Thermoelectric energy harvesting is less popular than the three leading forms of energy harvesting - photovoltaics, electrodynamics and piezoelectrics.
Thermoelectrics have tended to be used where a small device is managing large temperature differences because they are relatively expensive to make and inefficient.
However, Micropelt in Germany reports a surge in interest in its thermoelectrics recently, notably in industrial applications. There is also great interest in thermoelectrics in the human body and in the possibility of large area, flexible thermoelectrics in various potential applications such as on the surface of an engine or a vehicle traction battery.
One typical requirement with thermoelectrics is a layer that electrically insulates them from a hot metallic surface yet has excellent thermal conductivity. The following development may be useful here and it is flexible.
Polyethylene normally acts as a thermal insulator: it transmits just 0.35 watts per Kelvin per metre but previous studies have shown that its heat conducting abilities can be increased by stretching it, creating long, straight polymer chains that heat can travel along more easily. Gang Chen at the Massachusetts Institute of Technology and his team therefore set out to see how much heat they could make polyethylene conduct.
They first made a gel by dissolving samples of the plastic in an industrial solvent at 145º C and quenching the mix in water. The gel was then heated to 120º C so that long threads could be drawn from it. Once dried, the threads were stretched, resulting in strings several centimetres long but just tens of nanometres thick.
These were found to have a thermal conductivity of 104 watts per Kelvin per metre - better than pure iron, which conducts 80 watts per Kelvin per metre.
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