Author Topic: Peltier dumpload (Read 2040 times)

(unknown) · « **on:** June 10, 2007, 06:03:14 AM »

I just received 2 350 watt peltier modules from ebay. $20 each shipped. Yes they are very power inefficient but since I am going to be dumping power I might as well get some parasitic cold :-) I am thinking of mounting them in a 120 quart 5 day cooler to keep the all important beverages cold and the fridge door shut so it uses less energy. Going to water cool the hot side so some extra hot water or heat in the winter. Anyone ever run across a good control circuit for temperature? Probably will need to cycle back and forth to the dumpload still until I get the rest of the house lights wired for 12 volt.

ruddycrazy · « **Reply #1 on:** June 10, 2007, 01:53:52 AM »

Hi Mary,

Those sound like nice peilters

Now they do hog lots of current to operate properly and they don't like fast switching. Most importantly you need to dissapate the heat to let the cold side operate efficiently. Personanly I don't think they would be a good dumpload as the current draw would drop the battery voltage instantly causing the peltiers to switch on and off. As I'm on the laptop I dont have my bookmarks but if you google chemelec his website has some good info on peltiers.

Cheers Bryan

ghurd · « **Reply #2 on:** June 10, 2007, 06:28:42 AM »

I never saw one that big!

They take some massive heat sinks, ie: item 190121283447 is listed as 70W.

A muffin fan on the hot side will help. Maybe a laptop fan on the cool side?

Might want that on a LVD, set kind of high.

A friend made this circuit without any problems. Maybe replace the fan with a relay or bank of fets?

http://www.rason.org/Projects/fancont/fancont.htm

G-

Nando · « **Reply #3 on:** June 10, 2007, 08:13:39 AM »

MaryAlana:

With the Peltiers you need to choose either heating or cooling but NOT both,

Normally, to cool one side you need to reduce the "heat" in the other side and more reduction, better cooling on the other side.

The Peltiers do not like "snap" On power, it should be incremental and in reality they are current devices for best life, so your controller should be a variable constant current source, indeed you may turn ON/OFF but the best is variable sources.

If compared to a gas refrigerator with the same electric power, the gas wins by several factors, like 6 to 9 times more cooling capacity per watt.

The circuit offered by another member needs a change to insure that the switching doe snot go ON/OFF continuously and it is done by adding a resistor to generate a hysteresis band, also to change the small transistors for MosFets or to replace the motor with N type mosfets and place the Peltier from + to the Drain of the added N Fet Drain.

If you want to use this type of circuit, let me know to make the additions, though we need to know the type of thermal resistor needed for your circuitry.

Nando

SamoaPower · « **Reply #4 on:** June 10, 2007, 09:01:16 AM »

Mary,

Doing just that except I use mine for a little extra air conditioning. We can always use a little more cool here in the tropics.

It's shown with the cold side fan and shroud removed. It takes about 45 Amps at 13 Volts using nine 50 Watt modules and doesn't seem to mind 200 Hz PWM from a C40 controller. A small pump circulates the water to the hot side which gets used for dish washing. It's about 6" x 8" x 2".

I don't think that you would want to cycle it with a thermostat since that would remove the load from the turbine.

DanG · « **Reply #5 on:** June 10, 2007, 09:03:09 AM »

Wow - left this screen open during chores and the replies poured in...

Peltiers can not be counted on as dump loads - they are massively parallel devices which by design auto-protect the other junctions failing to an open circuit that eventually cascades into a dead peltier. Some are more reliable then others, but for unattended operation a current limiter for each module will extend their life; using both to obtain 300-350 watts is likely to give good service...

I got about five weeks of constant duty from a pre-assembled OEM module w/ heavy (mass is more important than area) heat sinks using a regulated & filtered PS, pre-assembled hopefully meant having the heat sinks tourqed to correct specs, could be 12 in-lbs or could be 10 ft/lbs depending and has to be evenly distributed across its surface planes ... 1/8th turns of each clamping fastener in a star pattern after contact is a good method. If adding them to a resistance dump load as a shunt any switching noise will wreak havoc on their junctions, some filtering is in order.

ghurd · « **Reply #6 on:** June 10, 2007, 09:20:40 AM »

Dan, 5 weeks and they are shot?

Mary, this was not intended as the only dump load, was it?

I thought it was more of a way to use a bit of extra power, but not the dump load.

G-

(unknown) · « **Reply #7 on:** June 10, 2007, 11:52:24 AM »

It is to be run in parallel with the dump load, hopefully use some mosfet pwm switching to set the cold point and how much current the dump load sees. Can't see the sense in just dumping excess as heat in resistors and cooling water/beer/pop is not a critical application so a failure wouldn't be disaster. Just need to make sure that in a failure the dump load switches in to take the current load.

(unknown) · « **Reply #8 on:** June 10, 2007, 11:54:40 AM »

Going to make a water cooling block for the hot side. Forced air is a no way with ones this large. And as I mentioned the extra hot water can come in handy :-)

(unknown) · « **Reply #9 on:** June 10, 2007, 11:58:53 AM »

Interesting, I have one of the Coleman coolers with one in it. Think it is going on 10 years old and has sat in the shed all summer long connected to a cheap cb type regulated supply. Manufacturers spec is 200,000 hour life but they do not like fast on/off thermal cycles. Thats why I was thinking pwm, as the peltier gets less duty cycle the dumpload gets more and vice versa.

(unknown) · « **Reply #10 on:** June 10, 2007, 12:01:46 PM »

Interesting circuit. 741 op amps are not very temperature stable themselves plus pwm is the best way to go I think.

Ungrounded Lightning Rod · « **Reply #11 on:** June 10, 2007, 05:07:27 PM »

Why not use a small, low-voltage, refrigeration compressor?

Much more BTU for the watt and doesn't mind cycling as long as you don't restart too soon after stopping (which is not hard to arrange).

SamoaPower · « **Reply #12 on:** June 10, 2007, 06:46:48 PM »

One reason is that if you dump via PWM the voltage at the compressor will be quite variable and sometimes low. The compressor wouldn't like that where the thermoelectric modules don't much care.

Bruce S · « **Reply #13 on:** June 11, 2007, 10:07:08 AM »

MaryAlana;

To help with the wishes to keep those important items cool here is a very nice write up that has gotten very good results for those who have tried this.

http://misc.hackaday.com/2005/07/04/peltier-beer-cooler/

From past experiences, I would make use of a lrage Al heat sink for the hot side and use copper for the water jacketed thermal siphon. You won't get it as hot as possible using it in the cool mode, but with this setup you will be able to bring the water up to warmer that cool.

These units are also switchable , by a very simple trick of reversing the leads going to it. This would help the hot side become the dominant side. You could in the winter then use to outside cold to keep it cold and thus making the hot side stay hot.

Make sure and use a good silicon grease between the Al and the peltier it is ceramic and may not look like it , but full of holes making for a not-so-smooth surface. We smear the grease on the ceramic and then using a small twisting motion , make sure the grease is fully in contact with the Al, and only a thin layer is needed, in this case more is not better.

Question? are they 12 voltage rated? using a small constant current control to keep them at the desired voltage and current you could certainly extend their life. We have built a few small table top beer can coolers and they've lasted 2 years now. We use the NiCd sub-c batts setup for the rated voltage and relatively sized for the current and then let Ghurd's up coming dump controller charge them.

Nice package. Power hogs ...sure.... but so cool and the softball games(:->.

Those newer AAs with 2500mAr could be used to setup a very nice voltage level them just add enough to take the dump and use a constant current to keep your Peltiers happily cooling or heating.

Hope this helps

Bruce S

(unknown) · « **Reply #14 on:** June 11, 2007, 11:06:39 AM »

15 volt rated at 30 amps each :-) so small batteries are out. Any hot water is a maybe side benefit. Cooling to keep beverages cold and the fridge door shut is the main objective. Hot side is going to get a copper waterblock, cold side a small aluminum heatsink with fan to help move the cold around.

Bruce S · « **Reply #15 on:** June 11, 2007, 02:12:30 PM »

If you don't mind going down this train of thought, I will go further...

Since these are 15V max volts. and most battery packs are 12 which while in charging mode is actually 13.8 at a full charge. You could actually build a dump controller that is setup to charge rechargables :-) they can handle it and they won't die if you take them down with only 20% left, actually I have taken them down to 90% empty with no seeable damage, just long recharge times..

Since they at rest 1.25V, then it would only take an even dozen of these to link together for the proper voltage. The NiCds are known for keeping their voltage levels until the bitter end:-) with todays levels both NiCds and NiMh could be a good choice. You could setup a dump controller that outputs 15V with a constant current to charge the Batts and using the same type of controller run the pelts off the voltage at a rate of constant current. The numbers you have are actually the max and can certainly run them at a current rating much lower for longer battery life. Using a small thermysistor you could also turn off the cooling to allow battery recharging. The same circuitry could have a timer setting to not allow sudden turn on once the circuit has turned off once a temp has been met. I have not had any problems turning them off and back on with a timer set at 120 second time outs.

Just a thought

Bruce S

(unknown) · « **Reply #16 on:** June 11, 2007, 08:39:29 PM »

take to many extra batteries that cost a lot of money. The C60 charge controller I have is pwm so I could use that to drive the mosfets to regulate the peltier. Maybe use a timed thermostat that switches to the dump load when the cooler reaches 35 degrees or so.

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