I've been playing with these little Peltier modules for a little while now, and have come to realize their potential for use in the land of RE (and real life as well) for use as a source for chilling.
Translation? Refrigeration.
While they have been in use for many years now for just that, there's been a fatal flaw with them that for whatever reason was never addressed.
They are horrendously inefficient in their 'native' design specification, but I have found a cute little combination of hacks that make them potentially viable for use in this arena, even in RE.
I held out forever with this until I could prove to myself that it was viable. First hold up was that the idea of running them on reduced input didn't really cross my mind until recently. The other was time to set aside to play with them. Not that it was difficult to do the experiment, just hadn't gotten around to it.
Last night, I found out that they can be run in a 'maintenance' mode, using between 1/4 and 1/6 the power they are rated for (and even less with the right circumstances), and still keep things cool.
I have two of these modules, one is buried inside it's OEM implement, a new Coleman "44 can" cooler that I got from wally world recently, that can stand upright just like a micro-fridge. The other was removed from a much smaller cooler that I purchased years ago. The barebones assembly is the only thing intact, with the module sandwiched between the two heatsinks.
The Coleman module appears to use quite a bit more power than the other one, and so this was one of my motives for trying this. The power supply that came with the unit gets outrageously hot in operation, and I couldn't see the need for it.
Both modules are designed to run on 12V nominal.
Turns out, reducing the input to approximately 5V results in moderate chilling capability with
much less power consumption than it is designed for. It was able to 'hold the cold' overnight, only gaining a couple degrees C over the entire span. On full power, it will hold the temp lower, but the return is nowhere near what it is given to do so.
I'm in the process now of designing a thermostat that will switch between 5, 12, and 15V (the output from the power supply that came with it) in an effort to economize this thing as much as possible.
I'm also looking into using very large heatsinks with multiple modules to work my way closer to the abilities that a carnot cycle fridge can provide, even though I'm aware that the overall efficiency will still be much lower. Efficiency, surprisingly enough, isn't the primary concern here. The lack of moving parts is what makes it so appealing. With a large enough hot-side heatsink, I may be able to even eliminate the fans altogether from the design. More on that at a later time.
The relationship between power consumption vs cooling ability is non-linear:
@ 5V : ~1.8A (~9W)
@ 12V: ~4A (~48W)
@ 15V: ~4.6A (~70W)
The couple of degrees of 'cool' that are lost by running it at 5V is trivial compared to the power savings.
I'm not sure if the module can handle much more than 15V, and at the moment, I don't really care to find out. Power consumption is apparently exponential as the input increases, with only marginal additional heat pumping abilities.
In 'maintenance' mode, the module cannot quickly recover much in terms of heat that has entered the chamber due to the door being opened, etc. So, I am working out a multi-stage thermostat (maybe even just a PWM design) to provide power based on how close the temperature is to the set (desired) temperature.
At the moment, this means 5V when holding, 12V for normal cycling (heat creep recovery), and a switch on the door to engage 15V to prep the heatsinks for maximum recovery ability when the door is open. Considering a timer for this as well, to hold the 15V for a short time after the door has been closed, at which point control would be returned to the thermostat.
Here are some pics:
The Coleman cooler, holding at 7 deg C (~45F). It flirts with 6 degrees C (~43F) regularly, even after sitting all night. This is cold enough to keep many foods from spoiling, at least in any immediate term. Not bad for 9 watts.
The inside, showing the bags of dihydrogen monoxide provided for thermal mass. This makes a big difference.
The immediate rise in temperature as seen by the probe in the short amount of time the door was opened to take the picture above. The probe is dangling below the tray, not really visible. The meter actually peaked out at 16C (~61F), and had already recovered to 14C (~57F) by the time I could take this pic.
My other Peltier module, with heatsinks. This one uses a little less power by design, but doesn't have quite the cooling capacity either. Good for getting an idea of what to expect from the bigger module in the Coleman, however. They operate on the exact same principle, and so far, hasn't led me astray. The small sensor wedged in the center chunk of aluminum is some sort of temperature limiting device (I'm assuming for automatic defrost - this one had some minor issues with that when it was in it's original implement.
The probe's temperature recovery is good (within a minute or so) with the bags of water and soda bottle as shown. Recovery is very poor without these present.
Figured someone might find this interesting... More to come as the experiments continue...
Steve
EDIT -
Maybe even just as a supplement to a carnot system, a hybrid.
Haven't run the math to get an idea of how this pins up against a comparable sized conventional mini-fridge's average usage. No samples.
May be worth it - a deep sleep that uses almost no juice.
FWIW