A team of researchers from Columbia Engineering, Cornell, and Stanford have demonstrated heat transfer can be made 100 times stronger than had been previously predicted by theory, simply by bringing two objects to within about 40nm of each other, without touching.
Radiative heat transfer by infrared light is usually much smaller than heat transfer by conduction and convection. Now a team led by Professor Michal Lipson of Columbia and Shanhui Fan of Stanford have made a mechanical system that transfers heat using light between two parallel wires.
“At separations as small as 40 nanometers, we achieved almost a 100-fold enhancement of heat transfer compared to classical predictions,” said Professor Lipson, in a statement. He added that his team is the first to reach levels of performance that could be used for energy applications, such as directly converting heat to electricity using photovoltaic cells. This would be done by radiating heat energy exactly at the bandgap frequency of the photovoltaic cell.
Professor Lipson’s team was able to demonstrate near-field radiative heat transfer between parallel silicon carbide nanobeams in the deep sub-wavelength regime. They used a MEMS actuator to control the distance between the beams and exploited the mechanical stability of the nanobeams under tension to minimize thermal buckling effects, and thus keep control of the nanometer-scale separation even at large thermal gradients.
Using this approach, the team was able to bring two parallel objects at different temperatures to within 42nm of each other and observed that the heat transfer between the objects was close to 100 times stronger that what is predicted by conventional blackbody radiation laws. This was repeated for temperature differences of up to 260 degrees C. Sustaining such high temperature differences is important as conversion efficiency is proportional to the thermal difference between the hot and cold objects.
Raphael St-Gelais, lead author on the study, said that being able to control heat flow using such optical transfers has implications for electricity generation in photovoltaic cells.
The researchers raise the possibility of converting heat into electricity by creating nano-grid arrays of thermal wires in close promixity to a photovoltaic cell. Such modules could be used to convert waste heat from the automobile engines back into useful electricity.
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