 So, in talking about thermally electric coolers or Pellchier coolers, typically nowadays they are not manufactured using the two bead method that we just saw. So, typically nowadays what they do is they'll use semiconductor material in the fabrication of the Pellchier coolers. So what I'll do now is I'll draw a schematic of what one of these Pellchier thermally electric coolers looks like. So this is a schematic of what a typical Pellchier cooler or thermally electric cooler would look like. Nowadays, as I mentioned, it's made with a semiconductor device and so we have the N type semiconductor and the P type junction, the N having more electrons becomes negative and the P deficiency of electrons becomes positively charged. There is an interconnect between the N and the P junctions as you can see in the schematic and they're mounted onto a ceramic substrate and the ceramic substrate then prevents any kind of current flowing into that ceramic. But with that, what you do is you put a current, you provide a current source that runs through our N and P junctions and consequently what will happen is we will have a cooling effect or heat flowing into the top and we will have a heat generation or a heat source and queue out on the bottom. So what we're going to do now is we're going to take a look at a quick video of an example of one of these thermally electric coolers operating. We're now going to conduct an experiment where we have a thermally electric cooler mounted in a vice and there are the two leads of the thermally electric cooler coming out here and you'll notice that on each side of the thermally electric cooler, I have a thermocouple. So there's one on each of the sides of the thermally electric cooler. The displays from the thermocoupler showing down here now there's a little bit of an offset between the two thermocouples mainly probably because the zero is not perfectly correct. But what we're going to do, we're just going to take a standard 9 volt battery and we are going to apply the connections to the thermally electric leads and in a way this is probably not the wisest because we're draining a lot of current out of the battery or forcing the battery to source a lot of current in a very short period of time which is essentially almost like a short circuit so not the best thing for the battery. But nonetheless what I'd like you to do is to watch the two thermocouple displays and watch the temperatures that are being read out. Now the thermocouples aren't perfectly mounted on the surface they are just taped in place but they are providing an indication in terms of what the temperature is. I'll try to keep my hand out of the way as I connect the other terminal and there they are now connected and you can see one side is getting hotter and hotter and the other side is dropping down in temperature and so I don't want to do this for too long because I'm drawing a lot of current out of the battery itself but you can see we're up at 50 it's getting hotter so I will release the terminals now 60 degrees down to about 18. So what we see exhibited here now we'll let it record for a little longer and you'll notice when we remove the current source it becomes or goes to a more uniform temperature whereby the entire thermoelectric module itself goes to a higher temperature. However while we were applying current we were able to have a fairly strong temperature differential and so what I'll do is I'll let this go now and then I'll fast forward it so that you can see the final steady state temperature that this will come to. And so if you recall the original offsets that we had with the thermocouples we're probably in thermal equilibrium now on either side of the thermoelectric cooler and if we were to let it go for a long period of time it will come back to room temperature. So that gives us an idea as to the thermoelectric effect. You can see the thermoelectric cooler is a very small device just this white module here with all the PN junctions that are embedded within and you apply the current and you get the temperature differential occurring on the module. So that gives us an idea as to how the thermoelectric cooler behaves you can see the temperature on the right hand side went up to 60 degrees and on the left hand side of the thermoelectric cooler down to 18. And it's not the most efficient form of refrigeration however it is very compact. Now the thermoelectric coolers themselves are usually made of bismuth telluride and with standard semiconductor concepts the N type is doped with excess electrons and consequently that's why we say that it's negative in the P type there's a deficiency of electrons making it positive and the one thing that we can say is that in these devices the heat transfer is related to the amount of current applied across the terminals. So applications where are these devices used? Well we did talk earlier about uses of the thermoelectric generators used within the space program so one application space program another one is for cooling of electronic equipment it's a very compact cooling source no moving parts or anything like that however I mean it does have the downside that it draws a lot of current. Currently on the market there are quite a few products that involve coolers small coolers that you might put in your car if you want to have a drink or food while you're driving on a long drive and you plug those into the cigarette lighter in your car they would also use the same sort of cooling mechanism they may have a fan on the top in order to reject the heat but the thermoelectric cooler on the inside is what is cooling so the advantages of the thermoelectric cooler so the obvious advantages they're quiet no moving parts lightweight fairly robust the chances are won't break down that quickly no pressure vessel is required disadvantages probably the biggest disadvantage is that they draw a lot of current they're not the most efficient devices and so the current draws anywhere from 1.2 amps up to 36 amps for 7 to 128 NP couples and that's a lot of current when you consider a typical household circuit for example will source at least in North America on the average about 15 amps that's a heck of a lot of current that we have to flow through these and that's why for your car the alternator on your car can generate that kind of current although it does put more load on on the engine and so it does you know consume more and more fuel however that that's the data that the disadvantage would be the amount of current required so that's a thermoelectric cooler and that concludes the section on refrigeration it also concludes this lecture thank you very much