Thermal diode
Encyclopedia
The term thermal diode
is sometimes used for a (possibly non-electrical) device which causes heat to flow preferentially in one direction. Or, the term may be used to describe an electrical (semiconductor
) diode in reference to a thermal effect or function. Or the term may be used to describe both situations, where an electrical diode is used as a heat-pump or thermoelectric cooler.
Due to the second law of thermodynamics
a passive system cannot move heat from a cold source to a hot destination. But the law allows to avoid or minimize the flow in this direction. If the source is hotter than the destination heat flows with a low thermal resistance towards the destination.
This application of thermal diode is based on its property to change voltage across it linearly according to temperature. As the temperature increases, diodes forward voltage decreases. Microprocessors having high clock rate encounter high thermal loads. To monitor the temperature limits thermal diodes are used. They are usually placed in that part of the processor core where highest temperature is encountered. Voltage developed across it varies with the temperature of the diode. All modern Intel CPUs have on-chip thermal diodes. As they are right there in the middle it provides most relevant CPU temperature readings. The silicon diodes have temperature dependancy of -2mV per degree celsius. Thus we can determine the junction temperature by passing a current through the diode and then measuring voltage developed across it.
, or less efficient metal, i.e. thermocouple
s, working on the principles of the Peltier-Seebeck effect. The other relies on vacuum tubes and the principles of thermionic emission
.
This can be used in construction of engines or in power production.
The efficiency of present thermal diodes is about 18% between the temperature range of 200-300 degree celsius.The researchers have found various ways of improving the efficiency
of the thermal diodes.Basically, they incorporated two changes-
1.The researchers found an efficient way of moving the electrons across the device.
2.They found a way of blocking the electrons from returning back.This efficiently increased the voltage of forward current . It made the energy conversion more efficient.
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
is sometimes used for a (possibly non-electrical) device which causes heat to flow preferentially in one direction. Or, the term may be used to describe an electrical (semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
) diode in reference to a thermal effect or function. Or the term may be used to describe both situations, where an electrical diode is used as a heat-pump or thermoelectric cooler.
Introduction:
The diode is temperature sensor. It is a pnp bipolar junction transistor whose base terminal is grounded externally and constant current is pumped through emitter which develops voltage across base emitter junction.One-way heat-flow
A thermal diode can be:- a heat engineHeat engineIn thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...
which converts a heat difference directly into electric powerElectric powerElectric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
. - a heat engine working backwards as a refrigeratorRefrigeratorA refrigerator is a common household appliance that consists of a thermally insulated compartment and a heat pump that transfers heat from the inside of the fridge to its external environment so that the inside of the fridge is cooled to a temperature below the ambient temperature of the room...
, such as a Stirling engineStirling engineA Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work.... - a type of Heat pipeHeat pipeA heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
which will only allow heat to flow from the evaporator to the condenser. When the condenser is hotter than the evaporator, the coolant in the heat pipe condenses in a reservoir at the evaporator end. This reservoir does not have a capillary connection with the condenser, preventing the liquid from returning to the condenser. During normal heat pipe operation, when the evaporator and reservoir become hotter than the condenser, the reservoir is emptied by evaporation, and heat is transferred to the condenser.
Due to the second law of thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
a passive system cannot move heat from a cold source to a hot destination. But the law allows to avoid or minimize the flow in this direction. If the source is hotter than the destination heat flows with a low thermal resistance towards the destination.
Electrical diode thermal effect or function
- a sensor device embedded on microprocessors used to monitor the temperature of the processor's die.
This application of thermal diode is based on its property to change voltage across it linearly according to temperature. As the temperature increases, diodes forward voltage decreases. Microprocessors having high clock rate encounter high thermal loads. To monitor the temperature limits thermal diodes are used. They are usually placed in that part of the processor core where highest temperature is encountered. Voltage developed across it varies with the temperature of the diode. All modern Intel CPUs have on-chip thermal diodes. As they are right there in the middle it provides most relevant CPU temperature readings. The silicon diodes have temperature dependancy of -2mV per degree celsius. Thus we can determine the junction temperature by passing a current through the diode and then measuring voltage developed across it.
Thermoelectric heat-pump or cooler
There are two types. One uses semiconductorSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
, or less efficient metal, i.e. thermocouple
Thermocouple
A thermocouple is a device consisting of two different conductors that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a...
s, working on the principles of the Peltier-Seebeck effect. The other relies on vacuum tubes and the principles of thermionic emission
Thermionic emission
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...
.
Peltier devices
- a heat engine working backwards as a refrigeratorRefrigeratorA refrigerator is a common household appliance that consists of a thermally insulated compartment and a heat pump that transfers heat from the inside of the fridge to its external environment so that the inside of the fridge is cooled to a temperature below the ambient temperature of the room...
, such as a Peltier device (diode)
Advancements
A team at MIT is working for construction of thermal diodes that convert heat to electricity at lower temperatures than before.This can be used in construction of engines or in power production.
The efficiency of present thermal diodes is about 18% between the temperature range of 200-300 degree celsius.The researchers have found various ways of improving the efficiency
of the thermal diodes.Basically, they incorporated two changes-
1.The researchers found an efficient way of moving the electrons across the device.
2.They found a way of blocking the electrons from returning back.This efficiently increased the voltage of forward current . It made the energy conversion more efficient.