Turning Waste Heat into Power

Silicon, in the form of photovoltaic cells, is good at generating electricity from sunlight. New research shows that it could also make a good thermoelectric: a material that converts heat into electricity and vice versa. Since silicon is more abundant than the leading thermoelectric materials and has a vast manufacturing infrastructure behind it, it could eventually yield cheap devices for generating power from engines’ waste heat or from solar heat.

In this week’s Nature, University of California, Berkeley, chemistry professor Peidong Yang and his colleagues report having fabricated silicon nanowires that generate electricity when a temperature differential is applied across them. Until now, silicon has been considered a bad thermoelectric material. But according to Yang, “the performance of the nanowires is already comparable to the best existing thermoelectric material.”
Yang and his colleagues reduced silicon’s thermal conductivity by using silicon nanowires. They fabricated an array of silicon nanowires that are between 20 and 300 nanometers in diameter. Nanowire synthesis often involves liquefying a nanoparticle and inducing it to grow, much like a hair. But that produces nanowires with smooth surfaces. The chemical etching method that Yang’s team uses results instead in nanowires that have rough surfaces. The researchers found that wires that are about 50 nanometers wide retain electrical conductivity but have only one-hundredth the thermal conductivity. This results in a thermoelectric efficiency close to that of some commercial bismuth telluride materials.

Technology Review