Thermoelectric Rotational Ornament

Background:

This is another thermoelectric experiment/ornament where the whole construction (candle, hot side, module and cool side) is rotating and both heating and cooling itself with a perfect balance between module output power, motor torque & rpm, candle efficiency, heat transfer, cooling efficiency, air flow and friction. A lot of physics are going on here but with a very simple construction. I hope you enjoy this project!

See videos for final result:
Youtube Video 1
Youtube Video 2
Youtube Video 3

Some other of my thermoelectric projects can be found here:

Thermoelectric Fan
Smarthphone Charger
Emergency LED


Concept:

The heart of the construction, the thermoelectric module, is also called a peltier element and when you use it as a generator it's called seebeck effect. It has one hot side and one cold. The module generates power to drive a motor which axis is attached to the base. Everything will turn and the air flow will cool the upper heat sink faster than the aluminium plate below. Higher temperature difference => increased output power => increased motor RPM => increased air flow => increased temperature difference but decreased candle power. As the candle also follows the rotation the heat will be less efficient with increased speed and this will balance the RPM to a nice slow rotation. It cannot go too fast to put out the fire itself and it can't stop until the candle runs out of fuel.

http://en.wikipedia.org/wiki/Thermoelectric_effect

Result:

My original plan was to have stationary candles (see video) but I found this construction was both more advanced and fun. You could run this with stationary candles but it will require 4 of them if you don't use two modules or larger aluminium heat area.

The speed is between 0.25 and 1 revolution per second. Not too slow and not too fast. It will never stop and the fire will burn until candle runs empty. The heat sink will be quite hot over time. I used a high temperature TEG module for this and I cannot promise a cheaper TEC (peltier module) will make it. Please be aware if temperature exceed the specification of the module it will be damaged! I don't know how to measure the temp but I cannot touch it with my fingers so I guess it´s somewhere between 50-100C (on the cold side).

Materials:

• Aluminium plate: 140x45x5mm
• Plastic rod: 60x8mm [from a venetian blind]
• Electric motor: Tamiya 76005 Solar Motor 02 (Mabuchi RF-500TB). [Ebay].
• Thermoelectric module (hight temp TEG): TEP1-1264-1.5 [from my other project, see below]
• Heat sink: Aluminium 42x42x30mm (single directional air channels) [from an old computer]
• 2x Screws + 4 washers for motor: 10x2.5mm (not sure about threading)
• 2x nails for heat sink attachment: 2x14mm (cut)
• 2x springs for heat sink attachment
• Counter weight: M10 bolt+2 nuts+2 washers+magnet for fine adjustment
• Thermal paste: KERATHERM KP92 (10 W/mK, 200C max temp) [conrad.com]
• Steel wire: 0.5mm
• Wood (birch) (final base is 90x45x25mm)

TEG spec:

I bought the TEP1-1264-1.5 at http://termo-gen.com/
Tested at 230ºC (hot side) and 50ºC (cold side) with:

Uoc: 8.7V
Ri: 3Ω
U (load): 4.2V
I (load): 1.4A
P (match): 5.9W
Heat: 8.8W/cm2
Size: 40x40mm

Tools:

• Drills: 1.5, 2, 2.5, 6, 8 and 8.5mm
• Hacksaw
• File (metal+wood)
• Wire brush
• Steel wool
• Screwdriver
• Abrasive paper
• (Soldering iron)

See drawings for all measurements.

1. Draw on the aluminium plate or use a template.
2. Use hacksaw to cut out the piece.
3. Use file to fine adjust
4. Drill two 2.5mm holes for motor (22mm between) plus 6mm hole for motor center
5. Drill two 2mm holes where the nails will be (for heat sink attachment)
6. Drill one 8.5mm hole for counter weight (will be threaded as M10)
7. Finish the surfaces with wire brush and wool

I used a cut in half fire wood.

1. Use file and abrasive paper before cutting it (easier to fixate)
2. Drill a 8mm hole in the top center for the rod (20mm depth, not all the way through)
3. Cut the piece at 90mm length
4. Finish the surface
5. Use oil or wood stain for nice surface color (I applied dark wood stain after all photographs for better look)

This is the most tricky part i guess. Maybe easier if you do this at the end when everything is finished and working. I used a thin wire to bend it by using just two pieces. It was difficult to photo all angles. This part will hold the candle beneath the thermoelectric module at a distance so the flame does not touch the aluminium plate.

1. Bend two identical parts to fit the candle
2. Glue the two parts together

1. Use one washer at each side of the plate
2. Make sure the screws are correct length (to long will damage the motor)
3. Screw the motor

The washers will separate the motor a little from the plate and make sure it doesn't get overheated later on.

It's a critical part to use thermal paste in order to get a good heat transfer between the parts. I used high temperature (200C) thermal paste but it "might" work with regular CPU thermal paste. They can usually take between 100-150C.

1. Make sure the surfaces of the plate, module and heat sink and clean from dirt (must be good contact)
2. Apply thermal paste on the "hot side" of the module
3. Attach module hot side to the plate
4. Apply thermal paste on the "cold side" of module
5. Attach heat sink on top of the module
6. Attach springs to hold the heat sink steady (high pressure results in better heat transfer)

1. Drill 1.5mm hole in the rod (3mm depth)
2. Attach motor axis to the rod
3. Attach rod to the base wood

1. Attach module cables to the motor (soldering iron is good)
2. Attach candle hanger to the same nails as the heat sink springs are attached to
3. Place a candle in the hanger
4. Mount counter weight and tilt the construction to make sure you have right balance

Please be aware that the heat from the candle can damage your module if the specification has low max temp. Even the cold side will be pretty hot! Another step that you might want to do is to prepare the heat sink with electrical tape and fill it with water. That make sure the cold side will never reach over 100C! My planB was to do this but I didn't need it.

1. Lit the candle (detached)
2. Place the candle
3. Wait 10sec and maybe try to help it spin to get it started before the cold side gets overheated
4. Enjoy!

Main formula:
Energy=Energy+fun

Detailed formula:
RPM=mF(tegP)-A*(RPM^2)

RPM="motor revolutions per minute"
mF()="motor characteristics formula"
tegP="module power"
A="air resistance + motor friction constant"

tegP=mod(Tdiff)
mod()="thermoelectric module characteristics formula"
Tdiff="temp difference"

Tdiff=sink(RPM)-fire(RPM)
sink()="heat sink characteristics formula based on air velocity"
fire()="candle fire efficiency formula based on air velocity"

Finally:
RPM=mF(mod(sink(RPM)-fire(RPM)))-A*(RPM^2)


Alternative Solutions (Feel free to make suggestions):

1. Two modules and heat sinks (symetrically) on each side of the motor for more power
   • Connect the modules in parallel or in series with the motor (stronger vs. faster)
2. Use stationary candles on the ground or fixed in the base
   • I had to use 4 candles to get sufficient power
   • See vid