Peltier Help - (Solar Powered Cooler?)

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Peltier Help - (Solar Powered Cooler?)

Postby PseudoNym01 » July 27th, 2011, 3:31 pm

Hey All,

I saw this video - http://www.howcast.com/videos/470062-Ho ... olar-Power and it looked like a project I wanted to undertake myself however I couldnt help but notice that near the end the guy has managed to fit two beers into his tiny cooler which seems to be a bit of a waste to me. I was wondering if this project could be scaled up to bigger cooler, as in one big enough to encompass a six pack (to start with :D).

My question to the hackaday community a large is what size peltier do you think I should go with? I could probably run the numbers on the physics but what the numbers should and what actually happens is usually what screws me over a lot of the time. I realize I wont get my six pack down to 33 degrees (fahrenheit) but I would like to keep an approx 8in x 4in (32 sq inches total area) at a decent drinkable temperature.

Lastly I would like to be able to use solar power on this so while I realize I could get a massive sized peltier plate and make it work I would like to contain everything to the size of the cooler and keep as low a budget as possible.

I know Im kinda asking for the moon but still in the planning phase so thoughts/comments/suggestions would be helpful.

-Psuedo
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Re: Peltier Help - (Solar Powered Cooler?)

Postby k-ww » July 27th, 2011, 4:09 pm

Not to dash your hopes, but it's best that you research just how much power you can get out of a solar cell array and how much power even a small peltier needs - [don't forget that there is heat leakage thru the walls of the cooler]
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Re: Peltier Help - (Solar Powered Cooler?)

Postby UAirLtd » July 27th, 2011, 4:29 pm

Ah, beer-related hack, good choice!

Ok, so I have had some experience on the topic of peltiers and solar panels that I'd like to share. First off, let me run through some solar panel calculations.

I'm guessing on average in the summer you'll be getting solar insolation of about 1kW/m^2, and low-cost solar panels that you might be able to get relatively cheaply is about 15% efficient. This indicates you'll be getting about 150W of electrical power per square meter of solar panel.

Your average low cost peltier device is rated at 120W, so very roughly, you're looking at a square meter of solar panel per peltier you run at full power. (Or you could go with a 60W peltier hand use half a square meter). This would mean the panels would make up most of the cost of the system if you use them for power.

Because peltiers are so cheap, my advice to you is to just use as many as you can afford to power.

For example if you only have budget for half a square meter of solar panels, use one peltier.
If you are ok with carrying around a car battery (15kg), then you can power 7 to 10 peltiers (rough estimate based derated peltier power rating when running at 12 - 13V, typical capacity of car battery, and maximum safe continuous discharge current) for an hour, or one peltier for several hours. (Or you could get a smaller sealed lead acid battery (2kg) and power one peltier for an hour or two).
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Re: Peltier Help - (Solar Powered Cooler?)

Postby PseudoNym01 » July 27th, 2011, 5:13 pm

Hmm what about the usb powered drink cooler as in disassembling that? Also is one peltier is going to cool what amount of area? Maybe add in a PC Case fan? And I was planning on working with something similar to the solar panels featured in the movie. Whatcha think?

-Pseudo
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Re: Peltier Help - (Solar Powered Cooler?)

Postby PseudoNym01 » July 27th, 2011, 5:22 pm

Also @ k-ww I understand Im just trying to put out a feeler for some info from some more experienced types.

-Pseudo
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Re: Peltier Help - (Solar Powered Cooler?)

Postby PseudoNym01 » July 27th, 2011, 5:24 pm

UAirLtd wrote:Because peltiers are so cheap, my advice to you is to just use as many as you can afford to power.



Just curious what is your definition of cheap and where do you get your peltiers?

-Pseudo
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Re: Peltier Help - (Solar Powered Cooler?)

Postby Osgeld » July 27th, 2011, 9:25 pm

PseudoNym01 wrote:Also is one peltier is going to cool what amount of area?


about 1 can in time hehe, your probally going to need some form of battery, car has been tossed out there but lawn mower and motorcycle? also peltier coolers are not magic, one side gets cold, but less than a ~dozen MM away is a scalding hot side, in order for max use you need to heatsink and possibly fan the hot side while keeping it isolated from the cold side

Best idea is out there already for decades at truck stops, hooks up to a car battery, peltier the size of your face and a chunk of finned aluminum bolted on the outside. if nothing else its worth a look.
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Re: Peltier Help - (Solar Powered Cooler?)

Postby UAirLtd » July 28th, 2011, 4:45 am

Cheap like $12 on ebay (120W device)

USB powered drink cooler not good - power rating on a single USB 2.0 port is 2.5W, can't imagine a device rated for USB power is going to do much good with a six-pack.

You definitely want a heatsink/fan on the hot side - peltiers pump heat from one side to the other, whatever heat gets sucked out of your beer will end up on the hot side PLUS the electrical power used. e.g. if you use a peltier at 120W (electrical), and it's sucking up approximately 50W of heat from the beer, it will chuck out 50W+120W of heat out the other side that you'll need to dissipate.

If you let the hot side gets hot, the amount of energy transferred by the peltier is reduced (less cooling for the same energy usage), and if it gets too hot, the thing breaks.

I can supply some calculations for you, see next post
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Re: Peltier Help - (Solar Powered Cooler?)

Postby UAirLtd » July 28th, 2011, 5:30 am

For cooling a six-pack in a well insulated container, it would depend on how long you want to wait. Here's some calculations:

Say you've just returned from the store with a six-pack that was kept at 24 degrees (approximately the temperature you might find indoors on a warm day), and you want to get a six-pack down to a refreshing 4 degrees (sorry, I can't do these calculations in Fahrenheit). You must therefore reduce the beer temperature by 20 degrees centigrade or kelvin.

Volume of an American tin can is 0.355 litres, six of these is 2.13 litres. Assuming beer is mostly water with a c of 4.12kJ/l K. Thus the amount of energy you need to remove from 2.13 litres of 4.12kJ/l K to reduce its temperature by 20 K is 2.13*4.12*20 = approx 175 kJ

And now this is where I make a lot of approximations. Say you slapped an inexpensive computer heatsink/fan which was designed to keep a 100W thermal load under 50 degrees. So I'm going to make the big assumption here that the hot side of the peltier is at 50 degrees. If the cold side of the peltier is going to be between 24 degrees and 4 degress, I'm going to approximate the average temperature gradient to be 35 degrees.

The datasheet for a cheap 60W peltier shows the thermal exchange around about this point is about 30W (again these are approximations), this value is fine because this means 90W of heat to be disspiated by a heatsink/fan (this indicates a 120W peltier needs a really beefy heatsink/fan).

So 30W of cooling power to transfer 175kJ takes 5833 seconds or one and a half hours. (I'm assuming your cooler container results in only negligible thermal transfer, typically a good cooler should only result in a few watts). If you used a 120W peltier with the same heatsink, the hot side will run hotter, so you might only get 45W of cooling.

So 1hr 30 minutes to cool a six-pack with a 60W peltier, maybe about 1 hour with a 120W peltier.
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Re: Peltier Help - (Solar Powered Cooler?)

Postby asheets » July 28th, 2011, 10:38 am

Maybe this article will help a little to give you some context... http://www.afrotechmods.com/fuxx0red/sn ... nooler.htm
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