Energy Harvesting involves utilizing low-grade energy that would otherwise go to waste, mainly to power electronics with low power requirements. It provides an environmental benefit by doing more useful work with the energy we already produce. There are many applications where it is advantageous to not have batteries or wiring – everything from ground sensors to body implants. Innovations such as piezoelectric materials cultivated in the form of virus can help provide scalability to the technology.
Energy in transition
Energy can never be destroyed, as we know, without going through a chain of conversions between different forms. Some forms we value higher because they are easier to exploit and we can use for example the potential energy in a reservoir to drive turbines in a hydroelectric plant, and generators convert the motion into electrical energy that is easy to deploy and use for many different purposes. Electricity, for example, can drive an electric motor and be converted back into kinetic energy. From our perspective, the last link in many energy chains is to use low-grade heat, which usually is not utilized.
When we use energy there are always losses along the way in the chain – for example, the frictional heat in a machine or transformation losses in the electricity grid. The proportion of the added energy that makes use what we call efficiency: a gasoline-powered car can have an efficiency of around 25% and then uses a quarter of the fuel’s energy to move itself and its cargo, and a muscle converts chemical energy into motion with similar efficiency.
From an environmental perspective, the energy issue is central and ever-present, and there are several ways to increase the environmental benefits by focusing on different stages of the chain. One way is of course to invest in cleaner forms of energy. Another is to switch to more energy-efficient appliances. A third is to choose technologies that minimize the losses in the distribution and transformation – and there are big gains to be made. Over a quarter of the energy produced is lost on the way to the user.
To recycle waste energy
Yet another approach is something called energy harvesting. It involves various methods to take advantage of the ambient energy that is being wasted, and try to exploit it in the form of electricity. Waste energy can be small differences in heat, vibration, radio waves – or simply mechanical work from people in motion. The basic principle in energy harvesting is based on the piezoelectric, thermoelectric or pyroelectric effects of some materials.
Piezoelectric materials have the property of being able to extract electricity from movements, sounds and vibrations. The mechanism for this is that the materials have charges separated in a symmetric crystal structure; and when subjected to mechanical stress, a charge asymmetry arises that gives rise to voltage.
Thermoelectric and pyroelectric materials build an electric charge by absorbing and converting heat energy. In the former, it occurs when the crystals in the material experience a difference in temperature between its different sides. Pyroelectric materials generate electricity while the material undergoes a change in temperature.
Low power but great potential
Since energy harvesting works with diffuse energies near the limit of what is meaningful to exploit, the effect is generally small. The primary use is therefore electronics with low power requirements – which, however, can be quite significant. One example is the network of wireless sensors – a technology that can bring great efficiencies in both industry and agriculture (see the article “Effektivare resursanvandning i jordbruket, or Efficient resource use in Agriculture”). In time, energy harvesting could go hand in hand with nanotechnology and allow self-guided micro-machines that are not limited by batteries. The possible applications of it would be almost unlimited, especially in biotechnology.
Another important area where energy harvesting can be useful is to replace batteries in medical implants, where the limited operating time otherwise poses a big problem. Heartbeat, internal organ motion or the body’s glucose reserves can serve as sources of energy.
Energy harvesting has the potential to reduce the need for batteries in general. That in itself can be beneficial for the environment, for example, in view of the fact that some batteries contain toxic heavy metals.
Firms Pavegen and PowerLeap are examples of players who are experimenting with piezoelectric flooring and pavement coatings that can extract electricity from waste energy from those who go on them. Trial installations on a small scale are already being tested worldwide. Integrated in environments such as railway stations, and other places where large crowds are moving, they may provide significant contributions to the local electricity supply. A variation of this could be roads that pick up vibrations from the road surface to drive traffic signals. Other perhaps less revolutionary but still practical applications may be contributing to the power of mobile devices, thereby reducing the power draw from the grid.
Solar, wind and wave power in general can of course also be seen as energy harvesting, albeit on a larger scale and higher up in the energy chain. For example, wind turbines could also be constructed on a micro scale by using piezoelectric materials, to meet local needs. A proposed such design includes long, swaying stems of carbon fibre reinforced pipes – as in Wind Talk project. The stems are made up of piezoelectric discs stacked on each other, which are deformed and build up when the wind blows.
Genetic engineering offers new material
For energy harvesting to have a major breakthrough requires that the materials be developed so that the technology becomes easier to scale up. One promising development is to cultivate piezoelectric materials in the form of genetically modified viruses. Many biological materials have piezoelectric effects, and they can be further enhanced in the laboratory. Virus-based materials would be easy to work with due to their ability to self-replicate and their capacity to naturally arrange themselves into a film-like structure.
Our need for energy is large and growing, and it’s easy to just turn our attention to the major issues associated with bigger energy producers. But to recover any of the energy that is about to ooze out of our hands can do their part in energy performance, while contributing to both environmental benefits and new technological opportunities.
This article was published in February 2014