Post Rad Chiller Concept

[Uncle Jimbo]

mmm... i was hoping to keep the res colder then the return on the cpu. :T

In your diagram, the res basically acts as a 'capacitor' - it stores energy and stabilizes the system. If it is insulated, and your TEC capacity is greater than the heat load, you can certainly drive the temp below the CPU return.

I did a quick experiment using a setup like this, but with a dummy heat load.
- 1 gallon res, insulated (a Styrofoam portable cooler)
- heat load variable from 0 to 200W
- 100W to 300W TEC cooling in the second loop
- 1.5 GPM in both loops
1. With no heat load, I ran the TEC loop at 200W. Water temp dropped from 25C to 15C in 15 minutes.
2. I set the heat load to 140W. Res temp continued to drop, but much more slowly, reaching 12C in 15 minutes.
3. I set the heat load to 200W. Temp began to rise very slowly, reaching 13C in 15 minutes. That probably reflected pump heat in.

I did the same test, but with the TEC set at 100W. Results were:
1 - water temp dropped from 25C to 20C in 15 minutes with no load.
2 - with 140W heat load, temp started moving up - reaching 23C in 15 minutes.
3 - ran with no heat load until res was at 15C. Ran 200W heat load. temp rose to 20C in 15 minutes.

So the res acts as I would expect - it dampens the rate of change. For a system where you have relatively short spikes of high power, your dual loop design should maintain the average temp for quite a while.

This also opens up an easy way to control the temp - use a bang-bang controller (such as a standard heating control thermostat and relay) to turn the TEC on and off based on res temp. The latent heat will keep the TEC from cycling quickly, and if the TEC transfer power is more than the heat load, you will maintain your load cooling temps nicely.

I tried that using the setup above, but with an old mercury thermostat mounted on a copper plate with most of the plate in the water. The thermostat controlled an air conditioning relay rated at 60A which I happened to have around. That switched the TEC on and off.

With the TEC at 300W and a 200W heat load, and the thermostat set to 18C (the lowest it could go), I powered up the system. The res got to about 17C in a little over 20 minutes, and the TEC switched off. After about a minute, the water in the res got to 18.5C and the TEC switched on. It stayed on for about 2 minutes, the temp went back to 17 C, and the TEC switched off. I ran it for another 20 minutes or so and the duty cycle was very stable, running 2 minutes on, 1 minute off and maintaining temp between 17 and 18.5C with a 200W load.