Peltier device

[meatpie]

This one is to try to help a retired mate.

He builds bird brooder boxes to earn some very much needed extra cash.
The boxes are constructed from plywood and he makes them in 6 sizes. They are designed to be either plugged into 240v or used in the car with an inverter. Up till now, he has used heaters of various types, lamps, elements etc. Now he's trying to create a fully portable 12v model using a peltier device, so that it will heat in winter and cool in summer(or in a hot car). He is controlling the peltier with an electronic thermostat.

The problem is : The peltier will heat quite well but on cooling, the temp will not drop any more than about 2 deg. below ambient. It should be able to freeze moisture on the cold side.

The peltier is sandwiched between 2 processor heatsinks, with a fan on the hot side.
Heating or cooling is achieved by the direction of current flow. I have checked it and he has wired it correctly.

Does anyone have any experience with these here peltiers ?

Meatpie

 

[kchriste]

TThe peltier is sandwiched between 2 processor heatsinks, with a fan on the hot side.
Heating or cooling is achieved by the direction of current flow.

If he is reversing the current through the peltier, then the fan is not always on the hot side. Try using fans on both sides because it is important to distribute the cold as much as it is the heat.

 

[meatpie]

If he is reversing the current through the peltier, then the fan is not always on the hot side. Try using fans on both sides because it is important to distribute the cold as much as it is the heat.

Oh sorry...What I meant to say is that he does have fans both sides. In my interpretation I was trying to emphasize the need satisfied for the fan on which ever side is hot.

As for distributing the cold, that's not a problem as the circulation from the heatsinks and through the brooder box has been well designed. It's the fact that the cold side doesn't get cold in the first place.

 

[Hero999]

It depends on how hot it get in the car and how well insulated the box is?

You location is Australia so I wouldn't be surprised if it gets to >60°C inside a car unless you're in the mountains, which is hot enough to cook.

Peltier coolers are not very efficient, compared to traditional phase change coolers (the kind used in normal air-conditioners or freezers), they're just lighter and cheaper on a smaller scale.

 

[meatpie]

Thanks for the reply

It depends on how hot it get in the car and how well insulated the box is?

The box and airflow has been well designed and proven on past constructions. We are measuring at the heatsink, not the air some distance away.

You location is Australia so I wouldn't be surprised if it gets to >60°C inside a car unless you're in the mountains, which is hot enough to cook.

The box is not in a car yet. We are looking at the peltier and heatsinks arrangement on the bench and not even in the box, at the moment.
1 peltier, 2 heatsinks, 2 fans, 12v power supply, that's all.

Peltier coolers are not very efficient, compared to traditional phase change coolers (the kind used in normal air-conditioners or freezers), they're just lighter and cheaper on a smaller scale.

We know they are not very efficient, but there are 12v portable cooler boxes (esky) available that use the exact same setup that we are trying to use and they will near freeze a drink can.
We are trying to understand why my mates configuration doesn't work.

 

[kchriste]

there are 12v portable cooler boxes (esky) available that use the exact same setup that we are trying to use and they will near freeze a drink can.
We are trying to understand why my mates configuration doesn't work.

Most cooler use stacked Peltiers to get the required temperature differential. Maybe your mates doesn't have enough in the stack.

 

[meatpie]

Most cooler use stacked Peltiers to get the required temperature differential. Maybe your mates doesn't have enough in the stack.

Hmm...well the 3 units that I have pulled apart in the past only had 1 in them, and they seamed to work fine.

The stacked ones would prob have around 50w peltiers in them. They are available in a wide range of wattages. The one he's using at the moment is 170w, it should definately be able to drop the temp more than 2 deg below ambient. Can't touch the hot side heatsink base but the cold side does nothing.

He only want to cool the birds a little on a hot day, not freeze them, so 1 device should be plenty.

I have seen a hack site where a guy used 1 device in his car centre armrest to keep his drink cold. He used a small peltier and 1 small heatsink with a fan on the hot side only.

 

[imix500]

A 170W device will need a very large heatsink. Are you sure you have enough? Unless you are using one of the large Xeon or Mac Pro units a cpu heatsink may not cut it.

 

[Boncuk]

Can't touch the hot side heatsink base but the cold side does nothing.

That might be the problem.

Peltier elements produce a temperature difference of 65deg/C between hot and cold plate.

If the hot plate gets to e.g. 80deg/C the lowest temperature will be 15deg/C on the cold plate not considering losses.

I suggest to use 3 to 4 elements (20W each) and large heatsinks (not those small CPU heatsinks) to get the hot plate to a temperature of 70deg/C resulting in a cold plate temperature of 5deg/C.

Use PWM for the plates to achieve the desired temperature on the cold side.

I used a 35W peltier to cool my wireless modem using a large tunnel type heatsink and a small fan blowing through the tunnel and got the modem frozen up well at ambient air temperature of 35deg/C.

Boncuk

 

[meatpie]

That might be the problem.

Peltier elements produce a temperature difference of 65deg/C between hot and cold plate.

Thanks Boncuk. That's something I didn't know
Unfortunately there I go again not explaining properly. When I said can't touch the heatsink, I meant when, one time, we tested without the hot side fan. Silly me.
After fitting the fan, we measured the hot side heatsink at 42 deg. at the base, using a quality IR thermometer, and with the fan running. This was on a 28 deg. day, and the best we could get the cold side was 26 deg.

I suggest to use 3 to 4 elements (20W each) and large heatsinks (not those small CPU heatsinks) to get the hot plate to a temperature of 70deg/C resulting in a cold plate temperature of 5deg/C.

The heatsinks he used are not small. They are nice and solid about 85mm square. Like for pent 4, 2.8 gig.

Use PWM for the plates to achieve the desired temperature on the cold side.

PWM ?.....Do you mean Pulse Width Modulation ?

I used a 35W peltier to cool my wireless modem using a large tunnel type heatsink and a small fan blowing through the tunnel and got the modem frozen up well at ambient air temperature of 35deg/C.

This is interesting cos my Belkin router overheats and has to be turned off occasionally to cool and reset.

 

[imix500]

The heatsinks he used are not small. They are nice and solid about 85mm square. Like for pent 4, 2.8 gig.

According to Intel, that processor dissipates about 68W. You would need three times the heatsink for the peltier you have.

 

[Boncuk]

Thanks Boncuk. That's something I didn't know
This was on a 28 deg. day, and the best we could get the cold side was 26 deg.

.

Something must be wrong with heat exchange. Did you also measure the hot side of the arrangement that day?

Did you also check the current flow through the peltier? There might be a wiring problem as well causing the peltier to work at "idle".

The heatsinks he used are not small. They are nice and solid about 85mm square. Like for pent 4, 2.8 gig.

When I said large I meant 80X150mm (for lousy 35W). Look for heat sinks with a thermal resistance <0.7K/W.

PWM ?.....Do you mean Pulse Width Modulation ?

Yes, I guess it the best way for controlled temperature.

This is interesting cos my Belkin router overheats and has to be turned off occasionally to cool and reset.

Don't try it. As soon as the router freezes up you'll have the "digestionals" full of ice. When switching off the "cooler" it melts and floods the circuit board.

I returned to fan cooling after I purchased a new modem.

Boncuk

 

[meatpie]

.
Something must be wrong with heat exchange. Did you also measure the hot side of the arrangement that day?

Yes...we measured the hot side heatsink at 42 deg. at the base, using a quality IR thermometer.

Did you also check the current flow through the peltier? There might be a wiring problem as well causing the peltier to work at "idle".

Yes we did...Draws the correct current and all wiring has been tripple checked.

When I said large I meant 80X150mm (for lousy 35W). Look for heat sinks with a thermal resistance <0.7K/W.

The one he's using is fine considering the hot side runs at only 42deg with the fan on.

Yes, I guess it the best way for controlled temperature.

No need to worry about PWM. It's only bench test at the moment running flat out. Then when it's fitted to the box and working correctly it will be electronically temp controlled.

 

[Boncuk]

One more way to measure: Get the temperature probe in the slot between heatsinks and measure both temperatures. They should be significantly different.

 

[meatpie]

One more way to measure: Get the temperature probe in the slot between heatsinks and measure both temperatures. They should be significantly different.

Yep...that's where the measurements were done, right at the base of the heatsinks, using the IR thermometer.

 

[imix500]

A couple questions...

You are using grease on both sides of the peltier right?

How are you attaching the heatsinks to the peltier?

 

[meatpie]

You are using grease on both sides of the peltier right?

Yep

How are you attaching the heatsinks to the peltier?

Nylon screws through the heatsinks at the peltier perimeter.