Login |  Register |  FAQ
   

Post new topic Reply to topic  [ 6 posts ] 

Peltier requirements for fastest cooling...

 Post subject: Peltier requirements for fastest cooling...
PostPosted: Mon May 02, 2016 3:14 am 
Offline

Joined: Mon May 02, 2016 2:58 am
Posts: 3
Dear TEC fanatics,

First I've tried the TEC Calculator but it doesn't let you enter custom TEC values...

I have two TEC rated a Qmax 95w Imax 11 Vmax 15.4 dTmax 68C 40x40x3.3mm in parallel

The hot side is cooled at a stable temperature of 10c

I have to cool 250ml of water as fast as possible to 0c

Image

My current time to cool to 0c 250ml of water is 4 minutes.

I've noticed my PSU only give 11v to the TEC (mesured 8.5 amps)... will buying and adjustable psu and increase to 15.4v cut in half the cooling time?

Also how does it work with the heat load of water ?

Many thanks for your help :)


Top
 Profile  
 
 Post subject: Re: Peltier requirements for fastest cooling...
PostPosted: Mon May 02, 2016 5:16 pm 
Offline

Joined: Fri Jan 23, 2015 7:42 am
Posts: 18
Location: Sanford, FL.
The specific heat of water is 4.197 Joules per gram per degree K (or C). At 250ml, you have 250 grams of water since water is the basis of the definition. To take your water from room temperature (say 23degC) to 0degC will require the removal of 4.197 * 250 * 23 = 24,133 Joules. And that's assuming your container is perfectly insulated which is, of course, impossible.

Since a Watt is one Joule per second, and it's taking you 4 minutes (240 seconds) to move the temp to 0degC, you are cooling at a rate of 24,133 / 240 = 100.55 Watts ... again, if your insulation were perfect.

I assume you're also pumping the water to circulate it through the cold side heat exchanger. The act of pumping using a centrifugal pump will heat the water. So you're fighting that as well as heat infiltration from the surrounding environment. Of course, it looks like your pump is actually submerged in the cooled water container so in that case you can calculate the power being delivered to your pump (volts x amps = Watts) as being all added back to the water and not just that transferred by the pump impeller.

Also, getting to 0degC with regular water presents some challenges. Since you're pulling the hot side of the TEC to near 10degC, your cold side is probably well below freezing. I would bet that you're getting ice buildup inside of your cold side block and it's is restricting your flow and your efficiency.

In any case, at 11v and 8.5 amps you're driving the TECs each with 93.5 Watts. At that power level, not having the datasheets handy, I suspect you're getting 60 Watts of cooling or so from each TEC. Unless you're saying that 8.5 amps is shared between them. Then you're only driving 93.5 Watts in total, which seems low to be able to get the water to 0degC in 4 minutes. Unless, of course, it started at something lower than room temp.

I would be hesitant to drive the TEC's too much harder. The COP will go down and you'll be left with diminishing returns. In general, I wouldn't drive a TEC to more than 85% of it's rated power due to the rolloff in efficiency. But each situation varies. You could probably get them to 13V and still be drawing good heat from the water.

Given the icing in your pictures, I'm thinking the inside of your cold side block is nearly frozen closed.

Do you have the TEC's stacked together between your heat exchangers? Holy cow, that's going to be difficult to calculate. You'd be way better off running them side by side than stacked. You don't get to double the cooling power by stacking them. You get a higher deltaT but at a much lower efficiency.

Get two more blocks and split the water flow between them with each TEC doing 1/2 the water.


Top
 Profile  
 
 Post subject: Re: Peltier requirements for fastest cooling...
PostPosted: Tue May 03, 2016 2:35 am 
Offline

Joined: Fri Jan 23, 2015 7:42 am
Posts: 18
Location: Sanford, FL.
Oh, I see. You have two hot side blocks with a TEC between each and the one cold block. So ignore my last paragraph above. It was hard to make out at first due to all of the ice buildup.

So yes, I suspect you're getting about 120 watts of cooling and you're losing some of it to a lack of insulation and what looks like the pump submerged in the liquid ... a small pump, maybe 5 Watts. I think my estimates hold up in that case.


Top
 Profile  
 
 Post subject: Re: Peltier requirements for fastest cooling...
PostPosted: Sat May 07, 2016 8:23 am 
Offline

Joined: Mon May 02, 2016 2:58 am
Posts: 3
Thanks again for the reply! :)

The picture is a bit confusing, I was pumping absinthe alcohol (55% abv) to see how low it can go (-28C)

I uploaded the picture just to give an overall view of what kind of setup with the water blocks I have.

For some reason, I never received the email that warn of new post on that thread so I purchased the 15V adjustable psu but like you said the performance decreased at maximum 15.4v / 11 amps each TEC (2x) so instead of 4 minutes (23C->0C) I got 5 minute 20 secs!!!

I think I need to upgrade the TECs to double the qmax to cut time in half?

I looking at some nice 24V 172W Qmax 40x40 Datasheet----> http://tetech.com/wp-content/uploads/20 ... .4-0.8.pdf

What do you think?


Top
 Profile  
 
 Post subject: Re: Peltier requirements for fastest cooling...
PostPosted: Sun May 08, 2016 6:33 pm 
Offline

Joined: Fri Jan 23, 2015 7:42 am
Posts: 18
Location: Sanford, FL.
Those are nice, just make sure you're power supply can push them up there. Seems like a fun experiment.


Top
 Profile  
 
 Post subject: Re: Peltier requirements for fastest cooling...
PostPosted: Sun May 15, 2016 4:31 am 
Offline

Joined: Mon May 02, 2016 2:58 am
Posts: 3
bbragg wrote:
Those are nice, just make sure you're power supply can push them up there. Seems like a fun experiment.


Thanks I've bought 2x slim Mean Well LRS-350-24v 350w for each peltier is enough :)

Hey where is everybody?

No answer from owner too!


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 6 posts ] 


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  

cron

Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
CoDFaction Style by Daniel St. Jules of Gamexe.net