George Herold wrote:> On Jul 19, 7:57 am, Richard Rasker <spamt...@linetec.nl> wrote: >> 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....>> is it better to connect these Peltier elements in parallel or in series?> TEC's in series as other have suggested. What's the temperature > range?Ideally from 10 to 60 degrees centigrade, with a 20-25 degrees ambient temperature. This should not be a problem.> 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.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 -- http://www.linetec.nl
Muliple Peltier elements: parallel or series?
Started by ●July 19, 2012
Reply by ●July 19, 20122012-07-19
Reply by ●July 19, 20122012-07-19
On 7/19/2012 3:37 PM, Phil Hobbs wrote:> Phil Hobbs wrote: >> >> Martin Brown wrote: >>> >>> On 19/07/2012 12:57, Richard Rasker wrote: >>>> 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. >>> >>> 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. >>> >>> -- >>> Regards, >>> 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.)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.>> 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, >> http://electrooptical.net/www/book/thermal.pdf . There's also an >> incomplete draft that was intended to accompany the second edition, >> which has some more stuff in it, at >> http://electrooptical.net/www/book/draftthermal.pdf . >> > > 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.-- Bill Sloman, Nijmegen
Reply by ●July 19, 20122012-07-19
"George Herold" <gherold@teachspin.com> wrote in message news:68a5592b-2d41-467f-aa95-1748583a3e2a@s1g2000vbj.googlegroups.com... On Jul 19, 9:50 am, Martin Brown <|||newspam...@nezumi.demon.co.uk> wrote:> On 19/07/2012 14:29, Phil Hobbs wrote: > > > > > > > Martin Brown wrote: > >> Current. They are in essence a bunch of high power diode PN junctions. > > > 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, > >http://electrooptical.net/www/book/thermal.pdf. There's also an > > incomplete draft that was intended to accompany the second edition, > > which has some more stuff in it, at > >http://electrooptical.net/www/book/draftthermal.pdf. > > 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... > > -- > Regards, > Martin Brown- Hide quoted text - > > - Show quoted text -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
Reply by ●July 19, 20122012-07-19
On a sunny day (Thu, 19 Jul 2012 14:50:00 +0100) it happened Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote in <1gUNr.50376$iI7.22431@newsfe03.iad>:>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 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 by ●July 19, 20122012-07-19
On Jul 19, 10:31=A0am, Richard Rasker <spamt...@linetec.nl> wrote:> George Herold wrote: > > On Jul 19, 7:57=A0am, Richard Rasker <spamt...@linetec.nl> wrote: > >> 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. > ... > >> is it better to connect these Peltier elements in parallel or in serie=s?> > TEC's in series as other have suggested. =A0What's the temperature > > range? > > Ideally from 10 to 60 degrees centigrade, with a 20-25 degrees ambient > temperature. This should not be a problem. > > > And most important what's on the other side of the TEC's? =A0In > > 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. > > 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 feelin=g> that this might cause faster temperature changes in the heat sink than th=e> control loop is capable of compensating. If necessary, I'll use a bigger > heat sink. > > Richard Rasker > --http://www.linetec.nlI'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 by ●July 19, 20122012-07-19
Martin Brown <|||newspam|||@nezumi.demon.co.uk> writes:> On 19/07/2012 14:29, Phil Hobbs wrote: >> Martin Brown wrote: > >>> Current. They are in essence a bunch of high power diode PN junctions. >> >> 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, >> http://electrooptical.net/www/book/thermal.pdf . There's also an >> incomplete draft that was intended to accompany the second edition, >> which has some more stuff in it, at >> http://electrooptical.net/www/book/draftthermal.pdf . > > 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...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. -- John Devereux
Reply by ●July 19, 20122012-07-19
On Jul 19, 10:47=A0am, "tm" <No_one_h...@white-house.gov> wrote:> "George Herold" <gher...@teachspin.com> wrote in message > > news:68a5592b-2d41-467f-aa95-1748583a3e2a@s1g2000vbj.googlegroups.com... > On Jul 19, 9:50 am, Martin Brown <|||newspam...@nezumi.demon.co.uk> > wrote: > > > > > > > On 19/07/2012 14:29, Phil Hobbs wrote: > > > > Martin Brown wrote: > > >> Current. They are in essence a bunch of high power diode PN junction=s.> > > > 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 p=ut> > > 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 actio=n> > > at low drive current. > > > > If you push them, or you lose cooling water to your heat exchanger, t=he> > > sign of the gain can invert and the whole thing turns to lava very > > > fast. (That can happen with individual control or series-connection a=s> > > well, but the melty ones can't hog current from the cooler ones, so i=t> > > isn't so unstable.) > > > > The other thing is that you have to allow the Peltiers to slide aroun=d 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 1=00x> > > better than ordinary thermal grease.) > > > > I talk a lot about Peltiers in my free thermal chapter, > > >http://electrooptical.net/www/book/thermal.pdf. There's also an > > > incomplete draft that was intended to accompany the second edition, > > > which has some more stuff in it, at > > >http://electrooptical.net/www/book/draftthermal.pdf. > > > 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. O=n> > 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... > > > -- > > Regards, > > Martin Brown- Hide quoted text - > > > - Show quoted text - > > Interesting, =A0I take your question to be, "Where along the I/V curve > is a TEC most efficient when run as a generator?" =A0(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 sa=y> 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 mor=e> 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- Hide quoted text - > > - Show quoted text -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 by ●July 19, 20122012-07-19
On 19/07/2012 15:45, Jan Panteltje wrote:> On a sunny day (Thu, 19 Jul 2012 14:50:00 +0100) it happened Martin Brown > <|||newspam|||@nezumi.demon.co.uk> wrote in > <1gUNr.50376$iI7.22431@newsfe03.iad>: > >> 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 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... >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 -- Syd
Reply by ●July 19, 20122012-07-19
On a sunny day (Thu, 19 Jul 2012 16:44:52 +0100) it happened Syd Rumpo <usenet@neonica.co.uk> wrote in <ju9a13$jor$1@dont-email.me>:>On 19/07/2012 15:45, Jan Panteltje wrote: >> On a sunny day (Thu, 19 Jul 2012 14:50:00 +0100) it happened Martin Brown >> <|||newspam|||@nezumi.demon.co.uk> wrote in >> <1gUNr.50376$iI7.22431@newsfe03.iad>: >> >>> 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 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... >> > >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.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 by ●July 19, 20122012-07-19
On Thu, 19 Jul 2012 14:50:00 +0100, Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:> >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... > >-- >Regards, >Martin BrownYou 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. http://infinitebeautysupply.store.buy.com/p/heat-powered-wood-stove-fan/223536999.html The Peltier is horizontal and sandwitched between the two aluminum plates (below the heat sink).
