Thursday, December 3, 2015

Thermo-Electric Generators

One of the regulars on our Facebook page posted a short video from the BBC showing a way to charge your cell phone or other small electronic device from a home-made device added to a clay firebox or stove. There was some interest in how this device worked, so I took a closer look at the video to try to figure out which method they were using.

For those who can't or haven't seen the video:
  • The generator is two metal plates, about 4 inches square, bolted together, with three pieces of rebar welded to one of the plates (the fire side) and a square metal box (about 2x2x6 inches) filled with water welded to the other plate.
  • The generator is inserted into a hole in the side of a fired clay cook stove that is burning wood.
  • The captions on the video mention that the fire creates a heat differential in the metal rods sticking into the fire to “create” electricity. This is incorrect. The metal rods are there to provide more surface area in the flames.
  • There is a pair of twisted wires coming from the middle of the metal plates, indicating that some part of it came from a factory.
  • There is a white substance between the two plates, probably conductive paste as is used on computer heat-sinks.
  • The wires are run directly to a cell phone and an LED flashlight, which means it is putting out around 5 volts of DC power.
  • Since there are commercial versions of this on sale in America (Powerpot and similar), this was something designed for use by people with little to no money.
All of these points lead me to believe that they are using a Seebeck junction (sometimes called a Peltier junction) placed between two locally-fabricated pieces of steel.

Thermo-Electric Junctions
A Seebeck junction is a sandwich of dissimilar materials that will generate electricity when placed between a hot surface and a relatively cold surface. As the heat moves from the hot area to the cold area, electrons are knocked off of the material, creating an electrical flow. If you reverse the conditions and push electricity into the junction, one side will get hot and the other side will get cold.

Solid-state coolers are normally called Peltier junctions, while the thermoelectric generators are called Seebeck junctions even through they are the same thing.

Seebeck thermoelectric generators are not terribly effective -- about 5% efficiency -- but they are perfect for use in waste heat reclamation when placed in the exhaust or combustion chamber of wood-fired heaters.

Seebeck generators are fairly easy and cheap to buy. Since the main uses involve the Peltier (cooling) function, it can be hard to get (and understand) good data on the amount of energy you'd be able to get out of a given module, but I'll give it a try.

In Practice
Amazon has a cheap one if you want to try making electricity from fire. A bit over 1.5 inches square, it weighs less than an ounce and costs less than $10.00,  making it perfect for experimenting.
  • Power output will vary with the difference in temperature between the two sides. The higher the difference in temperature (not the higher of actual temperatures), the higher the voltage and amperage that will be produced. This one has a maximum temperature of 150° C (302° F), so it would work with small wood fires or placed on the surface of a metal wood stove. 
  • The cold side on the generator shown in the BBC video used a container of water to keep the temperature at 100° C (212° F). The hot side was the metal rods inserted into the flames, which in a wood fire is about 150° C (302° F). The difference is 150-100= 50° C. This is a small difference when looking at Seebeck generators. 
  • Unfortunately, the seller doesn't give enough information to be able to estimate the power output. One of the buyers, however, mentioned getting around 7 volts DC by placing the hot side on a stove and the cold side on a ice pack.

A more expensive version can be found here.

  • These modules can handle up to 350° C (662° F). 
  • Wiring a few of these modules together to get the voltage you desire (around 5V for cell phones) would be the best route.
  • Looking at the charts for a module the same size as the one from Amazon, a 50 degree differential would put out about 2.5V at a little more than 0.5A. Not bad for something the size of a pack of matches. 
  • Wire two of them in series (connect positive of one to the negative of the other and draw power from the unused connections) and you'd have 5.0V at 0.5A. 
  • To put those numbers in perspective, most older cell phone chargers (and USB 2.0 ports) are rated at 5V and 0.5A while the newer chargers are still 5V but 2.0A.
If you don't want to buy a module, you can find them in the common 12V coolers sold at Walmart, and most truck stops, and in high-end desktop computers (active cooling for the CPU-mounted between the chip and the cooling fins). Salvaging one and using it to make a generator would require a bit of knowledge of electricity and a way to determine which wire is positive and which is negative. Voltage output will vary according to the temperature difference between the hot and cold sides, so a cheap voltmeter would come in handy.

Thermo-electric junctions are interesting ways to generate small amounts of power from fire, something you'll probably already need in an emergency. Since there are no moving parts, maintenance is minimal and they're pretty fool-proof once assembled. I hope I was able to answer a few questions, but if you have more I will do my best to answer them. Feel free to leave comments here or on the Facebook page.

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