UBC researchers have discovered a new method of trapping heat, called the ‘heat trap’ effect, and used nanotechnology to create a novel device for converting sunlight to electricity.
Solar cells are now found all around us, from rooftops to portable electronic devices to experimental solar-powered vehicles. These cells typically operate based on what is called the photovoltaic effect, where the absorption of light leads to the creation of mobile electrons in a semiconductor, which generates electric power. However, among other factors, costly materials and complex manufacturing processes have limited the commercial success of many of these devices.
An alternative approach consists of using focused sunlight to heat a metal to thousands of degrees. At such elevated temperatures, electrons literally boil out of the material. If another piece of metal is placed nearby, it can collect these electrons and create electric current, thus delivering electric power. This method, called thermionic conversion, is, in principle, simple and inexpensive. Theoretical studies show that thermionic devices can be quite efficient and generate very high electric power relative to their size and weight. These devices have been of interest for decades for organizations like NASA.
Still, progress has been limited. The few existing prototypes use elaborate heating systems involving large, complex, light-focusing and heat-reception mechanisms only available in specialized test facilities. The reason is the fundamental challenge of heating a conductor to the high temperatures needed for boiling off electrons. Although one can easily heat an insulator using sunlight and a simple lens (remember the favourite childhood trick of burning paper using a magnifying glass), this does not work for conductors, where the generated heat dissipates to a wide area. The result is that impractically large amounts of light are needed to reach the required temperatures.
But this was before our UBC team observed the ‘heat trap’ phenomenon. Through this effect, the heat generated in a carbon nanotube array illuminated with light can become trapped, easily increasing the surface temperature to thousands of degrees. Our team has been able to build a small, compact thermionic solar cell prototype and successfully demonstrate its operation.
Carbon nanotubes, tubular carbon structures with tiny nanoscale diameters, are materials with rich and promising electrical, optical, thermal and mechanical properties. A carbon nanotube array is made of millions of individual nanotubes and looks like an unremarkable, regular black material to the naked eye. Yet, this discovery shows how the novel properties of nanostructured materials may provide solutions to decades-old energy problems.
Though still a long way from commercial application, the UBC device is a breakthrough toward a new, potentially inexpensive way of creating clean electricity. The invention was recently reported in the journal AIP Advances and, together with the University Industry Liaison Office, we are exploring its commercialization potential.
Read Part One of this story.