Researchers claim to have found a way of making cheap thermoelectric materials that could harvest waste heat from a range of scenarios.
A team led by Dr Ole Martin Løvvik of Oslo University’s Centre for Materials Science and Nanotechnology in Norway has been studying the thermoelectric effect at the nanoscale for several years.
Discovered in 1821, it essentially describes the generation of a voltage arising from a temperature difference across a material — generally made up of two different metals.
‘It looks easy on the outside, but on the inside the electrons are doing all the work,’ Løvvik told The Engineer. ‘It’s essentially a heat engine but the working fluid is the electronic gas, because the electrons are free to move all around.’
However, the technology been limited to specialist applications — for example, deep-space missions use radioisotope thermoelectric generators based on plutonium.
Attempts to bring the technology into the mainstream, in order to harvest waste heat from industrial and everyday scenarios, have been limited by cost and practicality.
Løvvik said the key to the problem is that a good thermoelectric material ought to have high thermal resistance but low electrical resistance. Therefore, perhaps counter-intuitively, it is important to prevent heat dissipation through the material.
The team achieved this by introducing nanoscale barriers into various common semiconducting materials, which reflect waves of vibrating ‘hot’ energetic particles of certain frequencies.
‘It’s possible to choose your frequencies with care and then you can maintain the electronic conductivity while dramatically changing the heat dissipation — that’s what we aim for,’ Løvvik explained.
The fabrication method involves cooling down blocks of semiconducing materials to -196°C with liquid nitrogen to make them more brittle and less sticky, then grinding them down into nanoscale particles using a ‘mill’. These particles are then essentially compressed back together in a controlled fashion, leaving the essential nanoscale barriers.
‘We use the same kind of mill they use to make paint, it’s a well-established technique, it can be upscaled and it’s cheap, so that’s important,’ Løvvik said.
The team’s calculations suggest it could recover around 15 per cent of all energy losses in a variety of scenarios. The team is in talks with a major automotive manufacturer with a view to first placing the material in the exhausts of cars.
‘This is just the starting point for using this technique to exploit the vast amount of waste heat that is available almost everywhere in society,’ he added.