Polymer sandwich harvests electricity from waste heat

IN 314 BC the Greek philosopher Theophrastus noticed something unusual: when he heated a black crystalline rock called tourmaline, it would suddenly attract ash and bits of straw. He had observed what we now call pyroelectricity - the ability of certain crystals to produce a voltage briefly when heated or cooled. Now the same phenomenon is being used to convert waste heat into electricity.

Nearly 55 per cent of all the energy generated in the US in 2009 was lost as waste heat, according to research by the Lawrence Livermore National Laboratory in California. There have been many attempts at using this waste heat to generate electricity, so far with only limited success.

Pyroelectricity could be the key, say Scott Hunter and colleagues at Oak Ridge National Laboratory in Tennessee. They have built an energy harvester that sandwiches a layer of pyroelectric polymer between two electrodes made from different metals. Just a few millimetres long, the device is deployed by wedging it between a hot surface and a cold surface - between a computer chip and a fan inside a laptop, for example. Crucially, the device is anchored to the hot surface alone and so acts as a cantilever - a beam supported at one end.

As the device warms, the polymer expands more than the electrode close to the cold surface, and the whole device bends like the bimetallic strip in a thermostat. It droops toward the cold surface, where it cools and then springs back toward the hot surface, warming up again. Soon the cantilever is thrumming between the hot and cold surfaces like the hammer of a wind-up alarm clock. Each time it is heated, the polymer generates a small amount of electricity which is stored in a capacitor (Proceedings of SPIE, DOI: 10.1117/12.882125).

Previous attempts at using pyroelectric materials to recycle waste heat have only managed to turn 2 per cent of the heat into electricity. Hunter believes his device could achieve an efficiency of between 10 and 30 per cent.

Hunter says the device can also convert heat in exhaust gases into electricity. It might even be used to capture the energy that solar cells lose as heat, he says. Energy generation aside, he adds that the devices could soak up enough heat to play a significant role in cooling laptops and data centres.

Laurent Pilon of the University of California, Los Angeles, who also studies pyroelectric energy harvesting, says he likes the compactness of the device and its relative simplicity, but has some doubts about the potential efficiency. "I think some of their expectations are a little exaggerated," he says. "They are relying on conduction to heat the device, which is a slow process." He and other groups have used fluids to heat or chill a pyroelectric material. This is much quicker, though the need to pump the fluid around does consume some of the energy generated.

So much going to waste.