How to control Peltier/Thermoelectric units

Discussion in 'The Projects Forum' started by PJ51, May 10, 2009.

  1. PJ51

    Thread Starter New Member

    May 7, 2009
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    Hi I'm new to the forum and would appreciate any help you can give me with this idea i'm working on.

    My idea is to run upto 6 peltier units off of 2 - 4 battery packs for an item of clothing that cools the body, the units have got to read the temperature of the skin and respond accordingly by either heating up or cooling down. I've got no experience with the technology however and i'm not sure what sort of circuit to use to control the units.

    I know the units can be very power hungry but i'm hoping to get upto 1 -2 hours use out of the cells (i'm thinking of using lythium polymer cells) by introducing some kind of timing element so that the units are not powered constantly but switch on and off over a set time. The temperature that i'm hoping to maintain should only see a difference of + - 10°C.

    I would appreciate any advice you could give, also on battery configurations and sensors.

    Thanks for reading.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The unit that I had used 6A @12V, 1.5X1.5X 3/16 in, Getting heat in to and out is a problem. Might be best to use heat exchangers and fans.Puse width modulation PWM should work to control power, thermisters for sensing temperature.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    PWM is good, you will also need something called an H-Bridge, it will feed current either way. I would google H-Bridge to find out more.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Have a look: http://hyperphysics.phy-astr.gsu.edu/HBASE/thermo/coobod.html This gives a rough idea of how much energy needs to be moved, and the range of skin temperature to be measured. (More like +/-3°C instead of +/-10°C.)

    Taking the average from several thermistors in proximity to the skin might work.

    LiPo batteries don't work so well in the cold (no batteries do) so I would put them inside the suit. Make darn certain your charger is specifically designed for LiPo batteries - LiPos are easily and spectacularly destroyed by overcharging.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Sorry to be the proverbial "wet blanket", but this is just not a practical idea.

    Peltier devices are very power-hungry. If you intended to power them by Lithium-Polymer (LiPo) batteries, you would need quite a few of them to power a Peltier "suit" for an hour or so; the person wearing the suit would need to be towing a large and heavy trailer filled with batteries.

    Peltier devices won't last long if subjected to PWM; that's somewhat akin to beating on them with a hammer. They will last much longer if they are supplied with a constant current.
     
  6. PJ51

    Thread Starter New Member

    May 7, 2009
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    Could u possibly suggest somthing else to use in replacement of the peltier units?... the initial reason for the peltier units is that there small and there simplicity makes them very reliable... would it be more practical with another form of battery? possibly ni-cad or ni-mh?

    Would using a separate heating element be less power hungry?
     
  7. villacherman

    New Member

    May 11, 2009
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    I have to agree with SgtWookie here, the power required to run TECs is not an efficient use of energy (they are in the range of 15% efficient). The materials in Peltier units have not advanced far enough yet to be useful for man-portable systems. Instead, people are using them at IMEC are using them for generating power from human heat.

    I've worked on adsorption heat pumps for cooling soldiers in high heat environments. This is quite a system to build and they have been using fueld cells - chemical combustion seems to have enough power density for these power hungry applications.

    I do have to disagree with SgtWookie on the PWM - electrical pulsing is fine as long as you don't have too much thermal pulsing. Thermal cycling reduces the efficiency over time.
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Dewar of liquid nitrogen in the backpack, perhaps? You could regulate a slow drip from the tank into a fan system. Not exactly a fashion statement, but might work for a class project.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    My thought was more chilled water, but that still begs the question what chills the water.

    What part of the body needs cooling?

    Evaporative cooling is very efficient, if the humidity isn't too high.
     
  10. Darren Holdstock

    Active Member

    Feb 10, 2009
    262
    11
    I have to agree with SgtWookie too - the human body generates far too much heat to be removed by inefficient Peltier devices. Bill has a good point with the evaporative cooling - maybe some sort of air pump to circulate air through the clothes and make the most of all those free sweat glands. Solar-powered hats have been around for years that utlise this principle.

    It is possible to drive a peltier device with PWM if one is very, very careful. Peltiers don't like thermal shocks (just as SgtWookie and villacherman said), so as long as the PWM frequency is way above 1/peltier_thermal_time_constant and the current setpoint is ramped gently up and down then all should be well.
     
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