Heat transfer efficiency

[fell8]

I did a search, so don't kill me if this is a repost.

I am very curious about, watt for watt, which will move more heat--a Peltier or a conventional vapor-phase cooler.

To clarify (in case of cofusion): If I run a 100W Peltier and a vapor-phase with a 100W motor under otherwise identical conditions, which will move more BTUs?

Thanks

Doug

 

[Mingon]

Peltier should, but depends on how they are rated

 

[fell8]

Thanks for the reply. I kinda figured there would be different ways of rating them that I don't know about, so I tried to break it down to the simplest absolute terms (electrical energy input vs. amount of heat transfer accomplished). It would be nice if I could just find some nice technical papers with charts and everything, but I guess this isn't the most pressing question on most scholars' minds. Oh well.

 

[CTho9305]

Uh, a 100W peltier draws a CRAPLOAD more than 100 watts (200?). Peltiers are VERY inefficient.

I don't know how much energy refrigerators use normally, but I saw a commercial for one that says it uses less power than a lightbulb. Assuming a 100 watt bulb... I don't think a 100 watt peltier would keep your fridge AND freezer cool very well at all, even running constantly, and cooling a 100 watt peltier is not easy. I think you'd need at LEAST as much surface area as a refrigerator radiator.

For example, an air conditioner with an EER of 10 might consume 1,000 Watts of power with a cooling capacity of 10,000 BTU; it would be superior to an average 20-year-old air conditioner with an EER of 7 that would use 1,000 Watts to cool with a potential of 7,000 BTU

7000 BTU/hr = 2000 watts (assuming I can do math). Therefore, the 20-year-old AC (phase change) uses 1000 watts to cool 2000 watts. source

Here, we see that this fancy peltier pumps 160 watts, and draws about 250 watts (14.2*17.6).

 

[Mingon]

The problem is in the rating, is the 100 watt rated peltier cooling 100 watts or is it using 100 watts? is a comparable phase change using the same rating? You have to compare apples to apples. Peltiers are very efficient due to their simplicity, which should give them the advantage - but I cant prove this so take from it what you will.

 

[Fencer128]

The only way to do a fair and balanced comparison is to use power in - power out wall-plug like efficiencies. A 100W peltier may dissipate 100W of heat, but what kind of power are you supplying in order to do that? Same goes for phase change tech. I'm quite interested in this because I'm building a peltier based laser cooling system right now.

Let us also not forget that a peltier is a heat pump. It dumps heat from one plate to another. It's only as effective as it's cold sink (heatsink, water cooling line, etc). I guess a peltier system that uses a phase change cold sink would work rather nicely.

Cheers,

Andy

 

[CTho9305]

Mignon / Fencer128 did you read the numbers in my post? Peltiers draw 1 watt and cool less than 1 watt. Phase change coolers draw 1 watt and cool more than 1 watt.

Simplicity does not necessarily equal efficiency. Bubble sort is simple, quicksort is efficient.

A peltier is a heat pump, but so is a phase change cooler. Both take heat from one place (cold side / evaporator) and move it somewhere else (hot side / condensor). You have to cool the "somewhere else" no matter which cooler you use.

 

[wacki]

CTho9305 is correct

peltiers are very inefficient
compressors are much better, and can get much colder

the most efficient coolers are sonic condensors, but they are still being developed.... can't wait

 

[fell8]

Thanks for the info, guys. What a bummer Peltiers are so inefficent, I was hoping to make some "improvments" using them.

It's true about refrigerators using less power than a lightbulb, due to highly effective insulation. In one of his work horror stories, my uncle (a head service guy at Sub-Zero) described a particularly gruesome scene (rotting food, flies, unbearable odor) resulting from, not mechanical failure, but simply the owners leaving the door slightly ajar before going on vacation. It wasn't the cold air leaking out that caused the problem, rather it was the heat given off by the lightbulb. Given that the average appliance lightbulb runs 40W, it was a mere 35W or so of heat that led to the catastrophe. Go figure.

 

[ET2nuke]

Originally posted by: wacki
the most efficient coolers are sonic condensors, but they are still being developed.... can't wait

Got any info?

 

[Mingon]

Mignon / Fencer128 did you read the numbers in my post? Peltiers draw 1 watt and cool less than 1 watt. Phase change coolers draw 1 watt and cool more than 1 watt.

But their is more to efficiency than just electrical draw, how much heat do phase change pump out ? I am not saying you are wrong, just pointing out their is more than just electrical draw in the equation.

 

[Mingon]

Technically, the word efficiency relates to the amount of energy one gets out of a machine versus how much energy one puts into it. In heat pumping applications, this term is rarely used because the energy-in is very different from the service provided. We supply electrical energy to a TEC, but we get heat pumping. For TECs, it is standard to use "coefficient of performance" not efficiency. The coefficient of performance (COP) is the amount of heat pumping divided by the amount of supplied electrical power. In other words, COP tells you how many units of heat pumping you will get for each unit of electrical power you supply. It is possible, in special situations, to pump more watts of heat than the watts of electrical power input. COP depends on the application, heat pumped, and temperature differential required. Typically, the coefficient of performance, heat pumped then divided by input power, is between 0.4 and 0.7 for single stage applications. However, higher COPs can be achieved with optimized custom TECs.

Blah blah from google gives a COP of 0.4-0.7 typical for TEC, and Vapor phase is 4>. So this make Vapor phase technically more efficient, BUT, practically its not always the best solution due to size constraints. (last comment added to stir up CTho9305 )

 

[CTho9305]

Originally posted by: Mingon
So this make Vapor phase technically more efficient, BUT, practically its not always the best solution due to size constraints. (last comment added to stir up CTho9305 )

I completely agree. A lack of moving parts can also be a good thing for long term applications.

mingon: Any system has to put out what it takes in... so the hot side of a pelt will be power drawn + heat pumped, and it should be the same for the phase change cooler.

mmm peltiers (most of those use 1 172-watt pelt, or 1 or 2 74-watt pelts)

 

[AbsolutDealage]

Originally posted by: CTho9305
Phase change coolers draw 1 watt and cool more than 1 watt.

I'm surprised nobody has jumped on this little comment yet....

 

[CTho9305]

Originally posted by: AbsolutDealage

Originally posted by: CTho9305
Phase change coolers draw 1 watt and cool more than 1 watt.

I'm surprised nobody has jumped on this little comment yet....

Am I reading it wrong? Does it violate conservation of energy somehow? You move 2 watts from the cold side to the hot side, while spending 1 watt... so unless you can use the temp difference created to do more cooling, you don't have perpetual motion.

 

[AbsolutDealage]

Originally posted by: CTho9305

Originally posted by: AbsolutDealage

Originally posted by: CTho9305
Phase change coolers draw 1 watt and cool more than 1 watt.

I'm surprised nobody has jumped on this little comment yet....

Am I reading it wrong? Does it violate conservation of energy somehow? You move 2 watts from the cold side to the hot side, while spending 1 watt... so unless you can use the temp difference created to do more cooling, you don't have perpetual motion.

No, but I don't think I've seen a computer based phase change cooler that dissipates more wattage than the system consumes. To be honest, I don't know if that's even possible... maybe someone out there can enlighten me?

 

[CTho9305]

Originally posted by: AbsolutDealage

Originally posted by: CTho9305

Originally posted by: AbsolutDealage

Originally posted by: CTho9305
Phase change coolers draw 1 watt and cool more than 1 watt.

I'm surprised nobody has jumped on this little comment yet....

Am I reading it wrong? Does it violate conservation of energy somehow? You move 2 watts from the cold side to the hot side, while spending 1 watt... so unless you can use the temp difference created to do more cooling, you don't have perpetual motion.

No, but I don't think I've seen a computer based phase change cooler that dissipates more wattage than the system consumes. To be honest, I don't know if that's even possible... maybe someone out there can enlighten me?

My statement was based on the air conditioner numbers posted above... but that was my only source because I'm too lazy to look for more. It might be incorrect.

 

[AbsolutDealage]

Actually, I would venture to guess that I am the one who is wrong here. I guess it makes sense that an active system can dissipate more energy than is put into it... If a heatsink dissipates such large loads with only a couple of watts driving the fan, it would make sense that a more complicated system can take that to the next level.

I guess I don't quite have a grasp on HVAC... maybe its time to head over to trade school...