by Phreejak



In 1834, Jean Charles Athanase Peltier discovered that the passage of a current through a junction formed by two dissimilar materials caused a temperature change, Dubbed the “peltier effect”, it was later clarified by Emil Lenz who observed that by passing a current through a bismuth-antimony junction, water could be frozen and that ice could be melted if the current were to be reversed.

Thermoelectric (Peltier) Modules are solid-state devices that convert an electric current into a temperature gradient. They consist of two sides – a hot side and a cold side. The module acts as a heat pump in that it moves heat from the cold side to the hot side. The hot side requires a method of removing that heat for the unit to function properly. The more efficient the means of removing this heat from the hot side, the colder the cold side will operate. 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.



A temperature differential is created when voltage is applied to two ends of the peltier. With the temperature difference, heat is moved from one end to the other. The absence of heat is simply cold. The heat is absorbed from the environment and is carried through the cooler by electron transport. It is released to the opposite side of the peltier (hot side) as the electrons go from a high to low energy state.


Actual Operation

All Peltiers are not alike.

They all look much the same; serried ranks of little semiconductor junctions soldered together and mounted between two thermally conductive white ceramic plates. They all work much the same; apply power to them and one side gets hot, while the other side gets cold. Reverse the input polarity and the hot and cold sides reverse.

Peltier modules behave pretty much like a simple resistive device from a power supply's point of view. Double the input voltage and you get almost double the current consumed. You don't get quite double the current, because as the Peltier gets hotter its resistance increases. And, for similar reasons, as voltage and current increase, the useful heat transfer you get doesn't increase as fast. Peltier efficiency, in other words, drops off as the input power rises.

We know from the second law of thermodynamics that heat will move to a cooler area. Essentially, the module will absorb heat on the "cold side" and eject it out the "hot side". This heat is, in turn, absorbed and moved by an attached cooling solution. In addition to the heat being removed from the object being cooled, the cooling solution must be capable of dissipating the heat from the electrical power applied to the module, which also exits through the modules hot side. The heat created is actually proportional to the current (amperes x volts) because of the flow of current is working in two directions (the Peltier effect). Therefore, the total heat ejected by the module is the sum of the current times the voltage plus the heat being pumped through the cold side.

The typical maximum temperature difference between the hot side and the cold side of a TEC is around 70°C.

Run a TEC above 80° Centigrade and it will degrade quite quickly; run it above 85° and it can physically fall apart in a few days.

Example

Before we get into the more technical aspects of pletier cooling, let's look at a generic module's charcteristics so that you might gain a better understanding of how one works.

First, let’s distinguish the difference between watts (in terms of energy) and watts (in terms of heat)

Electricity (watts): amps * volts
Heat (watts): Energy/Time (Joule per second)
4.18Joules = 1 Calorie
1 Calorie is the energy required to raise 1gram (or 1cc) of water by 1C.

Now, back to our generic module

80 watt module
16 volts
8.5 amp draw

What these numbers tell us is that we have a module that is rated to move 80 watts of heat. To calculate the amount of energy that it requires to do this, you multiply the amps times the volts

8.5 * 16 = 136 watts

So, to break this down, a module with an 80 watt rating (for heat, not energy) operating at 16 volts with a draw of 8.5 amps will require 136 watts of energy to do so.

Now, here’s where you determine the load that the cooling solution on the hotside must relieve. You add the wattage (heat) absorbed by the module (80 watts) and the wattage (electricity) that the module is using (136 watts) and your heat load comes to 216 watts.

That’s 216 watts of heat that the cooling solution on the hotside is going to face.

So, from this example, you've learned how to calculate the total heat load (THL).

THL= Module wattage + Energy (wattage)


What is Qmax, Vmax, Imax and Tmax?

A peltier modules rating is given by the manufacturer and consists of 4 factors - Qmax, Vmax, Imax and Tmax.

Qmax is the maximum heat load the peltier can transfer from the cold side to the hot side assuming it is running 100% efficient.
Vmax is the most efficient voltage,
Imax is the most efficient current (amps)
Tmax is the most efficient temperature difference between the hot side and cold side, under no load.

Generic Peltier module: 80 watt module, 16 volts, 8.5 amp draw, DeltaT = 64 degrees Celsius



From the above module we can conclude:
Qmax = 80 watts
Vmax = 12volts
Imax = 8.5amps
Tmax = 64 degrees

DeltaT is the actual temperature difference between the hotside and coldside of the module.

Delta T = (1 - (heat load/max cooling power))*max temp difference

Heat Load: The heat generated by the processor measured in watts
Max Cooling Power: peltier module rating
Max Temperature Difference: a modules Tmax

Assuming we use a magic proc that emits 40 watts of heat with our generic module above we can conclude:

DeltaT: (1-(40/80))*64 = 32 degrees (This means that there is a 32 degree difference between the hotside and coldside. )

The variable in the equation is the Heat Load because it reflects the heat generated by the processor. So, dependent on the actual rating of the processor and/or whether it is overclocked will determine its actual heat load.

* If the DeltaT rating is a negative number means that the processor has gotten warmer. This is typical of using an underpowered TEC module
(example: using a 60 watt module to cool a processor that generates 80 watts of heat)

* If the DeltaT rating is 0 then you’ve accomplished nothing
(example: using a 80 watt module to cool a processor that generates 80 watts of heat)

* If the DeltaT rating is a positive number then that indicates a decrease in the temperature of the coldside of the module – you’re headed in the right direction here.
(example: using a 120 watt module to cool a processor that generates 80 watts of heat)


MATERIALS

A variable of peltier cooling that seems to cause the most anxiety for enthusiasts is the issue of condensation. This is an understandable concern as the module can create sub ambient temperatures to cool a processor. However, the installation procedure for a TEC water block provides for ample protection against the possibility of this occurring.

There are 3 main components used for condensation prevention: conformal coating, dielectric grease and neoprene padding.



Conformal Coating is, typically, spray on lacquer and is used to provide a waterproof cover. It is sprayed on the back of the motherboard (or GPU PCB) in an area directly behind where the CPU socket would be. It is also used to cover the area around the socket in the front as well. Before spraying, it is recommended that all slots and connectors near the socket be covered with masking tape so that the coating does not interfere with any connections that may be used.









Dielectric Grease is a waterproof substance that is also conductive. It is squeezed, in abundance, directly into the CPU socket in such amounts that when the CPU is placed in the socket the grease will slightly ooze out of the edges.






Neoprene Padding is used to seal off the area around the module from the surrounding atmosphere as a measure to further prevent the possible temperature differences that would interact in an unsealed environment.











(UNDER CONSTRUCTION)