14-09-2013, 04:09 PM
Electronics Cooling Methods in Industry
[attachment=58090]
Historical Background
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 where by 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.
Introduction
Thermoelectric are based on the Peltier Effect, The Peltier Effect is one of the three
thermoelectric effects; the other two are known as the Seebeck Effect and Thomson Effect.
Whereas the last two effects act on a single conductor, the Peltier Effect is a typical junction
phenomenon.
Thermoelectric coolers are solid state heat pumps used in applications where temperature
stabilization, temperature cycling, or cooling below ambient are required. There are many
products using thermoelectric coolers, including CCD cameras (charge coupled device), laser
diodes, microprocessors, blood analyzers and portable picnic coolers. This article discusses
the theory behind the thermoelectric cooler, along with the thermal and electrical parameters
involved.
Peltier Effect
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 17.2, 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 (Ein) 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. 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.
Thomson Effect
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.
Thermal Analysis and Parameters Needed
The appropriate thermoelectric for an application, depends on at least three parameters. These
parameters are the hot surface temperature (Th), the cold surface temperature (Tc), and the heat
load to be absorbed at the cold surface (QC).
Powering the Thermoelectric
All thermoelectric are rated for Imax, Vmax, Qmax, and ∆Tmax, at a specific value of Th. Operating
at or near the maximum power is relatively inefficient due to internal heating (Joulian heat) at
high power. Therefore, thermoelectric generally operate within 25% to 80% of the maximum
current. The input power to the thermoelectric determines the hot side temperature and cooling
capability at a given load.
Comparison: Conventional Refrigeration
Because thermoelectric cooling is a form of solid-state refrigeration, it has the advantage of
being compact and durable. A thermoelectric cooler uses no moving parts (except for some
fans), and employs no fluids, eliminating the need for bulky piping and mechanical
compressors used in vapor-cycle cooling systems.
Such sturdiness allows thermoelectric cooling to be used where conventional refrigeration
would fail. In a current application, a thermoelectric cold plate cools radio equipment mounted
in a fighter jet wingtip. The exacting size and weight requirements, as well as the extreme g
forces in this unusual environment, rule out the use of conventional refrigeration.
Thermoelectric devices also have the advantage of being able to maintain a much narrower
temperature range than conventional refrigeration. They can maintain a target temperature to
within ±1° or better, while conventional refrigeration varies over several degrees.
Unfortunately, modules tend to be expensive, limiting their use in applications that call for
more than 1 kW/h of cooling power. Owing to their small size, if nothing else, there are also
limits to the maximum temperature differential that can be achieved between one side of a
thermoelectric module and the other.
Summary
Although there are a variety of applications that use thermoelectric devices, all of them are based on the
same principle. When designing a thermoelectric application, it is important that all of the relevant
electrical and thermal parameters be incorporated into the design process. Once these factors are
considered, a suitable thermoelectric device can be selected based on the guidelines presented in this
article.
[attachment=58090]
Historical Background
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 where by 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.
Introduction
Thermoelectric are based on the Peltier Effect, The Peltier Effect is one of the three
thermoelectric effects; the other two are known as the Seebeck Effect and Thomson Effect.
Whereas the last two effects act on a single conductor, the Peltier Effect is a typical junction
phenomenon.
Thermoelectric coolers are solid state heat pumps used in applications where temperature
stabilization, temperature cycling, or cooling below ambient are required. There are many
products using thermoelectric coolers, including CCD cameras (charge coupled device), laser
diodes, microprocessors, blood analyzers and portable picnic coolers. This article discusses
the theory behind the thermoelectric cooler, along with the thermal and electrical parameters
involved.
Peltier Effect
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 17.2, 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 (Ein) 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. 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.
Thomson Effect
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.
Thermal Analysis and Parameters Needed
The appropriate thermoelectric for an application, depends on at least three parameters. These
parameters are the hot surface temperature (Th), the cold surface temperature (Tc), and the heat
load to be absorbed at the cold surface (QC).
Powering the Thermoelectric
All thermoelectric are rated for Imax, Vmax, Qmax, and ∆Tmax, at a specific value of Th. Operating
at or near the maximum power is relatively inefficient due to internal heating (Joulian heat) at
high power. Therefore, thermoelectric generally operate within 25% to 80% of the maximum
current. The input power to the thermoelectric determines the hot side temperature and cooling
capability at a given load.
Comparison: Conventional Refrigeration
Because thermoelectric cooling is a form of solid-state refrigeration, it has the advantage of
being compact and durable. A thermoelectric cooler uses no moving parts (except for some
fans), and employs no fluids, eliminating the need for bulky piping and mechanical
compressors used in vapor-cycle cooling systems.
Such sturdiness allows thermoelectric cooling to be used where conventional refrigeration
would fail. In a current application, a thermoelectric cold plate cools radio equipment mounted
in a fighter jet wingtip. The exacting size and weight requirements, as well as the extreme g
forces in this unusual environment, rule out the use of conventional refrigeration.
Thermoelectric devices also have the advantage of being able to maintain a much narrower
temperature range than conventional refrigeration. They can maintain a target temperature to
within ±1° or better, while conventional refrigeration varies over several degrees.
Unfortunately, modules tend to be expensive, limiting their use in applications that call for
more than 1 kW/h of cooling power. Owing to their small size, if nothing else, there are also
limits to the maximum temperature differential that can be achieved between one side of a
thermoelectric module and the other.
Summary
Although there are a variety of applications that use thermoelectric devices, all of them are based on the
same principle. When designing a thermoelectric application, it is important that all of the relevant
electrical and thermal parameters be incorporated into the design process. Once these factors are
considered, a suitable thermoelectric device can be selected based on the guidelines presented in this
article.