Author Topic: A look at Peltier cooling  (Read 109050 times)

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Stainless

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Re: A look at Peltier cooling
« Reply #132 on: March 27, 2015, 11:51:51 PM »
Well, it appears that those of you who suggested that I be more patient with the larger heatsink are correct.

I put the big one back on and mounted the whole apparatus to the fridge for a full-scale test run. The fridge was empty, aside from an empty 1 L paint can that I was using to hold the temp probe aloft and to determine if containers inside the running fridge would actually feel cold. Here's what I found --

+0 minutes, ambient temp 23.5 C, fridge temp 23 C
+30 minutes, ambient temp 23.5 C, fridge temp 15 C
+60 minutes, ambient temp 23.5 C, fridge temp 12 C
+210 minutes, ambient temp 21.5 C, fridge temp 9C

The test is still going at +240 minutes, and it doesn't appear to want to drop below 9 C inside the fridge. I opened the door for around 5 seconds and things seemed pleasantly cool inside... the metal can was certainly cold to the touch.

Upon opening the door, the internal temp rose to 13 C, and the damned thing recovered very quickly. It was back down to 9 C in around 60 seconds. I opened the door for a full 15 seconds, and the internal temp rose to 15 C. It was back down to 9 C in 3 minutes 40 seconds. Not as good, but still not too shabby.

For this test, I only have 1 inch of imperfectly-sealed R5 rigid insulation in the fridge's back opening, and the temporary wiring is affecting the door seal to some degree. I can improve the overall air tightness and insulation quite a bit for the working version.

So, I've got a 9 degree "fridge" that draws 3.35 A. I will be happy with 5 - 6 C and it's somewhat rewarding to at least be in the same universe as my goal.
« Last Edit: March 27, 2015, 11:58:09 PM by Stainless »

kc7noa

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Re: A look at Peltier cooling
« Reply #133 on: March 28, 2015, 02:46:40 PM »
More insulation and finished off seal will go along way ......

Insulation is a money back rewards for the life of the fridge ..... just like a large antenna .. its costly up front but will pay for its self in the long run ..

Nautilus1

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Re: A look at Peltier cooling
« Reply #134 on: March 30, 2015, 10:22:29 AM »
The larger heatsink idea poked me to ask:

In our Peltier devices, we use a copper or aluminum heatsink to accumulate heat and release it to the air. People who use stacked Peltiers for computer cooling to near-freezing temps may even dump the heat further into a water cooling system.

In a Carnot vapor-compressor cycle, the air itself is the heatsink, to cool the coils at the back of the fridge, there are barely any metal fins on the coils. But air left to flow slowly over the coils and not moved by a fan is a poor and slow heat absorbing medium.

What if we dump the waste heat from the Carnot fridge into some fluid medium, let's say into a water tank / boiler? Water has best heat capacity among usual, non-toxic and non-regulated fluids.

So if we wrap the condenser coils around a metal water boiler, they release heat to the boiler and raise the water temperature inside (not much, maybe to 45-50°C / 113-122°F). When the need for hot water arises, it takes much less energy to raise the water temps to 80-90°C, compared to raising it from cold.

MattM

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Re: A look at Peltier cooling
« Reply #135 on: March 30, 2015, 06:56:32 PM »
The problem with peltiers turned off is the heat flows back to rebalance when there is a gradient.

Bruce S

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Re: A look at Peltier cooling
« Reply #136 on: March 31, 2015, 08:53:08 AM »
The problem with peltiers turned off is the heat flows back to rebalance when there is a gradient.

That is way the person working with it now is looking at a liquid cooling system, to remove even that heat and use it as a barrier.
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kc7noa

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Re: A look at Peltier cooling
« Reply #137 on: March 31, 2015, 12:06:44 PM »
I would try to build an insulation box around the hot side heat exchange ..... with a servo activated door.

And since this is micro controller controlled ... have a insulated flapper door open when cooling is called for .. and when its not, just close it off -- sure the hot side will cool off twards the cold side temp, but will not keep leaking cold because its now enclosed in an insulated box too ...


Bruce S

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Re: A look at Peltier cooling
« Reply #138 on: March 31, 2015, 02:13:46 PM »
After re-reading Stainless' posts, I'm certain he's wanting this to stay on the simple side, the less moving parts the better.
Remember , this is going into a camper NOT full on home use or meat locker.
His water-block system, that is currently under testings does move most of the heat away, along with its fan assisted cooling.


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Stainless

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Re: A look at Peltier cooling
« Reply #139 on: March 31, 2015, 09:31:23 PM »
kc7noa -- Heat backflow is a problem with TECs when they're shut down, which is why I never allow this one to completely stop cooling. When the fridge's internal temperature reaches a low setpoint, the microcontroller throttles the TEC down to a "maintenance mode," which involves sending just enough power to the TEC to prevent backflow. I currently have it set to about 2 V and 200 mV, which produces a 2 C delta between the TEC's hot and cold sides. In maintenance mode, the fans shut down completely, but the water cooler keeps running.

I haven't got far enough yet to determine if it works as I hope, but that should happen in the next few days.

Bruce S -- You're quite right about me wanting to keep this as simple as possible. I'm certainly not historically averse to adding complexity to things like this, but whatever I wind up with here will have to live through bouncing down crappy gravel roads, sitting over the winter, and considerable dust.

Did you pick up one of these liquid CPU coolers yet?

Bruce S

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Re: A look at Peltier cooling
« Reply #140 on: April 01, 2015, 09:15:41 AM »
Sadly , I have not  :(.
I went to my "go to place" to purchase one and they did not have one even close !
I had to fend off the gamers/salesperson and explain that I did not want to super-duper ultra quiet latest gizmo cooler for my computer! They do normally have them but I think it'll be easier just to hit up flea-bay and order one.

How goes the code writing?
I am horrible at writing code ,  (which is why I'm not a Dozen-aire like GHURD  ;D), but really good at following it.


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Bruce S

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Re: A look at Peltier cooling
« Reply #141 on: April 02, 2015, 01:48:03 PM »
Stainless,
I think I'm going to build my cooler piece meal , I keep looking at the all-in-one's and they just seem like the hoses aren't long enough for where I'm wanting to route mine.

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Stainless

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Re: A look at Peltier cooling
« Reply #142 on: April 03, 2015, 01:24:54 PM »
How goes the code writing?
I am definitely no programmer either, but the Arduino platform is pretty easy to write code for if you have any hobbyist development experience at all. The basic version of the software is done and works fine, but I can definitely see areas for potential improvement and refinement once testing determines if this is even a viable project in the real world.

I'm curious to see how your DIY liquid cooler turns out. This one I have looks like it might allow for longer hoses with a little creativity, so maybe that's an option for you as well if the piecemeal idea turns into a bigger nuisance than it's worth.

Stainless

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Re: A look at Peltier cooling
« Reply #143 on: April 03, 2015, 02:10:42 PM »
OK, so I mounted all the components into the fridge, connected the battery and sat down with a couple cold beers to see what happened. I did not have a multimeter temperature probe inside the fridge, so the door sealed properly, but the insulation on the rear is still "imperfect" because I don't want to plug all the tiny gaps with spray foam until I'm sure that it doesn't have to all come apart again.

At an ambient temp of 21.5 C, it took approximately 45 minutes for the fridge to reach 8 C. I set the cutoff temp to 8 degrees for testing, so the fans shut down as expected and the TEC went into maintenance mode. I added code to direct the ATTiny to wait until the fridge internal temperature rose 4 degrees above the low setpoint before putting the TEC back into active cooling mode, to prevent too many unnecessary starts and stops.

When the fans shut down, I started a stopwatch to see how long it would take for the internal temp to rise high enough to require more active cooling. Disappointingly, I wasn't able to go longer than 2 minutes before the fans and TEC fired back up. This cycle repeated several times until I ran out of beer and patience.

Now, I know I have small air leaks and insufficient rear insulation, but I still hoped for something in the 10 minute neighborhood. I am not satisfied with where I have the microprocessor's temperature probe installed -- it's near the cooling mechanism, which might be skewing its data and causing it to sense the temperature of the cold air coming from the cold side heatsink fan rather than the actual air temperature inside the box. Also, I'm struggling to cut and shape hardboard insulation precisely enough to avoid gaps and ugly edges with missing "pebbles" of insulation.

So, I'm going to pull everything apart and remake the cooling system insertion point out of ABS sheeting. Then, I can use spray insulation to fill the back area to a greater R value and with fewer gaps.

Here are a couple pics of the current setup that will probably be gone by the time you read this --


The test rig.


The rear of the cooling insertion point. The gaps are obvious, but I can't fill them without using spray foam, which I don't want to do for initial testing.


The cooling insertion point from inside the fridge. I circled the ATTiny's temperature probe in red to illustrate why I think it's in a crappy spot. It gets a direct hit from the wash of cold air when the heatsink fan is running, and probably freaks out when that cold air disappears.

I mounted it here to avoid drilling unnecessary holes in the fridge's plastic shell, but I think it's got to move.

DamonHD

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Re: A look at Peltier cooling
« Reply #144 on: April 03, 2015, 02:25:12 PM »
You can put some low-pass filtering in (in the OpenTRV control loop if temperatures are changing too fast), eg sample once per minute at most and take the mean of the last 2 or 3.

Maybe also put a little baffle between the sensor and the fan, both to stop the wash of air but also to stop the sensor from directly 'seeing' the cool place radiatively.

Rgds

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« Last Edit: April 03, 2015, 03:12:59 PM by DamonHD »
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Bruce S

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Re: A look at Peltier cooling
« Reply #145 on: April 03, 2015, 02:37:13 PM »
Until I settle on a coolant unit, I'm quickly moving forward on switching out the huge heat-sink on the back for a Heat pipe


These can be found in the older Dell desktops (GX280s Full sized desktops not slimline ) they are the biggest  one's I've found. Copper alone shows to be 6" along with a 120mm 12Vdc 2.5a 9-bladed fan that is near whisper quiet. They also have a nice air-flow tube too.
First run trials with the TEC set at 9V using a Walwart that maxs out at 750ma , and a 12Vdc 1.2mA NiCad batt pack similar to the ones Norm built I had the unit running smoothly with a laser therm from HF Heat-Sink on inside hit 8C while I checked heat pipe.
First run was kinda neat NOT having the fan running until I got a bit stupid  :o and touched the copper.

I'm going to wire up a push-pull fan system to see how well it goes.
I'll get a couple HF Meters on the system to get the numbers.
I do have a slight advantage, the fridge is already a SS unit, it just has the controller shot!
« Last Edit: April 03, 2015, 02:49:05 PM by Bruce S »
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Mary B

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Re: A look at Peltier cooling
« Reply #146 on: April 03, 2015, 08:56:11 PM »
Get some thermal mass in the fridge. Put some beer in, or water, something to add a cold sink like it would act in use. 4c temp rise is pretty steep too... 2c would be better. Get it cooled down then let it sit full of stuff and test.

Stainless

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Re: A look at Peltier cooling
« Reply #147 on: April 07, 2015, 11:19:22 PM »
I tore the cooling entry point apart and made a new rear baffle out of ABS. It's sealed much better than the previous effort, but there's no insulation in the rear at all now until I load the area up with spray foam. It definitely requires a greater commitment to the project than the last method because the head of the liquid CPU cooler will be pretty much buried with foam that is very difficult to remove, but I guess that's required sometimes.

I tried a test run, but the total lack of insulation made it pretty futile. So, I guess it's time to fill 'er up with foam and see what happens.

I also agree that 4 C is too much of a temp rise, and the final version probably will be around 2 C.

Current pictures --


The new ABS baffle. I used grommets for all wire passthroughs, and also siliconed the grommets.


Jugs of cold water as per Mary B's suggestion of adding thermal mass. It didn't make a difference for this test, but we already know why.

You can also see the curved ABS baffle that I added around the ATTiny's temp sender after DamonHD suggested it.

I probably won't get around to hosing the thing with foam until the weekend, so if anyone has any suggestions before then, lay them on me.

Frank S

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Re: A look at Peltier cooling
« Reply #148 on: April 07, 2015, 11:46:42 PM »
Just a thought,
 After you assemble everything but before you foam would it be prudent to use a thin plastic film semi vacuum-formed in the cavity and around the component  that the foam would be filling then you could remove the foam plug for servicing if you ever needed to without having to dig it out
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Bruce S

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Re: A look at Peltier cooling
« Reply #149 on: April 08, 2015, 08:56:21 AM »
I agree with Frank S on putting a plastic barrier around where the foam is going to be.
A use of plain cling wrap should withstand any heat buildup or  the wax paper used in kitchens which can withstand well over 400F .
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XeonPony

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Re: A look at Peltier cooling
« Reply #150 on: April 08, 2015, 02:38:02 PM »
I can tell you this is doomed from the start, a single stage pelt simply can not handle the thermal leakage from the box its self, I was considering this idea long ago but just no matter what I did the vapor compression option was the only sound one.

My design was based off 2 50w units inside with a large heat sink, then a copper heat spreader to a 100w final stage pelt, sort of like a heat diod if you will, then a thermo siphon heat sink (Larger scale heat pipe)

If this is simply a food cooler then it will work as well as most, but as an actual fridgs? Not so much once the temps get above 20 it will simply not keep up.
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MattM

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Re: A look at Peltier cooling
« Reply #151 on: April 09, 2015, 02:39:35 AM »
Without exotic materials of course it will not work as a substantial refrigeration unit.  Adding warm mass will overload it, sure.  But for an afternoon it should sustain something already cool to a certain degree. /pun intended.

Bruce S

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Re: A look at Peltier cooling
« Reply #152 on: April 09, 2015, 08:52:50 AM »
Given that Stainless isn't under any assumptions that this will end up being a meat freezer , and that it could only be a dry cooler along with a learning experience.
I don't see this as being doomed from the git go.
Take a look as his posts on page 5 of these posts, before being a nay sayer.
   
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XeonPony

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Re: A look at Peltier cooling
« Reply #153 on: April 09, 2015, 10:37:25 AM »
Oh I read this whole post theroughly, I can make my own heat pipes and am a refer guy by trade.

Fun project sure, just under no circumstances trust it to preserve food out in the world! Even as drink coolers I find the pelt systems a failure in any thing above 17c, I modified one by stacking another 50w TEC on it and got slightly better resualts.

Basicaly energy in Vs energy out leaves a huge gap and much to be desired.

Key points is his heat absorbing heat sink inside needs to be massive to provide a thermal ballast, and heat rejection needs be very agressive and at least 2 stacked for any real world performance.
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Bruce S

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Re: A look at Peltier cooling
« Reply #154 on: April 10, 2015, 10:25:02 AM »
XP;
Perhaps you could lend some info on building heat pipes?
Relying on units out of older Dell desktop computers could be a losing game , given their age.
I understand thermal dynamics relating to aluminum and copper, BUT could always use extra insights.
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DanG

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Re: A look at Peltier cooling
« Reply #155 on: April 10, 2015, 11:00:06 AM »
A very power hungry $85 Store bought Coleman cooler with the heat/cool option & 120VAC adapter froze groceries and a gallon of milk solid in 20 hours down in Arkansas with 102ºF daytime temperature, with the heat remaining above 90ºF until 1 or 2AM; using the AC power adapter at a State Campground with the cooler standing mostly shaded from sun on a picnic table. Prices mentioned have nearly doubled since then.

I used the same unit to flue cure tobacco  (digest starches & reduce nicotine content), one week at 95ºF,  115ºF, and 130ºF for six or seven batches until the peltier gave out in its little fail-safe fashion - then it was a $32 module swap from Coleman.

It would be interesting to dedicate an efficient variable constant voltage - constant current (CVCC) supply to explore that type of units efficiency curve.

An interesting detail of the cooler is dissimilar fan sizes on one dual shaft DC motor: as well as the external heatsink massing almost twice as much as inner.

If I were going for long term use of a unit like this I'd look at reducing/eliminating the 'smokestack' thermal convection across the peltier by guiding heat AWAY from what it wants to do once the unit has powered off - make an air trap of dead air over the inner/outer heatsinks where: the outside element will pool cold air around itself by keeping it from sinking & the inside element will pool warm air around itself by keeping it from rising; and so stagnate to some degree heat racing back into the unit.
« Last Edit: April 10, 2015, 11:04:11 AM by DanG »

joestue

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Re: A look at Peltier cooling
« Reply #156 on: April 12, 2015, 05:51:36 PM »
This might have to be filed under complete waste of time.. but I wonder if anyone has done it.
Take the refrigerant loop out of an office water cooler.. they typically draw about ~1 amp from the 120vac line
Cut the compressor open and rewind the 2 pole induction motor for a 4 or 8 pole three phase winding. (most of them are 24 slots and a 6 pole winding is not an option)
Power the compressor directly from your car's alternator, which is a 12 pole machine, giving you a 1:3 or 2:3 reduction in rpm at the compressor relative to the alternator.
seems to me it should work but, may run into some other issues, saturation at idle, drop out at high revs..

Alternatively you may be able to find a BLDC motor driver that can power an induction machine.. they are very cheap now.
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OperaHouse

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Re: A look at Peltier cooling
« Reply #157 on: April 12, 2015, 06:02:24 PM »
I had a NORCOLD 12V RV  refrigerator and it had a 24V AC motor.   Two transistors drove a center tapped transformer to create the 24V.  There ought to be plenty of these around used or new.   What I could never understand is this low voltage motor never drew more than 6.5A at startup and settled down to less than 5A.   Inverter draw for my tiny freezer is 120A at startup. 

XeonPony

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Re: A look at Peltier cooling
« Reply #158 on: April 13, 2015, 02:25:32 PM »
XP;
Perhaps you could lend some info on building heat pipes?
Relying on units out of older Dell desktop computers could be a losing game , given their age.
I understand thermal dynamics relating to aluminum and copper, BUT could always use extra insights.

Basicaly it is a pure working fluid in a staurated vapour.

IE, an evacuated tube (500Micron or better vacuum) with a few Ml of your working fluid, this is selected based on max heat expected at the cool end.

if you need wicking to be an option using wire mesh from cable grounding works well.

Most the tube needs to be cooled while you use very high heat to braze the end.
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XeonPony

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Re: A look at Peltier cooling
« Reply #159 on: April 13, 2015, 02:31:36 PM »
I had a NORCOLD 12V RV  refrigerator and it had a 24V AC motor.   Two transistors drove a center tapped transformer to create the 24V.  There ought to be plenty of these around used or new.   What I could never understand is this low voltage motor never drew more than 6.5A at startup and settled down to less than 5A.   Inverter draw for my tiny freezer is 120A at startup.

They're called swing compressors, basicaly it is an alternating solenoid with a return spring on it. So there is no surge load, as all it will do is hum till it can get to a full stroke at which poitn it runs per normale, and if no back pressure just gets going.

I have 3 of those compressors I keep on hand for projects, one of my goals is to make a 24V dc to 24V AC out via toleroid transformer.

120V ac is put through a transformer and droped to apx 26V or so then fed to the compressor directly, the 12V is run through a crude inverter that uses the same transformer for the inverter circuit.
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XeonPony

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Re: A look at Peltier cooling
« Reply #160 on: April 13, 2015, 02:43:21 PM »
XP;
Perhaps you could lend some info on building heat pipes?
Relying on units out of older Dell desktop computers could be a losing game , given their age.
I understand thermal dynamics relating to aluminum and copper, BUT could always use extra insights.

Basicaly it is a pure working fluid in a staurated vapour.

IE, an evacuated tube (500Micron or better vacuum) with a few Ml of your working fluid, this is selected based on max heat expected at the cool end.

if you need wicking to be an option using wire mesh from cable grounding works well.

Most the tube needs to be cooled while you use very high heat to braze the end.


Tools:
Charge and check cylinder (For refrigerant)
Pipe pinch off tool
Oxy Acetyle torch
Process tube adapter kit
Good quality guage set
2 stage quality vacuum pump
A good quality micron guage
hand tools (Assorted)

Viable refrigerents:
R-152a (Found in air duster cans)
R134a (Found in both air duster cans and air horn cans)
Very pure water
Isopropyle alcohol

I'd recomend starting with water, for that you use some capilery tube and a needle valve to charge into the tube, put the water under partial vacuum in advance to degass it, from then on it is trial and error to get the ideal charge for your needs.
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

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Stainless

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Re: A look at Peltier cooling
« Reply #161 on: April 13, 2015, 10:35:58 PM »
Just for the record, I'm more than aware that this isn't the optimal way to build a "fridge" for my little trailer. I'm also aware that it could use a larger inside heatsink, and that there are probably better ways to do just about everything that I've attempted here so far.

It's no secret that I'm winging this whole thing, but it's been fun. I am into this project for far less than a hundred bucks so far, which is roughly a tenth of what it would cost to buy a small DC compressor fridge, and I've learned some interesting new stuff while working on this. That's more important to me than throwing money at something that just works with no real effort on my part. If that's what I wanted above all else, I probably wouldn't have built my own trailer in the first place. I'm always a tinkerer before being a consumer... sometimes it's a positive character trait, sometimes it's not. We'll see which side of the fence this adventure lands on.

Anyway, I used Frank S's excellent suggestion of making a mold for the foam rather than just spraying it into the rear cavity. I used tinfoil. A couple pics --


The first layer of spray foam. This was actually the second attempt... when I globbed the cavity full of foam the first time, the foam was too thick and wouldn't dry even after three days. So, I relined the cavity with foil and tried again in thinner layers. This dried in a few hours, so I piled more atop.


Dried, removed and trimmed. It doesn't conform perfectly to the cavity's shape, so I may still have to touch it up with more foam if or when this becomes the permanent configuration. It should be more than enough to continue testing.


Nice and clean. I'm just holding the foam plugs in place with duct tape during the next round of tests.

And speaking of testing, it's running right behind me in the garage as I type this (the fridge is empty this time). At a 20.5 ambient, it's down to 9 C in roughly 20 minutes. The performance seems to have improved considerably with a better seal and more insulation, but we'll see what happens over the next hour or so.

Just as I typed that last sentence, the fans cut out at 8 C and I started a timer to see how long it would take to require more active cooling. Still using the overly-generous 4 C allowable rise before active cooling restart, I got a rather shocking 21 minutes out of it. I was starting to wonder if my crappy circuit board had failed or something just as the fans restarted. I am quite surprised at how much better it seems to work in the new configuration, so it's time to start getting more aggressive with the target temperature and allowable rise.
« Last Edit: April 13, 2015, 10:41:01 PM by Stainless »

Stainless

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Re: A look at Peltier cooling
« Reply #162 on: April 13, 2015, 11:34:43 PM »
DanG
Quote
If I were going for long term use of a unit like this I'd look at reducing/eliminating the 'smokestack' thermal convection across the peltier by guiding heat AWAY from what it wants to do once the unit has powered off - make an air trap of dead air over the inner/outer heatsinks where: the outside element will pool cold air around itself by keeping it from sinking & the inside element will pool warm air around itself by keeping it from rising; and so stagnate to some degree heat racing back into the unit.

Yeah, I've considered something like what you describe. I'm a little limited in what I can build on the back of this cooler while still having it fit into its spot in the trailer (nothing more than 3.5 inches off the fridge's outer rear wall), so I've been a little hesitant. As you can also see, things are already getting congested on the rear side of the fridge, what with the plug of spray insulation, the cooling hoses and a bunch of wires. Those things aren't really movable, so I'm not sure how I'd enclose the hot side. The cold side is more logistically doable, so I'll keep it in mind if things get that far.

Thanks for the suggestion.

Frank S

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Re: A look at Peltier cooling
« Reply #163 on: April 13, 2015, 11:46:57 PM »
It's a little late now for this I guess , but I find that some of the canned foam products seem to work better in high humidity.
 a lot of the time when I use canned foam I will use a spray bottle with water on what ever I am going to foam and again a light spray while the foam is rising and curing
I live so far outside of the box, when I die they will stretch my carcass over the coffin

Stainless

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Re: A look at Peltier cooling
« Reply #164 on: April 14, 2015, 10:08:46 AM »
It's a little late now for this I guess , but I find that some of the canned foam products seem to work better in high humidity.
 a lot of the time when I use canned foam I will use a spray bottle with water on what ever I am going to foam and again a light spray while the foam is rising and curing

Yeah, I learned that as well, and I hit the foam with a few mists of water while it was drying. It did seem to make a pretty big difference.