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Harvesting electricity from temperature difference

30.09.2015 industrial internet , industrial , defence , prototyping and studies , consulting , eero heikkinen , electronics

New processors use less and less electricity, so Internet-connected industrial sensors can be installed in even more diverse and exotic places. Energy harvesting makes this possible without electrical wiring or batteries that either need to be replaced or recharged.

Energy harvesting will therefore play an important role in sensors of the future. The electricity they need is either diverted from the processes they measure or generated by scavenging energy from their surroundings, of which the most common sources are solar energy, motion, vibration, and temperature differences.

Due to the low power consumption electronics, energy harvesting makes sense nowadays. In the past, processors needed huge batteries, but now a small capacitor that is recharged once in a while is sufficient.

IoT devices need electricity mostly when they communicate. Power is therefore usually stored in some temporary storage, a battery or capacitor.

But you can’t get something from nothing. If a non-electric device is immobile in a dark place without vibrations, motion, or temperature differences, no amount of harvesting can generate an electric current.

Solar panels only work with direct or indirect light. In the future, it may become common to convert temperature differences into electricity. This thermoelectric process is known as the Seebeck effect.

The Seebeck effect is an old discovery

A better known thermoelectric effect than the Seebeck effect is the Peltier effect, which works the other way around: when electricity is fed into a Peltier module, one end of it begins to cool and the other end begins to heat. This is used in travel fridges, for example.

The effects were discovered by Thomas Johann Seebeck and Jean Charles Athanase Peltier in 1821 and 1834 respectively. In the first, a temperature difference produces an electric current; in the second, an electric current produces a temperature difference.

The Seebeck effect cannot be used to produce large currents, which is why it has not been used much in the past. Now the situation is different because IoT processors need so little power that enough can be produced using the Seebeck effect.

In the Seebeck effect, the current increases exponentially with greater temperature differences. So the greater the temperature difference, the better the system works. A difference of 10 degrees already produces some current, and a difference of 100 degrees ensures a good power source.
 

Electric power as a function of temperature difference in a Seebeck module

Seebeck modules are most useful in various sensor applications. Sensors can, for example, measure temperature, humidity, and acceleration. The data can be stored locally and transmitted using for example a Bluetooth connection.

Seebeck modules don’t look special

A Seebeck module doesn’t look interesting; a small, square pieces of ceramic with two wires. It produces an electric current when the difference in temperature between its two outer surfaces is large enough.


 
The efficiency of such a module is fairly low, and it doesn’t make sense to use a very large module. The low efficiency is partly due to heat escaping past the module. A larger module of course produces the same current using a smaller temperature difference.

The process is very effective, for example, when one side of the module is attached to the side of a combustion engine’s exhaust pipe. A device’s waste heat can indeed usually used by energy harvesting.

The modules are mainly produced in the Far East and mostly used for cooling, in other words, they are actually Peltier modules. They are used, for example, in cooling computer CPUs, travel fridges, and anything for which a compressor would be too big.

Although the commercially available small modules are in general designed mainly for cooling and are thus Peltier modules, they can also be used as Seebeck modules to produce electricity.

Useful applications

One potential application is as a sensor attached to the rotor of an electrical machine to measure for example vibration, acceleration, and temperature. Such a sensor of course cannot be connected to electrical wiring. But if some part of the motor gets hot, a Seebeck module can be attached to it to generate the power the sensor needs.

The measurement data can be transmitted using a short-range radio connection.
 

About the author

Eero Heikkinen is an electronic design guru at Espotel with more than 30 years of experience in various electronics design tasks ranging from large telecom systems to small, cost efficient consumer products. Eero has been fortunate as he has turned his beloved hobby as a youngster into a career as a highly respected professional.

Eero can be reached at +358 50 559 3125 or by email: firstname.lastname@espotel.com


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