Originally Posted by
Liam_G
Ok, so you've just posted a bunch of different stuff up there, i can't be bothered trying to interpret or explain it via your messages, so i will try to explain to you how i understand the "relationships" between all the variables and how i choose a tec and how i know it will cool the load i require.
So,
Find out the load you want to cool, so lets say i want to cool 300+ watts from a 3930k at 5ghz, so this lets me know i need enough Qmax to cool 300+ watts,
but i need it to cool that at a dT of greater than 20, otherwise i will end up right back where watercooling had me before, basically ambient and no lower. Now, it is quite easy to find a TEC that will cool 300watts, but it is not so easy to find one or a combination that will give you 300watts Qmax at greater than 20*dT and not consume a ridiculous amount of power. So thats why we go with multiple tecs and undervolt them. I would also ignore the CoP for now, optimise that later, if you try to choose your TEC based on optimal CoP you will probably never be able to cool your heat load, i would say the best you will ever get with CoP is about 1.5 with tecs, and only then with a small load.
So from this i now know what to look for when i am choosing a TEC, i need a combination of TEC's that will give me 300wQmax, i would aim for a 30*dT, to give you room to lose dT because of thermal resistance of waterblocks and radiators etc. and you want to do it with a decent amount of power, however for such a large heat load i would say you will not achieve a CoP any better than about .75 without using a rediculous amount of tecs, and being reasonable anything more than 8 large tecs is prob not ideal for space and cost, even if it makes sense for power usage.
So at this point your power supply voltage comes into play, lets stick with 12v for now, even though 24v or greater actually makes more sense. so 12v limits you in that you can only run 1 to 4 TECs in series realistically, i would even say no more than 2 in series is realistic, i personally think 16 tecs is too many and any less than 6v will not net you enough qmax for each individual tec to be able to use 8 tecs or less.
so lets see where we are at, so 300w load, target of 30*dT, 12v PSU.
So at this point i would say, browse through a bunch of tec performance graphs and apply either 12v or 6v to them and see what amperage that uses at 30*dT. So at 30*dT you need an amperage that will give you 75wQmax per tec if using 12v per tec and 4 tecs, or 37.5wQmax per tec if using 6v per tec and 8 TECs. (by the way, applying 3v to that 400w tec will not give you 13amps if you use four tecs in series, it will actually use less than 4amps and give you no cooling whatsoever, as far as i understand these things, when you combine tecs in series, the voltage doubles and the amps stay the same, so if you apply 12v to one tec you get 12 to 13amps, so to get 12-13 amps with 2 tecs in series you would have to apply 24volts, if you apply 12v to 2 tecs each tec will get 6v and you will have a current of 4 amps through the 2, once again, no cooling power there at all. if you want to keep the amps the same, double the volts.
so when playing with 12v you can either chose a high Vmax and high Imax tec if you want to run them singly, or a low Vmax high Imax tec if you want to run them in series.
this is all getting a bit confusing but hopefully you can understand me enough to help you. one key thing to remember is the only thing you can control with a tec is the voltage you give it, everything else is a consequence of that, the amps, dT and Qmax are all dependent on choosing a tec that suits the voltage you will apply to it. also remember when running tecs in series you need to double the voltage applied to keep the same cooling capacity from each tec.
so to try and sum up my initial point, find out your heat load (you can do this by finding how much power your cpu uses, that is a good approximation of how much heat your cpu puts out, same for gpu's. so if your cpu uses 300watts at full load then that is your target for qmax. so, find out your heat load, decide on your PSU voltage, take that voltage and combined with your target dT you can see the amps you will use and from that you will find out the qmax of each tec at that volts and amps. if you happen to find a combo that gives you good CoP then great, you also now have a ball park figure to work to to try and achieve a better CoP from a different combo of tecs, but really tyhre only way to achive great CoP is to have low heat load, say about 200, or lots and lots of tecs like that group of 16 i described and even then it was only about a CoP of 1.2 (which is great in my books)
i hope this is understandable at all, bit all over the shop but i'm too tired to make more sense of myself :P
so here is a quick example of my system at work:
- 300w+ heat load for tec to transfer
- 12v PSU
- 30*dT target
- so, 12v > 30dT gives me 12 amps for that 400w tec
- 12 amps gives me about 90wQmax per tec at 30*dT
- so i need 4 tecs at 12v each to give me greater than 300w Qmax at 30*dT (4 x 90 = 360wQmax) note that they are running at full 12v so this means 4 tecs in parallel, not series.
- so 12v x 12a = 144watts power usage/tec, so 576w power usage all up
- so 360w Qmax and 576w usage gives a CoP of .625, not bad considering only using 4 tecs and cooling up to 360watts, but not the greater than CoP of 1 you are looking for, you will only achieve that with a larger number of tecs if you want to cool a large load.
- now don't forget, that for your tec to be able to transfer 360w from the cold side to the hot side you need to be able to cool your total heat load on your hot side of tec, this heat load is the sum of your transfered heat (360w) and your power usage, so 360+576 = 936 watts that you need to be able to cool to even achieve these numbers, so your looking at 2x360 or 480 radiators to cool that load, and after all thermal resistance is taken into account you will end up about 20* below ambient.
there is so much more to this than i know and also more than i even thought possible, ultrasonic is your guy for all the nitty gritty, but that is the level of understanding that i am at and it is enough to know how to do it for now.
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: 16 TECs in a 4x4 par/ser config. if i remember ohms law correctly this would effectively cause each TEC to operate at 25% voltage and current - and would provide a 2-4 CoP (coefficient of performance). here is my question: sinec the total power draw - in watts - would essentially be that of a single TEC, what would the Qmax be with a overall CoP of 2-4? i'm fairly certain dTmax would remain unchanged from single (for example 70C).
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