The Royal Society

Energy harvesting

Posted by on 3 June 2011

Posted by Raiders of the lost amp! Laurie Winkless, National Physical Laboratory

The world is still dependent on fossil fuels, which are both finite and environmentally costly. Sustainable and so-called “carbon-neutral” energy can be derived from nuclear fission or captured from ambient sources – and the first of these is ever-present in the media.

We are all familiar with large-scale ambient energy capture, such as collecting sunlight using solar cells, wind from turbines and tidal energy from tidal generators.

But at the other end of the scale, there are small amounts of ‘wasted’ energy that could be useful if captured. Recovering even a fraction of this energy would have a significant economic and environmental impact. This is where energy harvesting (EH) comes in.

Energy harvesting, or ‘energy scavenging’, is a process that captures small amounts of energy that would otherwise be lost as heat, light, sound, vibration or motion. It can use this captured energy to improve the efficiency of systems, by reusing some of the waste energy, or even to power new technologies, such as wireless sensors.

There are several promising microscale energy harvesting materials (including ceramics, single crystals, polymers and composites) and technologies utilising them are already in development. The aim of EH is not to generate large-scale power, but to capture small amounts of energy that is ‘wasted’ during industrial and everyday processes.

Our research focuses on two classes of energy harvesting technologies:

  • Piezoelectric materials
  • Thermoelectric materials 

Vibration, movement and sound can be captured and transformed into electrical power using piezoelectric materials. One area of interest is in harvesting the kinetic energy generated by the footsteps of crowds to power ticket gates and display systems – this technology has already been implemented in Japan’s busiest train stations, and UK companies are aiming to do the same for London’s 2012 Olympics. This technology can be used to power wireless sensors too – replacing the need for batteries.

The biggest energy loss in industry is HEAT. Heat engines generate nearly all of the world’s electrical power. These are gas or steam-powered turbines that convert heat to mechanical energy, which is then converted to electricity. But approximately two-thirds of the energy input is not converted to electrical power but lost as heat. And in the car industry, the figures are just as bad – more than 70% of the energy produced by a car engine is lost, and most of that, in the form of heat. Thermoelectric materials can capture some of this heat, and produce electricity. Cars and lorries equipped with a thermoelectric generators (TEG) would have significant fuel savings (especially with the increasing cost of petrol). In 2009, VW demonstrated this proof of concept Link. The thermoelectric generator of their prototype car gained about 600W from running on a highway, reducing fuel consumption by 5%.

At NPL, and along with our colleagues across Europe, we aim to fully characterise piezo- and thermoelectric materials. The physics of these materials is relatively well understood on the microscales, but the same is not true for the final devices. We hope to improve their efficiency and to understand how properties on the nano- and microscales relate to the operation of a fully-fledged energy harvesting device. 

Even though energy harvesting technologies can capture just a small percentage of the world’s “waste” energy, they could make a REAL difference to energy efficiency across a range of industries.