Any problems with using Peltier junctions for heating?

[Ryland]

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, solid state heat pumps, not the most efficient form of heat pump but they are small and no moving parts, someone on Ebay as a 545 watt 12 volt peltier junction for $30 with shipping that seems like a pretty good price, my thinking is 4 of them to run off my 48V battery pack, drawing 2,200 watts but being able to put out more then 2,200 watts worth of heat! and in the summer being able to cool.
The idea is to heat, so even if the hot side only gets 10% hotter then a resistive wire it's still more efficient then a resistive wire, the cold side could also be used to cool the air going in to the motor, thus taking low temp heat and amplifying it.
Any one want to pick this idea appart?

[comptiger5000]

For heating, they'll work fine. For cooling, there are more efficient ways (they make much better heaters than coolers).

[Ryland]

Originally Posted by comptiger5000

For heating, they'll work fine. For cooling, there are more efficient ways (they make much better heaters than coolers).

I realize that they make better heaters then coolers, altho my plan is to someday ad 40 to 60 watts of pv to the roof of the car, being able to switch the PV between battery charging and vehicle cooling would just be an added bonus, primary goal tho is vehicle heating, the current cooling by opening a window works just fine.
I'm really liking the idea of being able to suck heat out of the motor, keeping the motor cool instead of letting it just cool down as it gets hot.

[saand]

ryland sounds like a good concept, typically peltier devices at 2200 watts are very expensive but sounds like what you found on ebay is fairly cheap
so some things to think about
- how are you going to power the peltiers, if you have an electric car i assume it is running at a high voltage and your peltiers are likely 12v or 24v so you may need a converter which will reduce efficiency unless you reclaim the heat from it
- Have you considered all the places you are currently loosing energy (loosing energy usually means heat generation), Some energy loss areas you might be able to get "free" heat from
- Motor
- brakes
- motor controller

if you are after some out there kinda ideas you could also consider
- plug in heater
- heat bricks, warm up bricks before your trip, put them in your car when you start, they will keep the car warm, if you insulate them well and control the release of heat you may keep them warm all day

regarding your comment about the motor and trying to keep it cool, motors will typically work with no issue up to 80 degrees or more (Depending on materials). From your comments it sounds like your motor is developing heat so if i was you i would just run air over the motor and feed it into the cabin. Louvers could be made to give you more control, put more air over motor that doesn't feed into the cabin ...
using your excess motor heat is free

[gone-ot]

...any electrician will tell you a simple, high-wattage, wire-wound, RESISTOR makes an excellent 'heater.'

[Ryland]

figure that my motor is 90% efficient and draws 6,000 watts gives me 600 watts of heat from the motor, my electric car came stock with duct work to cool the motor with a fan, after a few miles the motor gets hot enough that it starts to put out a little bit of warm air but not much, turn the fan off and you can smell that it is getting warm so active cooling for the motor is a must, but as I said it does not put out enough heat to be useful, a heat pump like this would help to concentrate the heat by cooling the motor and give more heat faster and with an electric car that is built for short trips instant heat would be nice.
As for a converter to run 4 12v peltier junctions off a 48v battery pack... a serise of 4 would be a perfect match for a 48 volt battery pack, no converter needed, just a switch that could handle 40-50 amps and for that I think I have a spare contactor that will work just fine.

Sure, I could run a resistive wire but for a 2,000 watt input I would only get 2,000 watts of heat out of it, I would get no cooling of the motor or other electronics and the only advantage that I could see would that it would be cheap, $120 for 4 junctions seems pretty reasonable for what it can do.

I'm thinking I might just get one junction for now, build an insulated box and try comparing a 500 watt resister wire to a 500 watt pelteir junction to see the heat out put in a controlled environment, a project like that is going to have to wait till winter fully sets in but is the best idea I have come up with for testing the heating ability.

[cthermo]

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.

Originally Posted by Ryland

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 ....

[Ryland]

Originally Posted by cthermo

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.

I'm a little confused, you said it becomes [Bold] less then[/Bold] 100% efficient, a resistive wire would put out heat at 100% efficient, correct? so it sounds like you are saying that if it's 50C cooler outside then what want the hot side of the plate to be that I would at that point be using the peltier as only a resistive plate, getting one watt of heat out for every watt of energy in? or is it that at those lower temps some of the heat loss would be to the cold side as well giving me a net loss.

[Automcdonough]

there there a max temperature difference it can generate? Could be an issue if it gets too cold out. I know the hot side can only reach 100ºC, maybe 150 if you get a nice one. You should dig up a datasheet on it. Ceramic heaters run much hotter than that, helps transfer heat quickly to the air.