Remote Living > Heating
A look at Peltier cooling
Madscientist267:
Basically, yes. All of you are right from what I can tell.
Its not that they are more efficient overall, it's that when used to just 'keep the edge off', they can be throttled back to next to nothing. Very much like how an LED can 'break darkness' for very little juice. Like LED's, a Peltier's efficiency is determined by how much current is being forced through it. The harder it gets pushed, the more it will do (to a point), but the less efficient it is at doing it.
It doesn't take very much power at all to keep the heat where you (don't) want it once it's (not) there when using a Peltier module. For that, it appears that the a Carnot (phase change) would use much more power doing the same job.
The inverse is true for raw cooling power; Carnot is much more efficient at pumping large amounts of heat than Peltier is. So for quick initial cool-down and heavy loading caused by open doors, etc, a compressor is the way to go, and Peltiers would still be painfully chugging along trying to move the heat load long after the compressor got done with it.
--- Quote from: dnix71 ---Peltier fridge with an "insulation bridge" to limit heat flow backwards
--- End quote ---
This is basically what I'm looking at doing, only with multiple modules, instead of a single. Just barely enough juice to prevent heat transfer from outside to in when in 'sleep' mode.
They will collectively also be able to pump more heat when needed to handle door loads.
First rendition of the unit would be only Peltiers (for that whole lack of moving parts thing), but eventually, I'd even like to see this combined as a hybrid with a standard Carnot cycle fridge. I think it would make a huge difference in the amount of energy the unit uses when it's just maintaining a temperature.
Another factor that affects both types of heat pump is external ambient temperature. The lower the external temp, the more efficient the system is, because it does not need as much power to hit such a high diffferential. In extent, ambient temps too high make the Peltier impractical, when it can not reach the desired cold side temp. Carnot has a little better response for this; depends on the refrigerant used.
So, G -
--- Quote ---Am I following this correctly?
My take on it is 9W is better than 70W, and the things cost $4 each, and RE is expensive.
So $40 saves a lot of RE power.
--- End quote ---
Absolutely. If it's true. Need more data to determine the truths here, but it's looking that way by a rough gander from a distance. :)
Steve
kevbo:
In a previous life, I got paid to play a bit with Peltier modules. I found that they were inappropriate for what we were trying to do, so the boss gave the project to someone with a "better attitude" who went full speed ahead into a product that never worked. Essentially they were trying to make a high power processor (a DEC Alpha) work in an environment that did not have sufficient cooling air available.
The OP is pretty good information. The only big flaw I am seeing in this thread is in regard to using PWM to "throttle" the devices:
--- Quote from: Madscientist267 on February 09, 2011, 08:41:03 PM ---This is where that whole PWM/Active Insulation idea comes in... if the duty cycle is continuously variable, it will only use as much energy as it needs to based on how much it needs to change the temperature at any given moment.
--- End quote ---
Be sure to filter the PWM supply to nearly pure DC. Raw (unfiltered) PWM is a loser with Peltiers because during the "on" part of the waveform they are running full power, which as you noted, is not very efficient, and during the "off" part they are not pumping any heat, but _are_ still leaking it in the reverse direction. Running a Peltier cooler on raw PWM actually decreases the efficiency below the full power mode.
Also, if the raw PWM is low frequency, then you are thermal cycling the modules which can lead to reduced life due to mechanical failure.
To avoid resistive losses the filters need to be L-C type. To keep the inductor (L) small you need to keep the frequency pretty high. Not too high though, or switching losses become a problem. I'd probably shoot for 25 KHz or so...just high enough that humans can't hear it.
A filtered PWM controller is essentially a switch mode power supply, but there is one big difference: Power supplies are designed to run at one output voltage, but this thing needs to vary it's output voltage as commanded by thermostat. The effective loop gain depends on the output voltage, and becomes very high at low output voltages. This means that if you are using feedback to control the output voltage, it becomes very difficult to maintain loop stability at low output voltages.
Fortunately, great precision in the output voltage is not needed, and you can run it open loop or feed-forward, and just close the overall loop with the thermostat.
ghurd:
Good call!
I think I recall someone else telling me raw PWM and Peltiers did not get along.
Its been a long time, but seems like even with low duty cycle the junctions failed?
G-
Madscientist267:
Yep, filtered. Thought I mentioned it... LOL
PWM referring more actually to 'buck'... ::)
Was more or less eluding to integrating the thermostat with the natural characteristics of the feedback loop.
Tomato, Tomato.
(damn, that still doesn't work...)
???
Start out with 15V/module and back it down to ~ nothing. I'm not going to be too worried about a little instability at the bottom, probably won't see it; chances are I will run these things in series anyway with a set or group collectively running at a higher voltage together (4@60V at 'full' throttle). Might make switching 'cheaper' too; less current. Thinking maybe just going line-to-module, direct conversion for the 120/240V version...
The only thing I wonder about is would current interactions (due to temperature differences among modules) cause some form of runaway (ie, one getting too cold, or never cold enough)... ?
Steve
dnix71:
There is a video on the web of a regular hack-a-day group that took 4 Peltiers and sandwiched them between two cast iron skillets. They were wired 2 series x 2 parallel, to double both the voltage and current out. The skillets were back to back with the Peltiers in between. Water was poured in the top and the bottom was set over a campfire. The thing only produced 2 or 3 volts.
That suggests that they don't need much voltage at all to work. If you had 12 in series that were identical and fed the string 12vdc they would each see 1 volt. That might be efficient at the expense of initial cost.
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.
One other hack would be to place a paperboard wick under and around the cold side to transfer condensation to the hot side through a fine slit in the back wall of the fridge. All normal a/c's shuttle condensate to cool the hot side because cold water represents a lot of energy spent to make but has no practical value otherwise. Most of the energy cooling air like that is just getting it past the dew point.
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