View Full Version : Build your own refrigerator for $25
08-06-2012, 08:47 PM
Plus the cost of shipping, of course. Before we begin, you folks all understand there's no such thing as cold, right? Just the presence or absence of heat. And you also probably know that a refrigerator (or icebox, in case you grew up in Arkansas) works by removing the heat from the things you put in there. You may have learned a refrigerator (or airconditioner or freezer) needs things like a compressor and freon and coils of copper or aluminum to work as a heat exchanger. But there are other ways to exchange heat and one of those is the Peltier Junction (which is a lot different than the Petticoat Junction, believe me). The Peltier Junction uses dissimilar metals and electricity in what is known as the "thermoelectric effect" to move heat away from one side of the device and to the other side.
Now, you may at this time be asking yourself "why would I want to build my own refrigerator?" I can think of a few reasons...maybe your company has gone paperless and you have a vacant drawer in your desk that once held files. With just a little work you could build a fridge that would fit neatly in there and keep all sorts of things nice and cold. Like beer. I'll bet there are no signs where you work saying "NO DRINKING." So go ahead. If they have a problem with this, look them in the eye and tell them Jon said it would be ok. Remember this is your refrigerator. You can make it any size you want...seriously.
Or maybe you have a son or daughter who is of the age to build a science project and they need a clever idea. Something a little better than the mud volcano 79% of their friends are going to build after they've waited until the last night before it is due. Well, the Peltier is at once simple and then again pretty complex. But the beauty of it is the simplicity and it should be a very good and educational science project. It might stimulate a child to want to learn more about electronics, and that isn't necessarily a bad thing. Actually it is a good thing. After all, they'll be in the workforce before you know it, and they may need to build a beer fridge for their desk, too.
Or you could build an interesting water cooler, maybe a cold tray to keep dips, shrimp, cold cuts, vegetables, etc cool at a party...you're only limited by your imagination. I don't know if they're still being made, but these were once used to build suits to keep firemen cool while working and they're used frequently in medical and manned space applications. What else can you think of that supplies 12 volts DC and plenty of amps? That's right...the alternator in your car.
For $25 plus postage, I'll send you:
(a) a used but working Peltier thermoelectric chip
(b) the aluminum spacer connecting it to the cold sink
(c) the aluminum cold sink
(d) the aluminum heat sink
(e) the electric fan to dissipate heat...a muffin fan I selected because it was quiet and powerful and fit the perimeter of the hot sink well.
You'll need: a 12 volt DC power supply that can deliver anywhere from about 4 to 8 amps of current, something to line your fridge (sheet aluminum is excellent), basic tools like screwdriver, electric drill, maybe plywood or you could use a small insulated cooler. You're basically going to need a relatively airtight box with insulation to keep the heat out. (or the cold in, but remember cold doesn't actually exist so you're already ahead of the game).
I hooked this up to a transformer to check it. The ambient temperature in the room was 80 degrees and in about 10~12 minutes the temperature on the cold exchanger side had dropped to about 50 degrees (f). You can get a temperature differential of about 50 degrees (f) as I recall. Maybe more if you can figure out how to dissipate heat more efficiently from the hot side.
Thermometer and power supply not included.
Questions? Please send them to firstname.lastname@example.org
08-07-2012, 02:15 AM
I must point out here the electrical cost of such a device. The power you use to run this thing would pay for a commercial chest freezer every year if you leave it on to cool drinks or film or something.
However, they do make an awesome science/technology demonstrator and are good for getting *huge* temperature differentials, which means you can do stuff like get semiconductors down near the point of liquid nitrogen and therefore build stuff like far-IR (longwave) infrared cameras that can see body heat, etc.
I actually had one of these in my PC for about a year for overclocking purposes; of course waiting a year for the next model gives a performance boost just as good as all the refrigeration and plumbing that I built, but you learn a lot about thermodynamics and how control-systems can interact badly with physical systems - a traditional bang-bang thermostat will destroy a Peltier by thermal cycling shocks so you need to build a PID controller with PWM output if you want to have a controlled temperature. You don't want to discover that failure by smell on your PC...
If you like a challenge, try building a multi-stage cooler and keep both stages running in their efficient zone.
08-07-2012, 07:41 AM
Thanks for the reply. Actually...you're right...the energy cost is higher than a typical compressor refrigeration circuit, however there are varying schools of thought on this as I'm sure you already know. My study showed it depended completely on the transformer you used. As long as you could convert 110 to 12v and 4+ amps effectively, you could run one of these for the same cost as a 60 watt light bulb. Once I ventured into use of a less efficient transformer, the figures begin to change, though.
As for a science project, I think this would be pure dynamite...and something a younger person could understand (the bi-metal concept) and then also explain adequately.
As for use in a computer, we've done that over here also. The problem most people had was humidity and condensation. If you notice the foam around the center of the one in the picture, that's partly to keep the chip/aluminum spacer from being exposed to humidity. In a humid environment, you have to do that or you'll pour water on your motherboard constantly. I'm interested to hear more about your thoughts on the PID controller. It makes sense, but honestly I used a common old (as you called it bang-bang) thermostat and never had any trouble. One of the things I always liked was the speed at which these can move temps. I have toyed with the idea of a drink cooler that either had a large chunk of aluminum with holes drilled in it to match the diameter of the cans or (more practically) aluminum bent to match the can's diameter. At any rate, a cold side which would contact the aluminum can as solidly as it could.
No question these are specialty devices for specialty applications. I've also thought a solar panel delivering the right voltage & amp output would be an interesting combination.
As for a multi-stage cooler, do you mean by stacking the Peltiers in a tower approach? You can easily do that with devices of different "powers" as I'm certain you know. About 20 years ago I saw a tower constructed such that water would freeze solid in one second or so when set on the top. This fellow was trying to build a thermoelectric cooler for his Volkswagen, but I never knew if that worked for him or not.
08-07-2012, 09:18 AM
So if I made a vacant desk drawer or something similar into a mini-fridge, I would want to seal it. I wonder where I might be able to find some material to seal it...who here knows something about seal material... ;)
08-07-2012, 09:51 AM
I bet we can find somebody in the rogues gallery...
Good question, though. If you're serious, I'd suggest using a coated foam or at least figure out how to cover open-celled foam with vinyl or something. The foam I handle is open-celled and while it won't allow air nor light to pass through it, open-celled foam is like a miniature sponge and will absorb moisture and possibly let that drip onto your desk drawer. We wouldn't want you to damage company property, and the particle board your desk drawer is likely constructed of will (just like your liver) self-destruct if given a drink on a frequent and regular basis.
Actually, I've thought for a while one of these might work well as a dehumidifier, especially if you could power it with a solar panel. I'm also looking into re-directing the condensation drain line from our A/C unit to pour into a bucket so it can be used to water plants, etc. I've wondered the same thing about our shower (and other gray water) drains, but that involves a lot of work under the house...which I shy away from in 100+ temps.
08-07-2012, 12:20 PM
For those of us that live near Hell in the Summer and have water coming out of the tap at 85 degrees (F) or so, could one build an efficient in-line chiller with one of these?
08-07-2012, 01:03 PM
Yes...you could definitely build an in-line chiller. Whether it would be efficient or not depends on a lot of variables, though. There are challenges. Actually I did this several (maybe 15) years ago. I had the idea of incorporating a water cooler into a mini fridge or incorporating a mini-fridge into a water cooler, depending on your point of view. A salesman I knew at Market Hall had one of those around 1961, and I thought it was a really neat combination. You can see what I came up with below. The heat exchanger side of the device was made of brass and I made a serpentine-like affair that fed water in one end and out at the other. It worked but the trick was feeding water through slowly enough to extract heat and then storing the cold water until you needed it. What I ended up deciding was that the best arrangement would be a copper canister which would hold a quart or more and sit inside a typical refrigerator with a supply line going in and a delivery line going back out. That will work, but you have some plumbing challenges and the delivery line must be insulated if you are planning on bringing it back to a faucet as it will condense water around here like crazy. That is essentially what a fridge that serves water from the front door does. It might be something best done in a house which had a wet bar (which was equipped with a mini-fridge or an ice machine). At least that way the delivery line wouldn't have far to go.
08-07-2012, 01:08 PM
I think some of us are missing an obvious use. This would be a great cooler for an automobile. Convert a picnic cooler and connect via cigarette lighter socket for power. Sounds like an affordable way to keep film and drinks chilled whilst touring.
08-07-2012, 03:07 PM
That has been done, but badly. The company who made the car cooler engineered it after drinking too many Pearl beers and they created something that could keep cold things cold for a while but could not make warm things cold. But that certainly didn't stop them from producing and selling thousands. Nosireebob.
These things are really amazing when they work at their best and when they're kept within their limitations. I know it is hard to get your mind around a concept like the thermoelectric device at first. It was for me, too. The first time I worked with one I could not believe anything could move heat so rapidly. But similar to what Clint Eastwood said, you gotta know your limitations. That is really important. The idea that you could aircondition your house with several of these is folly. The idea you could aircondition a very small car or better yet a small pickup with several of these might be more possible, but still very unlikely and challenging. The idea you could (if you were clever and creative) build a small countertop icemaker is actually possible. In fact I'll prove it. Please look at the image below. That is a 1 inch diameter circle of ice about 1/8 inch thick and it took me about 5 minutes to make it. The ambient temp in here is 80 degrees...maybe 79 but the thermometer says 80. The cold sink was removed exposing the 1 inch thick aluminum spacer onto which I spread a film of water so I'd know when the freezing point was hit. The heat sink was set atop the fan and the fan was given about 1.5 inches of space between it and the top of my desk. The unit was started and within 5 minutes it had completely cooled the aluminum spacer (about 2.5 inches by 1.5 inches by 1 inch thick) to less than 32 degrees and had frozen the water I had dropped onto it at the start. The more water I dropped on to the spacer, the more it froze. And remember this is out in the open at an ambient temp of 80. I stopped at about 1/8 of an inch (which did not take long to build up). I'd expect a clever person could design either a small cube ice maker or better yet a flaker...that's where the real money would be had. After all, I just showed you a monkey can make ice cubes with one of these. A countertop ice flaker could be sold quite easily because flaked ice has always been special. And ice flakers are not cheap...go price some Scotsman flakers. You can part with $5,000 easily. The key with that type of machine is to separate the ice from the chiller before it becomes hard. You have to use either a screw blade wiping a cylinder or something similar to separate the ice just at the point where it freezes and then move it up and out of the way so more can follow it. But I believe it could be done.
Now here is the other thing I haven't told you yet. If you reverse polarity, what originally was the hot side now becomes the cold side and what was originally the cold side becomes the hot side.
I second the motion of beer at work...
08-07-2012, 03:44 PM
Ah Corona, la cerveza mas fina. Have you ever tried Modelo Especial or Carta Blanca? Corona beer is a sad story for me. In the 1960s (when Corona was totally unknown in the USA) a friend and I had gotten in the habit of drinking it. We were buying it (in Mexico) for 60 cents a six pack. One day we were having lunch and I said "you know this beer we've been drinking all summer is very good and I think it would sell in America. Maybe we should see about getting the rights to import it." He agreed and then we did nothing at all. Well, that's not entirely true. At the end of the day we did meet some girls and go over to the Caverns of Music (Juarez) and drink salty dogs and eat dried shrimp until about midnight, but we never went forward with the Corona idea. And we never saw those girls again, either. Que lamentable es!
08-07-2012, 03:48 PM
A flat surface cooler like this might be nice for pouring carbon tissue in warm climates. Turn on the juice after one pours the pigmented gelatin, quickly cool down the pouring surface (nowhere near freezing is needed...40 to 50F) so that the gelatin sets, turn off the juice, and lift off the tissue.
One might be able to rig the surface to be cooled or warmed (80 to 90F) as needed. Warm for the pour, cool to set. Pour tissues year-round no matter what the room temp is!
However, my low-tech way works in my mild climate: opening and/or closing the window until the room is the right temp.
08-07-2012, 04:00 PM
That is actually a very good idea, Vaughn. You can warm by reversing polarity (a simple switching function) and you could then reverse to cool. Aluminum conducts heat (or lack of it) uniformly. It should work.
08-08-2012, 02:32 AM
Many of the cheaper car-fridges are already based on these.
Efficiency of a peltier is inherently low, before you even get started on inefficiencies of the power supply. Example characteristic curves (http://pcbheaven.com/wikipages/The_Peltier_Thermo-Element/) and a good FAQ (http://www.tellurex.com/technology/peltier-faq.php). A typical "freezer" application requires a temperature difference of about 40C to ambient, and many devices will have an efficiency coefficient down near 0.5 for that temperature gradient, i.e. you need to put in 60W of electrical power to suck 30W of thermal power from the cold side, then dissipate 90W from the hot-side. A good compressor-based system will achieve an efficiency coefficient of about 3 though, e.g. put in just 10W to transfer 30W and then dissipate the 40W total from the hot side.
A 12V 4A peltier is about 50W (plus power supply inefficiencies), which is 438kWh/year and it will struggle to keep a few drinks cold. In comparison, my 150L chest freezer uses only 220kWh/year running at -20C.
A bang-bang thermostat running very slowly (large hysteresis) could probably work well for lack of thermal shock. I set my first attempt to have a 5C window and it was sitting on top of a 65W CPU with only about 50g of Cu as thermal mass, so it was cycling every few seconds. The thermal cycling stress cracked one (of the about 400) peltier elements which meant the whole thing went open-circuit and ceased to cool. Bad-times. I put in a PID controller running in a PIC16F84 and PWM'd the current to the peltier at about 30Hz; it was all good after that and held temp within 1C regardless of CPU load.
You really don't want to know the power consumption of that PC - turning it off paid for my next machine ;) The heat exhaust was a 4x6" car-heater radiator core and there was a continuous warm blast issuing from it. Great in winter and with near-unbeatable bragging rights on 15C core temperature (above dewpoint!) under load.
An often-overlooked feature is that they're physically symmetrical, i.e. pushing heat through a Peltier device gives you electrical power! Attach one side to a big heatsink and the other to a few candles and you can get measurable power from it to run LEDs and the like. I believe you can now buy portable wood-burning cooking stoves that produce 5V USB-power for phone-charging while camping (http://biolitestove.com/). Most phone chargers are only 10W and a small woodfire is a couple kW, so a bit of waste-heat off the side will do that nicely.
Edit: what I *meant* to say before all that bullshit is, "these things are huge fun".
08-08-2012, 08:15 AM
Polyglot has made some good points...especially the final part about "huge fun". They're definitely fun. As I said early on, these are at once simple and under the surface more complex. If not abused, they will work for a long, long time. I would personally suggest that a person design a machine or appliance which would be an occasional use thing...that being something you need to turn on to accomplish a chore and then turn off when no longer needed. Efficiency can be a slippery thing when you step into the world of refrigeration, and common sense is a good yardstick to remember. Example: I showed you how I created ice in 5 minutes. This also included the burden of 80 degree ambient air and cooling about 4 cubic inches of aluminum to a temperature below freezing. Now, let's consider this: the ambient temperature inside the freezer compartment of your refrigerator is ideally 0 degrees (f) but more realistically it varies between around 10 to 18 degrees (f) ... in a typical appliance, that is. Mine hovers around 14 degrees most of the time. How long will it take to make ice if you place the same quantity of water I froze in 5 minutes in an aluminum cup in there and then set that aluminum cup on top of another piece of aluminum which is at a temperature of 80 degrees (f)? Don't worry about going to try it...I can answer that question right now. It took me over 25 minutes. How about if we forget the 1" thick aluminum spacer? How about if we just put the same quantity of water in the aluminum cup and set it in there? With the fan running (which will improve efficiency of your freezer obviously), it took me almost 15 minutes to get the ice as firm as it was on the test yesterday. So, if it takes you between three and five times longer to freeze that quantity of water using a more traditional appliance, which approach is more efficient? I'm not arguing...I'm just adding the idea that factors other than a simple measure of wattage should be included. You know, often it will take one man all day to do what 5 men can do in a couple of hours.
The "thermoelectric effect" is really 3 separate effects. The Seebeck effect which has to do with temperature differences converting into electricity, the Peltier effect (which we've been discussing to this point)...the heat transfer and all and the Thomson effect which as I recall wasn't discovered by Thomson but Kelvin. It has more to do with coefficients and is more complex than I care to wander into this early in the morning.
But yes...you definitely can generate electricity by moving heat through the device and waste heat can be put to good use that way. Thanks for pointing that out, Polyglot.
08-08-2012, 10:38 AM
For those of us that live near Hell in the Summer and have water coming out of the tap at 85 degrees (F) or so, could one build an efficient in-line chiller with one of these?
Practically speaking, no. The amount of heat in water and the speed at which you need cold water to flow thru a line would necessitate a compressor. Remember the line will even have heat in it that must be removed. Even with a loop to a cold water storage box it would take time for a compressor, of course depending on the amount of water and the temperature needed. Small chillers are available in different capacities and in small sizes like cold water drinking fountains.
For peltier cooling buy one of those ice chests that run on ac/dc or just use your refrigerator or ice.
08-09-2012, 10:19 AM
I need something that will keep a substantial mass of fish frozen on my annual migration from Newfoundland to Ontario. Packaging densely without air pockets in stryrofoam sealed boxes gets me about 48 hours or a little more. With good planning of ferry, etc. that will almost get me there. However, it doesn't allow any stops enroute to visit friends.
Can I build something that will work? Can you estimate the number of hours that I would need to run the vehicle every 24 hours to maintain frozen fish?
08-09-2012, 10:38 AM
This would not be a good option for keeping a substantial mass of fish frozen, Ralph...the challenge is size and efficiency. I would suggest dry ice and good ice chests might be a better and more economical alternative. Sorry, but I would not encourage you down the wrong path.