Generating electrical power from waste heat

2 mins read

A small silicon-based device that can harness ‘waste heat’ and turn it into DC power has been created by a team from Sandia National Laboratories.

"In the short term,” says Paul Davids, a physicist and the principal investigator for the study, “we're looking to make a compact infrared power supply, perhaps to replace radioisotope thermoelectric generators (RTGs)." These generators are used for tasks like powering sensors for space missions that don't get enough direct sunlight to power solar panels.

Davids' explains that the device is made of common materials, such as aluminium, silicon and silicon dioxide, which are combined in uncommon ways.

The aluminium top is etched with stripes roughly 20 times smaller than the width of a human hair - this pattern serves as an antenna to catch the infrared radiation. Between the aluminium top and the silicon bottom is a very thin layer of silicon dioxide. This layer is about 20 silicon atoms thick, or 16,000 times thinner than a human hair. The patterned and etched aluminium antenna channels the infrared radiation into this thin layer.

The infrared radiation trapped in the silicon dioxide creates fast electrical oscillations, about 50 trillion times a second. This pushes electrons back and forth between the aluminium and the silicon in an asymmetric manner. This process, called rectification, generates net DC electrical current, the Sandia team explains.

The researchers call the device an infrared rectenna, a portmanteau of rectifying antenna. It is a solid-state device with no moving parts and doesn't have to directly touch the heat source.

Because the team makes the infrared rectenna with the same processes used by the integrated circuit industry, it's readily scalable, says Joshua Shank, electrical engineer and the paper's first author.

Rob Jarecki, the fabrication engineer who led process development, explains that the biggest fabrication challenges the team faced lay in inserting small amounts of other elements into the silicon, or doping it, so that it would reflect infrared light like a metal. "Typically, you don't dope silicon to death, you don't try to turn it into a metal, because you have metals for that. In this case we needed it doped as much as possible without wrecking the material."

The version of the infrared rectenna the team reported in Physical Review Applied produces 8 nanowatts of power per square centimetre from a specialised heat lamp at 840 degrees. For context, a typical solar-powered calculator uses about 5 microwatts, so they would need a sheet of infrared rectennas slightly larger than a standard piece of paper to power a calculator.

To improved efficiency, the team are investigating a few ideas: making the rectenna's top pattern 2D x's instead of 1D stripes, in order to absorb infrared light over all polarisations; redesigning the rectifying layer to be a full-wave rectifier instead of the current half-wave rectifier; and making the infrared rectenna on a thinner silicon wafer to minimise power loss due to resistance.