pashiran's workspace

(Step 1 was research oriented, so it is not included here).

Step 2: Characterizing the Peltier Tiles:  

Two sets of cheap, different sized tiles were obtained⁶. Tile 1 had an area of 1.36 cm² and an internal resistance of 5 ohms, and Tile 2 was 4 cm² and had an internal resistance of 2.4 ohms. I tested the power generated by each Peltier stack on a per cm² basis.

To do this, I taped each tile onto a square aluminum tube. One side of the Peltiers was cooled by an ice pack, and the other  was heated with a 12-volt light bulb connected to a variable power supply. The temperature difference between the sides of the Peltier tile was measured. Both Peltier tiles produced enough power to light an LED, but only at 50 and 73 mV.

I needed 2500 mV to light the flashlight LED.

Step 3: Boosting the Voltage.

Direct Current cannot be multiplied, but if the DC is changed to AC, the voltage can be stepped up with a transformer . The answer lay in constructing a simple oscillator circuit with a step-up transformer.
To do this I constructed a feedback oscillator with a field effect transistor and wound my own transformers with step-up ratios of 5:125.

The oscillator worked, but the LED did not light up until the Peltier voltage was 120 mV.  I needed it to light up at about 50 mV (voltage produced at a 5°C temperature difference).

Searching the Internet, I came across an article about energy harvesting⁷ and the use of a power converter integrated circuit, LTC3108⁸. The circuit contained FET’s that would oscillate at voltages as low as 20mV. When used with a recommended   transformer, the IC would provide well over 2.5 volts AC.
The IC also worked fine as a very low voltage transistor oscillator. My circuit now had only 4 components: The IC, the step-up transformer, a 47µF capacitor, and the LED. With the LED across the transformer, I was able to obtain good LED brightness with only 50 mV DC input across the oscillator.  The efficiency of the converter to be about 50% at 100 mV. 

Step 4: Physical Flashlight Design.

I decided to make the flashlight with dimensions of 25mm in diameter and 125mm long. Four of the large Peltier tiles covered 16cm^2, and four of the smaller tiles had a combined area of 5.4cm² .

Tiles were mounted on a milled area of 25mm diameter aluminum tubing, and placed inside a larger PVC pipe, insulated from it by air. The hand griped the tiles through an opening in the PVC pipe. Air flowing through and around the aluminum tube cooled the flashlight. The circuit was mounted in the front, and the LED was centered in the middle of the tube. The PVC pipe was wrapped with insulating foam. I made two flashlights, the F1 with 4 smaller tiles, and F2, with 4 larger tiles.

Havesting Thermal Energy to Agricultural Sensors .pdf