Everything You Wanted to Know About TECs: but were too afraid to ask.
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This guide was written for those who are already familiar with the basics of air and possibly water cooling and are looking to integrate TECs into their cooling system. I take full responsibly for everything in this guide and if you can find me you can sue me.
-Prerequisites: Whatt is a Watt? -
Watt (in terms of electricity) = Current * Energy (Amps * Volts = Watt)
Watt (in terms of heat) = Energy/Time (Joule per second); 4.18Joules = 1 Calorie; 1 Calorie is the energy required to raise 1gram (or 1cc) of water by 1C.
- Appreciating the Watt in terms of heat -
A heat source radiating 60-watts radiates 60-Joules per second. It would take the typical processor (generating 60-watts) 1 hour and 54 minutes to bring 1 liter of water at 1C to the boiling point (100C). Whereas a heat source generating 70watts would only take 36minutes. That’s a 2 liter coke bottle of water near freezing to boiling in 73minutes!
- TEC? -
When a low voltage DC current is applied to a thermoelectric module, heat can be moved through the TEC from one side to the other. One side is therefore cooled [cold-side] while the opposite side is simultaneously heated [hot-side]. The temperature differences can be upwards 50C in real world application. TECs help enhance your cooling ability by creating a temperature differential that can be more easily moved out of the system. Water-cooling systems can only cool an object to ambient temperatures (room temp), but they still have excess cooling capacity (provided they have sufficient flow-rate and a capable radiator). TECs allow more of the cooling capacity to be utilized and therefore achieve lesser-than-ambient temperatures.
- Playing with Ratings -
TECs have two basic ratings that determine overall cooling ability. It’s maximum temperature difference (in degrees C) and it’s maximum cooling capacity (in watts).
Maximum Temperature Difference (degrees C)
TECs work by creating a temperature difference between the cold-side and hot-side. The maximum temperature difference lets you know how many degrees C that difference can be under ‘perfect theoretical’ conditions.
Maximum Cooling Capacity (watts)
The maximum cooling capacity lets you know how close to the TEC’s maximum temperature difference, at a given heat load, the TEC can achieve.
-) Many 40-watt TECs can achieve a maximum temperature difference of 69C.
-) Many 226-watt TECs can also achieve only a maximum temperature difference of 69C.
-) This leads some to speculate that a 40-watt TEC is more efficient than a 226watt TEC with both having the same maximum temperature difference. This is not the case.
-) The maximum temperature difference can ONLY be achieved under a theoretical setting in which the heat load (what the TEC is trying to cool) is zero. This never happens.
-) The higher the number of watts the TEC is rated for – the higher percentage(%) of the theoretical maximum temperature difference (69C) it can achieve per increase in heat load. That percentage of the theoretical temperature difference is called Delta T. Time to illustrate the point by using a little friendly math…
- The Formula to Determine the Delta T (bet you wished you had paid attention in algebra) -
Delta T = (1 - (Heat Load/Max Cooling Power)) * Max Temp Difference
*) Delta T = The actual difference in temperature between the hot-side and the cold-side that you should experience in the real world.
*) Heat Load = The heat generated (in this case) by the processor measured in watts.
*) Max Cooling Power = Maximum TEC rating in watts. (e.g. a 221-watt TEC or a 80-watt TEC)
*) Max temp = TEC’s maximum temperature rating in degrees C (theoretical Delta T, a.k.a. Delta T max).
- So Show Me -
Here are a few examples with difference watt TECs cooling a modern processor which usually generate around 60+ watts of heat. Practice the formula on at least one of these examples before calculating Delta T for your own project!
*) A 40watt TEC cooling a modern processor generating 60-watts of heat.
Delta T = -34C (negative 34C). That’s an INCREASE in temperature on the cold side, so the processor would have been warmer than before! Using an under powered TEC can destroy the processor. Cooking your CPU with your TEC is NOT a good thing, avoid this common mistake.
*) A 60-watt TEC cooling a modern processor generating 60-watts of heat.
Delta T = 0 (no change in temp). Congratulations, you’ve accomplished nothing!
*) A 176-watt TEC cooling a modern processor generating 60-watts of heat.
Delta T = 46C. That’s a decrease in temperature on the cold side, making the cold-side 46C cooler than the hot-side. Not bad.
*) A 226-watt TEC cooling a modern processor generating 60-watts of heat.
Delta T = 51C. That’s a decrease in temperature on the cold-side, making the cold-side 50C colder than the warm-side. Even better, but it took a lot more energy from the TEC (28% increase from 176-watt TEC) to get an additional 5C (only 11% increase in Delta T).
Conclusions from the Examples
The larger the wattage of the TEC in proportion to the watts of heat generated by the heat load (CPU) the closer delta T approaches the theoretical limit of 69C. But, the closer you get to the theoretical limit (in this case 69C) the more and more watts it takes for the TEC to achieve it. Thus the less efficient it becomes. Fortunately it’s not like TECs were the kings of efficiency to begin with.
-Use the Formula –
There are various resources where you can find the thermal properties of various processors (e.g. the amount of heat they generate in watts).
Click here to find out how many watts of heat your CPU will generate at both stock and overclocked speeds!. You can also check the documentation online at AMD or Intel for your specific CPU’s heat output and it’s maximum safe operating temperatures.
-Dangers to be Avoided (stuff you don’t wanna do) –
The biggest danger is attaching a TEC that is underpowered; many have cooked their brand new processors doing this. :cry Of course, this is easily avoided; just don’t attach a TEC will less cooling power than your CPU generates. Duh.
The second best method to frying your CPU is to have an insufficient method of cooling the hot side of the TEC. The hot-side can become so hot that it begins to warm the cold side which adds to the heat load of the cold-side which adds to the hot-side’s heat and the cycle begins. Not a pretty thing. I’ll go into more detail about TEC cooling further down.
- Finding the Actual Temperature of the Cold-Side (after finding Delta T) -
As you probably can tell from the title, you’ll need to have figured out the value of Delta T for your system before you can determine what the final temperature will actually be on the cold-side of the plate. Also, if you thought that Delta T IS the temperature of the cold-side then you must be very confused right now so I’ll quickly explain.
Delta T is the difference in temperature between the hot-side and the cold-side. If Delta T was 50C and the hot-side was 25C, then the cold side would be -25C (that’s negative 25C). If the hot-side were raised to 50C then the actual temperature on the cold-side would be 0C. Moving right along…
- The Critical Element: Controlling the Temperature of the Hot-Side -
As we have seen from the example above, you must control the hot-side’s temperature to achieve a given temperature on the cold-side. This, however, can be more difficult than it seems.
Let’s say Bob wants to cool his XP 2000+ which generates 60-watts of heat. Bob just so happens to have a ‘Magic Heatsink’ that magically cools a heat load of 60-watts to room temperature (in this case room temp is 25C, remember that). Not a bad heatsink (must be a Thermalright). So Bob figures, “If I attach this bad boy to the hot-side of my 226-watt TEC then I can maintain room temperature on the hot-side of 25C, then the cold-side will be -26C assuming a Delta T = 51C.” Bob is about to get a rude awaking.
What’s Bob’s problem? The heat load generated on the hot-side of the 226-watt TEC is not just 60-watts anymore (the original heat output of his CPU). It is the original heat output of the CPU (60-watts) plus the Cooling Power of the TEC (226-watts). 60watts+226watts = 286-watts of heat generated on the hot-side of the TEC. That’s enough watts to heat 1 liter of water by 4.1C in one minute. See Bob’s problem? Well, it’s about to become yours.
THL = Heat Load + Max Cooling Power
-THL (Total Heat Load) = The heat load generated on the hot-side of the TEC in watts.
-Heat Load = The heat generated (in this case) by the processor measured in watts.
-Max Cooling Power = Maximum TEC rating in watts. (e.g. a 226-watt TEC)
So even with the TEC, your back to playing the same cooling game as you had been before. Except this time you have a few more watts of heat to deal with. Remember, you can get much better results using a TEC with good water cooling than you could with good water cooling alone.
- How to Cool the Hot-Side (the real game) -
This is where my little TEC guide becomes less specific. Beyond this point is the realm of more traditional cooling in order to cool the TEC. It’s a whole different ball game. Remember, cooling the hot-side of the TEC is as important as choosing the appropriate TEC for your heat load in order to obtain your temperature expectations.
Where did the math go?! Well, this is where things start getting theoretical too. Yes, you can determine the thermal resistance of a given water-block at a given flow rate and then determine the amount of watts you can dissipate in a given radiator at a given flow rate at a given CFM (air flow) with a given static pressure. It can be done, and I wish you much luck. Don’t forget to carry the 1.
Your best bet is to find others on the forums with similar setups you’re considering and find out how they’re performing versus your needs/expectations. If you don’t have a lot to go on or are unsure, think big; big radiator, high flow rate, etc.
Air Cooled TEC? Can we say terribly inefficient? You might be able to slap a SLK-800 and 120CFM fan on the TEC and see what happens, but generally you should stick with water cooling when it comes to TECs.
- Condensation -
When using a TEC to bring temperatures down significantly below ambient temperatures, condensation becomes an issue. Condensation is the process whereby water condenses and forms on your expensive computer equipment and thus ends up costing you a small fortune when it all shorts out. Proper insulation methods are a must.
Whether or not condensation will occur depends entirely on what temperatures you achieve and what the dew point is for where you live, which by the way changes with every day, and even every hour. The dew point is the temperature at which condensation will occur for a given ambient temperature (outside temperature), pressure, and humidity. The dew point is a measurement you can find alongside your normal weather report for your area. You will want to find the normal and record dew point lows for your area (make sure to consider all four seasons). After you have taken this into account, compare the lowest temperature you will achieve on the cold-side of your TEC to the lowest dew point expected [plus any necessary room for error], and make the determination if you need insulation.
In other words: if expected low dew point exceeds the cool side of the TEC, then condensation will occur and will require proper insulation.
To give you an idea,
here's a link to show you the current dew points across the US.
- Other Important Points to Consider -
A 226-watt TECs can use up to 25-amps to run. Even if your PSU has enough current to run it, you might want to check the rail you’ll be running it off of (usually 12V) to ensure it has enough current minus various other components drawing current from it. More than likely you’ll need to get another power supply specifically for the TEC. Remember nothing is 100% efficient in this world so a TEC with the cooling capacity of 226-watts can pull upwards 300-watts in energy (the volts*current kind), hope you’re not on a strict electricity budget.
Yes, if you cross the charges (the red(+) and black(-) wires) on you're TEC, you will cause the TEC to operate inversely. In other words, you'll cook your CPU. So pay attention to what you're doing.
Yes, you can stack multiple TECs to for additional cooling. No, I’m not going to into that now.
Common TEC ratings: 40-watts, 80-watts, 176-watts, 226.1-watts all at maximum temperature difference of 69C.
Although I’m currently unsure of the exact heat output of GPUs are (video card processors) popular opinion states that a 80watt TEC should be sufficient.
Finally note, that just because the cold-side of your TEC is at 0C, don't think your processor's die will be as well. Essentially your touching the CPU to a freezing piece of metal so don’t expect your math to give you the final temp of your processor’s die.
Many TECs can not be used in ambient conditions below -40C. Generally adding a TEC to a phase-change system or using very extreme water cooling will not yield any additional benefit.
Helpful link for your calculations. - Let's you know what your heat load will be (in watts) given your specific processor, clock, and Vcore. The site also has other helpfull calculator programs. - Check it out!
Product Links:
http://www.swiftnets.com – Pre-built TEC and Waterblock
http://www.dangerden.com – Some Assembly Required
Just remember to spend plenty of time doing your homework and you’ll be able to deploy a cooling system that knows no limit to Vcore increases [note: the Mobo still will so…]. Good Luck! :thumbup:
[Originally written 2/9/03]