Getting the Basics of Thermoelectric Cooling
A thermoelectric module may be used for both heating and cooling, making it highly suitable for precise temperature control applications. Sometimes called a thermoelectric cooler or Peltier module, a thermoelectric cooler is a semiconductor-based electronic component that functions as a small heat pump. By applying a low voltage DC power source to the module, heat will be moved through the module from one side to the other. Therefore, one module face is cooled while simultaneously the opposite face is heated. This phenomenon may be reversed by a change in the polarity (plus and minus) of the applied DC voltage, which will cause heat to be moved in the opposite direction.
If a typical single-stage thermoelectric module was placed on a heat sink maintained at room temperature, and the module then was connected to a suitable battery or other DC power source, the "cold" side of the module would cool down to approximately -40oF (-40oC). At this point, the module would be pumping almost no heat and would have reached its maximum rated temperature differential (∆T). If heat was added gradually to the module's cold side, the cold-side temperature would increase progressively until it eventually equaled the heat sink temperature. At this point, the thermoelectric cooler would have attained its maximum rated heat pumping capacity.
Although thermoelectric coolers and mechanical refrigerators are considerably different in form, the same fundamental laws of thermodynamics govern both refrigeration systems.
Mechanical vs. ElectronicIn a mechanical refrigeration unit, a compressor raises the pressure of a liquid and circulates the refrigerant throughout the system. In the evaporator or "freezer" area, the refrigerant boils and, in the process of changing to a vapor, absorbs heat, which causes the freezer to become cold. The heat absorbed in the freezer area is moved to the condenser, where it is transferred to the environment from the condensing refrigerant.
In a thermoelectric cooling system, a doped semiconductor material essentially takes the place of the liquid refrigerant, a finned heat sink replaces the condenser, and a DC power source takes the place of the compressor. The application of DC power to the thermoelectric module causes electrons to move through the semiconductor material. At the cold end, or freezer side, of the semiconductor material, heat is absorbed by the electron movement, moved through the material and expelled at the hot end. Because the hot end is physically attached to a heat sink, the heat passes from the material to the heat sink and then, in turn, transfers to the environment.
The physical principles upon which modern thermoelectric coolers are based are not new science (see sidebar, "Speaking Historically.") German scientist Thomas Seebeck discovered that an electric current flows continuously in a closed circuit made up of two dissimilar metals, provided that the junctions of the metals were maintained at two different temperatures.
To understand how the Seebeck Effect works, look at the simple thermocouple circuit shown in figure 1. The thermocouple conductors are two dissimilar metals denoted as Material X and Material Y.
In a typical temperature measurement application, thermocouple A is used as a reference and is maintained at a relatively cool temperature (TC). Thermocouple B is used to measure the temperature of interest (TH) that, in this example, is higher than temperature TC. With heat applied to thermocouple B, a voltage (VO) will appear across terminals T1 and T2. This voltage, known as the Seebeck EMF, can be expressed as:
VO = SXY x (TH - TC)
Part-time physicist Jean Peltier also contributed to the development of thermoelectric coolers. Peltier found there was an opposite phenomenon to the Seebeck Effect, whereby thermal energy could be absorbed at one dissimilar metal junction and discharged at the other junction when an electric current flowed within the closed circuit.
In figure 2, the thermocouple circuit is modified to obtain a different configuration that illustrates the Peltier Effect, a phenomenon opposite that of the Seebeck Effect. If a voltage (VIN) is applied to terminals T1 and T2, an electrical current (I) will flow in the circuit. As a result of the current flow, a slight cooling effect (QC) will occur at thermocouple junction A (where heat is absorbed), and a heating effect (QH) will occur at junction B (where heat is expelled). Note that this effect may be reversed whereby a change in the direction of electric current flow will reverse the direction of heat flow. The Peltier effect can be expressed mathematically as:
QC or QH = PXY x I
Joule heating, having a magnitude of I2 x R (where R is the electrical resistance), also occurs in the conductors as a result of current flow. This Joule heating effect acts in opposition to the Peltier Effect and causes a net reduction of the available cooling.
William Thomson, who described the relationship between the two phenomena, later issued a more comprehensive explanation of the Seebeck and Peltier effects. When an electric current is passed through a conductor having a temperature gradient over its length, heat will be either absorbed by or expelled from the conductor. Whether heat is absorbed or expelled depends on the direction of both the electric current and temperature gradient. This phenomenon is known as the Thomson Effect. PCE
Sidebar: Applications for thermoelectric coolers run the gamut from industrial to commercial to laboratory use. Here's a quick look at just some of places where they can be a benefit.
Putting Thermoelectric Modules to Use
- Charge-coupled device (CCD) cooling.
- Charge-induced device (CID) cooling.
- Cold chambers.
- Compact heat exchangers.
- Electronics cooling.
- Electronics package cooling.
- Integrated circuit coolers.
- Immersion coolers.
- Instrumentation, sensors and detectors.
- Laboratory cold plates.
- Pharmaceutical processing.
- Silicon-wafer cooling.
- Small portable and stationary refrigeration.
- Thermal cycling devices (PCR Cyclers and blood analyzers).
- Thermal management.
- Wafer thermal characterization.
- Wine coolers.
Sidebar 2: Although commercial thermoelectric modules were not available until almost 1960, the physical principles upon which modern thermoelectric coolers are based actually date back to the early 1800s.
The first important discovery relating to thermoelectricity occurred in 1821 when German scientist Thomas Seebeck found that an electric current would flow continuously in a closed circuit made up of two dissimilar metals, provided that the junctions of the metals were maintained at two different temperatures. Seebeck did not actually comprehend the scientific basis for his discovery, however, and falsely assumed that flowing heat produced the same effect as flowing electric current.
In 1834, a French watchmaker and part-time physicist, Jean Peltier, while investigating the Seebeck Effect, found that there was an opposite phenomenon whereby thermal energy could be absorbed at one dissimilar metal junction and discharged at the other junction when an electric current flowed within the closed circuit. Twenty years later, William Thomson (eventually known as Lord Kelvin) issued a comprehensive explanation of the Seebeck and Peltier Effects and described their relationship. At the time, however, these phenomena were still considered to be mere laboratory curiosities and were without practical application.
In the 1930s, Russian scientists began studying some of the earlier thermoelectric work in an effort to construct power generators for use at remote locations throughout their country. This Russian interest in thermoelectricity eventually caught the attention of the rest of the world and inspired the development of practical thermoelectric modules. Today's thermoelectric coolers make use of modern semiconductor technology in which doped semiconductor material takes the place of the dissimilar metals used in early thermoelectric experiments.
The Seebeck, Peltier and Thomson effects, together with several other phenomena, form the basis of functional thermoelectric modules.