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Mike.jpg

Novel? idea for sub-ambient liquid cooling

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Posted · Original PosterOP

A couple years ago my team and I were working on a proposal for our final year engineering project. We were heavily considering a novel (we think/thought) method of sub ambient water cooling, similar to using a chiller. Instead of a water chiller or TEC (garbage efficiency), we were considering a free piston stirling cooler (FPSC from now on). Most engineers are familiar with the stirling cycle, usually input of heat (fuel being burnt for instance) and outputting mechanical work (torque @ RPM). However, if you invert the cycle and INPUT work, then this creates a temperature differential which will "pump" heat. This is what is known as a Stirling cooler. What is free piston? Instead of a rotating crank and flywheel, a solenoid is used to actuate the cycle. Working fluid is helium or similar gas IIRC (completely sealed). This has a benefit of making the FPSC compact, very low maintenance and quiet. Efficiency depends heavily on the hot side and cold side temperatures, but is much better than TECs. I've heard of FPSC's used to cool: IR cameras and quantum computers. Cold side temperature can be as low as -150C, although its efficiency decreases with temperature decrease (efficiency 0 at minimum cold side temp). 

 

Where do you get a FPSC? This is the most difficult part. The most easily available was used in a small electric cooler sold on the commercial market, as well as for medical transport (search "Twinbird FPSC Portable Freezer"). When we were doing the research for this project we bought the 2004 coleman cooler for 400 CAD. Unfortunately I did not keep it and left it with my school. 

 

Whats cool about this cooler is it could be implemented in a cooling loop, see below. It will need a water block like device to transfer the heat from its cold end to water. We didn't end up doing much with the cooler, due to us thinking it was underpowered for PC cooling (draws 12V 4A). But maybe someone here is willing to give it a go and find out if there is a way to get more performance out of it.   

 mpgudAJGTatrOXe2owZBmwzXxzeLk_yp4buAt_UJkmxLJz6tAAyxcdcyAyL8VFBeTMB-JNYNLf1kopvBxBOgdaLk1gE43NdTxZUYgi1xaREWgtxbKvPoFU0ElMIash6azyj5IxA

Note: PV is proportioning valve for bypassing FPSC, not necessarily required.

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28 minutes ago, Mike.jpg said:

A couple years ago my team and I were working on a proposal for our final year engineering project. We were heavily considering a novel (we think/thought) method of sub ambient water cooling, similar to using a chiller. Instead of a water chiller or TEC (garbage efficiency), we were considering a free piston stirling cooler (FPSC from now on). Most engineers are familiar with the stirling cycle, usually input of heat (fuel being burnt for instance) and outputting mechanical work (torque @ RPM). However, if you invert the cycle and INPUT work, then this creates a temperature differential which will "pump" heat. This is what is known as a Stirling cooler. What is free piston? Instead of a rotating crank and flywheel, a solenoid is used to actuate the cycle. Working fluid is helium or similar gas IIRC (completely sealed). This has a benefit of making the FPSC compact, very low maintenance and quiet. Efficiency depends heavily on the hot side and cold side temperatures, but is much better than TECs. I've heard of FPSC's used to cool: IR cameras and quantum computers. Cold side temperature can be as low as -150C, although its efficiency decreases with temperature decrease (efficiency 0 at minimum cold side temp). 

 

Where do you get a FPSC? This is the most difficult part. The most easily available was used in a small electric cooler sold on the commercial market, as well as for medical transport (search "Twinbird FPSC Portable Freezer"). When we were doing the research for this project we bought the 2004 coleman cooler for 400 CAD. Unfortunately I did not keep it and left it with my school. 

 

Whats cool about this cooler is it could be implemented in a cooling loop, see below. It will need a water block like device to transfer the heat from its cold end to water. We didn't end up doing much with the cooler, due to us thinking it was underpowered for PC cooling (draws 12V 4A). But maybe someone here is willing to give it a go and find out if there is a way to get more performance out of it.   

 

Note: PV is proportioning valve for bypassing FPSC, not necessarily required.

So how does this compare to a regular VCC or cascade phase change system? LN2 is still the king of sub-ambient CPU cooling.


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I don’t get why you would have a radiator in a sub-ambient loop, is the intention to cool the room down?

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Posted · Original PosterOP

Honestly no idea, but way less cooling potential than LN2 and any gas refrigeration type cooling. This could be a daily driver type thing though. In a tier list it would be LN2>Waterchiller>FPSC+Liquid cooling loop>Liquid cooling loop. Another option would be to place the FPSC directly on the CPU die, but this increases risk. More something you add to a liquid cooling loop to drop 5-10C below ambient. I never got far enough to test the effectiveness of the idea.

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Posted · Original PosterOP
1 minute ago, For Science! said:

I don’t get why you would have a radiator in a sub-ambient loop, is the intention to cool the room down?

Note the direction of flow arrows. Hot Liquid from heat source first travels to radiator in order to cool to near ambient levels. Then it is run through the cooler which brings it down to sub ambient levels. Placement of the reservoir and pump could just as easily be after the radiator.

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3 minutes ago, Mike.jpg said:

Note the direction of flow arrows. Hot Liquid from heat source first travels to radiator in order to cool to near ambient levels. Then it is run through the cooler which brings it down to sub ambient levels. Placement of the reservoir and pump could just as easily be after the radiator.

The pump would have to be REALLY slow for that to be true, any pump suitable for watercooling will equalise the fluid temperature throughout, the whole loop is cooled and chilled as a whole, not by distinct sections.

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Posted · Original PosterOP
10 minutes ago, For Science! said:

The pump would have to be REALLY slow for that to be true, any pump suitable for watercooling will equalise the fluid temperature throughout, the whole loop is cooled and chilled as a whole, not by distinct sections.

Wouldn't there be no heat transfer without delta T across heat source and radiator?

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Just now, Mike.jpg said:

Wouldn't there be no heat transfer without delta T across heat source and radiator?

Yes, but the fluid is always warmer than ambient, so there will be a delta across the ambient air and the radiator. The heat source (CPU) will also be substantially hotter too, with modern systems chewing through several hundred watts when overclocked.

 

To put it into context, a conservative loop flow would be 3.8 L/min, with a loop volume of about 1 L for a CPU-only loop. So any section of water is whizzing through the system at all times, and so its not like once segment of water is heated, then passed onto the radiator to be cooled.

 

Slowing down the flow only hurts performance for a standard loop as the volume surrounding the CPU waterblock microfins is only a few mls, and would go very quickly in undesirable territory. So good flow is a must.

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As for the novelty, this ultimately is a "chiller alternative" in my eyes.  You still have a waterblock/heat exchanger between a standard watercooling loop and the chiller element. Whether this is a peltier, standard chiller, a pot of liquid nitrogen, or a free piston Stirling engine; its all a question of efficiency and convenience. As a non-expert I have no idea whether the FPSC is more or less effecient than what is implemented in an aquarium chiller, but I do not think that the idea of "lets replace the chiller with something more efficient" to be a novel concept especially if FPSCs are generally used as chillers already.

 

In this paper abstract, do I understand that to get 350 W worth of cooling, it must consume 3.57 kW of power? I understand this to hold the system at LN2 temperatures, and so wonder if it flexible enough that you can still achieve cooling to lesser temperatures (say -10 degrees C). But from the figures it looks like the effeciency drops with decreased input power.

https://www.sciencedirect.com/science/article/pii/S1876610219308136

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Posted · Original PosterOP

The CPU and radiator are both elements of a watercooling loop which create ΔT. The radiator decreases and the cpu cooler increases water temperature. This must be the case in order to transfer heat. The FPSC in this case is a third ΔT creating element, one which also decreases temp. 
 

I understand that it isn't a new loop idea or anything and stated as such in my original post, saying that it is similar to a chiller loop.

 

As for how efficient it is? Not sure, IIRC efficiency of the stirling cooler increases when hot and cold end temperatures converge. Most studies i've seen are dealing with efficiency at very low cold side temps (~80C or less). 

 

There are huge disadvantages to FPSC cooling in computers, but I just wanted to put it out there in case anyone else is looking for a different idea for a project.

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10 minutes ago, Mike.jpg said:

The CPU and radiator are both elements of a watercooling loop which create ΔT. The radiator decreases and the cpu cooler increases water temperature. This must be the case in order to transfer heat. The FPSC in this case is a third ΔT creating element, one which also decreases temp. 
 

I understand that it isn't a new loop idea or anything and stated as such in my original post, saying that it is similar to a chiller loop.

 

As for how efficient it is? Not sure, IIRC efficiency of the stirling cooler increases when hot and cold end temperatures converge. Most studies i've seen are dealing with efficiency at very low cold side temps (~80C or less). 

 

There are huge disadvantages to FPSC cooling in computers, but I just wanted to put it out there in case anyone else is looking for a different idea for a project.

Just remove the radiator and all is fine, you don't want to have a radiator for a sub-ambient loop - period. The ΔT of concern is always between the heat/cooling source (CPU/Radiator) and the water. But the water temperature measured at any point in the loop will be 1-2 degrees of each other. Across the inlet and outlet of a radiator the watertemperature will be within 1 degree of each other (outlet being 1 degree cooler), and the across a CPU block the inlet and outlet will have a temperature difference of 1 degrees (where the outlet is 1 degree hotter). 

 

I've dug out this old jayztwocents video when he used to still be informative for me. This thermal imagining timelapse demonstrates that I mean that the whole loop warms up as a whole, and cools down as a whole. Its not that the water leaving the CPU loop is piping hot, and then comes out of the radiator cool, that's jsust not how it is.

 

 

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Posted · Original PosterOP

Interesting, didn't realize the delta T across components was so small, even with less than 1 degree the theory behind a FPSC will still work. You'll be decreasing the inlet temperature into the cpu by a tiny amount which will then increase heat transfer rate. Probably not significant tbh, but would be an interesting experiment. 

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4 minutes ago, Mike.jpg said:

Interesting, didn't realize the delta T across components was so small, even with less than 1 degree the theory behind a FPSC will still work. You'll be decreasing the inlet temperature into the cpu by a tiny amount which will then increase heat transfer rate. Probably not significant tbh, but would be an interesting experiment. 

For me the idea of the PV to have a section of fluid bypass the chiller is the more interesting concept that I hadn't seem on this forum before. I assume this is so that you can mix chilled and unchilled so that you can regulate the temperature of the fluid. Is this because the FPSC cannot be regulated (as in its nothing or full blast), or can you just "lower the power of the FPSC" instead of having a bypass.

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