Device harvests energy from temperature fluctuations
Researchers developed a novel thermoelectric device which harnesses temperature fluctuations to generate electricity. The power generated is limited—yet it is continuous and unaffected by short-term changes in weather conditions—making the device ideal for powering electronics which require small, but long-lasting power.
The device, called thermal resonator, was developed by a group of researchers at the Massachusetts Institute of Technology (MIT, Cambridge, MA, USA) whose findings were reported last week in the Journal Nature Communications. The operating principle is the thermoelectric effect which has been known since the 18th century: a difference in temperature between two sides of a conductive material induces a flow of charge carriers—i.e. current—from the hot to the cold side. The novelty of the device is thus not in the mechanism to generate electricity but rather in its efficiency.
An ideal thermal resonator has a very high thermal effusivity. This is the property of a material to both conduct and store heat efficiently. Usually materials are either good thermal conductors or high thermal capacitors—if they conduct heat well, they are likely not good at storing it and vice versa. What the MIT researchers did was to develop a new material that would be both a good heat conductor and a good heat capacitor. The material consists of a metal foam (copper or nickel) coated with a layer of graphene and infused with an alkane hydrocarbon called octadecane. The metal and graphene provide the high thermal conductivity, while the octadecane (which can change between the solid and the liquid phase within a particular range of temperatures) is responsible for storing the heat efficiently. “The phase-change material stores the heat,” says Anton L. Cottrill, lead author of the study, “and the graphene gives you very fast conduction”. Essentially, one side of the device captures the heat, which radiates slowly to the other side. One side always lags behind the other and the system cannot reach thermal equilibrium. This persistent difference between the two sides is harvested through conventional thermoelectrics to produce electrical power.
In the proof-of-concept experiment, the researchers were able to measure persistent energy harvesting from diurnal temperature fluctuations, extracting voltages and powers as high as 350 mV and 1.3 mW from approximately a 10 °C excursion—enough to power simple, small environmental sensors or communications systems. The harvested energy was more than a factor three higher—in terms of power per area—than that of identically sized, commercial pyroelectric devices (these generate a temporary voltage when they are heated or cooled and are used for converting temperature fluctuations to electricity). The initial testing was done over a 24-hour daily cycle of ambient air temperature, yet tailoring the properties of the material could make it capable of harvesting energy from temperature cycles at shorter time scales (e.g. from the on-off cycles of motors or industrial machines).
It has to be said that the harvested energy is quite modest, but the main advantage of the device is its long-lasting and continuous performance, ideal for powering—for instance—networks of small field sensors over long periods of time. The device works based on temperature excursion and is hence not directly affected by lack of sunlight; in fact, it could be placed underneath a solar panel, in its shadow, and harvest energy by drawing away waste heat.
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