Skip to content

A Knee-Jerk Reaction?

An article was published in Science on Friday (8 February 2008) reporting the results of a study on the generation of energy via an ‘energy harvester’ mounted on the human knee. The authors of the article begin by noting that humans are a rich source of energy. Indeed it seems that the average person stores the same amount of energy in fat as a 1000kg battery. There have been previous attempt to harvest this energy, including a device that generates energy from the compression of the sole of the shoe and a device that generates energy by harnessing the vertical oscillations of a spring-loaded back pack. However, it seems that none of these are as effective as the new knee-mounted device, which generates an average 5 watts of electricity per knee-mounted device. This is ten times more effective than the shoe-worn alternative and more effective than the spring-loaded backpack, which weighs 38kg. Each knee-mounted device weighs 1.6kg. However, the devices studied are prototypes and researchers working on the project are hopeful of making energy harvesters which are smaller and lighter.

      Wearing the energy harvester does require some extra effort from the wearer but this is minimal compared to other forms of human power generation. In a curious way the energy harvester makes walking easier. It is selectively engaged at the end of the swing phase of a stride, drawing power from ‘negative work’ in a way that is analogous to regenerative braking in hybrid cars. Energy that would otherwise be dissipated during the deceleration of the knee joint at the ends of a stride is harnessed and the process of doing this assists the deceleration of the knee joint.

      The scientists who have developed the device anticipate that it would be of use to generate power to recharge portable devices such as iPods, GPS locators and mobile phones. They also suggest that it could be used to help power medical devices such as pacemakers. These are all devices that may be away from conventional power sources for long periods of time. One motivation for using the energy harvester is to be able to generate power at times when we are away from sources of conventional power. However, there may be another motivation. In much the same way that people choose to take their own bags to supermarkets – in order to reduce waste – people may choose to use devices such as the energy harvester to reduce their drain on conventional sources of power and thereby help reduce carbon emissions.

      If the use of energy harvesters was to become common, and the threat of global warming remained present, we could imagine that there would be social pressure bought to bear on all able-bodied people to use energy harvesters or other devices to harness energy from their own bodies. Perhaps in some societies people would be required to use energy harvesting devices whenever they walked or otherwise expended significant amounts of energy. New technologies open up new possible ways of structuring society and it is worth our while to try to conceive of ways in which societies may develop as a result of technological innovations. That way we may be able to shape our future technology, instead of simply allowing future technology to shape us. But of course we should be wary of ‘knee-jerk reactions’.

Share on

1 Comment on this post

  1. I think this is a very interesting development. My fear would be that this new energy source would just spawn new ways of using that energy – witness the modern trend in fibre-optic clothing that can display the users messages. If these generators were used for this and nothing more, then their help in tackling issues of carbon emissions, for example, would be minimal. So I would be interested to hear how this new source of energy can be used to provide for our existing energy needs, rather than creating new energy markets.

    I guess this is the general message that looking at new ways of generating energy is not enough to tackle issues of carbon emission – we have to look at how that energy is used aswell. Input and output are both elements of the equation.

Comments are closed.