Quote Originally Posted by leuler View Post
I plan to run the TECs at 12 volts. I bought some server PSUs from eBay
that have three 12 volt rails rated at 15 amps each.

My worst case calculations are darn close ( about 2 degrees higher ) to
the temps you have given. I've been working out the performance of my
hotside cooling from the theoretical side ( using a heat transfer model
that I think applies in this case ). To have someone with experience
give numbers that are in the neighborhood of what I came up with is
reassuring. However, my calculations don't include the effect of the
copper mesh. My hope is that it improves the heat transfer by factor of
2.5, which would lead to a hotside rise of 14 - 15 degrees above ambient.
In the end, the performance is going to depend on how well I have
constructed the chiller. I lack the tools and experience to get the very
best results. We just have to see if I did things good enough to get
good performance.

I would like to thank you Uncle Jimbo. Your knowledge of TECs has been
valuable to me. I studied for this project for about 3 months, and I
have fairly strong foundation in math and physics, so most of the stuff
I have researched has sunk in. But, a good dose of experience is just
as important. And you have supplied that to us.

As far as your offer for help on a power controller, if this chiller works
pretty well, I will take you up on it. It would take me fair amount of
time figure out how to build one by myself. I have thought about using
a couple of 3 position switches and switch between 12 and 5 volts,
but a power controller would be much more versatile.
I have also used server supplies for this work - ProLiant DL380 supplies are 12V at 32A, and you can also buy the redundant power sharing board to make it plug-in. I have gotten those for as low as $15 each.

The risk of failure in doing power control of TECs is from thermal cycling. Since you have both the hot side and cold side on water, they won't have much risk, because the heat won't change quickly between power on and power off.

That means you can use a bang-bang controller. A simple thermostat actually does a good job. I used a $10 mercury thermostat on one test. Because it was designed to work in air, I put it in an insulated box mounted on a piece of copper that was stuck into the reservoir. It didn't really matter that the temp switched a little above the 'real' temperature, as long as it was consistent I could put it where I wanted. I had to bend the sensor a little to get it to read down where I wanted since it was designed for 18C to 40C and I wanted to control around 5C.

When the water temp rose above the setpoint, the thermostat triggered a 12V relay which turned on the TEC power, and when the water got cold enough, it turned off. There is a degree or two of hysterisis in that kind of thermostat, so it doesn't turn on and off very fast. In one setup I tested, the whole body of water in the cold loop was about 2.5G and the cooling power was 250W for a 140W heat load. Once water got to 5C, the relay opened, and when it heated up, it closed. It was on for 2 min and off for 30 sec under load, and at idle it was about 1 min on and 1 min off. Water was controlled to between 4.5C and 6C.

It is fairly easy to construct a PWM control with an IC and a big power MOSFET, and for precise control that's the best way, but for what you are doing, I think the simple thermostat control works just as well.