Waste not, want not
[This is my latest Muse column for Nature News.]

We will now go to any extent to scavenge every last joule of energy from our environment.

As conventional energy reserves dwindle, and the environmental costs of using them take on an apocalyptic complexion, we seem to be developing the mentality of energy paupers, cherishing every penny we can scavenge and considering no source of income too lowly to forgo.

And that’s surely a good thing – it’s a shame, in fact, that it hasn’t happened sooner. While we’ve gorged on the low-hanging fruit of energy production, relishing the bounty of coal and oil that nature brewed up in the Carboniferous, this “spend spend spend” mentality was never going to see us financially secure in our dotage. It’s a curious, almost perverse fact – one feels there should be a thermodynamic explanation, though I can’t quite see it – that the most concentrated energy sources are also the most polluting, in one way or another.

Solar, wind, wave, geothermal: all these ‘clean’ energy resources are vast when integrated over the planet, but frustratingly meagre on the scales human engineering can access. Nature provides a little focusing for hydroelectric power, collecting runoff into narrow, energetic channels – but only if, like the Swiss, you’re lucky enough to have vast mountains on your doorstep.

So we now find ourselves scrambling to claw up as much of this highly dispersed green energy as we can. One of the latest wheezes uses piezoelectric plastic sheets to generate electricity from the impact of raindrops – in effect, a kind of solar cell re-imagined for rotten weather. Other efforts seek to capture the energy of vibrating machinery and bridges. Every joule, it now seems, is sacred.

That applies not just for megawatt applications but for microgeneration too. The motivations for harnessing low levels of ‘ambient’ energy at the scale of individual people are not always the same as those that apply to powering cities, but they overlap – and they are both informed by the same ethic of sustainability and of making the most of what is out there.

That’s true of a new scheme to harvest energy from human motion [1]. Researchers in Canada and the US have made a device that can be mounted on the human knee joint to mop up energy released by the body each time you swing your leg during walking. More specifically, the device can be programmed to do this only during the ‘braking’ part of the cycle, where you’re using muscle energy to slow the lower leg down. Just as in the regenerative braking of hybrid vehicles, this minimizes the extra fuel expended in sustaining motion.

While advances in materials have helped to make such systems lightweight and resilient, this new example shows that conceptual advances have played a role too. We now recognize that the movements of humans and other large animals are partly ‘passive’: rather than every motion being driven by energy-consuming motors, as they typically are in robotics, energy can be stored in flexing tissues and then released in another part of the cycle. Or better still, gravity alone may move freely hinging joints, so that some parts of the cycle seem to derive energy ‘for free’ (more precisely, the overall efficiency of the cycle is lower than it would be if actively driven throughout).

There’s more behind these efforts than simply a desire to throw away the batteries of your MP3 player as you hike along (though that’s an option). If you have a pacemaker or an implanted drug-delivery pump, you won’t relish the need for surgery every time the battery runs out. Drawing power from the body rather than from the slow discharge of an electrochemical dam seems an eminently sensible way to solve that.

The idea goes way back; cyclists will recognize the same principle at work in the dynamos that power lights from the spinning of the wheels. They’ll also recognize the problems: a bad dynamo leaves you feeling as though you’re constantly cycling uphill, squeaking as you go. What’s more, you stop at the traffic lights on a dark night, and your visibility plummets (although capacitive ‘stand-light’ facilities can now address this). And in the rain, when you most want to be seen, the damned thing starts slipping. The disparity between the evident common sense of bicycle dynamos and the rather low incidence of their use suggests that even this old and apparently straightforward energy-harvesting technology struggles to find the right balance between cost, convenience and reliability.

Cycle dynamos do, however, also illustrate one of the encouraging aspects of ambient energy scavenging: advances in electronic engineering have allowed the power consumption of many hand-held devices to drop dramatically, reducing the demands on the power source. LED bike lights need less power than old-fashioned incandescent bulbs, and a dynamo will keep them glowing brightly even if you cycle at walking pace.

Ultra-low power consumption is now crucial to some implantable medical technologies, and is arguably the key enabling factor in the development of wireless continuous-monitoring devices: ‘digital plasters’ that can be perpetually broadcasting your heartbeat and other physiological parameters to a remote alarm system while you go about your business at home [2].

In fact, a reduction in power requirements can open up entirely new potential avenues of energy scavenging. It would have been hard, in days of power-hungry electronics, to have found much use for the very low levels of electricity that can be drawn from seafloor sludge by ‘microbial batteries’, electrochemical devices that simply plug into the mud and suck up energy from the electrical gradients created by the metabolic activity of bacteria [3]. These systems can drive remote-monitoring systems in marine environments, and might even find domestic uses when engineered into waste-water systems [4].

And what could work for bacteria might work for your own cells too. Ultimately we get our metabolic energy from the chemical reaction of oxygen and glucose – basically, burning up sugar in a controlled way, mediated by enzymes. Some researchers hope to tap into that process by wiring up the relevant enzymes to electrodes and sucking off the electrons involved in the reaction, producing electrical power [5]. They’ve shown that the idea works in grapes; apes are another matter.

Such devices go beyond the harvesting of biomechanical energy. They promise to cut out the inefficiencies of muscle action, which tends to squander around three-quarters of the available metabolic energy, and simply tap straight into the powerhouses of the cell. It’s almost scary, this idea of plugging into your own body – the kind of image you might expect in a David Cronenberg movie.

These examples show that harnessing ‘people power’ and global energy generation do share some common ground. Dispersed energy sources like tidal and geothermal offer the same kinds of low-grade energy, in motion and heat gradients say, as we find in biological systems. Exploiting this on a large scale is much more constrained by economics; but there’s every reason to believe that the two fields can learn from each other.

And who knows – once you’ve felt how much energy is needed to keep your television on standby, you might be more inclined to switch it off.

References
1. Donelan, J. M. et al. Science 319, 807-810 (2008).
2. Toumazou, C. & Cass, T. Phil. Trans. R. Soc. Lond. B Biol. Sci. 362, 1321–1328 (2007).
3. Mano, N. & Heller, A. J. Am. Chem. Soc. 125, 6588-6594 (2003).
4. Logan, B. E. & Regan, J. M. Environ. Sci. Technol. 40, 5172-5180 (2006).
5. Logan, B. E. Wat. Sci. Technol. 52, 31-37 (2005).