Muliple Peltier elements: parallel or series?

Hello,

I'm designing a 'thermo table', consisting of a 150 x 150 x 4 mm aluminium surface (6" x 6" x 1/6"), which has to be kept at a precise temperature throughout (+/- 0.1 degree centigrade). Since both heating and cooling are required, the whole design is based on one or more Peltier elements.

Ideally, one would use a Peltier element of comparable size as the aluminium surface, to minimize temperature differences due to small local heat losses. However, for reasons of cost and available electrical power (100W max), I'm limited to commonly available Peltier elements with a 40 x 40 or

50 x 50 mm surface area.

So I thought I'd use four identical 25W Peltier elements (40 x 40 mm) instead of one 100W Peltier element (50 x 50 mm). My question: is it better to connect these Peltier elements in parallel or in series? In other words: is a Peltier element's heat transport mainly determined by voltage difference or by current? Any ideas are appreciated.

And oh, I thought about using much thicker aluminium to minimize local temperature differences, but with the above dimensions, I already have a heat capacity of > 200 J/C, so with 100W power it already takes at least 2 seconds to heat it up by one degree. Doubling the thickness also doubles this time, and that quickly becomes a problem: users will want to change the temperature often, and don't want to wait several minutes every time until the temperature settles to its final value.

Thanks in advance for any ideas.

Richard Rasker

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http://www.linetec.nl
Reply to
Richard Rasker
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Current. They are in essence a bunch of high power diode PN junctions.

Their efficiency as a cooler isn't all that great so insulation is important if you are to obtain good thermal stability as is a suitable heatsink on the dump side. Good for having no moving parts thought.

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Regards,
Martin Brown
Reply to
Martin Brown

I'd control them individually, or wire them in series. Compound Peltiers have a nasty thermal instability, where they start getting hotter as you go to higher current instead of colder.

If you put them in series, they all see the same current, which along with a spreader plate top and bottom) helps prevent runaway. If you put them in parallel, the ones with higher delta-T will draw less current than the lower delta-T ones, which gives you a nice stabilizing action at low drive current.

If you push them, or you lose cooling water to your heat exchanger, the sign of the gain can invert and the whole thing turns to lava very fast. (That can happen with individual control or series-connection as well, but the melty ones can't hog current from the cooler ones, so it isn't so unstable.)

The other thing is that you have to allow the Peltiers to slide around a bit as the cold plate cools down, because otherwise they'll crack. So use Arctic Silver or a very small amount of very good thermal grease on one side, and solder on the other. IIRC you can solder the 30 mm ones, but not the 50 mm ones--it'll be in the datasheet. (Solder is about 100x better than ordinary thermal grease.)

I talk a lot about Peltiers in my free thermal chapter,

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. There's also an incomplete draft that was intended to accompany the second edition, which has some more stuff in it, at
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.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal Consultant
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Phil Hobbs

aluminium

One other thing about Peltiers that's often overlooked: compared with heaters, they can make the system far less vulnerable to thermal forcing. With just a heater, you have to have a big heat leak in order for the loop to work at any reasonable speed, so the bandwidth and the thermal forcing go up together. With a Peltier you don't have that constraint, because it can pull as well as push.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal Consultant
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Phil Hobbs

Handy info thanks. Although the TECs are available their datasheets are often inadequate and miss out details needed for real world designs.

Can I pick your brains for a slightly odd requirement? Ideally one that can be done with at most a pair of TECs one high temp and one normal. On paper they were rated for 3A and 14v or so each but in practice I was struggling in reverse to get 2v and a few mA out with a candle flame heated plate at 250C on one side and a slab of aluminium at -18C on the other. I gave up in the end as it was taking too long and safety concerns of very hot metal plates and children scuppered it in the end.

The aim would be for an Xmas science demo to harvest some of the ~100W waste heat of a candle flame and drive a ~1W LED. This seemed a modest aim when I started out but in practice it proved impossible. To be any good the LED must be a *lot* brighter than the candle flame!

I am guessing that to stand any chance I also need a fan assisted heat sink and to be authentic it must all be powered by the TECs. It is for a physics demo so hiding a button cell somewhere is just not on...

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Regards,
Martin Brown
Reply to
Martin Brown

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TEC's in series as other have suggested. What's the temperature range? And most important what's on the other side of the TEC's? In my limited experience you need to spend at least as much time thinking about how you keep the temperature of the 'other side' of the TEC roughly constant.

George H.

Reply to
George Herold

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If you NEED the surface to be at a uniform temperature, I'd look into getting a heat pipe made up for it, or at least talk to those people.

Can't find their contact info. But, at trade shows they hand out little 'hockey stick' coppercolored heat pipes as promos. The surface temperature of those sticks is uniform, fast.

Probably sell to multicore processor cooling too. transfer heat out to fan area.

Reply to
Robert Macy

People do this over camp stoves, between a pot of water and the burner. Peltiers are really stinky at power generation--their maximum efficiency is only about 3% iirc.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
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Phil Hobbs

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Wonderful information, thank you very much! I already have a small-sized (30 x 30 mm 10W) single-Peltier setup working properly, but this information will be very useful when scaling things up.

Richard Rasker

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http://www.linetec.nl
Reply to
Richard Rasker

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Interesting, I take your question to be, "Where along the I/V curve is a TEC most efficient when run as a generator?" (heat to electric power.) That should be fairly easy to determine experimentally.

Perhaps you need more elements in series?

And how did you get the -18C? If this is in the winter time perhaps you'd allow for the cold end to be dunked into an ice bath... cooled with snow from the great outdoors.

George H.

Reply to
George Herold

...

Ideally from 10 to 60 degrees centigrade, with a 20-25 degrees ambient temperature. This should not be a problem.

There's a rather big (1kg) aluminium heat sink on the other side, perhaps with a fan. I'm not certain about the fan though, because I have a feeling that this might cause faster temperature changes in the heat sink than the control loop is capable of compensating. If necessary, I'll use a bigger heat sink.

Richard Rasker

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Reply to
Richard Rasker

It's all very predictable. The current through the Peltier junction transfers heat from one side to the other at so many watts per amp, but it also resistively heats the junction, at so many watts per amp squared.

There's a maximum current above which more heat is generated in the junction than gets pumped across it; if you do the detailed calculations, this maximum useful current depends on the thermal resistance to ambient of the exhaust side.

Sloman A.W., Buggs P., Molloy J., and Stewart D. ?A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor? Measurement Science and Technology, 7 1653-64 (1996), includes the formula in Appendix A.

I also managed to get it into a comment in Rev. Sci. Instrum, a few years later.

Sloman A.W. ?Comment on ?Implementing of a precision fast thermoelectric cooler controller using a personal computer parallel port connection and ADV8830 controller?[Rev.Sci. Instrum. 74, 3862 (2003)]? Review of Scientific Instruments, 75 788-9 (2004).?

It's a very good idea to monitor the temperature of the exhaust side of the junction as well as the temperature you are trying to control - you don't have to do it all that accurately to avoid getting into trouble, and with a microcontroller based solution, you can figure in the net heat-transfer per watt and keep the control loop critically damped over a range of thermal loads.

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Bill Sloman, Nijmegen
Reply to
Bill Sloman

On a sunny day (Thu, 19 Jul 2012 14:50:00 +0100) it happened Martin Brown wrote in :

I would use a thermocouple (or some i nseries) and a JFET oscillator with step up transformer. JFET oscillators oscillate from about next to zero volts up. Have not tried it on a thermocouple, but did try the low voltage JFET oscillator. Only a few parts needed...

Reply to
Jan Panteltje

Interesting, I take your question to be, "Where along the I/V curve is a TEC most efficient when run as a generator?" (heat to electric power.) That should be fairly easy to determine experimentally.

Perhaps you need more elements in series?

And how did you get the -18C? If this is in the winter time perhaps you'd allow for the cold end to be dunked into an ice bath... cooled with snow from the great outdoors.

George H. _______________________________________________

It's been a while but I have worked with thermal electric generators (TEG) that used propane and a catalytic heater as a power source. The unit was made by Teledyne Energy Systems. I don't know if they are still around.

When I met with their engineers, I ask about the TEC make-up. They did say that it was optimized to be a generator. It used different junctions than what was used for heating/cooling applications. I don't remember much more other than it was made up of five modules, each running about 6 volts and 2 amps. Overall, it did run at 28 volts and provided 50 watts of power. It consumed a 100 gal tank in about two weeks.

Please don't hold me to exact numbers, it was more than 20 years ago.

Regards, Tom

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I'd suggest running the numbers on the heat sink. (Wakefield and other heat sink makers will give you some numbers.) If it's just air cooled and has to dissipate 100 Watts, it will need a lot of surface area or have a fan... Unless you are doing single shot cooling. (Cooling for a short time and tehn allowing the heat sink to cool down again.)

George H.

Reply to
George Herold

Hi Martin,

It was 20 years ago but IIRC...

In my old flat I was surprised to discover than the boiler gas valve was powered directly from a thermocouple assembly in the gas flame. (I only realised this after finding what I assumed was a "thermal switch" had a permanent short circuit and the valve had no external power source...)

I wonder if one of these could be used? Sort of thing you could get off ebay to try.

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John Devereux
Reply to
John Devereux

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Interesting thanks.... punching numbers...

1 gal gas ~ 33kW*hr 100gals =3D 3,300 kW*hr. 50 watts for two weeks ~17 kW*hr eff. ~ 0.5%

George H.

Reply to
George Herold

up transformer.

oscillator.

I'd go with several thermocouples in series. The advantage over a Peltier is that you can have the cold junctions some way from the heat.

Say use Iron/Constantan at about 55uV/K so for 1.8V at 200C you'd need about 160 thermocouples which is doable I reckon, probably many fewer depending on flame temperature. You can just twist the ends together to make the junctions, should last a while.

Cheers

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Syd
Reply to
Syd Rumpo

On a sunny day (Thu, 19 Jul 2012 16:44:52 +0100) it happened Syd Rumpo wrote in :

step up transformer.

oscillator.

I think that is how those RTGs work. But 160 is a lot, JFET oscillator works from about 20 mV (in my test).

16 theromocouples is doable. I was also wondering if one could make a low voltage oscillator with a depletion mode power MOSFET, just to touch that subject again (I still have to order some).
Reply to
Jan Panteltje

You can buy fans that sit on top of a stove. I picked one up at a fellow's cottage and it looked like it was a little toy motor run by a bog-standard 30x30 or 40x40mm Peltier.

I would not expect much efficiency.. maybe a couple percent.

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The Peltier is horizontal and sandwitched between the two aluminum plates (below the heat sink).

Reply to
Spehro Pefhany

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