Muliple Peltier elements: parallel or series?

[Richard Rasker]

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

 

[Martin Brown]

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.

 

[Martin Brown]

Martin Brown wrote:

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...

 

[Richard Rasker]

On 19/07/2012 12:57, Richard Rasker wrote:

....

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.

[snip]

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 .

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

 

[Richard Rasker]

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

 

[tm]

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

 

[John Devereux]

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.

 

[Syd Rumpo]

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

 

[Spehro Pefhany]

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 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).

 

[Martin Brown]

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).

 

[Spehro Pefhany]

Martin Brown wrote:



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

Those must be thermopiles, a single Cu-Fe thermocouple would produce
MUCH less voltage.

 

[tm]

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

 

[[email protected]]

Of course, TECs are 60-year-old technology....

Yup. My father designed a TEC refrigerator in the late '50s or very early
'60s, complete with a solid-state power supply (string of Germanium DO-3
PNPs). I had a couple of them in the basement to play with when I was a kid.
It was designed to go in a car but never made it past prototype. It got beat
out by Styrofoam disposables. ;-)

 

[Don Lancaster]

Hey, 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.

Peltier Coolers are almost always totally worthless.
Their delta T across the heatsink usually EXCEEDS their intended delta T
drop

See < http://www.tinaja.com/glib/hack68.pdf >

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[[email protected]]

you can get plenty of 12V TEC refrigerators/coolers but I don't think
they have
much if any smarts in them, they just say something like "up to 20'C
below ambient"

Fifty years ago you couldn't. ;-)

 

[tm]

Nonsense.

If you try cooling a CPU or a voltage regulator with one, you deserve
what you get. But many kinds of instruments would be far harder to
build without them. Oh, and of course the telecom infrastructure
wouldn't work without them either, because Peltiers are what keep all
the DWDM lasers from scribbling across neighbouring channels.

I think he implies that in his paper. Certain special applications are ideal
for TECs.


tm

 

[tm]

Setting up a straw man and then knocking it down is an old rhetorical
amusement, but IMO it's beneath engineers of Don's calibre. Of course
he's been in the biz for probably 50 years or more, and so feels
entitled to take the occasional potshot from his porch, and who's to say
that isn't OK.

I bought a copy of his TTL Cookbook in about 1974, when I was 14 years
old, and it helped me a lot back then, and for several years
thereafter. I was also intrigued by his TV Typewriters, but since I
couldn't type they weren't that compelling. They were really just a
tease anyway, aimed at those of us who didn't have our own private
PDP-10.

However, saying that "Peltier coolers are almost always totally
worthless", when in fact they enable most of modern communications and
the Internet, is just plain nonsense, and needs to be called out. Maybe
it'll improve Don's aim.

Cheers

Phil Hobbs

--

Well, saying "they enable most of modern communications and the internet" is
a bit Algoreish in its own right.

I have yet to see a 10 gig SFP long haul module that is cooled. I would
guess a good bit of the Internet uses them.


tm

 

[John Devereux]

John Devereux wrote:


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.

Pretty clever really, you can see why I was confused. I had even
designed a temperature controller with a thermocouple input by then. But
I was not used to thinking of them generating amps!

 

[Martin Brown]

CANDLE-powered Sterling. Oops.

Trouble is I had already done a candle powered heat engine and then a
more sophisticated coffee cup powered rotary motion one in previous
years. I was trying to find yet another candle based demo for Xmas.

 

[Martin Brown]

Martin Brown wrote:



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.

My back of the envelope suggests that for a candle flame delta-T of
1000K there would be around 12mV per junction pair to play with.

I presume the boiler thermostat is a big bunch in series. A pair of
those would provide enough power capture with a bit of luck.
How big are they?

I did try messing about with iron and copper wire too, but it didn't
look good and produced a miniscule voltage for the amount of effort.
Whoever said 160 reliable thermocouple wire junctions in a candle flame
is "doable" is something of an optimist. I got fed up at about a dozen.

I didn't know about these gas boiler thermopile contraptions.