Two fundamental principles in the relationship between heat and electricity are the Peltier and Seebeck effects. Both relate temperature to voltage intensity and current direction and have been essential in many cooling / heating applications. This is especially true in high cooling environments such as refrigeration units, advanced computer system cooling, and cooling for high-powered lasers, etc. Read below for information on the two effects.
The Peltier Effect (Thermoelectric Effect)…
The Peltier effect, first exhibited by Jean Peltier in 1834, is viewed as the compliment to the Seebeck effect – outlining the ability to generate a heat variation due to a voltage difference across a two dissimilar metals at the junction. It is easy to see potential uses for such a device in cooling applications as P and N type materials can be made to be exhibit the Peltier effect in very small packages (often the order of several millimeters) and come with all the benefits of being solid-state.
The formula that governs Peltier heat transfer is:
Peltier = Qp = P*I*t
Comparing this to the standard Joule Heat equation:
Joule Heat – Q =R*I*I*t
we can see that the Peltier case includes an added coefficient (P) and is a factor of current (not the square of current). These differences explain why the direction of current input factors into the determination of heat absorption or release and the formation of the ‘Hot’ and ‘Cold’ Sides of the Peltier device.
One characteristic of Peltier coolers is a fast switching ability in the definitions of these ‘Hot’ and ‘Cold’ sides. The effective time change is very rapid and in many applications must be controlled for. Another fundamental characteristic in Peltier cooling design is the ability to magnify the temperature differential by using a parallel device configuration. By placing the devices back-to-back, essentially we have a Peltier cooler cooling another Peltier cooler, etc. and the effective temperature at the cold end can be reduced.
As mentioned above, Peltier coolers operate as functions of current direction. Thus, such devices can serve in both cooling and heating applications. In the cooling condition, it is important to pump away as much heat as possible on the ‘Hot’ side to ensure that the effect will be exhibited. If heat is not pumped away from the Hot side (oftentimes this is done with a heatsink/fan setup) then the temperature on the ‘Cold’ side will slowly rise. With both sides approaching a median temperature – neither hot nor cold – the usefulness of the device decreases. The Peltier device is used to transfer heat from one side to another, but the ultimate transfer of heat away from the device must be performed by an external carrier.
Peltier cooling devices are commonly fabricated using Bismuth Telluride (BiTe) and are capable of running at below-freezing temperatures. When such is the case, it is important to consider ambient moisture in the air (as water condensation on your device is a possibility). The introduction of water may cause severe damage to both the device being cooled and the Peltier cooler itself. There are several possible solutions to this, the best being to run your setup in a vacuum.
One final note of interest in relation to Peltier coolers comes in problems posed by measuring a device with a standard ohmmeter. Given that the ohmmeter introduces an external voltage (for its testing purposes), a slight temperature change will be exhibited in the device (due to the Peltier effect). Subsequently, since the temperatures have changed a resultant change in voltage will be presented to the ohmmeter (the Seebeck effect). This relationship makes it impossible to measure a Peltier cooler using standard laboratory tools (resistance tests must be run).
The Seebeck Effect (Thermoelectric Effect)…
Thomas Seebeck is credited with the discovery of the ‘Seebeck Effect’ in 1821. The discovered phenomenon illustrated a relationship between heat differential and a conversion into an electromotive force (voltage). Seebeck first noticed this relationship by observing the reaction of a compass needle in the presence of a closed metal loop with a distinct temperature difference. Seebeck proposed that the temperature difference was inducing a magnetic field in the immediate area of the compass and used this idea to explain the needle deflection.
Eventually a formula was created to describe the relationship between the voltage and temperature difference:
For SB and SA are Seebeck coefficients. Seebeck generators are oftentimes created using PbTe or SiGe. This formula is straightforward and easy to analyze, however, such an inspection is unnecessary for the purposes of our experiment.