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Is a single loop TEC hybrid cooler feasible?

post #1 of 27
Thread Starter 
I'm new to TECs.

I'm wondering if it's possible to get some benefit in a single loop system by coupling the cold side of a TEC into the loop before the CPU and coupling the hot side of the TEC into the loop after the CPU. So the TEC effectively shunts some heat to bypass the CPU and dumps its working power into the same loop after the CPU as well.

If the radiator can handle the extra load I think this should self regulate so the water going through the CPU is a bit colder than it would be without the TEC. The water can't get too cold though because the temp before the cold side of the TEC can't go below ambient (the TEC and the CPU are both dumping net heat into the water which is only being removed by the radiator which is dumping it to air at ambient temp).

I think it might mean there would be an optimum flow rate since decreasing the flow rate would decrease the temperature of the water going to the CPU. Too slow and the TEC won't shunt any heat, too fast and there would be little difference in the water temp to the CPU.

Do people do this?

I'm asking because I'm planning a small build and this wouldn't take up much space.
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post #2 of 27
I have thought about exactly what you're describing before and it seemed super logical. However I never tried it
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post #3 of 27
Thread Starter 
I wrote a little bit of code.

I'm not confident the TEC or heat exchanger models in it are correct but it's generating some plausible results.

EDIT: This is the original model. The latest ones are at the bottom of the post.

TEC.c.txt 5k .txt file

Here are some results:

results.txt 70k .txt file

The bottom line is that it looks to me like this idea works provided you tune it properly.

Some notes and observations:

The model is numerically unstable when the flow rate is too low (this might be a bug in the code or perhaps just that the TEC is on fire in that region).

The model does show that if you provision enough TEC capacity then there is an optimum flow rate for minimum CPU temperature which is quite low --- plausibly achievable with available watercooling pumps I think. See the flow rates in the results - 19g of water per second for 300W CPU dissipation (EDIT:D5 pump goes up past 400g/s when unrestricted so 19g/s should be doable).

The optimum flow rate is a function of CPU dissipation, number of TECs and TEC parameters but if you set the flow rate at the optimum for a design maximum CPU dissipation then the system is stable for CPU dissipation from 0W up to the maximum.

So, you could just run it at a fixed flow rate optimized for the design maximum CPU dissipation and either run the TECs continuously at their design maximum power (which would be extremely inefficient) or alternatively increase the TEC voltage with increasing CPU temperature up to the design maximum TEC voltage (which would be merely very inefficient).

The example parameters I plugged into the model resulted in a system with 4 TECs that could hold a CPU dissipating about 280W at the same 25C temperature as the water flowing out of the radiator using 430W power for cooling resulting in water to the radiator at 34.75C. The radiator would have to be capable of dissipating 710W to ambient to cool the 34.75C water back to 25C. (the model calls the 25C radiator outflow "ambient" but the real ambient would need to be lower according to the performance of the radiator).

For the parameters I chose, the water leaving the CPU block is still below ambient temperature so there is no advantage in putting an intercooler radiator between the CPU block and the hot side of the TEC. It's better to put all the available radiator capacity between the TEC hot side outflow and the TEC cold side inlet.

If you find bugs in the model, please post them to this thread.

EDIT:

Here are the two final models I ended up with:

This one for driving the TEC at constant current is similar to the original model but also takes into account the variation of TEC voltage with deltaT when calculating the TEC dissipation. It might be a bit more accurate and less conservative than the original model.
TEC_constant_current.c.txt 7k .txt file

This one for driving the TEC at constant voltage subject to current limits:
TEC_constant_voltage.c.txt 8k .txt file

The models have not been checked against reality. You have been warned.
Edited by heb1001 - 7/18/16 at 8:37pm
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post #4 of 27
Since TECs are not 100% efficient(they always use more watts then they cool) something like this would have a net negative effect on coolant temps. You will be dumping the TECs heat into the loop, but the cold side will cool less then the amount of heat the other side is adding.

Unfortunately, it doesn't work...BUT if you use multiple modest sized TECs(to keep the heat output of each TEC low), isolate the hot side and cool it with either a separate loop or a large powerful air cooler, then you will get the benefit of the TECs smile.gif.
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post #5 of 27
Thread Starter 
Quote:
Originally Posted by Puck View Post

Since TECs are not 100% efficient(they always use more watts then they cool) something like this would have a net negative effect on coolant temps. You will be dumping the TECs heat into the loop, but the cold side will cool less then the amount of heat the other side is adding.

Unfortunately, it doesn't work...BUT if you use multiple modest sized TECs(to keep the heat output of each TEC low), isolate the hot side and cool it with either a separate loop or a large powerful air cooler, then you will get the benefit of the TECs smile.gif.

It's true that the system as a whole has to dissipate more heat because of the power dissipated in the TEC but provided the radiator is big enough to do so then it will work. You need to make the radiator about three times larger than it would have to be without the TEC and in return you can get the CPU temp below ambient.

You can take a look at the numbers.

The whole point of the exercise was to evaluate a single loop system to save cost and space. I'm aware that it has been done successfully with dual loop systems.
Edited by heb1001 - 7/13/16 at 2:12pm
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post #6 of 27
Quote:
Originally Posted by heb1001 View Post

It's true that the system as a whole has to dissipate more heat because of the power dissipated in the TEC but provided the radiator is big enough to do so then it will work. You need to make the radiator about three times larger than it would have to be without the TEC and in return you can get the CPU temp below ambient.

You can take a look at the numbers.

The whole point of the exercise was to evaluate a single loop system to save cost and space. I'm aware that it has been done successfully with dual loop systems.

It will not work. A TEC is a heat pump, it moves heat from the cold side to the hot side, and creates its own heat in doing it, any heat you move from the cold side will immediately be added back into the loop once the coolant passes through the hot side water block and then some because of the power consumption of the TEC being added into the loop as heat.

The TEC itself uses power and this is added to the heat it is moving from the cold side and needs to be cooled on the hotside. If you add a TEC in to your loop with the cold side plumbed into your loop prior to the CPU and the hot side plumbed into your loop after your CPU all you will do is increase the overall temperature of your loop because of the added heat load of the TEC and will have no effect on the CPU temp.

It is a very slow process to chill water, ie if the all the coolant in your loop passes through your TEC cold side block once, it will barely change temperature, maybe 0.1*c, and it will be immediately negated and in fact heated above where it was prior to entering the cold side once it then passes through the hot side water block due to the added heat load from the TEC's power consumption. So your CPU may be 0.1*c cooler than the rest of the loop, but your overall temperature will be warmer than without the TEC in your loop at all because it is actually adding heat into the loop from the TEC's power consumption.

You would get better results from moving to a larger radiator and not using the TEC at all as this will drop your overall loop temp. If you want to go further than that you can incorporate an air cooled TEC into your loop. ie plumb the cold side into your loop, and cool the hotside of the TEC with an air cooler. I tried this with 2 x 12715 TECs at 12v cooled be some very small GPU heat sinks on the hot side, this dropped my loop temperature about 0.5*c, it didn't make much difference because my loops heat load was about 400w and the heat sinks were inadequate, it would have been much more effective if I used large CPU coolers or a separate water cooling loop on the TEC hotside. It is very hard to actually do much with radiators in the same loop with the TEC cold side block, as the radiator is very effective at removing heat when the temperature is above ambient, and progressively worse as it gets closer to ambient, with radiators in the loop its virtually impossible to get less than 1*c above ambient. I tried my experiment with the 12715 TEC's as I had all the bits laying around to make it and it seemed like a free radiator basically, ie adding an extra radiator would have dropped my loop temp about the same but that would have required purchasing a radiator, I did it with parts I had, and it worked, but it used a lot of power for 0.5*c temp drop so I took it out.

You have some options;
1, play around with adding a TEC cold side into your loop, but cool the hot side separately either by air, or with a liquid loop, even an AIO. Not very effective, but fun.
2, Look into placing a large TEC directly on your CPU and either control it to stay above dew point or insulate your motherboard and socket. Very easy, not very costly, very effective, just make sure you insulate well or get a good controller.
3, Or, build an actual chiller with no radiators in the cold side and water cool the hot side of the TEC's. Quite complex but most effective and most costly.
post #7 of 27
Here is a very basic paint picasso showing the usual sort of temperature fluctuations within a loop. ie the temp change in each part of the loop is extremely small, whether the coolant is passing through something hot or something cold, it has to pass through it many many times to change the temperature of the loop. It is harder to chill the water than it is to heat it though, so 1 pass through the cold block might equal half a pass through the hot side block in terms of temp change in the loop. On top of this, you need to add your loops delta above ambient, ie 2*c above ambient for extremely high performance loop, 5*c above ambient for a good loop, or 10*c above ambient for a poor loop, or silent fan loop, or an AIO.



This pic shows the nominal 50w that the cold side will remove and the minor temp drop from it, then the CPU heat load of a nominal 120w and temp increase, then the 50w moved from the cold side added back in to the hotside, and the power consumption of the TEC added in and the temp increase from that. You can see you are operating a loss, so your loop will be warmer overall by having the TEC hot side in the loop, and it will continue to get warmer and warmer with every pass of the coolant through the blocks. Effectively, you just added 100w of heat to your loop, with no real benefit, unless you think extra power consumption and heat load is worth an immeasurable temp drop for your CPU
Edited by LiamG6 - 7/13/16 at 5:41pm
post #8 of 27
Thread Starter 
Quote:
Originally Posted by LiamG6 View Post

Here is a very basic paint picasso showing the usual sort of temperature fluctuations within a loop. ie the temp change in each part of the loop is extremely small, whether the coolant is passing through something hot or something cold, it has to pass through it many many times to change the temperature of the loop. It is harder to chill the water than it is to heat it though, so 1 pass through the cold block might equal half a pass through the hot side block in terms of temp change in the loop. On top of this, you need to add your loops delta above ambient, ie 2*c above ambient for extremely high performance loop, 5*c above ambient for a good loop, or 10*c above ambient for a poor loop, or silent fan loop, or an AIO.



This pic shows the nominal 50w that the cold side will remove and the minor temp drop from it, then the CPU heat load of a nominal 120w and temp increase, then the 50w moved from the cold side added back in to the hotside, and the power consumption of the TEC added in and the temp increase from that. You can see you are operating a loss, so your loop will be warmer overall by having the TEC hot side in the loop, and it will continue to get warmer and warmer with every pass of the coolant through the blocks. Effectively, you just added 100w of heat to your loop, with no real benefit, unless you think extra power consumption and heat load is worth an immeasurable temp drop for your CPU

The picture you drew is good but as I said previously there is an optimum flow rate for this to work at. If the flow rate is too high then the effect will be negligible apart from the extra heat added by the TEC as you show. If the flow rate is significantly lower then the water moves slower through the TEC and so is cooled more, hence arriving at the CPU much cooler. At the optimum flow rate, the fact that the water at the CPU is much colder is more significant than the fact that it is moving much more slowly and so there is a net beneficial effect on keeping the CPU temp down.

Of course, there might be another reason why it won't work but it doesn't not work for the reasons you are stating :-).

I have run the model and seen that the effect tends to nothing as the flow rate increases. It is essential to use a flow rate in the right region but it's not a notch effect, it's not super sensitive.

In your picture, if you slow the water down by a factor of 1000, you get a 10C drop at the CPU.

The water going to the rad would be 20C hotter but that's a good thing because it helps it dissipate the extra heat.
Edited by heb1001 - 7/13/16 at 6:50pm
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post #9 of 27
well just for some information to be added here, a TEC's hot side produces a lot of heat. i have a TEC water chiller setup, and on my test build i had two separate water loops for the setup. i had a hot side water loop, and a cold side water loop. the hot side water loop had two 360mm radiators on it, and with both of the radiators, a heat load from a CPU on the cold side water loop. the radiators could only hold the hot side water temps at 40C, the ambient temp of the house is 28C, so that is a 12C delta T. now this just my opinion here, but i really think that the final outcome of this setup, will be the same outcome you would get, if there was not a TEC and two extra water blocks in the loop at all.
post #10 of 27
Quote:
Originally Posted by heb1001 View Post

The picture you drew is good but as I said previously there is an optimum flow rate for this to work at. If the flow rate is too high then the effect will be negligible apart from the extra heat added by the TEC as you show. If the flow rate is significantly lower then the water moves slower through the TEC and so is cooled more, hence arriving at the CPU much cooler. At the optimum flow rate, the fact that the water at the CPU is much colder is more significant than the fact that it is moving much more slowly and so there is a net beneficial effect on keeping the CPU temp down.

Of course, there might be another reason why it won't work but it doesn't not work for the reasons you are stating :-).

I have run the model and seen that the effect tends to nothing as the flow rate increases. It is essential to use a flow rate in the right region but it's not a notch effect, it's not super sensitive.

In your picture, if you slow the water down by a factor of 1000, you get a 10C drop at the CPU.

The water going to the rad would be 20C hotter but that's a good thing because it helps it dissipate the extra heat.

The problem with that is CPU blocks these days are designed for high flow, yes, chilled water doesn't need as high flow to cool the CPU, relying more on thermal mass to hold the chilled temp rather than high turbulent flow rate to remove the heat, but that would require a custom CPU water block with a large thermal mass.

If you use a standard CPU water block with an extremely low flow rate I think you will have big problems

You are also missing the point that with each pass through the entire loop your overall loop temp will increase, ie first pass 0*c-10*c+20*c=10*c, second pass 10*c-10*c+20*c=20*c, so on and so forth. You need to remove the heat from the loop, not try to have differing temps within the loop. And to actually have any temp change that large your loop is virtually not even flowing, meaning while your coolant is just "chilling out" (pardon the pun) in the cold block, the rest of the coolant is hanging out in the sauna of the CPU block and hotside block, heating up far faster than the cold block can chill the block down on the CPU.

The only possible way to do that would be to split the flow into a highly parallel low flow setup through many many cold side blocks, and run that through a CPU block in series. ie 10 TEC, 10 cold side blocks, parallel flow, 1gpm overall flow rate / 10 blocks = 0.1gpm/block, then series into CPU block. This way you get the coolant to dwell in the cold side block for longer, and you still get a high flow of chilled liquid to the CPU block without having to have an extremely low overall flow rate that would be detrimental for the CPU block. But if you went to the trouble of that, you may as well build a chiller without rads in cold side and do something that actually works quite effectively, rather than try to prove a point.

TEC's are designed to move heat from A to B, ie cold side to hot side, if you move the heat from A to B, back to A + the heat from A + B, you get a runaway loop that will get hotter and hotter. If you move heat from A to B and dump it to C, ie air, you get a loop that gets cooler and cooler until it reaches the max dT the TEC can provide with the heat load you are cooling.
Edited by LiamG6 - 7/13/16 at 7:13pm
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