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Mini Generator Cheaply Converts Heat Into Electricity

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Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockApr 27 2020, 16:50 UTC

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The laws of thermodynamics don’t allow us to get something out of nothing. Whenever you spend energy, you have waste, more often than not as heat. Trying to “recycle” some of that heat has been a goal for a long time. Now researchers have developed a new approach that might be employed for small devices such as remote sensors or wearable tech. The development is reported in Nature.

Researchers from the University of Tokyo have designed a thin iron-based thermoelectric generator that converts heat into electricity and can power a device with low-energy demands. The newly announced generator employes iron and either aluminum or gallium. This is advantageous for three reasons: the metals are non-toxic, the material can be molded into many different shapes, and the elements are quite common, making its production affordable.

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"So far, all the study on thermoelectric generation has focused on the established but limited Seebeck effect," senior author Professor Satoru Nakatsuji said in a statement. "In contrast, we focused on a relatively less familiar phenomenon called the anomalous Nernst effect (ANE)."

The ANE allowed the team to generate a current perpendicular to the temperature gradient, rather than parallel. This is advantageous as one can shape the mini generators in ways that make them ideal for wearable tech. In both ANE and Seebeck effect scenarios, the generator is placed between a hot and a cold body, but there is a key difference. Just imagine the generator on your skin for example: When your body radiates heat out, this produces a current. In the Seebeck setup, the current generated goes in the same direction as the heat (it's pointing out), so the device needs to be of a certain thickness to make it worth its while. In the ANE setup, the current goes perpendicular to the heat and moves parallel to the skin, which allows for the construction of much thinner generators. Previous attempts to employ ANE required toxic and/or expensive material, which is why it has not been a major focus so far.

Thermoelectric devices based on the anomalous Nernst effect (left) and the Seebeck effect (right). (V) represents the direction of current, (T) the temperature gradient and (M) the magnetic field. 2020 Sakai et al

"We made a material that is 75 percent iron and 25 percent aluminum (Fe3Al) or gallium (Fe3Ga) by a process called doping," lead author Dr Akito Sakai explained. "This significantly boosted ANE. We saw a twentyfold jump in voltage compared to undoped samples, which was exciting to see."

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Designing new materials to take advantage of some quirky physical law is often a laborious process of trial and error. Repeated iterations are necessary and frequently use materials at first too expensive and time-consuming to make on a large scale. The team took advantage of the latest powerful computer simulations for the planning, allowing them to find the right materials to test.

"Numerical calculations contributed greatly to our discovery; for example, high-speed automatic calculations helped us find suitable materials to test," said Nakatsuji. "And first principles calculations based on quantum mechanics shortcut the process of analyzing electronic structures we call nodal webs which are crucial for our experiments."


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