Diy peltier water chiller

[Bonnie Floyd]

I am not a physics or engineering person. But I read and teach myself all sorts of things. I am trying to grow strawberries in hydroponics. The hydroponic side is going well. Plants growing and blooming a little. My problem is with the water in the resevoir is too hot for the root systems.

I found examples online of chillers made from a peltier module and a gamers heatsink. I used a water block to pump the water across the cold side of the Peltier and return it to the reservoir.

It will work, but I built it way too small for my needs. I am trying to cool a 12 gallon reservoir which is setting outside. I am using a shade cloth over the whole set-up. I need it to kick on when the water heats up to 78 degrees and be able to chill it down by 10 degrees, then shut off.

How big of a peltier and heatsink do I need. I am running the parts on 12volts right now.

 

[Pommie]

Assuming you bought the most common peltier which moves 50W of heat, to cool 60L (12g) by 5.5C will take approx 60,000*4.2*5.5/(50*3600) = 7.8 hours. Not very practical as you'd need about 10. I think what you need is a proper water cooler - check out marine tank coolers or evaporation coolers.

Note also, the heat sink required on the hot side needs to be big and fan cooled as it needs to dissipate 110W without getting too hot.

Mike.

 

[cowboybob]

Welcome to ETO, Bonnie Floyd!

Taking Mike's info a step further:

You'll also need to overcome the average ambient temperature of the area surrounding the hydroponics tanks since that heat will be "un-doing" the cooling you will be applying.

Assuming your tank(s) is (are) insulated, this site (http://www.discount-hydro.com/active-aqua-water-chillers/) will at least give you an idea of the cooling needed for your application and region. I think you'll see that a peltier based solution would be considerably more expensive to build and operate.

 

[Bonnie Floyd]

Thank you both for such a quick response.

I have come to the same conclusion, peltiers are too small and inefficient. I have a cube refrigerator, like is used for
camping or in a vehicle. We were truck drivers and used it that way. I have an idea to make a coil of copper tubing, pretty
cheap at Lowes, and run it in the refrigerator. I have rubber grommets used to put tubing in things in hydroponics and they seal
very well.

I was incorrect in the temperature requirements. I live in Texas and it is almost July. It is hot! The reservoir is insulated,
but the water is too hot. So I have to do som I'm thinking of 1/2" copper tubing. I believe the cube is about 24" inside height. I hope that is enough
tubing to chill the water. I cant remember how cold the little box gets. I'll check that out tomorrow. I'll also check out
that website. Thank you for that. More news after next attempt.

 

[AnalogKid]

Referring to an earlier post, 7.8 hours sounds like a long time. but that is the worst case number. If the cooler kicks in when the water temperature is 73 degrees, then it has to remove only 1 degree. So the next questions are the flow rate and timing. Do you pump water only to irrigate? If so, what is the flow rate? If low enough, the TEC can cool the water on the way to the plants. Sure it might enter the cooler 10 degrees too hot, but the cooler is cooling only what is moving by, not the entire tank.

ak

 

[dr pepper]

Welcome.
Not quite my kind of thing however you can work this out with btu's, from memory 1 btu is the amount of energy to reduce the temp of 1 pound of water by 1 degree over a certain time.
You can approximate btu's into watts, google probably does it.
So if you know how much water you have, how much you want to cool it by and how quickly you can back calc that into watts, which will tell you the size of cooler.
Tec's like other marketed items are probably well overated, plus the advertised power rating is probably the input power, so choosing something bigger is better.
Small tecs are cheap in 5's or 10's and 12v ones are common, pc processor heatsinks which have a built in fan can also be got cheaply, and pc psu's are inexpensive too, so a multiple system like this might be a good way to go, plus you could switch in and out heat pumps to control the overall power.

 

[cowboybob]

1. You have 12 gal of water. That weighs 96 lbs (we'll use 100 lbs to ease the math)
2. 1 BTU will raise (lower) the temperature of 1 lb of water 1°F
3. 1 BTU (added or removed) requires 1055 Joules of energy (at 100% efficiency)
4. 1055 Joules is equivalent to 0.29 Wh
5. A 1/10HP electric motor consumes ≈75Wh.

In your case, to lower (and maintain with external heating) 100lbs of water 1°F would require ≈29Wh (29 watts for 1 hour or 1 watt for 29 hours).

Scaling the math (higher/lower temps, shorter cooling times or more water) should be easy.

The elephants in the room are cooling efficiencies and reheating effects (transfer losses), both which are very difficult to estimate. My SWAG would be a generous 50%.

Taking that into consideration, I believe the 1/10th HP unit I cited above (or an equivalent system) would work. Keep in mind that what you're considering will not be cheap, irrespective of how you achieve it.

 

[Bonnie Floyd]

Phew!! I said I wasn't in y'alls league!! I am sticking with my copper coil in the refridgerator. If I use 1/2"
pipe I think I can pump water through it inside the box and cool the water. Update and pictures soon.

 

[AnalogKid]

Assuming that the copper coil does not have a zillion thin fins brazed onto it like a car's radiator, you'll need a way to increase the coil's contact with the cold air. I recommend a 4.7" (120 mm) fan, either AC or DC, something around 100 CFM. Sit it anywhere inside the refrigerator and let it run with the door closed. The idea is to create as much air movement and turbulence as possible within the cold chamber to increase the cold air contact with the coil. It's called a stirring fan and it really does work.

ak

 

[Bonnie Floyd]

Thanks, AnalogKid! Good info. I have a computer fan, 3", is that big enough? They are cheap, I can get a bigger one. I am going outside right now to get some measurements.

 

[AnalogKid]

Any stirring is better than none. Start with what you have and see if you need more.

ak

 

[tcmtech]

One often overlooked source of a small but fairly efficient heat pump system is a common dehumidifier unit.

They have the correctly matched compressor heat exchange systems and fans all in one package and most are built to be as simple as possible which makes them fairly easy to tear down and modify to work as mini air conditioner units or water chiller units.

Just something to think about.

 

[dr pepper]

I have a mini dehumidifier that uses a peltier pump, good source of bits too.

 

[Colin]

Get some sheets of thin copper or brass and wrap them around the copper pipe by half a turn and keep them touching the pipe with rivets through the sheet.
This is the only way to get the transfer of heat you need with such a short length of pipe.
A fan in the fridge will also add to the transfer.
But don't forget a fridge at 300 watts is normally only on 30% of the time. You may be overtaking the fridge. Feel the compressor.