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.
A thermal diode can be:
- a heat engine which converts a heat difference directly into electric power.
- a heat engine working backwards as a refrigerator, such as a Stirling engine.
- a type of heat pipe 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 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 dependency 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. In addition to processors, the same technology is widely used in dedicated temperature sensor IC's.
Thermoelectric heat-pump or cooler
There are two types. One uses semiconductor, or less efficient metal, i.e. thermocouples, working on the principles of the Peltier-Seebeck effect. The other relies on vacuum tubes and the principles of thermionic emission.
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 electricity production. The efficiency of present thermal diodes is about 18% between the temperature range of 200-300 degree celsius.