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Old 10-31-2010, 02:38 PM   #7 (permalink)
cthermo
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Peltier devices are good for heating...

Ryland,

Your idea is sound.

Peltier devices (also called thermoelectric modules, TECs etc) are the only heating source I know of that will actually deliver more wattage of heat than you put into it electrically.

heres a long explanation, but its necessary.

It works like this; A peltier is a heat pump. You give it electrical power and it literally moves heat from one face of the device to the other. Lets use an example to show some numbers. You give a peltier 12 volts and 5 amps = 60 watts of DC power. The Peltier device in turn will pump up to 50 watts of heat from the cold face to the hot face. But here is the kicker. Also coming out on the hot face is the input power you put in coming out as heat. So to summarize, you input 60 watts electrically, the cold face is absorbing 50 watts, and the hot face is giving off 50 + 60 = 110 watts of heat. As you can see the cooling is not very efficient, but the heating is.

Like all heat pumps, the efiiciency gets worse as the difference in temperatures gets greater. If you are trying to heat cabin air using outside air at only 10 C degrees cooler then the Peltier is very efficient giving numbers like above. If the outside air is 30 degrees cooler, then the efficiency drops because the peltier can only pump 25 watts at that point, so 25 + 60 = 85 watts of heat. Heating becomes less than 100% efiicient between 50 and 60 degrees delta T (difference in temp of hot and cold faces of TEC). Thats when resistive heaters would be more practical.

You can increase efficiency by connecting the cold face of the device to waste heat coming from the electric motors or batteries. This will avoid some of the loss of efficiency of very cold outside air.

A second way of increasing efficiency is to operate the Peltier device at a much lower input point where the heat pumping to power input ratio is much higher. In the example above, 12 volts at 5 amps was about 75% of the TECs maximum allowable input point. Lets say we use two of the same devices and put them in series and operate each at 6 volts. At six volts, the Peltier will need 2.5 amps = 15 watts input power. The heat pumped at this point is about 33 watts. So the heat output is then 15 + 33 = 48 watts of heat. You have 2 devices, so that gives a total of 96 watts of heat for only 30 watts of input. Much better than the previous example (60 watts in for 110 watts out) BUT, you now need two Peltier devices, so your cost has doubled.

These numbers are all based on a common Peltier device configuration, a 127 couple 6 amp device. Look at part number 12711-5L31-06CQ under Standard TECs at the Custom Thermoelectric website. The spec sheet has curves on the bottom where you can see the above relationships of power versus heat pumping (Qc).

You are also right that you would want to use a much more powerfull peltier devices than the example I used above. Be carefull when comparing Peltier devices by wattage alone. Some people quote the wattage as what the device can pump (Qc) and others quote it as the maximum input power so as to inflate the wattage value.

I work at Custom Thermo. I'm an engineer. I'm not trying to sell Peltiers here, but I can pass on what I know.

Feel free to ask me any questions and Ill do my best to answer them.



Quote:
Originally Posted by Ryland View Post
As it gets colder I've been thinking about how I would add heat to my electric cars and I started reading up on Peltier junctions ....
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