Nuts & Volts - September 2004

Java Power

several tens of watts of heat.

In this project, we’ll supply heat to the converter and draw a current from it. When heat is flowing through the converter (which will have an efficiency of typically 1% if the temperature difference across it is about 30° C), negative charge appears at the heatsink side (the “cold end”) of the converter in the N-type legs. Note that an effective heatsink is crucial — it is no use making the whole converter hot: one side must be hot and the other cold.

Improving Performance

Converters wired in series

Switch (optional)

MUG

Motor or other load

Thermal conduction is like electrical conduction — the flow of heat is like a current and temperature is like voltage. The thermal circuit is a chain of resistances from the high voltage (the hot liquid) to “ground” — the cool air. If the thermal resistance is low, then the “current” or heat flow will be highest.

One of the resistances in the circuit is the thermal converter itself. Ideally, this is the largest resistance in the circuit, so that most of the power is dissipated in the converter and not in useless resistances elsewhere.

So, the other resistances (thickness divided by thermal conductivity) should be minimized — this means a thin-walled mug made of a thermally-conducting material like metal. Similarly, the heatsink should be effective. The better the heatsink, the cooler it will be and, thus, the temperature difference across the converter will be maximized.

Poor contacts between surfaces can introduce very high thermal resistances — unless surfaces are ground exceptionally flat and perfectly aligned, a lot of airgaps fill the contact area with contact being made only through little bumps in the surfaces. The way to minimize these resistances and the temperature drops they cause is to use a thermal grease or heatsink compound that fills these little gaps with a relatively conductive material.

The voltage produced by the converter is proportional to the temperature across it — this is, after all, the way thermocouples are used as temperature sensors. The current produced by the converter is proportional to the heat flow through it. The heat flow through the converter is

Circuit diagram. Five converters are wired in series (note polarity!). They are in thermal parallel (i.e., the same temperature difference is driving them all). Heatsinks are omitted for clarity.

proportional to the temperature difference across it. So, the power produced by the converter (current times voltage) is, therefore, proportional to the square of the temperature difference. This is why it is vital to maximize the temperature difference by hot liquid, conductive mug and contacts, and a good heatsink.

To extract the most electrical power from the converter, it is important to choose the load impedance carefully

Resources

Thermoelectric Cooling Modules can now be found in many electronic parts stores and catalogs. One manufacturer/vendor is Magaland Technology, Inc. ( www.leadingtechnologysales.com). I used several of their ICE-IT TEC 1-12705S cooling modules ( 40 x 40 x 3. 8 mm, rated max 15. 4 V at 5.2 A for cooling).

Another manufacturer is TE Technology ( www.tetech.com) with a wide range of modules ($100.00 minimum order). I have also used their TM-TB-127-1.4-1.05(P) modules ( 40 x 40 x 3. 8 mm, rated 15. 7 V 8. 6 A). TE Tech’s website also has a lot of technical papers you can download.