• Chiller and Thermoelectric Cooling for Tanks

    In 1821, J. T. Seebeck (1770-1831) discovered that different metals junctions will develop a small voltage potential when both of them are at different temperature. This effect is known as the "Seebeck effect". Thermocouples are based on “Seebeck effect”. In 1834, a scientist called Peltier discovered the inverse of the Seebeck effect, known as the "Peltier effect". He found that applying a voltage to a thermocouple would induce a active heat pump. This is now known as Thermoelectric Cooler (TEC). I have decided to start this thread due the renew interests in TEM cooler. There are quite a few thing you need to know before everyone rush into buying TEM.

    Ask yourself the following questions :

    1. What is your target temperature of your tank ?
    2. How much heat (Qc) you intend to remove from your tank ?
    3. What type of heatsink to use ?
    4. Are you willing to fork out extra $$ to power your chiller ?

    Using a 1ft x 1ft x 1ft tank of 27L as a case study ,

    Lets say the temperature of my 30L tank is to be cooled to 24C. The ambient temperature of my room is 28C. The temperature difference ( dT = 28 -24 ) is 4C . We can determine the heatload (Qc) this can be easily done by using any heatload calculating software. Values of Qc for typical tank volumes is plotted on the graph below for your convenience.


    Step 1 : Determine the value of Qc from the graph below.

    From the graph, Qc of 1ft x 1ft x 1ft tank is about 40W



    Step 2 : Choosing a suitable TEM

    With Qc known we can proceed to the next step of choosing a suitable TEM for the Job. Don’t be confused by the power ratings of TEM. The power rating is the maximum electrical power the TEM will “consumed” from you power supply. Qc is the “Cooling Power” of the TEM its usually about 50% of the electrical power you supplied. Some retailers sell "80W peltier element", without stating what this value actually means. This is misleading - what you want is a high transport capability, but a low power consumption.

    For Example

    TEM rated at
    62W will give Qc = 35.1 W (model CP1-12704)
    77W will give Qc = 44.5 W (model CP1-12705)
    93W will give Qc = 53.1W (model CP1-12706)

    Choosing CP1-12705 77W TEM seems suitable for the rated Qc. But in this design consideration , choosing a higher rated TEM at 77W to give a some excess cooling power to play with. TEM can easily turn off when the set point temperature of 24C is reached.

    dT = (1 - (heat load/max cooling power)) * max temp difference

    where
    heat load Qc = cooling requirement
    max cooling power = TEM rating
    max temp difference = TEM max dT under no load.

    For example, the 77 watt TEM can cool a tank with ambient at 28 C as follows:

    dT load = (1 - (40/44.5w)) * 68 C = 15.9 C

    Theoretical tank temperature = 28 C – 15.9 C = 12.1 C

    With an ambient of 28C, the TEM theoretically will cool the tank to 12.1 C. The actual temp will depend on how well it's insulated, power supply amperage, etc. I also took some shortcuts by using the TEM's spec rating, so actual temperature will be much higher.


    Step 3 : Choosing a suitable Heatsink.

    Heatsink performance is measured in °C/W (or K/W ) We refer to this as thermal resistance. For this design , the lower thermal resistance the better .An example for what these values mean: if a thermal load of 20W is applied to a heatsink, and this causes the temperature of the heat source to raise by 10°C, the heatsink has a rating of 10°C/20W = 0.5°C/W. What does this number mean? A resistance of 0.5 °C/W means that if one Watt of heat goes through the object, the temperature drops by 0.5 degrees.

    In this case a “forced Convention” type is heatsink is most suitable.

    Below shows the theoretical temperature of heatsink in the micro chiller using this equation ,

    THS = TA + RT Qh

    where
    THS = Heatsink Temperature
    TA = Ambient Temperature 28°C
    RT = Thermal resistance of heatsink = 0.15°C/watt

    Thermal load on heatsink Qh = Qc + Pin
    = 40 watts + (5 amps) * (15.4 volts)
    = 40 watts + 77 watts
    = 117 watts

    Therefore THS = 28°C + (0.15°C / watt) (117 watts)
    = 28°C + 17.55°C
    = 45.55°C
    That is to say , 45.55C is the temperature of the heatsink when the micro chiller unit when running. This heatsink temperature will play a part in the actually temperature in the tank.

    As a general guide for choosing a heatsink for the TEM , look for heatsink with
    - big heatsink with lots of fins and surface area.
    - the fan should be higher power rating. i.e. at least above 0.4amps


    Step 4 : Getting a Power Supply Unit (PSU)

    There are 2 common types of PSU on the market. Switch mode power supply and Linear Power supply. The latter is common know to us as the “set down transformer” . Switch mode is more efficient .At least 80% of the input power is converted while the linear PSU stands at only 60% meaning 40% is lost as heat.

    When choosing a PSU, look for one that at least 1.25 times the power of the chiller. The 77W TEM, would need a PSU rated at least 77W x 1.25 = 96W