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Forum Index : Other Stuff : Peltier cooler-heater questions
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| Georgen Guru  Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Thought to make little air cooler – heater using Peltier devices. Have couple of problems. My 40W solar panels in open circuit produce over 20V Would it be OK to connect two Peltier devices in series to solve too high voltage problem? Or it is better to use Step-Down Buck Converter (bigger one would be needed for 6A current) Also I got impression that depending on +/- connection, either side can be heater/cooler? Is it true, or I got it wrong? George |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Found that Peltier devices can be connected in series. 
Also looks that most probably they cannot be connected the other way as while looking for answers found that semiconductors are used and they to my knowledge work in one direction. George |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
They can be reversed in the wiring to swap between hot and cold, its how it is done with those cheap car fridge/warmers, some have a switch for hot and cold, others just have a 2 pin plug you reverse to select hot or cold. Pete. Sometimes it just works |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Thank you Pete, Great news. (I was going to cautiously try it anyway, but now I will do it with confidence) So my little project will be reversible by just swapping wires or 3 position switch. George |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Just remember that with these devices one surface gets hot while the other gets cold. Your biggest challenge will be to keep those two different temperature areas from interacting with each other. Klaus |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Need thin (less than 4 mm) layer of material that is good insulator. Have two ideas: Marine quality plywood – not sure if they make it that thin. Closed cell foam - actually have some thin packing foam somewhere, that is probably suitable. George |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
What are you insulating? not the cell i hope. Sometimes it just works |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Plywood comes in sizes down to 1mm or less  but I would not suggest its a good insulation material for your project.
You have to remember that the cold surface will have condensation moisture on it so anything that soaks up moisture will have its insulating value quickly degraded. Only 4mm thick is a tall ask, you might try non absorbing foam. A long time ago I built a tiny refrigeration chamber for some work related scientific experiment, the hot side used a massive heat sink (75W Peltier module) and the cold side had foam insulation about ten times the thickness you are aiming for. With this the unit cycled on and off at a reasonable ratio to maintain the cold temperature. With thin insulation it will be on all the time and ambient temperature will have a much greater influence. Good luck Klaus Klaus |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Understand that 'Marine quality (grade)' plywood does not absorb moisture, but not sure that insulating quality will be good enough. I want to cut hole in whatever insulator I will use and this will be separation of cold and hot side. Will have to make picture of sketch to show what I have in mind. George |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Sadly, your understanding about marine plywood is incorrect. I built a 8m sailboat from that stuff and know a bit more than you on this subject  .
"Marine quality" just states that the plies (layers) of the plywood are of a much better grade than ordinary plywood, in particular the inner layers have no voids or unfilled knot holes. Marine grade is the highest quality - and most expensive - of the plywoods available. *All* grades of plywood are subject to moisture absorption unless fully encapsulated in something like epoxy resin. Klaus |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Klaus, Most appreciate your warning, will prevent me from finding it the hard way. Closed cell foam will be my insulation layer. Had some cloth that was recovered from old stove. Fabric that covered stove, most probably to prevent heat escape. Pity that I was made to throw it out as 'useless' that was kept for years. Suppose encapsulated asbestos would be not bad either, but better not play with danger. George |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Learned another thing about Peltier Device's Symbols TEC - Should be used for cooling with very good ventilation on hot side as temperature above 100 deg C will kill it. TEG - Can withstand temperatures 150 Deg C, so can be used for cooling or heating and is more forgiving if cooling of hot side is not up to scratch. George |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Found out one more little information about Peltier device. If connected ( + ) to red wire, hot side is where stamped letters are. Not as important since it can be connected either way. George |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
And I found out something long ago which you don't want to find out .
The wires attached to the device can easily break off  ., They appeared to be soldered so I tried to solder it back, it kind of worked but several of the little peltier junctions melted in the process - that material has a very low melting point.Klaus |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I tried something like this a while back with some interesting results. As others have already said, you can connect these in series, and they are symetrical devices, reversing the connections just swaps over the hot side and cold side and they work exactly the same when connected either way around. I did some very careful monitoring and there are several things you will very quickly discover for yourself. They are over 100% efficient when used as a heater. Heating power equals dc watts in, plus whatever heat it can suck from the cold side. They are absolutely fantastic for heating. Cooling not so good...... Efficiency rises very quickly from zero input power up to a peak cooling efficiency of about 35%, then falls off very quickly as input power is further increased. A one hundred watt cell works with highest efficiency at about two to three watts input !!! So you might expect a 100 watt peltier device to shift about one watt from the cold side with about three watts input power, and dissipate about four watts on the hot side with about one third power efficiency. Unfortunately the advertising can be very misleading, 100 watts max rated input powewr, 35% max efficiency, might lead you to some wrong conclusions. So beware. Increasing the dc input power above three watts actually reduces the cooling capacity. What happens is the hot side gets very hot, and the heat leaks back through the cell to the cold side. They are HORRIBLE thermal insulators within themselves. Efficiency falls dramatically with incresing differential temperature, and heat pumping efficiency reaches zero at about 20C differential. Now one watt of cooling sounds a bit poor, but if your insulation is really good, it might be able to pull a temperature difference of about 20C maximum across one cell. But you can stack cells in a pyramid. One cell might suck out one watt of heat, and dissipate four watts working at its best. One cell sitting on top of four more cells in a pyramid will suck out one watt, the entire stack will then dissipate 16 watts on the hot side, and require 15 dc input watts, but the achievable temperature differential will probably double to about 40C maximum. All five cells can be connected electrically in series. The tip is to buy the largest most powerful peltier cells and only run them at a few percent of rated power right in the highest efficiency zone. The most difficult problem is cooling the hot side. As you only achieve a 20C temperature differential, if the heatsink runs at 20C above ambient, you won't be able to get any cooling at all with only one stage ! So the heatsink needs to be absolutely huge and fan cooled. Its all fun and quite interesting, but hardly practical for any serious cooling application. One other point to be cautious of. If the hot side gets too hot, maybe above about 70C the Chinese peltiers seem to fail open circuit fairly readily. So be a bit careful in heating mode. Cheers, Tony. |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Tony, thank you very much for so much information Have two units: TEC – 12706 12V, 60 or 70W – not sure, looks now it doesn’t matter I was going to connect them in series and experiment with my 40W Solar panel that has 21.6V open circuit. This way I thought that Peltier’s will not overheat, not so sure now. Will have to make sure that heat sinks are big and maybe even I will start my experiments with my little 10W solar panel to see what happens. Plan was to get 80W or even 100W solar panel to run them, looks to be on hold now, until I get better understanding of what I have and what it can do. My way of thinking was that if I keep TEC Peltiers relatively cool I might be able to use free Sun energy and assist to heat-cool the air. I know that this is drop in the ocean of need, but should it make any difference I can set up 10 or 20 units to make their output more noticeable. During summer panels can be set up to give shade on West facing wall and also give a tiny quantity of cool air (Fingers crossed I will be able to put DC cables through the wall the way that everybody home is happy, luckily it will be a while) George |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Your best bet might be to start out using an adjustable voltage power supply, and start measuring some voltages, currents, and temperatures that are achieved with your particular peltier cells. You will probably find that with one peltier cell fitted to a huge heat sink, you will be able to get some condensation on the cold side, but no ice. Try adjusting the dc power up and down for minimum exposed surface temperature on the cold side. Next cover the cold side with a giant block of polystyrene foam to insulate it as best you possibly can. It still won't get all that cold, and that is the problem. Pouring more power into it, only makes things worse. There will be a sweet spot, probably around a volt or two where it works reasonably well, but at quite low power. Cheers, Tony. |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Connected 2 Peltier devices in series to one of my 40W panels and looks that they work OK. Not much Sun rays today, but measured about 2.6 V for 2 Peltiers in series and about 1.5V with just one. Which is bit of surprise, as I thought Voltage should be lower with 2 in series compared to just one. I could feel the cold and warmth and warm side would not be more than 40 deg C. I think that 45 and more is too hot to hold. So even without massive coolers, simple fan blowing should remove enough heat from hot side. Will have to try again during Sunny day. Now have to design some kind of arrangement that will take air from outside to push across hot side to be back outside. Fan on cold side will move inside air past cold side. It will kind of work as micro-mini AC. Will probably get one rectangular plastic downpipe from Bunnings and few corner pieces, or just cut and glue it to get desired shapes. Have to put pencil to paper to draw what I have in mind. George |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
Rough concept 
George |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Sounds like a plan George. Don't expect too much from it, it takes a lot of watts to heat/cool a room, especially if its not well insulated. The Peltier devices don't like sudden changes in temp, it causes the junctions to fail, so make sure you have some mass attached to each side, and don't cycle them off and on to quickly. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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