On 19/07/2012 18:08, Spehro Pefhany wrote:> On Thu, 19 Jul 2012 14:50:00 +0100, Martin Brown > <|||newspam|||@nezumi.demon.co.uk> wrote: > >> >> 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... > > 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.Yes. I know. I made one from the bits I had left over from this failed Xmas demo after seeing one at a neighbours house. Expensive for what they are - though his is prettier than mine. A cheap "solar" motor will run happily off the output voltage and a light weight aluminium fan. To be fair it does a good job of moving warm air out into the room that would otherwise be trapped in the fireplace alcove above the stove.> 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).A couple of percent off 100W would suit me fine. But I was getting less than 0.01%. Problem I ran into was I could not get both voltage and current sufficient to drive a white LED (or a red one for that matter). -- Regards, Martin Brown
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 Thu, 19 Jul 2012 13:32:34 +0100, 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.I think that you are not thinking your physics through in a thorough way. Consider your 150x150mm table, being oh-so-carefully held at temperature by one gawdaful big Peltier device (or a bunch of them in parallel, whatever). Now consider some smartass (me, if I'm in your lab), putting a 2cm cube of something good and warm (or good and cold) on your table. What happens to your average temperature? What happens to the temperature right underneath the hot thing? What happens to the temperature elsewhere? Me, I think I'd go with a bunch of small Peltiers glued to a moderately thin (I'm visualizing 1/8") plate, each with a thermistor or other easy to use temperature probe placed in a pocket of the plate, smack in the center of the Peltier device. Then a controller for each device, with whatever tuning is necessary so that the controllers don't get into fights and make temperature oscillations that are unstable spatially as well as in time. How many devices and coolers you use depends on how uneven the anticipated thermal load will be on the plate, how tightly you feel you need to control its temperature, and on the thermal conductivity of your plate. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●July 19, 20122012-07-19
On Thu, 19 Jul 2012 14:50:00 +0100, Martin Brown 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...You may do much better with a fan-powered Sterling engine. Or, for a complete demo, have one TE element complete with lots of hype, and one "150 year old technology" Sterling engine for comparison... -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●July 19, 20122012-07-19
On Thu, 19 Jul 2012 14:11:44 -0500, Tim Wescott wrote:> On Thu, 19 Jul 2012 14:50:00 +0100, Martin Brown 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... > > You may do much better with a fan-powered Sterling engine.CANDLE-powered Sterling. Oops.> Or, for a complete demo, have one TE element complete with lots of hype, > and one "150 year old technology" Sterling engine for comparison...-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●July 19, 20122012-07-19
Tim Wescott wrote:> > On Thu, 19 Jul 2012 14:11:44 -0500, Tim Wescott wrote: > > > On Thu, 19 Jul 2012 14:50:00 +0100, Martin Brown 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... > > > > You may do much better with a fan-powered Sterling engine. > > CANDLE-powered Sterling. Oops. > > > Or, for a complete demo, have one TE element complete with lots of hype, > > and one "150 year old technology" Sterling engine for comparison... > > --Of course, TECs are 60-year-old technology.... Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 19, 20122012-07-19
On Jul 19, 11:44=A0am, Syd Rumpo <use...@neonica.co.uk> wrote:> On 19/07/2012 15:45, Jan Panteltje wrote: > > > On a sunny day (Thu, 19 Jul 2012 14:50:00 +0100) it happened Martin Bro=wn> > <|||newspam...@nezumi.demon.co.uk> wrote in > > <1gUNr.50376$iI7.22...@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 wi=th 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 o=scillator.> > Only a few parts needed... > > I'd go with several thermocouples in series. =A0The 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. =A0You can just twist the ends together t=o> make the junctions, should last a while. > > Cheers > -- > SydHey, speaking of thermopiles I heard Hamamatsu has a new line of them. http://www.hamamatsu.com/news/2011/2011_09_21.html I can't find a spec sheet or price though. George H.
Reply by ●July 19, 20122012-07-19
Martin Brown wrote:> 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! >You are using the wrong junction for this purpose. There are far better junctions made specifically for this, although the thermocouple in a gas furnace will almost do it. About 3 or 4 in series will light an LED plenty brightly, in fact you'll need a current limiting resistor. I think they use copper and iron, nothing exotic at all, and in a gas flame, I think you get 600 mV out of these. A candle flame should be a bit less, so 4 junctions at 400 mV each should light a red LED. Jon
Reply by ●July 19, 20122012-07-19
John Devereux wrote:> 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...)This is called a millivolt system, still fairly common. Uses no mains electricity. It basically uses the same thermocouple used in the common gas safety controls on water heaters and furnaces. The solenoid coil is a small number of turns of heavy-gauge copper wire, and the current to operate the solenoid is up to several Amps. Jon
Reply by ●July 19, 20122012-07-19
On Thu, 19 Jul 2012 14:31:35 -0500, Jon Elson <jmelson@wustl.edu> wrote:>Martin Brown wrote: > > > >> 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! >> >You are using the wrong junction for this purpose. There are far >better junctions made specifically for this, although the thermocouple >in a gas furnace will almost do it. About 3 or 4 in series will >light an LED plenty brightly, in fact you'll need a current limiting >resistor. I think they use copper and iron, nothing exotic at all, >and in a gas flame, I think you get 600 mV out of these. A candle >flame should be a bit less, so 4 junctions at 400 mV each should >light a red LED. > >JonThose must be thermopiles, a single Cu-Fe thermocouple would produce MUCH less voltage.
Reply by ●July 19, 20122012-07-19
"George Herold" <gherold@teachspin.com> wrote in message news:8439768c-44bb-4288-820d-b4a862190f10@n33g2000vbi.googlegroups.com... On Jul 19, 10:47 am, "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 > > >> 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- Hide quoted text - > > - Show quoted text -Interesting thanks.... punching numbers... 1 gal gas ~ 33kW*hr 100gals = 3,300 kW*hr. 50 watts for two weeks ~17 kW*hr eff. ~ 0.5% George H. _____________________________________________________ Again, I may be mis-remembering the exact quantity of propane. It might have been 100 pounds. What do the numbers look like for 1000 gallons and one year? We used it to run a mountain top microwave relay site with no commercial power within miles. We refueled it in the summer. It lasted for more than 5 years until development brought power closer to the site (an old fire tower). The most important thing is to get the combustion products out of the shelter as a lot of water vapor is produced. It sure beat solar because of the problems with snow. I believe the efficiency was much better than 0.5 % , more like 5% or better. Back of the envelope shows 13% Regards, tm
