Generating electrical power from waste heat
ALBUQUERQUE, New Mexico: Researchers from Sandia National Laboratories have developed a tiny silicon-based device that can harness what was previously called waste heat and turn it into DC power. “We have developed a new method for essentially recovering energy from waste heat. Car engines produce a lot of heat and that heat is just waste, right? So imagine if you could convert that engine heat into electrical power for a hybrid car. This is the first step in that direction, but much more work needs to be done,” said Paul Davids, a physicist and the principal investigator for the study.
“In the short term we’re looking to make a compact infrared power supply, perhaps to replace radioisotope thermoelectric generators.” Called RTGs, the generators are used for such tasks as powering sensors for space missions that don’t get enough direct sunlight to power solar panels.
Davids’ device is made of common and abundant materials, such as aluminium, silicon and silicon dioxide — or glass — combined in very uncommon ways.
Silicon device catches, channels and converts heat into power
Smaller than a pinkie nail, the device is about 1/8 inch by 1/8 inch, half as thick as a dime and metallically shiny. The top is aluminium that is etched with stripes roughly 20 times smaller than the width of a human hair. This pattern, though far too small to be seen by eye, 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 very fast electrical oscillations, about 50 trillion times a second. This pushes electrons back and forth between the aluminum and the silicon in an asymmetric manner. This process generates net DC electrical current.
The team calls its device an infrared rectenna, a portmanteau of rectifying antenna. It is a solid- state device with no moving parts to jam, bend or break, and doesn’t have to directly touch the heat source, which can cause thermal stress.
Because the team makes the infrared rectenna with the same processes used by the integrated circuit industry, it’s readily scalable, said Joshua Shank, electrical engineer and the paper’s first author, who tested the devices and modelled the underlying physics while he was a Sandia postdoctoral fellow.
He added, “We’ve deliberately focused on common materials and processes that are scalable. In theory, any commercial integrated circuit fabrication facility could make these rectennas.”