Heat and waste heat

Have you ever touched the engine of your car after returning home from a long drive? Chances are, it was quite hot. Engines, be they in a car, a dishwasher, or an air conditioner, generate a great deal of heat.

Heat is a by-product in the conversion of one type of energy (i.e., the chemical energy in gasoline) into another (i.e., the mechanical work required to power your car). Although heat is a type of energy, it is often difficult to harness, and much of the heat generated by vehicles, factories, and appliances goes to waste.

Turning waste heat into electricity

Scientists and engineers are working to develop new devices that can convert “waste heat” into electricity. These devices rely on the thermoelectric effect, which is the ability of some materials to transform heat into electricity. If one region of these materials is heated, electrons will flow from the hotter region to the cooler regions, in-turn generating an electric current.

Developing thermoelectric materials

Although the thermoelectric effect was discovered in the nineteenth century, the idea of using thermoelectric generators as a source of clean energy has only gained traction in the last 20 years.

This is because most thermoelectric materials are expensive and not very efficient at generating electricity. Thermoelectric materials are most efficient when they have a low thermal conductivity and a high electrical conductivity – that is, when heat moves slowly through the material, but electrons are able to move quickly.

Unfortunately, thermal conductivity and electrical conductivity often to go hand in hand – that is, materials with low thermal conductivity tend to have low electrical conductivity, and vice versa.

With the advent of nanotechnology, however, materials scientists can now design and grow materials that have internal structures with dimensions that are as small as a few nanometers.

For example, they can manufacture layered structures in which each layer is only a few nanometers thick. They can also grow structures with more complex geometries, such as nanowires. Nano-scale structures can lower the thermal conductivity of a material by interfering with the movements of phonons.

Phonons are microscopic vibrations in a material that can carry heat. The wavelength of these vibrations is typically on the order of a few nanometers, thus when materials have nano-scale structures, the structure can interfere with the passage of phonons and significantly lower the thermal conductivity of the material.

Nano-structured thermoelectric materials with nearly double the efficiency of traditional thermoelectric materials have been reported.

However, these high-efficiency materials have yet to be successfully incorporated into devices. Nevertheless, interest in thermoelectric generation is growing. Major car companies have reported that they are testing thermoelectric generators in new models.

Scientists have also recently developed flat-panel solar thermoelectric devices, in which heat from the sun is converted into electricity. Though large-scale thermoelectric energy conversion remains to be implemented, new nano-structured materials show exciting potential.