Cooling (and warming) a container with a TEC (Peltier) - controller suggestion? - Page 1
Forum section: electronics / beginners

author: petreza
author_rank: Contributor
timestamp: August 25, 2015, 11:09:24 pm
content: Hi,I am going to use a TEC to keep a container (an ice cooler) at approximately 5C on my terrace. This means that during the summer (30C to 35C) the TEC has to cool the container and during the winter (-5C to -15C) it has to warm it.1. Could you recommend a controller that has a temperature probe and:- can lower the voltage as the temperature drops to near the target- cuts off the voltage at the target- reverses the voltage if the temperature drops below the target2. The outside dimensions of the ice cooler are 34"x18"x15" (86cm X 45cm X 38cm).Link:http://www.amazon.com/Coleman-Quart-Xtreme%C2%AE-Marine-Cooler/dp/B0019QD8HQ/ref=sr_1_1?s=sporting-goods&ie=UTF8&qid=1440543574&sr=1-1&keywords=Coleman+100-Quart+Marine+CoolerAt what rate would you expect the heat to leak into the cooler if the outside temp is 35C and the internal is 5C and the cooler is in the shade? What power rating TEC would you recommend to keep up with the load (I plan to use 4 lower power rating TECs instead of one high power rating TEC for redundancy (if that matters))?3. What brand of TEC is considered good, reliable one while not overly expensive?Thank you in advance for your help!!!

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 25, 2015, 11:29:35 pm
content: Quote from: petreza on August 25, 2015, 11:09:24 pmSeriously for this application you'll spend a fortune on electricity. TEC's are the worst technology for cooling available. Use a barfridge or even better, a bar freezer. Use an arduino to to switch the freezer on when the temperature reaches 6 degrees and back off at 4, then get it to switch a bank of resistors on when it dips below 4 for a few minutes. The arduino and it's power supply could go inside the freezer in a heated OR moisture proof enclosure if you like. It would help heat it in winter.The reason this works better is that a freezer has the best insulation and will cost the least to run. There are many ready made tutorials, and ready-built products and kits to turn freezers into super efficient fridges that keep drinks at 4degC or any other temperature you can think of. As for heating, nothing is more efficient than the cheapest resistors or scrap heating element you can find (short of a heat pump)

author: Chris C
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:04:23 am
content: I'm afraid I can't answer most of this, as I don't work with TECs. But I'll mention one thing I learned about them, which came as a bit of a surprise to me, and so I always remembered it:In general, the lower the current you run a TEC at, the more efficient it becomes. Here's a fairly generic chart, typical of most TECs:Outside temperature of 35°C vs inside temperature of 5°C means dT=30°C, so look at the orange line. 1 on the horizontal axis is 100% of the TEC's maximum rated current. Notice that if you run it at 50% of the rated current, efficiency doubles!But that doesn't mean it produces as much cooling as a TEC run at 100% current. To run it at 50% rated current, you need to reduce the voltage by half. Power = voltage times current, so at 50% current, you'll actually be running the TEC at 25% power, producing 50% as much cooling due to the efficiency increase, compared to the TEC run at 100%.A common way to do this is to take two TECs rated for maximum power at 12V, put them in serial, and drive them with 12V. Each gets half as much voltage, and due to the serial resistances, each gets half as much current. In the end, you get as much cooling as you would from a single TEC running at 100%, but using half as much power. Or put three in series and drive from 12V, you get the same cooling, but using a little more than a third of the power. That might even make  happy.So there is a good reason to use multiple TECs, but not for redundancy's sake.And don't downsize the TECs while doing this. Smaller TECs = less rated current, putting you back on the ugly parts of the efficiency curve. Use TECs sized so that any one could bear the normal cooling load alone, at 100% current.

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:10:30 am
content: Quote from: Chris C on August 26, 2015, 12:04:23 amNow how many in a row will I need so that it powers itself ?

author: petreza
author_rank: Contributor
timestamp: August 26, 2015, 12:13:14 am
content: Quote from: TheElectricChicken on August 25, 2015, 11:29:35 pmThanks for the suggestion TheElectricChicken but the landlord in my building does not allow us to have a second fridge/freezer because they pay the electricity. On the other hand I can blast the AC as much as I want. So I am stuck with a cooler with a hidden TEC in the back which I guess will cost them at least 4X as much. Go figure.Thanks!

author: Chris C
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:16:22 am
content: Quote from: TheElectricChicken on August 26, 2015, 12:10:30 amLOL! You jest, but something like that has actually been documented. Of course, it only works at VERY low power. Like in the pico-amps range. Not exactly useful!

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:19:22 am
content: sigh. Landlords, say no moreOther options are evaporative coolers. Placing a cloth or towel over a milkcrate or any kind of box and then keeping it damp will cool the contents on your veranda. I've had friends who do that, and it works extremely well. You won't believe it till you try it. I tried it myself with a bottle of drink in the car, by placing a cloth over it and splashing a little water on it from time to time on a long journey, it was WOW !and a heater is the easiest thing in the world to hide as it is silent

author: petreza
author_rank: Contributor
timestamp: August 26, 2015, 12:26:40 am
content: Quote from: Chris C on August 26, 2015, 12:04:23 amThanks for the info Chris C. I didn't know that.I was thinking of running 4 TECs in parallel but looking at the graph I might hook them 2 series-pairs in parallel. Better efficiency with some redundancy - I just need to get TECs with slightly higher max power rating.I can probably figure most of this stuff out but my main problem is how to control them - there I am a complete newbie.Thanks again!

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:31:58 am
content: Quote from: Chris C on August 26, 2015, 12:16:22 amOh my calculations show otherwise, an supervillan or superhero may want this setup, many applications I can see.

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:35:00 am
content: wait, I'm reading the graph wrong !!! argh !!!! plans for world domination foiled again !!back to the drawing board.

author: Chris C
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:42:49 am
content: Quote from: TheElectricChicken on August 26, 2015, 12:35:00 amDude. I just spit my coffee.

author: TheElectricChicken
author_rank: Frequent Contributor
timestamp: August 26, 2015, 12:47:44 am
content: Quote from: Chris C on August 26, 2015, 12:42:49 am

author: LukeW
author_rank: Frequent Contributor
timestamp: August 26, 2015, 04:15:18 am
content: "A common way to do this is to take two TECs rated for maximum power at 12V, put them in serial, and drive them with 12V. Each gets half as much voltage, and due to the serial resistances, each gets half as much current."Is it true to assume that the V-I curve is linear, like a resistor? At what point on the V-I curve is it most efficient?These are active devices (although the semiconductor crystals aren't arranged in PN junctions).Might be best to obtain data (including the V-I curve) for the particular thermoelectric array you're using.

author: rs20
author_rank: Super Contributor
timestamp: August 26, 2015, 04:24:21 am
content: The other thing to keep in mind is that TECs should not be (directly) driven by PWM, for the same reasons outlined above. If you compare using 5V at 50% PWM (i.e. 2.5V average) vs smoothed 2.5V DC; they'll both have the same amount of heat transfer from side A to side B, but the I^2 R heat dissipation in the PWM case will be twice as high. Since I^2R losses are what define the upper limit of TEC performance, this is very significant. Fortunately, sticking an inductor and a capacitor after your power PWM stage will do this smoothing for you (effectively creating an open-loop buck converter).As for the controller, why not just use an Arduino or equivalent, with a DS18B20+ on some wires for temperature measurement (or whatever standard Arduino temperature bit is standard), and a standard motor "shield"/controller (with the aformentioned L/C stage after it) to drive the TEC?Remember to put a very large, good heatsink on the hot side of the TEC... the performance of the TEC goes massively downhill if it can't dump the thermal energy on the hot side.

author: Chris C
author_rank: Frequent Contributor
timestamp: August 26, 2015, 07:26:46 am
content: Quote from: LukeW on August 26, 2015, 04:15:18 amI've always heard they were. But you piqued my curiosity, so I just checked a couple of datasheets, and found they lacked V-I curves. Although some other graphs not really organized for that purpose seemed to indicate linearity.In fact I was only able to find one bona fide V-I curve, measured by a hobbyist:From this page:http://www.procooling.com/index.php?func=articles&disp=43Which is close enough to linear, that I'd still feel comfortable informally calling it linear. And I wonder if this test might have been slightly flawed. As voltage/current goes up, so will the average temperature of the peltier; unless he has infinite heat sinks attached. Since resistivity of most metals goes up with temperature, that would account for some, or possibly all, of the non-linearity seen.Though if you know of any other graphs which show different results, I'm all ears.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 26, 2015, 03:58:21 pm
content: Having 2 or even more TEC thermally in series only makes sense if you need a rather large temperature difference. The second lower temperature TEC would need to be larger (e.g. 4 times) to transport the waste heat from the lower temperature one.Having 2 in series and 2 in parallel is very much as good as using just one. Just the stress to the joints might be little lower.The curve is correct - at lower temperature difference the best performace shifts to lower currents. However in such an application one needs to choose the TEC according to the highest outside temperature. If you choose it to large, maybe for redundancy, eficiency will drop. Due to the lower cooling requirement at lower outside temperature, the current will be lower in these cases. So its not that bad but sill will not hit the optimum curve - but there is little one can do about it.Likely the best bet would be going for a larger 1.st freezer inside. If it's a newer model current consumption may not be higher than the old one. 2 nd best option would be offering to pay the typical electricity costs for a second freezer (maybe $50 a year) - usually its noted somewhere in the documentation. If this one is just 5 C, so not makig ice cubes in large quanteties, the numbers are also usually quite reliable.

author: Chris C
author_rank: Frequent Contributor
timestamp: August 26, 2015, 04:59:19 pm
content: Quote from: Kleinstein on August 26, 2015, 03:58:21 pmAh, I should have clarified that. By in series, I meant electrically, NOT thermally. So the TECs would be side by side on the heatsinks, not stacked on top of each other. As you say, stacking them is useful for large dT; butonlyfor that, as the efficiency plummets.

author: rs20
author_rank: Super Contributor
timestamp: August 27, 2015, 02:29:56 am
content: Quote from: 3roomlab on August 27, 2015, 02:24:55 amAs I've already mentioned, you can use a motor driver, but you absolutely must to use an inductor to smooth the PWM into a DC voltage to reduce the otherwise massive I^2R dissipation in the TEC.

author: rs20
author_rank: Super Contributor
timestamp: August 27, 2015, 02:54:06 am
content: Quote from: 3roomlab on August 27, 2015, 02:40:24 amAgain, and again, this is a bad solution. It'll flip-flop between peltier off and peltier 100% ON, which is terrible because the peltier becomes dramatically less efficient as current increases. The control system should be aiming to find a steady-state, DC voltage that maintains the temperature just right. And this is coming from me, I'm normally the one most in favour of rail-to-rail digital solutions, but TECs are absolutely not the place to use this approach.By contrast, your solution is perfect for a traditional refrigerator.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 27, 2015, 09:41:20 am
content: The size / number of TEC modules should fit the heat load. A TEC that is to large will need more power, as the current will be below the optimum point. So the TEC(s) schould be matched to ther thermal insulation / size of the box. Neither a small TEC allways running on full power, nor a very large TEC, running on very low current is good. The heat resistance of the TEC is a large part of the heat leakage from inside to out. So one bad thing with TEC coolers is, that they don't hold the temperature very well in case of a power outage.Also thermal contact / cooling is important. This often means a fan at the hot side at least. If you don't get rid of the heat the TEC has to work much harder.I would not do the temperature control in analog mode any more. Typical the time constants are rather long , so this requires large caps / high impedance circuit. The TEC is a nonlinear beast - the cooling power is not a simple linear funktion of the voltage / current. Especially at higher temperature differences it helps to let the regulator knows that. At the upper llimit, or with poor heat sink (e.g. blocked air flow) the system could even get unstable and start heating instead of cooling. If high temperatre stability is not the ultimate goal, there may also be an extended range where the TEC is not needed. For a digital regulator (µC) it is realtively easy to take care of this.Energy efficiency is allready poor with a TEC, using hard ON/Off or linear regulation something like about doubles the power need. Even if you don't pay for the power, this means more cooling for the unit. A µC can realtively easy provide a PWM signal to drive something like a switching mode output stage. For switching from heating to cooling a bistable relais might be a good option. Usually there is not need to change that very often or fast.

author: petreza
author_rank: Contributor
timestamp: August 28, 2015, 05:04:05 pm
content: Thank you all for the replies!!!As I was expecting it quickly got out of my depth. Let me simplify the requirements so that hopefully the design becomes simple enough so I can implement it.1. the 5C internal temperature is not a very strict requirement. It can go to, lets say, 2C to 8C. I would like to avoid the lower range though since there would be some humidity in the container and it would freeze and accumulate on the cold heatsink.2. Forget about the variable voltage. I will use a straight 12V (from a computer power supply) with a variable resistor in series to calibrate the power.3. Forget about the reversal of voltage. I will do it manually as the seasons change.QUESTION: Is there some kind of 12V or 5V thermometer device (hopefully with approx. 2ft-3ft thermal probe) that cuts of the voltage (with a relay or something) when the temperature reaches the target 5C.So my new design is like this:_____________ ________________| |------------------------------| |-----------------------------------------|| Computer | 12V | | _________________ || Power Supply | | RELAY | | | | <-- To TECs|____________|++++++++++++++++| |+++++| Variable Resistor |+++|+ - |_______________| |_________________|+ - |+ - |+ - _______________ |+ -------| | | <--- Relay Control+ | | |+ 12V or 5V | Temperature |____________|+ | Control | |++++++++++| Device |_______________________________________________| <-- Temp. Probe|______________| |

author: Chris C
author_rank: Frequent Contributor
timestamp: August 28, 2015, 06:33:55 pm
content: Go on Ebay and search for "temperature controller".They are, in general, programmable digital devices with a nice readout, that control a relay. I know little about them beyond that. You may (or may not) need to buy a separate thermocouple probe, of specified compatible type.There are some with separate high and low limits. This could be used to drive four external relays in a H-bridge configuration, allowing automatic heating or cooling.I wouldn't bother with the variable resistor, personally. Using TECs in serial, or serial-parallel, as we earlier discussed will improve efficiency quite a bit. Not to the degree that something with full analog control of the voltage would. But a variable resistor itself is inefficient, and you'd have a hard time finding one that can handle the required power too. I think we should simply consider analog beyond your scope for now, more important to make some progress.

author: rs20
author_rank: Super Contributor
timestamp: August 29, 2015, 01:13:33 am
content: Quote from: petreza on August 28, 2015, 05:04:05 pmYou don't need to make these compromises at all. A dead standard motor driver + a single inductor = a variable voltage, reversible power supply ideally suited to driving a TEC.

author: petreza
author_rank: Contributor
timestamp: August 29, 2015, 01:22:54 am
content: Quote from: Chris C on August 28, 2015, 06:33:55 pmThank you Chris C,This is what I found:http://www.ebay.com/itm/12V-Double-control-Digital-Temperature-Controller-50-110-Sensor-Thermostat-/321603689558?hash=item4ae112dc56It even has the the relays built-in. I can short out the positive and the negative of both relays, BUT, of course, make sure in advance that the temperature settings are set properly so that only one relay is closed at a time. That way I can have heating and cooling with only one neat unit between the power supply and the TECs.What do you think?Thanks!

author: Chris C
author_rank: Frequent Contributor
timestamp: August 29, 2015, 05:46:00 am
content: Quote from: petreza on August 29, 2015, 01:22:54 amLooks fine, except one problem, which is easily fixed. To route current forwardorbackwards through the TECs, you'll needfourswitches in what is called an H-Bridge configuration. Wikipedia can describe that better than I can, so:https://en.wikipedia.org/wiki/H_bridgeFortunately there are only two combinations of those four switches than need to be produced. Which means you can use the controller's two built-in relays to drive two external 12V DPST relays:Giving you the four switches you need. As per Wikipedia's example diagram, one DPST relay would provide S1 & S3, the other would provide S2 & S4. You'd still need to set the high/low temperatures appropriately to avoid everything being turned on at once, causing a short.And this solves one other potential problem as well. That controller you linked is a no-name Chinese product. As such, the stated internal relay rating of 10A is probably optimistic, and they wouldn't last very long if actually used at 10A. If the TECs only draw 5A, that's better, but maybe not enough for years of reliable service. But since we're going with external relays, you can get quality relays, that will last a long time. The ones in the controller will then only be switching enough current to drive the external relays' coils, which will be a few tens or hundreds of mA, and should basically last forever.Quote from: rs20 on August 29, 2015, 01:13:33 amI like that. But that only solves the driver portion. Is there a similarly easy controller? I get the impression  is trying to avoid dealing with op-amps or MCUs.

author: rs20
author_rank: Super Contributor
timestamp: August 29, 2015, 06:06:27 am
content: Quote from: Chris C on August 29, 2015, 05:46:00 amEugh. If even a basic Arduino is not allowed, this becomes really lame. If that's the case, may as well just get a off-the-shelf peltier controller:http://www.ebay.com/itm/12V-Digital-Temperature-Controller-Peltier-WH7016K-thermoelectric-New-/271937968874?hash=item3f50c41aea

author: Chris C
author_rank: Frequent Contributor
timestamp: August 29, 2015, 06:37:48 am
content: Quote from: rs20 on August 29, 2015, 06:06:27 amLOL, I had no idea those existed. Simplest solution yet! Though I'm a bit surprised to see..."Relay current: Max DC 10A / 30V"...you reversed your recommendation, and suggested a non-analog driver. Is this because none exist off-the-shelf, at least not at a reasonable price?

author: rs20
author_rank: Super Contributor
timestamp: August 29, 2015, 08:04:19 am
content: Quote from: Chris C on August 29, 2015, 06:37:48 amNo: in this case you either buy off the shelf, or you write 50 lines of code and do a proper job. Since the latter option is apparently not allowed, one must suggest the former. It's interesting to speculate whether the engineers developing this device were aware of the peltier efficiency curves shown earlier in this thread. The controller I linked is probably just the one you linked with "Peltier" written on it! Having said that, I have no idea what's commercially available and at what prices, I had no idea those existed 2 hours ago either. My link is just the first that appeared on eBay under "peltier controller".

author: Chris C
author_rank: Frequent Contributor
timestamp: August 29, 2015, 10:17:23 am
content: Quote from: rs20 on August 29, 2015, 08:04:19 amHmm, yes it is. But probably irrelevant. If those curves aren't common knowledge, I figure few consumers would pay the few extra dollars to purchase one with a nice MOSFET H-bridge and filter inductor. Or realize they were paying more on their electric bill than necessary. I've seen a few commercial peltier coolers, have one in my car in fact, and there's not even a hint of attempt to provide more than mediocre efficiency. I'd upgrade if it were used for more than very intermittent purposes.Though today, if  is happy, so am I.If not ready for an Arduino yet, maybe a later revision. Three years ago I wouldn't have been comfortable with a MCU. Now they're almost trivial to put in things.

author: necessaryevil
author_rank: Regular Contributor
timestamp: August 29, 2015, 11:30:07 am
content: Well, it will be less economicall but it isn't impossible, especially if your container isn't to big. A peltier will be 4x less efficient compared to an 'normal' refrigerator, so you need 4 times the power to a fridge of similar size assuming the container is well isolated. If it is not terribly well isolated, you might still get away with only 4x the power, but it will take longer to get your drinks cold. Cheap peltiers may perform even worse by the way.Another option is using an ordinary fridge and put a small heater in it during the winter. Make sure the fridge can operate at low ambient temperatures, not all of them can! I know that you need special freezers for unheated places (freezers work the same way as a fridge)If you use peltiers, make sure to use decent cooling on the hot side, you 'll need a fan and a sink.

author: Andreas
author_rank: Super Contributor
timestamp: August 29, 2015, 01:16:27 pm
content: Hello,usually you get practically a maximum temperature difference of about 15-18 deg C(sometimes not more than 10 deg C) with a usual TEC (CAR) cooling box.The problem is that within the TEC element the thermal isolation is poor.And you hope to get down from 35 deg C to 5 deg C with a single stage?with best regardsAndreas

author: rs20
author_rank: Super Contributor
timestamp: August 29, 2015, 01:49:21 pm
content: So can you buy little miniature conventional refrigeration "stages" (I don't even know what the search terms would be)?

author: SeanB
author_rank: Super Contributor
timestamp: August 29, 2015, 01:58:34 pm
content: Smallest closed refrigerant systems I have seen are around 200W of compressor, the compressor does not really scale down well, unless you go to a Stirling cryocooler which is a very good small unit, but at an amazing price. That can provide sub 10W of cooling, but only a very limited range.

author: petreza
author_rank: Contributor
timestamp: August 29, 2015, 06:08:25 pm
content: Thank you all for contributing!Small refrigerator/freezer:As I explained earlier, this is not an option since the landlord does not allow it. I would have preferred to go this way.Arduino:The programming does not scare me - I know a little bit. But I have zero experience with Arduino or any other type of controller and do not have the time right now to overcome the initial learning curve just for this project. I will have to do it eventually since both I and some of my friends have come up with projects like this where an Arduino type of controller would have been very helpful.Chris C:Thank you for the H-bridge suggestion. I still might go this way despite rs20's controller suggestion. It all depends on what the maximum amperage is at 12V. I wrote the seller and am waiting for a reply.rs20:Thank you for the recommendation:http://www.ebay.com/itm/12V-Digital-Temperature-Controller-Peltier-WH7016K-thermoelectric-New-/271937968874?hash=item3f50c41aeaI saw a bunch of these controllers on eBay but none with an automatic switching. This controller is the closest to what I want to do but I am waiting for a reply from the seller as to the max amperage the relays will support at 12V.necessaryevil:The container is a 100qt (95 liters) ice cooler:http://www.amazon.com/Coleman-Quart-Xtreme%C2%AE-Marine-Cooler/dp/B0019QD8HQ/ref=sr_1_3?ie=UTF8&qid=1440870678&sr=8-3&keywords=coleman+100+qt+marine+white+coolerI plan to use maybe 4x 60W TECs (hooked to form 2 pairs in parallel each in series) with a heatsink like this with 2x 120mm fans for the hot side:http://www.ebay.com/itm/300x140x20mm-Aluminum-HeatSink-for-8x3w-20x1w-LED-Power-IC-Transistor-Module/131566824558?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D33097%26meid%3Dee6bbf86387f458b8fb4b11152fd8b7e%26pid%3D100005%26rk%3D4%26rkt%3D6%26sd%3D221662236925an aluminum block to squeeze the TECs to the hot heatsink (necessary because the cooler's lid has some thickness):http://www.ebay.com/itm/Aluminum-6061-T6511-Clean-Stock-9-75-x4-25-x1-4lb-3-3oz-Block-QTY-1-A4-/251813005978?hash=item3aa1398a9aand maybe two 90mm X 90mm heatsinks with 80mm fans on the cold sideAndreas:Quote from: Andreas on August 29, 2015, 01:16:27 pmThanks for the heads up!I will build it, test it, and if it is not sufficient I will double the the TECs - stack a second one on top of each original TEC.Again, thank you all for contributing!

author: Chris C
author_rank: Frequent Contributor
timestamp: August 29, 2015, 07:03:48 pm
content: Quote from: petreza on August 29, 2015, 06:08:25 pmI would not even attempt stacking. As it was explained to me, stacking allows it to reach a greater temperature differential, but ONLY if it's cooling a very small area. In stacked configuration the second TEC is using most of its heat moving capability to remove heat produced by the first, leaving little left over to remove heat from area being chilled. Putting this into hopefully more familiar terms, if a single Peltier can move 400 BTU of heat, a stacked Peltier might only be able to move 40 BTU. Those numbers are admittedly pulled out of my arse, but should serve well enough to demonstrate the point. also has a point about Peltiers having poor insulation. But I suspect that the numbers he used are representative of typical, single-Peltier coolers. I tried to find a non-stacked dual-Peltier cooler build to see what possible with the efficiency improvements we've discussed, but found nothing. Though here is a DIY single-Peltier that I know can achieve at least 20°C temperature differential:https://hackaday.com/2015/04/03/building-your-own-mini-fridge/That is without using two or more non-stacked Peltiers for improved efficiency. And without the tips  wrote in the comments, which I highly recommend reading. Perhaps you could contact the builder of that cooler, and ask what the maximum temperature differential is.Use of a 100qt. cooler puts it beyond the scale of any DIY Peltier cooler project I've seen. I wish I knew thermodynamics well enough to estimate how much heat you'll have to pump just to keep up with losses due to the imperfect insulation of the cooler! Maybe you can find someone who does. At any rate, if you go ahead with construction, please update us with results. I'm quite curious to see what happens.

author: edavid
author_rank: Super Contributor
timestamp: August 29, 2015, 07:17:35 pm
content: Quote from: petreza on August 29, 2015, 06:08:25 pmMaybe a dumb question, but is your landlord allowed to enter your apartment and search for extra refrigerators?

author: Chris C
author_rank: Frequent Contributor
timestamp: August 29, 2015, 07:42:39 pm
content: I just found another project that might be a useful example:This fellow built a Peltier refrigerator, very close to the size you're planning. It's solar powered, and during the day capable of producing an ice reservoir, sufficient to keep the temperature acceptable through a summer night so no energy storage is needed (though he lives in Canada, so summer will be cooler than for us, less dT). At the time the video was posted in 2013, it was using a single Peltier; but he was aware of using two Peltiers in series for better efficiency, and said he planned to try it. I saw no updates saying he actually did. He might be worth contacting to find out about this.

author: petreza
author_rank: Contributor
timestamp: August 30, 2015, 01:25:33 am
content: Chris C:Thanks for reminding me about this problem with stacking TECs. I used to know this years ago when I was reading about the use of TECs to cool CPUs. The trick was that the second stage TEC (the one cooling the first stage TEC) had to be much more powerful - something like 2-3 times the power rating of the first stage (if not more). It had to be powerful enough to pump the heat that the first TEC had pumped + the heat generated by the inefficiency of the first TEC; and then there had to be a lot of slack left to produce the temperature drop (and this temperature drop counts from the temp of the hot heatsink (not ambient), hence the need for good cooling on the hot side).Have I got this right?I know this would be very inefficient electrically but as I mentioned earlier I got free electricity and it is not my fault that I had to go this way - I would rather buy a fridge and be done with it.Thanks for the youtube video and the project link. This proves that my concept is at least theoretically feasible.In the project link I saw another problem I might have - condensation on the cold heatsink. Water will most likely freeze on the heatsink surface when the system is working and once it stops the water will start to melt on top of the fans. I will have to either get a protruding heatsink so that I can mount the fan on the side or more likely mount everything on the back surface of the cooler. Mounting on the back would make everything more hidden come inspection time.edavid:The landlord is allowed anearly(I mean) yearly inspection with prior notice. That is irrelevant though since my terrace is quite low and anyone passing in front of the building can see what I have there. An ice cooler under the table with stuff on top and in front of it will probably go unnoticed. On the terraces they are mostly interested in checking for open flame grills (electric is fine) which are illegal in apartments here.

author: Chris C
author_rank: Frequent Contributor
timestamp: August 30, 2015, 08:31:50 am
content: Quote from: petreza on August 30, 2015, 01:25:33 amYou have it exactly right.I understand your electricity is free, but I still recommend against stacking. A larger second stage TEC means much more waste heat, so the outside heatsink/fan will need to be much larger too. Making your project cost increase, and making it harder to pass off your ice chest as a normal ice chest.

author: Andreas
author_rank: Super Contributor
timestamp: August 30, 2015, 09:51:26 am
content: Quote from: Chris C on August 29, 2015, 07:03:48 pmYes it is one of those (good isolated) 25 or 26 liter single peltier coolers with 45-50W.The problems that they have:- the heat from the peltier itself heats up environment by 2-3 degress. -> you should get it away as far as you can. (heat pipe?)- to distribute the cold usually a fan is used which also heats 1-2 W against the 5-10 W transferred to the cold side.maybe you can get 20 degrees temperature difference when all is in new state.But after half a year the thermal grease gets dry.The fan bearings go harder and then you get with luck 10-15 deg Cbetween inside and outside.More than 20 deg is only possible if the surface to cool is relative small.One important thing. Some of the cheap peltiers have no sealing against humidity.Those do not last long when they get wet by condensating water.But for such a large box I would go for a absorber cooler (with camping gas).With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: August 30, 2015, 11:29:51 am
content: If your regular fridge is capable of making ice cubes or freeze water, one could use it for that, and move some ice to a regular passive cold box wherever you need it. Something like 1 kg (e.g. 2 bottles with 0.5 L) of Ice sould be enough for a day if you dont't put too much warm stuff in.Typical single TEC elements can diliver up to 50-60 K temperature difference in the limiting case of no heat load. At half the nominal cooling power this reduces to about half. This is at the optimum current, which is close to the absolute maximum.Reducing the current will reduce the maximum temperature and heat flow - though less than linear.

author: rs20
author_rank: Super Contributor
timestamp: August 30, 2015, 11:45:23 am
content: Quote from: Kleinstein on August 30, 2015, 11:29:51 amThese simplistic rules completely ignore the role of the heat rejection heatsink. As your heatsink gets smaller/worse, the maximum heat flux and heat power points shift downwards. The TECs I have a seriously affected by the limitation when used with remotely practical heatsinks.It's easy to figure out in your own specific case in a spreadsheet, just as long as you remember to factor in the heat rise on the heatsink due to the heat flux AND I^2R losses flowing through it.

author: Andreas
author_rank: Super Contributor
timestamp: August 30, 2015, 02:07:34 pm
content: Quote from: Kleinstein on August 30, 2015, 11:29:51 amThats right:with practical existing heat sinks that means:40-50 K on the hot side10-20 K on the cold sideWith best regardsAndreas

author: petreza
author_rank: Contributor
timestamp: August 30, 2015, 05:30:32 pm
content: I will let you all know how it goes when I build this in a little while (some parts ship from China and I am in the US).Meanwhile, I have a hard time finding a moderate quality and moderately priced power supply sold in the US. I was planning to use a regular computer power supply since those can be had for US$50-$60 in the range of 400W-600W (yes, I would probably need all that power - a second, smaller unit later). However, the max power rating of a Molex wire/connector is 8A and I would need more than that to pass through the relay.I saw someone recommending Meanwell but those are expensive.What would you recommend?Thanks!

author: bills
author_rank: Frequent Contributor
timestamp: August 30, 2015, 07:16:52 pm
content: try one of these.http://www.ebay.com/itm/400982411238?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

author: Chris C
author_rank: Frequent Contributor
timestamp: August 30, 2015, 11:38:11 pm
content: You can probably find a used computer power supply for less, maybe even free. I threw out several a few weeks ago simply because they lacked the appropriate connectors for modern hardware, and I had too many "just in case" supplies lying around. They probably all had 12A 12V rails, or greater.Bypassing the 8A limit for wire/connector is easy. Just rewire so that a single Molex connector has two separate 12V wires going into two pins, as well as two GND wires and pins. They'll carry the current in parallel, halving the effective resistance.That last Peltier controller linked stated the internal relays had a 10A limit though, and being Chinese, I would derate that. If you really need near 10A or above, maybe the temperature controller I linked would be a better choice, so that you can switching the power through external relays.

author: ez24
author_rank: Super Contributor
timestamp: August 31, 2015, 03:34:38 am
content: QuoteHow about solid state relays?