LAST resort for waste heat

Heat recovery is commonplace in many process plants. So-called 'waste' heat can be used for process heating purposes, and even to generate power.

It usually requires large amounts of complex piping and equipment. But imagine if a lump of material could convert that waste heat directly into electricity without any intervening equipment.

This is known as the thermoelectric effect, and materials which exhibit this property have been known for some time. However, they've tended to be regarded as curiosities, as they are too inefficient to be useful.

A team from Michigan State University has discovered that a semiconductor composed of ultrapure lead, antimony, silver and tellurium, known as LAST, is much more efficient.

'With this class of semiconductors, when you have a temperature gradient you can generate electric current,' explains Mihal Gross of the US Office of Naval Research, which funded the project. 'Or, if you pass an electrical current through the material, you can get a temperature gradient for cooling.'

LAST works so well because it has very high electricial conductivity, but very low thermal conductivity, explains lead researcher Mercouri Kanatzidis. This seems to stem from its structure, which contains nano-scale structures which impede the flow of heat through the material.

Theoretically, a solid-state LAST module could replace shipboard steam plants, which run generators, and produce electricity directly.

A similar technique could help use the energy thrown off by microchips, which currently has to be dissipated by fans, and on a large scale, could harness the waste heat from chemical reactions and other processes in chemical plants.