Peltiers... what pains... help please!

[Electric Rain]

Hello, I have been interested in using thermoelectric coolers, (Peltiers) for many applications. But, I don't know how to control them. I hear that simple on and off switching can be damaging to the module. But how else can I control it? If I use any kind of thermostat, I can't have it be anything too complex.

The application that I want to use one for now, is water cooling. I will be cooling about a 1/2 gallon of water with a 70 Watt module. Well, I'm gonna try at least... but I don't know if it will cool it enough, too much, or waste too much power. This is going to be drinking water, so the temerature needs to be from about 50-60 degrees Fahrenheit... or so. (Or 10-15 degrees Celsius). The water will be in a metal dish and I want to mount the module on the bottom of it. So, really I'm asking for the answers to the following questions:

1. How can I control it to keep the water temperature around 50-60 degrees Fahrenheit?

2. Will a 70 watt module be cold and powerful enough to cool a 1/2 gallon of water?

3. Will the module be overworking itself and killing my electric bill trying to keep the water cool?

4. Generally, how cold are these coolers? Can I even use them for applications like these or are they more for cooling electronic parts and stuff like that? If so, then what should I use to cool this water? I don't want to mess around with chemicals like freon, that's why I was so happy when I found these modules.

Can anyone please help me? :? Thanks.

Rain

Edit: P.S. Hey! This is my 300th post! Woo hoo!

 

[Klaus]

The peltier module I have played with and used for a project measured about 40x40x3 mm and was rated at 75W, from memory.
These things are as simple as it gets.
Supply power to it and one side cools while the other heats up.

Now, if you do that in free air (or, as you plan, by placing the module under a container) you'll be *very* disappointed by the results :wink: .
Why? The cold from one side will immediately get equalised by the heat from the other.
You HAVE to physically seperate the air at both sides so you will have to mount the module so it becomes part of an enclosure - like a cooler box.

Usually the hot side is mounted on a large heat sink, fan cooling this will improve its performance.
The cold side fits against a metal plate inside the cooler to radiate the cold (can one 'radiate' cold? - you know what I mean :wink: )
AND in between the two you should have as much insulation material as possible. The thicker and the better its thermal coefficient the better.

This last requirement is the hard part since the module is so thin, makes for some tricky engineering to get a reasonable efficiency. Have a good look at how peltier coolers are constructed and go from there. There is room for improvement as these coolers are built to a price.

I controlled it with a temperature sensor, turning the power on and off. Due the thermal lag of a good cooler this cycling is not very rapid at all.


Now, for your application, I think Its not going to be satisfactory if you drink a lot of water. And, its rather power inefficient when compared to other refrigeration methods. Its neat though .
The best way to use peltier coolers is to place things into them cold (or hot) and use the cooler (warmer) to keep the contends at this temperature. If you try to use it as a fridge it'll be disappointing.

If I remember right the max hot/cold plate temp difference is around 40 degrees C. The modules can be cascaded for bigger differences but at the price of even more complexity.

Have fun
Klaus

 

[Electric Rain]

Hmm... great... well, what do you think I should use instead then? :? Any suggestions? :?

Thanks for all of that info, helps me a lot.

Rain

Edit: Also, what are they good for it the cooling side is worthless anyway?

 

[bmcculla]

Edit: Also, what are they good for it the cooling side is worthless anyway?

TE coolers just move heat from one side to the other. The problem with them is that they also heat up like a resistor. So if you don't have a heat sink to dump the heat from the resistive heating the whole module will heat up - the cool side will still be cooler than the hot side but both will get quite warm.

What you are talking about would work if the water container was really well insulated but you probably would have to wait 10 or so hours to cool the water that much. I read about someone who used 2 90W coolers to cool a soda can. It eventualy was partialy frozen but it took about 12 hours.

About your electric bill: its easy to compare if you remember that an average light bulb will be 60-100W.

The coolers can get quite cold if you stack 3 of them together. Supposedly you can get down near the temperature of liquid nitrogen but that would only be for a very small area. An average cooler can get about 50 degrees C below the temperature of your heat sink if I remember correctly.

Brent

 

[TillEulenspiegel]

You have to remember thermodynamics says that intrinsically heat moves to cold , so its not really efficient, kinda like skiing uphill. They actually make small refrigerators with both sides being thermally sinked.and normally ( in the case of the fridge) cool air is circulated.

The most common use for them is direct contact with a CPU. The power specification depends on a few factors. Is the 70W a consumption figure ( of the device itself ) or a dissipation? The size and area of the heat and cold sinks , if you use a fan, what size is the enclosure............lota variables. I suggest start with your design parameters and the google for a device that fits your needs.

 

[Electric Rain]

Hey, what if I use a few of these? **broken link removed** :twisted: :twisted: :twisted: THAT'LL WORK!!! *sigh* I'm not sure what I'm gonna do. Alright, would it be safe for me to use some kind of chemical like freon? Now, I'm willing to do anything to cool this 1/2 gallon of water...

Edit: Hey, wait a sec... what about this power house? **broken link removed** Would that work? :?

 

[Nigel Goodwin]

Hey, what if I use a few of these? **broken link removed** :twisted: :twisted: :twisted: THAT'LL WORK!!! *sigh* I'm not sure what I'm gonna do. Alright, would it be safe for me to use some kind of chemical like freon? Now, I'm willing to do anything to cool this 1/2 gallon of water...

Why not just buy a fridge?.

The other two peltier devices you listed are basically the same as the original one, just higher power - you would still need to remove the heat from the hot side. By all means get one to play with, and see how you get on - as already suggested a heatsink and fan on the hot side, and insulation preventing the heat radiating into the water, would be a good idea.

 

[Electric Rain]

Yeah, you know what? I think I'll look into buying a nice, powerful one and play around with it a bit. I don't like to do that most of the time, I'm on a tight budget, (nice hobby to get into with low cash eh? :lol: ) so most of the time I like to make sure everything's planned out and I know exactaly what I need before I just start buying stuff. So as to save money. But shoot, if this doesn't work out, I can just slap a heatsink and fan on it and put the cold side on my cheek when I start to get hot and sweaty working outside. :lol:

Rain

Edit: Two more things though, what kind of adhesive/compound should I use to mount the heatsink? And two, do I NEED a coldsink? Or you know, do I need to mount the cold side on anything?

Second edit: Also, Nigel, a fridge won't do for what I need to do this for. But I can't share it. Someone might steal my idea and start making money on it. Gotta keep my ideas in my head and to my closest friends. :wink: Because... there's no telling, I might be able to sell them one day.

 

[chemelec]

Firstly, There is No Problem using PWM to control Thermoelectric Modules. I have been doing it for Many Years.

Second, the problem is Getting rid of the Heat on the Hot Side.

Where are you using this? Inside your house or outside?
What is the Maximum Ambient Temperature?

You can Contact me direct if you want. chemelec@hotmail.com
You MUST PUT AN APPROPRIATE SUBJECT LINE on your E-mail.

Take care.........Gary

 

[McGuinn]

The efficiency of a Peltier is determined somewhat by the ability of the heatsinks to conduct. A free air, passive heatsink, can manage a delta of ~20 degrees C, while an active (fan powered heatsink) can achieve up to about ~30 degrees C.
Using a copper block (shunt) of the same dimensions, cemented onto the Peltier, can increase the distance between the hot and cold surfaces, allowing better insulation between them. It is better to do this on the hot side, as otherwise, condensation may build up and cause in-efficiency due to the latent heat of condensation.

A peltier can easily achieve a drop to about -10 degrees C in a small space, provided the ambient temp is ~20 degrees C, and the hot side is cooled using an active heatsink.

Using an active heatsink on the cold side, can causes loss of efficiency also, as a lot of the power can go into drawing the heat away from the electric fan. As the fan motor cools, it may also tighten the bearings, causing more heat to be created, starting a vicious circle!

Anyhow, they are my €2 cents.

 

[evandude]

yes, peltiers can be controlled via PWM there is one hitch:
you have to keep the PWM frequency above a certain level. at a high frequency, PWM just looks like a variable voltage to a peltier, however at a low enough frequency the peltier starts to "see" the on/off switching, and that is bad for it.

hmm, maybe we should all start placing guesses as to what your big secret project is... to me, since you're cooling a somewhat small amount of water in a metal bowl, it sounds like it's a water dish for a dog or cat... (probably a dog if it's a half gallon, or a lot of cats... or a cat the size of a dog :wink: )
or making a chilly bird bath for those hot summer days?

 

[Electric Rain]

hmm, maybe we should all start placing guesses as to what your big secret project is... to me, since you're cooling a somewhat small amount of water in a metal bowl, it sounds like it's a water dish for a dog or cat... (probably a dog if it's a half gallon, or a lot of cats... or a cat the size of a dog :wink: )

:shock: Yikes... you got me... you're right. But I can't reveal any more of this to you guys. Listen, don't any of you guys steal my idea!! As soon as this thing starts showing up on TV I'll be showing up and all of your doorsteps first. :evil: And also, I'm using it inside. For 3 dogs and a cat. :lol:

Rain

P.S. Really, I can't trust one of you guys, or people that wander in from the net that aren't even members... but please, just, keep it to yourselfs and don't go trying to sell this. :cry: I should copywrite it...

 

[Oznog]

There's a lot on the web about using Peltiers. For each one, you just look for how many watts of heat it will pump with a given temp difference between the hot & cold side. Typical small ones pump around 20 watts with no temp difference, and 0 watts at 40 deg C difference (the max difference limit). This is not great, and cascading them is far, far worse. Cascading is only useful if a temp needs to be achieved for a special purpose with especially low temps, like a video imager, where only tiny amounts of heat need to be pumped.

Note that the difference is between the hot & cold side of the module. Even a great heat sink is going to be hotter than ambient, and there's a thermal impedance between it and the hot side of the module, and there's a thermal impedance between the cold side & the water too.

Assuming good heatsink design, the end performance is going to depend on how well insulated the cold water is, how cold you want to make it versus the ambient temp, and how fast you hope to cool down new water. It is difficult to say. The quickest calc is to ignore the insulation issues and look at how cold you want the water, the specific heat of the volume of the water, versus how many watts the device can pump.

Peltiers are great, but be aware they're fundamentally much less efficient than traditional compressor systems, less powerful, and have lower capabilities as far as temp differences.

Powering via PWM is not preferred, a constant voltage minimizes i^2*r heat generated in the module. It is more of a problem if you have high currents and short pulse widths.

Mounting the module is also tricky. The module can't support the heatsink in itself. If the temp variations are high enough, gluing the module to the hot or cold side can result in cracking from thermal expansion. So usually you want to bolt the high side to the low side and use thermal grease, but of course then there's thermal conduction between them through the mounting bolts themselves. Thus nylon bolts, thin stainless bolts, or thermally insulating nylon washers come into play. There's a lot of specs to look at if you want to maximize the pressure to minimize thermal impedance, but without cracking the module

 

[chemelec]

yes, peltiers can be controlled via PWM there is one hitch:
you have to keep the PWM frequency above a certain level. at a high frequency, PWM just looks like a variable voltage to a peltier, however at a low enough frequency the peltier starts to "see" the on/off switching, and that is bad for it.

I agree, but by simply putting a reasonable Filter Cap across the output of the PWM, it will even out the spikes and make it appear as a more even voltage.

I do this with very good results.

If your purpose is to use this as a Cooler for Pets Water Dishes, I doubt it will Fly. Too much Heat Loss to the air.

Take care...........Gary

 

[Electric Rain]

:roll: Yeah, I guess your right. If only peltiers were stronger. I guess I need to look into some other kind of cooling. *sigh* Well, what would consist of freon cooling? :?

Rain

P.S. Hey... does anyone know how to pronounce the word "peltier"? :lol:

 

[McGuinn]

P.S. Hey... does anyone know how to pronounce the word "peltier"? :lol:

Yeah, it's French so...
Pelt-eea.
'Pelt' like an animals fir, and drop the 'r' at the end to sound like you are saying 'e' and then 'a'.

 

[Electric Rain]

Wow, I'm glad I always call them "thermo-electric coolers" whenever I have to talk about them out loud. :lol: Never would have known that's how to say it. Thanks.

Rain

 

[Nigel Goodwin]

Wow, I'm glad I always call them "thermo-electric coolers" whenever I have to talk about them out loud. :lol: Never would have known that's how to say it. Thanks.

His description was excellent! - it's how I've always pronounced it as well, so perhaps school days French had some use after all? :lol:

 

[Electric Rain]

Anyway, anything I can do easy enough with freon? :? Also, should I move the freon talk over to a new thread since this one started about peltiers?

Rain

 

[Nigel Goodwin]

Anyway, anything I can do easy enough with freon? :? Also, should I move the freon talk over to a new thread since this one started about peltiers?

I didn't think you were allowed to use Freon any more?.

But assuming you were wanting to use it's modern (ozone friendly) replacements, it's simply a fridge!. You need a compressor, tubing, and a radiator to get rid of the heat.