Understanding Thermoelectric Cooling

Thermoelectric coolers (TECs), also known as Peltier coolers, are solid-state heat pumps that utilize the Peltier effect to move heat.

Passing a current though a TEC transfers heat from one side to the other, typically producing a heat differential of around 40°C—or as much as 70°C in high-end devices—that can be used to transfer heat from one place to another.

The Peltier Effect

The principle of thermoelectric cooling dates back to the discovery of the Peltier Effect by Jean Peltier in 1834.

All electric current is accompanied by heat current (Joule heating). What Peltier observed was that when electric current passed across the junction of two dissimilar conductors (a “thermocouple”) there was a heating effect that could not be explained by Joule heating alone. In fact, depending on the direction of the current, the overall effect could be either heating or cooling. This effect can be harnessed to transfer heat, creating a heater or a cooler.

Peltier himself did not appreciate the potential of his discovery, and it was not efficiently exploited until the end of the 20th century.

How it Works

When two conductors are placed in electric contact, electrons flow out of the one in which the electrons are less bound, into the one where the electrons are more bound.

The reason for this is a difference in the so-called Fermi level between the two conductors. The Fermi level represents the demarcation in energy within the conduction band of a metal, between the energy levels occupied by electrons and those that are unoccupied.

When two conductors with different Fermi levels make contact, electrons flow from the conductor with the higher level, until the change in electrostatic potential brings the two Fermi levels to the same value. (This electrostatic potential is called the contact potential.)

Current passing across the junction results in either a forward or reverse bias, resulting in a temperature gradient.

If the temperature of the hotter junction (heat sink) is kept low by removing the generated heat, the temperature of the cold plate can be cooled by tens of degrees.

Choosing Materials

At first glance metals with their low electrical resistance might seem like a good choice for TEC construction; however they also have high thermal conductivity. This tends to work against any heat gradient produced, and lowers their overall ZT value.

In practice semi-conductors are the material of choice. These are usually manufactured by either directional crystallization from a melt or pressed powder metallurgy.

The thermoelectric semiconductor material most often used in today's TE coolers is an alloy of Bismuth Telluride (Bi2Te3) that has been suitably doped to provide individual blocks or elements having distinct "N" and "P" characteristics. Other thermoelectric materials include Lead Telluride (PbTe), Silicon Germanium (SiGe), and Bismuth-Antimony (Bi-Sb) alloys, which may be used in specific situations; however, Bismuth Telluride is the best material in most computer cooling scenarios.

Bismuth Telluride has two characteristics worthy of note. Due to its crystal structure, Bismuth Telluride is highly anisotropic. Its electrical resistance is about four times greater parallel to the axis of crystal growth than perpendicular to it. Thermal conductivity, on the other hand, is about double parallel to the crystal-growth axis that perpendicular direction. Hence the anisotropic behavior of resistance is greater than that of thermal conductivity, and the highest Figure-of-Merit occurs in the parallel orientation. The thermoelectric elements must be incorporated into a cooling module so that the crystal growth axis is parallel to the length of each element (perpendicular to the ceramic plates), so that this anisotropy is harnessed for optimum cooling.

Another interesting characteristic of Bismuth Telluride is that Bismuth Telluride (Bi2Te3) crystals are made up of hexagonal layers of similar atoms. While alternate layers of Bismuth and Tellurium are held together by strong covalent bonds, adjacent layers of Tellurium are held together only by weak van der Waals bonds. As a result, crystalline Bismuth Telluride cleaves readily along these Tellurium–Tellurium layers (like Mica sheets). Fortunately the cleavage planes generally run parallel to the C-axis, so the material is quite strong when assembled into a thermoelectric cooling module.

TEC Construction

TECs are constructed using two dissimilar semi-conductors, one n-type and the other p-type (they must be different because they need to have different electron densities in order for the effect to work). The two semiconductors are positioned thermally in parallel and joined at one end by a conducting cooling plate (typically of copper or aluminium).

A voltage is applied to the free ends of two different conducting materials, resulting in a flow of electricity through the two semiconductors in series. The flow of DC current across the junction of the two semi-conductors creates a temperature difference. As a result of the temperature difference, Peltier cooling causes heat to be absorbed from the vicinity of the cooling plate, and to move to the other (heat sink) end of the device—see diagram.

The heat is carried through the cooler by electron transport and released on the opposite ("hot") side as the electrons move from a high to low energy state.

When the two materials are connected to each other by an electrical conductor, a new equilibrium of free electrons is established. Potential migration creates an electrical field across each of the connections.

When current is subsequently forced through the unit, the attempt to maintain the new equilibrium causes the electrons at one connection to absorb energy, while those at the other connection release energy.

In practice many TEC pairs (or couples), such as described above, are connected side-by-side, and sandwiched between two ceramic plates, in a single TEC unit.

The heat pumping capacity of a cooler is proportional to the current and the number of pairs in the unit.

Benefits of TEC

While refrigerators and air conditioners utilize compressors, condensers, and liquid refrigerants to lower temperature; solid-state cooling utilizes DC power, heat sinks, and semiconductors. This fundamental difference gives solid-state thermoelectric coolers the following advantages over compressors:

Quantifying the Effect

The amount of heat absorbed or released at the thermocouple junction is directly proportional to the current and its duration.

W = PIt

where P is the Peltier Coefficient (the amount of heat evolved or absorbed at the junction of a thermocouple when a current of one ampere passes through it for one second. Peltier coefficient depends upon temperature and the two materials of which the thermocouple is made.

The effectiveness of a thermocouple is given a “figure of merit” designated as ZT. It is calculated as follows:

ZT = S2T / r·k

S is the Seebeck coefficient, T is the temperature, r is the electrical resistance, and k is the thermal conductivity. S, r and k all vary according to the materials used.

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