We know that when you touch a hot cup of tea it can warm your hands. That’s heat conduction: Two surfaces of different temperatures make physical contact and heat is transferred from one to the other. We are also pretty aware of convective heat transfer, though it may not be quite as simple. In convection, the heat transfer occurs when a fluid—this can be air, some other gas, or even a liquid—is caused to move away from a source of heat and in the process carries energy with it. For instance, above the hot surface of a stove, the air being warmed expands, becomes less dense than the surrounding cold air, and rises.
The reason for this elementary explanation of heat transfer is to set them apart from another means of thermal energy transfer. Objects can also transfer heat to their surroundings using light, but that method of heat exchange has always been thought to be very weak compared with conduction and convection. Now, in collaborative research among researchers at Columbia, Cornell, and Stanford, they discovered that we just weren’t doing it right. Their conclusion: light could become the most dominant form of heat exchange between objects.
In research described in the journal Nature Nanotechnology, the scientists discovered that when two objects are really close together, heat transfer via radiation is 100 times stronger than had been predicted.
“At separations as small as 40 nanometers, we achieved almost a 100-fold enhancement of heat transfer compared to classical predictions,” said Michal Lipson, a professor at Columbia and one of the authors of the paper, in a press release. “This is very exciting as it means that light could now become a dominant heat transfer channel between objects that usually exchange heat mostly through conduction or convection. And, while other teams have demonstrated heat transfer using light at the nanoscale before, we are the first to reach performances that could be used for energy applications, such as directly converting heat to electricity using photovoltaic cells.”
When Lipson and her colleagues brought objects of differing temperatures in such close proximity (at distances of less than 100 nanometers), they observed near-field radiative heat transfer. They used a precision micro-electromechanical system (MEMS) to bring nanoscale beams of silicon carbide as close as 43 nm apart; the heat transfer far outstripped what could be predicted by conventional thermal radiation laws, specifically a phenomenon known as “blackbody radiation.”
“An important implication of our work is that thermal radiation can now be used as a dominant heat transfer mechanism between objects at different temperatures,” explained Raphael St-Gelais, the postdoctoral fellow at Columbia Engineering who was the study’s lead author, in the press release. “This means that we can control heat flow with a lot of the same techniques we have for manipulating light. This is a big deal since there are a lot of interesting things we can do with light, such as converting it to electricity using photovoltaic cells.”
This very strong, non-contact, heat transfer channel could be used for controlling the temperature of delicate nano devices that cannot be touched, or for very efficiently converting heat to electricity by radiating large amounts of heat from a hot object to a photovoltaic cell in its extreme proximity. And if we can shine a large amount of heat in the form of light from a hot object to a photovoltaic cell, we could potentially create compact modules for direct conversion of heat to electrical power.
The researchers imagine using this type of heat transfer to convert waste heat from a car’s combustion engine into useful electrical power, or do something similar inside homes and offices by converting renewables such as biofuels and stored solar energy into electricity.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.