Did you know that two-thirds of the energy that goes into an average family car is wasted, primarily through heat loss? Thermoelectric materials have been hailed as one solution to this energy problem. But what exactly are they? Well, as the name might suggest, thermoelectric materials have some interesting thermal and electrical properties. Generators based on these materials can use wasted heat to produce electricity, and without the need for complex mechanical parts.
To generate electricity, you apply heat to one side of a block of thermoelectric material. This heat gives energy to the electrons inside the material, causing them to flow from the hot side to the cold side. A flow of electrons is otherwise known as current electricity. So, by applying heat, we can produce electricity – simple right? Not so much. The trick is that you want the heat to stay at the hot end and only the electrons to flow.
But, most of the time, those materials that like to move electricity also like to move heat – and this is the problem. Designing materials to do one but not the other – to have a high electrical conductivity and low thermal conductivity – is a difficult task. Add to that, the fact that existing thermoelectric generators have low efficiencies, and you can see that there are big challenges surrounding the use of thermoelectrics.
The efficiency of an average thermoelectric generator is about 7%. In comparison, some domestic solar cells, like the ones seen on the roofs of houses, may be expensive but can easily reach efficiencies of 35 – 40%. The numbers don’t work. So, to make thermoelectrics viable, we need to be clever. Enter nanotechnology.
There is firm evidence to suggest that engineering these materials on the nanoscale can produce thermoelectrics with high efficiencies. Nanotechnology is all about size; think about filling a box with large particles or small particles – you need many more of the small particles to fill it. In a material, the size of particles – or more specifically, the interfaces between them – defines how heat flows through it. Using nanotech, the materials can be engineered to “trap” some of the heat and slow it down, reducing its thermal conductivity, while allowing electricity to flow. So, by being a bit clever with size and interfaces, traditional, inefficient thermoelectric materials could be used to produce a new generation of energy harvesters.