Remote Living > Heating
A look at Peltier cooling
Madscientist267:
--- Quote ---Run the fan as slowly as possible without frost to maximize heat transfer.
--- End quote ---
This applies more to 'high power' mode than anything right? Recovery etc. ?
Minimizing frost while maximizing transfer seems tricky anyway - two options I see:
Statically run the cold side sink as close to just above 32F as possible (via the buck regulation)?
Or keep enough heat going into it (regardless of heatsink temp) to prevent the frost from forming (by tweaking air flow rate, eg fan speed)?
Combination?
This end of thermodynamics is where I start to get a bit on the hazy side, and for pretty good reason too... :(
Under the right circumstances, the laws can twist and bend. This was illustrated to me by at least one 'magic trick' that I've experienced for myself up close and in person, and occasionally gets a good head scratcher going -
Heat pipes can do some wierd crap that isn't readily explained by logic. For example, when you heat one end, it's possible for the non-heated end to reach a higher temperature than the end with the heat on it! Found that one out trying to repair a heat pipe CPU heatsink assembly for a laptop one day... Blew my mind! Whiskey Tango Foxtrot Charlie?!?!
How can this be? LOL
Steve
dnix71:
I don't know about heat pipe behavior. Just keep the temp on the module at 35F and you should have no frost to deal with.
Or don't go cheap like most maunfacturers who use the inside wall as the cold side heat sink. Actually mount a decent heat sink on the inside and put a temp sensor directly in front of the sink and set it to 35F or so.
joestue:
@kevbo
You're right about the pwm.
but specifically its the resistive portion of the junction is what's creating your I^2R losses.
--- Quote ---Plus, there is a hack I see no one doing. Peltiers will frost over if you run them hard or without a fan on the cool side. A small fan on the cool side might bring big gains in efficiency if the fan was dc/variable speed and was controlled by a thermostat on the Peltiers. The best heat transfer occurs at the greatest Delta T between the air inside and the surface of the module, but any frost will insulate the module. Run the fan as slowly as possible without frost to maximize heat transfer.
--- End quote ---
err.. no, you want to run the fan at the upper limit of where the heat produced by the fan (.2 to 1 watt) is adding more heat to the fridge than the extra heat delivered to the junction.
basically, you want the temperature difference between the cold plate and the fridge and the junction as LOW as possible.
a nice big freon heat pipe/heat spreader inside the fridge is the best way to do this.
look at it this way:
At 45C temp difference you have a peltier moving 20 watts of heat from the fridge to the ambient air.
and lets say you have passive heatsinks on ether side.
you now add a 0.2C/watt heat sink to each side of the peltier chip, and it takes a 1 watt fan to get that. (figure a 5 heat pipe cpu cooler off newegg for $40 will get you that)
now the 45C temp difference is now reduced to say 35C, and the the peltier is pushing 27 watts of heat.
net gain of 5 watts.
if you can get the hot side of the junction to hardly 5C over ambient, and the cold side to no less than 5C cooler than the inside of the fridge, you'd be doing pretty good, and these fridges would be efficient enough to use in more situations
Madscientist267:
--- Quote ---inside wall as the cold side heat sink
--- End quote ---
Actually, this is exactly what I was going to do, only it wouldn't be just a flimsy piece of aluminum. Full on heat spreading sinks, with a small amount of finning on the inside to contribute as much to convection as possible. Both inside and out. The entire framework would be one giant heatsink more or less.
Thinking something like one module for every square foot of sink, with something like aerogel between the two sinks as insulation.
Might crop up first though as just several modules on one large heatsink.
Either way, actual bonafide heatsinks, yes.
--- Quote ---temperature difference between the cold plate and the fridge and the junction as LOW as possible
--- End quote ---
Gotta tend to agree on this part, however. Forcing the modules as close to frosting as possible might be a tactic for recovering from a door opening or warm item placed inside, but uses way too much power for what is gained out of it. This is why the spreading out among multiple modules. You can get very close to the same temperature differentials at very low power input, just without the ability to do it quickly. But for maintenance, pump rate is not very relevant. They just need to be able to keep the inside surfaces as cold as the target temperature of the items inside; ultimately, so that no heat can get in.
Doing this with as little power as possible is the goal, not Carnot-like performance out of a Peltier. Only insulation leakage will actually need to be continuously pumped out, so as always, the more effective the insulation, the better, but only will affect the amount of juice it takes to sustain the gradient between the two temps.
I've already proven that extended maintenance at reasonable temps and power levels can be done with just a single module. It's my theory that by spreading the juice even thinner, that the total energy requirements to maintain a temp with the door closed (ie sleep mode) will be even less, since Peltiers appear to do more with less, down to a certain point. Below a couple watts somewhere, they cease to function to any useful capacity, but exactly where that line is has yet to be discovered, particularly as it would apply to multiple modules working in tandem.
Need a really high quality cabinet to go full out with this... With some really nice heat sinks above and beyond that.
I'm thinking that modding an existing Carnot mini-fridge may be the way to go, especially since I can then directly compare apples to oranges and see which one is better suited for which contribution to the overall highest operating efficiency of the entire box. Benchmark both technologies with both in place, and come up with a controller that brings out the butter zone. ;)
Steve
Madscientist267:
Well, against my (ahem) better judgement, I went ahead and ordered the lot of 10 Peltier modules, they should be here in a couple of weeks.
Did that whole escrow thing on top of paypal, so we'll see how that works out.
Still in the process of hunting down some heatsinks. I have my eye on some, but have yet to determine the cost (need to call the company).
I also bought a Carnot cycle deep-freezer that will be leaving the warranty department almost immediately after it arrives in my presence, and I determine it works as designed.
It's a 5 cubic footer, not much of anything, but I think will fit exactly what I intend to do with it.
The interior is all sheet aluminum, with the freon coils apparently wrapped around the outside, in close contact with the aluminum.
The door is nothing more than just that, a door. Lots of insulation, and a seal. Plenty of real-estate for me to build my playground.
This of course means that I'm not going to be able to do the 'whole-unit' active insulation thing, but might give me a close enough idea of whether this is worth pursuing or not.
All in all, I'll have about $250 USD (if I'm calculating my heatsinks right), so worst case, I got a 'back to the future' looking deep freezer that can still be used. ;D
More to come as the parts arrive; the freezer is supposed to be here Saturday (along with the wife's new side-by-together fridge... why can't I just go in and get only what I intend to get?!).
Happy Valentines Day Baby!!! (not that she reads this hahaha)
Steve
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