2 brackets and opposite the can’s seam, that extends farther toward the top. The hole needs to be big enough to allow candles and fingers through. Using pliers, bend the edges of these cuts over, into the can, so that no sharp edges are exposed.
1c. Cut 2 more rectangular holes on either side of the can near the closed end (the top), to allow the light to shine out and air to flow in. Drill a hole above each of these, toward the closed end, that’s just large enough for the bolts to fit through.
2. Add the Chimney (Optional)
The chimney is not functionally necessary, but it adds a steampunk feel and balances out the design.
If you also want to paint the sides of the can, you can use a high-temperature paint, but I prefer the natural shine of the metal.
2a. Drill or cut another hole in the can, near the closed end and through the seam, which is the strongest part of the can. The hole should be the same size as the copper elbow, and should be a tight enough fit to hold the chimney in place.
2b. Fit the pipe elbow through the hole. If it doesn’t fit snugly, use high-temperature silicone glue or exhaust repair putty to secure it. Then, fit the chimney pipe onto the pipe elbow.
3. Make the Gasket
You could simply position the Peltier cell between the can and the heat sink, but framing it within a gasket limits the exchange of heat around it, which improves efficiency.
3a. Cut a circle out of circuit board material that’s the same diameter as the tin can. You can use a hole saw or a circle cutter if you have one. Otherwise, use a utility knife to cut a hexagonal shape, then file it round to fit. The circuit board is brittle, so score both sides deeply and snap off the excess with pliers.
3b. Cut a hole in the gasket for the Peltier cell to fit into. Draw around the cell with a marker, leaving some extra room for the cable contacts coming out of the cell. Then cut around the outline with a utility knife.
4. Put It All Together
4a. Bolt the springs to the top of the can by running each bolt through a spring end, a nut, the can, and the other nut inside the can, in that order. Fixing the depth of the bolt with an extra nut is neater than having a single nut on the inside of the can. Then, attach the heat sink to the 5V fan. I used thin copper wire, but you can also use glue or screws.
4b. Place the gasket on the top of the can, and place the Peltier cell in the gap. To improve efficiency, add a thin layer of thermal transfer grease (also called heat sink compound) to both sides of the cell.
4c. Secure the heat sink on top of the Peltier cell by hooking the tops of the springs to the heat sink’s tension bar. If it doesn’t have one, drill holes on each side to thread the springs through.
Position the can lid on the brackets inside. If the can is small, fix it to a sturdy base using silicone glue. Then make a candleholder that will fit inside the can. I used an anchovy tin with stiff wire poked through as a handle.
4d. Twist or solder together the red (+) and black (–) leads from the fan and Peltier cell, red to red and black to black, and also connect an alligator lead to each, for connecting the generator to other things.
Decorate as desired (I added a fancy door from a model steam engine to the candleholder), and you’re done!
Place a candle in the tin can furnace, and wait for the heat to build up. If the candle keeps going out, add more holes to the furnace.
If you’ve done everything correctly, the fan will begin to spin. If you have a voltage meter, you should start to see a reading soon after the candle is lit. If the fan doesn’t spin, check that you don’t have the fan wires connected backward.
Regulating the Voltage
The output from the Peltier cell is unregulated, and its voltage will vary along with the flame and fuel level. Unregulated power could kill some electronic equipment (although I’ve powered my MP3 player and radio on unregulated current with no ill effects).
You can rectify this with an LM317 voltage regulator. These cheap, readily available components can be configured to produce a constant voltage between 1.2V and 25V. You can wire a Peltier cell generating approximately 5V to an LM317, to produce a regulated output voltage up to about 3.8V that’s tuneable by turning a potentiometer. Output of 3.8V lets you run a high-power LED, charge a PDA or mobile phone, or power a radio or MP3 player.
To power higher-voltage devices, you would need to create a step-up regulator, which combines an oscillator circuit with a voltage multiplier to raise the voltage while lowering the current.
CAUTION: Step-up circuits are moderately complicated, and you should not attempt building one unless you are familiar with electronics, because they can give a nasty (and potentially fatal) shock.
Boosting the Power Output
For more power, use an alcohol burner instead of a candle, and raise the flame. Use a smaller can, a larger heat sink, and plenty of heat sink grease.
You can also increase output by connecting multiple Peltier cells together. Connect them in parallel to increase the current, and connect them in series to increase the voltage. Use LN4001 or LN4002 diodes to block current from entering the cells. With parallel cells, connect a diode to each cell’s positive lead, with its silver band facing away from the cell. With the cells in series, run a diode from the red wire on each cell to the black wire on the next cell, with the silver band facing black.