Peltiers

[DRGrim]

A peltier moves heat from one side of itself to the other using electric current. It is also possible to generate electricity with a peltier using different temperatures on each side.
My question is would it possible to use one peltier to cool a chip, and attach another peltier on its hot side, but in the current-generating mode. Would this get rid of the heat problem, and as an extra bonus, produce some extra power? Or would the heat still need to be disposed of in some way?
Thanks!

 

[rgwalt]

A peltier cooler uses electricity to generate a temperature difference or movie heat. A peltier cooler has a rated maximum for heat movement (Qmax). This maximum for heat movement is for a delta T of zero. Conversely, if no heat is being moved, the TEC has a maximum rated delta T of usually around 70 degrees C Now, a TEC doesn't get rid of heat, it simply pumps it across a temperature difference. However, if you were able to reverse the peltier cooler and make it a current generator, you would have to maintain one side at a lower temperature than the hot side. You would also need something to use the current generated. However, it is likely that the inefficiences of the TEC would lead to a very low return on the investment so to speak. Here is an exerpt from a company's TEC FAQ:

Yes. The 'flip-side' of the Peltier effect is the Seebeck effect?when thermal energy moves through an electrically-conductive material, charge carriers are transported by the heat. Thus when you create a temperature difference across a thermoelectric device, the movement of heat and charge carriers creates an electrical pressure (called Seebeck voltage). If an electrical load is connected across the device, current will flow?if not, the pressure builds to a steady state condition and a 'no-load' voltage will be present. While a standard Peltier unit can be used in this fashion, however, they seldom are. Usually when power generation is the objective, special thermoelectric modules are fabricated using materials which are optimized for that purpose.
Because TE power generation devices are fairly inefficient in converting thermal energy to electricity, their use is largely confined to applications where 'waste heat' is readily available or in remote areas where dependability is more important than efficiency. In these types of situations, the power conversion process may be less than ideal, but the 'fuel' is free. Furthermore, because their small size makes it possible to mount them in tight spaces, they can be used to reclaim energy in places where it would otherwise be impractical. Potential users should be mindful, however, that a Delta T still must be created acoss the device?there has to be a 'cold' side as well as a source of heat?and this can present a challenge to designers.

In using power generation devices, one of the principle objectives will be to extract as much power as possible from the thermoelectric modules. Because power generation devices have significant internal resistance, designers who want to employ this technology should review the principles of maximum power transfer in electrical circuits. It is essential to grasp that maximum power will be transferred when the load resistance equals that of the TE device configuration. In the end, a designer must come up with a series/parallel array of modules that will assure generation of the desired voltage while coming as close as possible to a 'matched load' condition. It is also critical to design to the worst-case Delta T?and to make sure that THot never exceeds the maximum rating for the device. Furthermore, if voltage regulation is important and the Delta T or load will be variable, a shunt regulator will be needed.

 

[CTho9305]

you theoretically could do it... but.
the "but" is that you need a temperature difference for the seebeck effect. what this means, is, say you stack two pelts, one cooling your CPU, the other generating power. the CPU is, at, say 10C and the hot side of the peltier 60C. The generating peltier sees 60C on one side and 20C (room temp) on the other side. It "uses up" this difference to generate power, as in, it will cool the 60C side and heat the 20C side.

You have to properly cool the "cold side" of the generating peltier.

However, peltiers are poor heat conductors when not being used as heat pumps, so in practice it won't cool the 60C side enough. (this is why a peltier failure is catastrophic to a CPU) The hot area will get very hot, destroying the peltiers and your CPU.

The amount of power you get out is minimal. In an experiment I did at school, 60C on one side, ice on the other could spin a small motor wih a fan on it fairly quickly. The voltage never exceeded 2 volts (I dont remember what it actually was... more like 1.5 I think).