Project using peltiers to cool water

[projectspotter]

Evening all,

As a quick run through I am looking to build an arduino controlled fluid cooling system that would make use of 4 peltiers/ TEC 12706 (12V / 60W). The minimum specification is:

• Can be used to cool 25 litres of water


• Have an operating range between 20degc-30degc


• Can reach target temperture +-1degc


• Adjust temperature within 60 minutes

The list of stuff I think I need:

• Power supply- will use lab one before switching to compact one


• 4 peltiers (TEC12706)


• 4 computer heatsinks with fans


• Water pump to pump water through the aluminium cooling heads


• At least 3 temperature sensors (on for input, output and main tank)- Can anyone offer links to arduino compatible temp sensors/ point in the right direction?
  

This is my first foray into arduino so I will start off by getting a peltier to simply turn on and off, then try to have control with ramping up/ down power, then trying to include sensor information. Once this has been achieved I would then try to employ a safety shutdown, followed by trying to get multiple peltiers to do the same thing at the same time. The final level of complexity would be if I could have a "polarity flip" to turn the cooling sides into heating, making it possible to also heat, as well as cool the water.

The cooling/ heating modules will be made out of 3 parts (in diagram)- the TEC module will provide the heat pump needed. This will be computer controlled. The aluminium cooling head will be fed by a "dumb" aquarium water pump. Finally, the heatsinks will be cooled using PC fans, again these would be dumb, simply on/off. IF I manage to get all of the control stated in the previous paragraph, I would consider looking into any control for these dumb parts.

Whilst this all sounds good in theory, I do have a problem- I do not know what type of arduino board to get. Any suggestions? Also, from reading a few different peltier systems and have seen they require "n- channel mosfets"  I don't have a clue what these are but from my understanding they protect the peltier's from the rapid on/off control of the microprocessor (correct?) the other question is surely there is some form of "you use this mosfet, for this amount of components etc."- I've had a good google and have started to at least grasp some concepts, but are there any useful must reads on the topic?

TLDR: Firstly, what type of Arduino board would I need (3 temperature sensors, 4 peltier modules, basic on/off button and maybe a LCD screen,) secondly any hardware I've overlooked, thirdly, any red flags/ pitfalls people can see?

Best,

Projectspotter  XD

[DVDdoug]

Any of the Arduino boards should be able to do it.   Your temperature sensors need analog inputs (one input per device). 

Any other "mode" switches (such as heat/cool or temperature up/down) will need a digital input.  If there is any kind of LED/LCD display, you'll need a digital input pin (more for a "more advanced" temperature display).

Your Peltiers can all work from one digital output if they all turn on-off together.  If you need to control 4 separately, you'll need 4 digital outputs.

If you need to control the fans independently from the Peltiers, you'll need one or more outputs for that.

In order for heating (3-states = hear, cool, off), you'll need one more digital output.  And, you'll need an "H driver" (4 MOSFETs plus a couple other things) to reverse the current.  (Or, you could use relays.)

Here is some information about MOSFETS.   You can't run a Peltier device directly from an Arduino output, because it takes too much power (more voltage and current than the Arduino can directly supply) .  The MOSFET is essentially being used as a switch that's turned on & off from a small electrical signal.   The power for the Peltier does not come from the Arduino.  You could use a relay in this application, since you are turning on & off the Peltiers at a slow rate.  If you choose to use a relay, make sure it can be run from 5V at less than 40mA because most relays won't work directly from the Arduino output, meaning that you need a transistor or MOSFET to power the relay coil.    (Most MOSFETs require more that 5V too, so make sure you get a "logic" MOSFET.) 

[radman]

The specific heat capacity of water is 4.181kJ/kg DegC.
A liter of pure water has a mass of 1kg at 4DegC.

You have 25l of water so call it 25kg, and you want to change the temperature by upto 10DegC.
You therefore need to extract (or add) 4.181kJ x 25 x 10 =  1045.25kJ of energy.

Since you have 60minutes or 3600s to do this in you need to remove (or add) energy at 1,045,250/3600 J/s = 290W

This answer sounded high to me so as a sanity check I looked at electric kettles and they seem to have a capacity of about 1.7l and a wattage of about 3kW.

Your spec. is for four peltiers at 60W each. It sounds like you might be a bit light on the power side they will not be 100% efficient, the environment will be heating your water as you attempt to cool it, and you will have more than the thermal mass of the water to cool.

Have I got this right?

[projectspotter]

The specific heat capacity of water is 4.181kJ/kg DegC.
A liter of pure water has a mass of 1kg at 4DegC.

You have 25l of water so call it 25kg, and you want to change the temperature by upto 10DegC.
You therefore need to extract (or add) 4.181kJ x 25 x 10 =  1045.25kJ of energy.

Since you have 60minutes or 3600s to do this in you need to remove (or add) energy at 1,045,250/3600 J/s = 290W

This answer sounded high to me so as a sanity check I looked at electric kettles and they seem to have a capacity of about 1.7l and a wattage of about 3kW.

Your spec. is for four peltiers at 60W each. It sounds like you might be a bit light on the power side they will not be 100% efficient, the environment will be heating your water as you attempt to cool it, and you will have more than the thermal mass of the water to cool.

Have I got this right?

Sir, you do seem to be bang on the money. What's slightly more worrying is that from a series of preliminary tests I've calculated that the actual power (as in heat removed from volume of water) I'm getting from a single module is actually more like 20.9W, if I need 14 of them to get these results I'll have to look for a different component. This value is calculated from a series of data points taken over a temperature range of 50-14 degc. This is however using a crappy little power 12v power supply which can only kick out 0.3mA when checked with a multimeter. I'm going into the lab tomorrow so will run some tests using a PSU that can give it the beans. Hopefully I can get a better heat extraction rate, if not, I'll have to hit ebay for some different poeltiers!

[mauried]

Peltier coolers in cooling mode are pretty inefficient, around 30% - 50% at best.
The issue is how well you can get the heat away from the hot side of the peltier
and the usual way of just connecting a heat sink with a fan doesnt work all that well
unless you have lots of air which means big fans.
Using a heat pipe is a better method , but makes the system more complex.