I am using a PIC as a controller for TEC (peltier) devices. I monitor
various temperatures and adjust a constant DC current to achieve the
required results. Eight (identical) peltier devices are assembled to a 'cold
plate' thermally in parallel. Electrically they are in series and they draw
approx 2A from the 12V supply.
When the design was presented for 'peer review', someone stated that the TEC
devices should be electrically connected in a series/parallel arrangement to
improve efficiency.
He couldn't state the reasons for this. I can't think why a simple series
connection should be less efficient. He didn't mean driver efficiency, just
thermal efficiency of the peltier devices. Does anyone out there know if I
ought to follow this up or just ignore it? Experiments could take a lot of
time which I do not have. I can't find anything in manufacturers data other
than keeping ripple low.
Any help appreciated.
>I am using a PIC as a controller for TEC (peltier) devices. I monitor
>various temperatures and adjust a constant DC current to achieve the
>required results. Eight (identical) peltier devices are assembled to a
'cold
>plate' thermally in parallel. Electrically they are in series and they draw
>approx 2A from the 12V supply.
>When the design was presented for 'peer review', someone stated that the
TEC
>devices should be electrically connected in a series/parallel arrangement
to {Quote hidden}
>improve efficiency.
>He couldn't state the reasons for this. I can't think why a simple series
>connection should be less efficient. He didn't mean driver efficiency, just
>thermal efficiency of the peltier devices. Does anyone out there know if I
>ought to follow this up or just ignore it? Experiments could take a lot of
>time which I do not have. I can't find anything in manufacturers data other
>than keeping ripple low.
>Any help appreciated.
>
>Regards,
>Gerry Cox.
Peltier elements are approximately a solid state equivalent of a
thermodynamic heat pump. Heat pumps pump the most heat power when the
temperature difference between the two sides is small. So, in general your
peltier elements will operate more efficiently if they are thermally in
series. On the other hand, I have read somewhere (perhaps the Melcor Corp
product info) that real peltier elements are most linear when operated
between about 40-80% of the maximum usable temperature difference.
>Peltier elements are approximately a solid state equivalent of a
>thermodynamic heat pump. Heat pumps pump the most heat power when the
>temperature difference between the two sides is small. So, in general your
>peltier elements will operate more efficiently if they are thermally in
>series.
Correct, if temperature difference is big AND the colder side peltier is
much smaller than the hot side.
This is because the hot side pump has to pump also the R*I loss (in form of
heat) generated by the cold side peltier, and that is usually much more
than the total cool effect.
Regards
/Morgan
>On the other hand, I have read somewhere (perhaps the Melcor Corp
>product info) that real peltier elements are most linear when operated
>between about 40-80% of the maximum usable temperature difference.
>
>Cheers,
>Art Bailey
>
>
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Morgan Olsson ph +46(0)414 70741
MORGANS REGLERTEKNIK fax +46(0)414 70331
H€LLEKS (in A-Z letters: "HALLEKAS")
SE-277 35 KIVIK, SWEDEN spam_OUTmrtTakeThisOuTiname.com
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
At 14:20 1999-01-22 -0000, you wrote:
>I am using a PIC as a controller for TEC (peltier) devices. I monitor
>various temperatures and adjust a constant DC current to achieve the
>required results. Eight (identical) peltier devices are assembled to a 'cold
>plate' thermally in parallel. Electrically they are in series and they draw
>approx 2A from the 12V supply.
>When the design was presented for 'peer review', someone stated that the TEC
>devices should be electrically connected in a series/parallel arrangement to
>improve efficiency.
>He couldn't state the reasons for this. I can't think why a simple series
>connection should be less efficient. He didn't mean driver efficiency, just
>thermal efficiency of the peltier devices. Does anyone out there know if I
>ought to follow this up or just ignore it?
Right, that is bullshit. It is about optimizing the drive voltage and poer
supply.
On the contrary: power supplies are more efficient at higher output voltahe
due to less percentage loss in the rectifier (and also in cables,
connectors etc), so connect them in series!
To get the best efficiency, you shall keep from PWM or on/off, instead
control a SPS to deliver just the reqiuered voltage, to minimize R*I losses
in the peltier.
OK, if you have a lot of elements another good method is to shift between
say 3 parallel x 2 series, and vice versa to have two modes with "lossless
conversion" (just a relay)
Good luck
/Morgan
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Morgan Olsson ph +46(0)414 70741
MORGANS REGLERTEKNIK fax +46(0)414 70331
H€LLEKS (in A-Z letters: "HALLEKAS")
SE-277 35 KIVIK, SWEDEN .....mrtKILLspam@spam@iname.com
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>I am using a PIC as a controller for TEC (peltier) devices. I monitor
>various temperatures and adjust a constant DC current to achieve the
>required results. Eight (identical) peltier devices are assembled to a 'cold
>plate' thermally in parallel. Electrically they are in series and they draw
>approx 2A from the 12V supply.
Sounds fine to me. I did some research into the application of peltier
devices used as generators several years back and don't remember any such
problems that your co-worker mentioned. The only thing that I found
non-intuitive was that TE coolers do NOT like PWM power - I always found
better efficiency if the PWM was smoothed to DC first.
Look at it this way. Standard coolers manufactured by Melcor consist of
(IIRC) 128 elements connected in series electrically but connected in
parallel thermally.
Come to think of it - the only problem I can see is if the modules are
somewhat different and do not drop the same voltage. This will cause the
higher resistance modules to dissipate more power than the lower. You
should be able to easily measure to see if this occurs and if it is a problem.
> The only thing that I found non-intuitive was that TE coolers do NOT
> like PWM power - I always found better efficiency if the PWM was
> smoothed to DC first.
This relates back to the thread on LED brightness. The difference is
that the series chain of Peltier modules achieves a higher voltage drop
and runs at significant power making it really worthwhile to use SMPS
techniques.
The equivalent curcuit for a Peltier device is a "back EMF"
proportional in significant degree to the temperature differential, in
series with an internal resistance. Power dissipated in the EMF
equivalent does useful work (cooling) whilst power disssipated in the
internal resistance is deleterious to performance (it actually heats the
"cold" side!).
The significant point is that the "back EMF" is very much a constant.
PWM without an inductor and commutation diode, preferably a capacitor
also, simply biases the internal resistance much harder for a fraction
of the time. Unfortunately, the power dissipated by this resistance
varies as the square of the current, whilst power achieving useful work
in the "back EMF" (situation is identical in a motor, BTW) varies
directly with the current.
IOW, delivering twice the current for half the time generates twice
as much waste heat. In a motor, this is a minor nuisance but can
perhaps be designed around. In a Peltier cooler however, it is
absolutely antithetical to the primary goal.
The other aspect of PWM, indeed as I am given to understand, frequent
and repeated cycling of the power, is that the thermal response of these
devices is instant and their thermal mass quite small. You will thus
produce a thermal alternation and through expansion, a mechanical
oscillation which is stressing the structure of the device.
Not a Good Thing, so I am told. It is recommended to drive them with
a power supply which ramps up the drive current relatively slowly and
keeps it constant. Raw DC from a battery charger is not advisable. A
well-designed SMPS, more to the point.
--
Cheers,
Paul B.
>Gerry Cox wrote:
>
>>I am using a PIC as a controller for TEC (peltier) devices. I monitor
>>various temperatures and adjust a constant DC current to achieve the
>>required results. Eight (identical) peltier devices are assembled to a 'cold
>>plate' thermally in parallel. Electrically they are in series and they draw
>>approx 2A from the 12V supply.
>
>Sounds fine to me. I did some research into the application of peltier
>devices used as generators several years back and don't remember any such
>problems that your co-worker mentioned. The only thing that I found
>non-intuitive was that TE coolers do NOT like PWM power - I always found
>better efficiency if the PWM was smoothed to DC first.
Could this be due to them acting as a generator when off ? Would
shorting the pins during the 'off' cycle help?
At 14:20 1999-01-22 -0000, you wrote:
>I am using a PIC as a controller for TEC (peltier) devices. I monitor
>various temperatures and adjust a constant DC current to achieve the
>required results. Eight (identical) peltier devices are assembled to a
'cold
>plate' thermally in parallel. Electrically they are in series and they draw
>approx 2A from the 12V supply.
>When the design was presented for 'peer review', someone stated that the
TEC
>devices should be electrically connected in a series/parallel arrangement
to
>improve efficiency.
>He couldn't state the reasons for this. I can't think why a simple series
>connection should be less efficient. He didn't mean driver efficiency, just
>thermal efficiency of the peltier devices. Does anyone out there know if I
>ought to follow this up or just ignore it?
Thermoelectric (Peltier) devices are often stacked in series (Mechanically)
in order to extend the temperature delta. They are also arranged
(mechanically) in parallel in order to extend the useful cooling surface.
Is it possible that your peer reviewer meant that a series parallel
arrangement of the peltier devices could extend the useful temperature delta
and usable surface area? Perhaps the remark was not intended to apply to the
electrical circuit configuration. I have created such cooling units myself
by using series and parallel peltier devices in this way. It may be worth
your time to check it out. I think it would benefit you more to respond to
a peer review comment rather than ignore it.
Best regards,
Richard
Right, that is bullshit. It is about optimizing the drive voltage and poer
supply.
On the contrary: power supplies are more efficient at higher output voltahe
due to less percentage loss in the rectifier (and also in cables,
connectors etc), so connect them in series!
To get the best efficiency, you shall keep from PWM or on/off, instead
control a SPS to deliver just the reqiuered voltage, to minimize R*I losses
in the peltier.
OK, if you have a lot of elements another good method is to shift between
say 3 parallel x 2 series, and vice versa to have two modes with "lossless
conversion" (just a relay)
Good luck
/Morgan
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Morgan Olsson ph +46(0)414 70741
MORGANS REGLERTEKNIK fax +46(0)414 70331
H€LLEKS (in A-Z letters: "HALLEKAS")
SE-277 35 KIVIK, SWEDEN @spam@mrtKILLspaminame.com
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Boy, this discussion is really heating up! (Groan!)
I trust my previous summation, to which I have privately received a
favourable confirmation by someone with practical experience, fully
clears up the matter of PWM including the corollary that grounding the
device during an "off" period would not only prevent the cooler from
working, but result in its rapid demise.
The matter of stacking coolers is fascinating. They must be stacked
in a "pyramid" with heat-distributing plena between ("heat pipes" come
to mind for this application), where successive lower layers have a
multiple of units inversely proportional to the efficiency of the units.
If for example, the efficiency of the device is 25%, then you will
require four devices in the second layer; 16 in the third etc. At this
rate, overall efficiency drops precipitously (exponentially to the
number of layers) and it is easy to see why cascaded Peltiers are rarely
used and *not* for primary cryo-cooling!
Even in a cascade however, the optimal performance and temperature
differential presumably relate directly to current, and for the same
device, each of the units in the same layer as well as each layer would
best be electrically wired, all in series.
In most applications, Peltiers are used purely for the convenience of
servo-control of temperature. For this purpose, it is often convenient
to use another system (conventional evaporative/ condensation
refrigeration cycle using mechanical compressor) as the primary source
of "cool" and the Peltier as the vernier.
Whether or not there is room for improvement of efficiency as was
hoped some 20 or 30 years ago to the extent that they could rival the
refrigerant compressor, at present they appear to be neither efficient
nor cheap.
--
Cheers,
Paul B.
iname.com
