My tec liquid chiller project

[Scarlet Infidel]

I think I worked out some numbers for all sorts of combinations of stacking/cascading peltiers. You would only ever do it to achieve higher dT than a single layer can provide.

I'm pretty sure you are always better off just raising the voltage of a single layer towards its maximum to achieve the higher dT. Only once you reach this maximum should you try more than one layer (I can't see why it would ever be worth it for our applications).

Edit: On second thoughts maybe there are situations where this is viable (just not how I'd use them). I'll get back on this tomorrow with an example if I can think of one.

[leuler]

I think I worked out some numbers for all sorts of combinations of stacking/cascading peltiers. You would only ever do it to achieve higher dT than a single layer can provide.

I'm pretty sure you are always better off just raising the voltage of a single layer towards its maximum to achieve the higher dT. Only once you reach this maximum should you try more than one layer (I can't see why it would ever be worth it for our applications).

Edit: On second thoughts maybe there are situations where this is viable (just not how I'd use them). I'll get back on this tomorrow with an example if I can think of one.

Cascades or multistage peltiers are used in situations where precisely
controlled low temperatures are needed and the heat load is low. Situations
that call for temperatures lower than a single peltier can achieve.
One situation I can think of is the use of a photomultplier, which can
detect a low flux of photons. I know that it is used in physics and astronomy.
It needs to be kept very cool because thermal noise can be misinterpreted
as a photon striking the sensor ( the speckles you see when using a digital
camera in very low light situations is caused by thermal noise ). Also,
cascades are used when cooling small samples ( biology, chemistry,etc)
to, let's say, -30 to -50 deg C.

For PCs, I can't think of a low heat producing part that needs to be cooled
to that low of a temp.

[Uncle Jimbo]

I think I worked out some numbers for all sorts of combinations of stacking/cascading peltiers. You would only ever do it to achieve higher dT than a single layer can provide.

I'm pretty sure you are always better off just raising the voltage of a single layer towards its maximum to achieve the higher dT. Only once you reach this maximum should you try more than one layer (I can't see why it would ever be worth it for our applications).

Edit: On second thoughts maybe there are situations where this is viable (just not how I'd use them). I'll get back on this tomorrow with an example if I can think of one.

As you say, it's done for very small TECs to create big dT - usually for instrument control. The problem with moving any power is that the second TEC in the stack is at a bad operating point because it has to move the heat load of the first, so the CoP of the whole stack is not good. But done carefully it can have some benefits.

Let's say you want to move 20W heat load and have the cold side at -50C in a 25C ambient. No TEC in the world will do that in one stage. so for the first stage, target a 40C dT, and we need to have Th at -10C. Max CoP on the 40C CoP curve is .25 or so at .625 of Imax, and pretty flat through 80% Imax. Using a 12715, we need 10A to move 20W at that Th. The 12715 needs 9V to get there, so total power on the hot side is 90W power in + 20W load heat for a total of 110W. If we put a 19933 on top of that, we need to move that load heat plus whatever power we put in. If we assume enough cooling to keep the hot side at 30C, then we are at the 50% Imax point, 16.5A at 12V for power in of 200W and total heat load of 310W.

That's obviously not real efficient, but if what you want is a -50C cold side, and don't need to move a lot of heat, that would get you there.