EVERY hour thousands of cars on one of the city鈥檚 busiest bypasses hurtle past your door. But rather than cursing them, you鈥檙e pleased, for each one that roars by shaves a little off your electricity bill. You鈥檙e also cheered by shoppers in the city centre. As they bustle about they generate power that helps light the streets. These people could even be helping themselves, recharging their laptops or mobile phones as they pound the pavements or simply eat their lunch.
This is a snapshot of a future that some scientists see as inevitable. They believe that traditional methods of generating and supplying power simply aren鈥檛 flexible enough. Why should we have to rely on cables linked to the electricity grid, they ask? And who wants the hassle of replacing or recharging batteries, especially when wireless gadgets take off?
They鈥檝e realised that there is a lot of unused energy flowing around in our local environment that we might be able to tap into. Speeding traffic, vibrating machinery, stray radio waves, even families on the high street are all fair targets for these power pirates.
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Of course, we鈥檙e already practised energy scavengers, having harnessed power from the wind and running water for millennia. And in the past 30 years, solar cells have spread everywhere from rooftops to watch straps. But these forms of energy collection just aren鈥檛 practical or versatile enough to provide for anticipated technological revolutions such as ubiquitous computing, in which tiny, ultra-low-power microprocessors will be built into everything from fridges, clothes and electronic door keys to street signs and bus stops. Researchers say that these devices will be able to communicate with each other, and with chips built into our phones and wallets.
So where will we get the energy to power all these devices, asks Thad Starner, an expert in power scavenging at Georgia Institute of Technology in Atlanta. Take smart bus stops or street signs, for instance. 鈥淵ou could put batteries in them, but who鈥檚 going to change them all as they die?鈥� The answer, Starner believes, is to power them using energy harvested from the very streets they serve.
It鈥檚 a strategy that appeals to Shad Roundy, a researcher at the University of California in Berkeley. He is pinning his hopes on converting the myriad vibrations that surround us every day into electricity. He has even set out to discover the best sources, by attaching sensitive accelerometers to a range of machines from microwave ovens to cars.
Roundy has found that most high-energy vibrations have frequencies in the range from 75 to 150 hertz. This has led him to focus on devices that produce these vibrations in abundance: car engines, industrial machinery and air-conditioning vents, for example. Kitchen appliances such as washing machines and food blenders also offer rich pickings.
To harvest the power, Roundy uses two thin strips of lead zirconate titanate (PZT) stuck together to form what he calls a 鈥渂imorph鈥� less than half a millimetre thick. PZT is a piezoelectric material: stretching or compressing it creates a voltage across its surface. When he tested his bimorph on a car engine, the vibrations set up a voltage that he used to push small currents around a circuit. Output was only about 80 microwatts 鈥� not much, but certainly enough to power a small sensor for monitoring oil pressure or engine temperature, for example. The sensor could send its readings as radio signals to the vehicle鈥檚 engine control system.
However, you don鈥檛 have to use high-tech components to scavenge power. For example, Gary Henderson, an engineer at Gravitational Systems in New York, wants states across the US to dig up their roads and install his pumps at regular intervals. The roadways themselves would then generate electricity as cars drove along them.
Each pump consists of a metal plate that sits on a liquid-filled bladder. When a car drives across it, the vehicle pushes the plate down by a centimetre or two. This forces liquid out of the bladder through a one-way valve and into a turbine. Each pump is capable of generating about 80 watts of electricity each time a car runs over it, and this power is stored in rechargeable batteries or capacitors while the fluid is returned to the pump via an inlet valve. On its own this output may not seem like a lot, but Henderson calculates that if 10 per cent of California鈥檚 drivers drove over two of these pumps per mile, it would generate 3 gigawatts of power.
Henderson has already sold a number of his pumps to farmers in Montana and Oregon, where power harnessed by cows walking across them will be used to pump water. But he also sees a world in which electricity generated by passing vehicles could benefit local communities. First, built as raised speed bumps, his pumps would help slow traffic down. Secondly, the electricity they generate can be stored for later use. 鈥淭raffic is such a source of power that you could actually run all of the street lighting and road signs from it,鈥� he explains. Instead of having toll roads, drivers would simply generate some energy as they pass. 鈥淐ommunities could be completely powered by the roads that run next to them.鈥�
Shopping for joules
Of course the energy isn鈥檛 free. Running over the pumps would raise fuel consumption, but Henderson insists the increase would be minimal. Offsetting that, other research suggests that fluid-filled speed bumps produce less wear on vehicles than solid ones. Trials on roads in Delaware, which should begin this year, will show how effective his system is.
Henderson鈥檚 hopes extend beyond motor transport. If cows can generate power as they walk, what about humans? By replacing paving stones around Macy鈥檚 department store in Manhattan, for example, with a flatter version of the pump, you could generate more than 2kilowatts of power from everyone who walked the length of the storefront, he says.
But why go to all the bother of digging up the streets in the first place? Ron Pelrine at research company SRI International in Menlo Park, California, is developing boots that generate power from every step the wearer takes. Inside the heel of the boot is a disc made from a modified silicone polymer that works much like Roundy鈥檚 piezoelectric PZT strip. Squeezing the polymer produces an imbalance of electric charge between its top and bottom surfaces. Connect electrodes to the faces of the disc and the resulting voltage difference can be used to drive current around a circuit. A similar device already powers LEDs built into some sports shoes.
Right now, Pelrine says, his modified shoes can produce up to 0.8 watts of power each, but he thinks they will eventually be able to generate 2watts, enough to run soldiers鈥� radios or navigation equipment, for example, and keep their rechargeable batteries topped up. And because the polymer disc replaces material inside the boot鈥檚 heel, it shouldn鈥檛 add any extra weight or be noticeable to the wearer.
It is even possible to scavenge power while sitting still. Starner鈥檚 latest project, in conjunction with researchers at Delft University in the Netherlands, is to make ceiling tiles that grab energy from ambient AM radio waves. His calculations show that he should be able to harvest twice the power needed to run a small microprocessor by intercepting the signal from a radio station 1 kilometre away. That鈥檚 not as simple as it sounds though. 鈥淚n the US this is frowned on by the regulator, the Federal Communications Commission, because if you use real AM radio, you鈥檙e interfering with someone else鈥檚 signal,鈥� Starner explains. 鈥淚t鈥檚 like there鈥檚 a big mountain in the way.鈥�
So he plans to generate his own radio waves and use them to drive an experimental network of low-power radio beacons that robots, initially, could use to find their way around his lab. Starner suggests fixing beacons like these to signposts and at road crossings to help partially-sighted people with special receivers to navigate around the streets.
Taking this idea a step further, Brian Berland at ITN Energy Systems in Littleton, Colorado, is developing special antennas that can absorb visible light as well as radio waves. In essence, he says, the antennas are ultra-efficient solar cells that can be tuned to collect electromagnetic energy over wide frequency bands. He鈥檚 already proved the idea works at some frequencies.
It鈥檚 an approach that has huge advantages over traditional solar cells. 鈥淭he problem with regular semiconducting solar cells is that there鈥檚 a fundamental limit to how much of the Sun鈥檚 energy you can capture,鈥� Berland says. In practice that means that usually only about 20 per cent of the energy can be harvested. By contrast, Berland calculates that his devices should be able to grab more than 90 per cent, perhaps over 90milliwatts per square centimetre.
The antennas are made from tiny wire grids forming six-by-six squares. Each side of each square acts as a mini-antenna capable of absorbing energy from short-wavelength electromagnetic radiation towards the visible end of the spectrum. But the trick is that these mini-antennas can add together lengthwise to form longer antennas, to absorb radio waves over a wide range of much longer wavelengths.
That鈥檚 the theory, at least. A device based on it is not fully working yet. To make the antennas work for visible light, Berland鈥檚 team must build grids that are about half a micrometre wide, barely visible to the naked eye. And to ensure the device produces a direct current, each mini-antenna in the grid must have a 10-nanometre-long diode so that current flows through the wire in only one direction. Berland says making diodes that small is not easy, but it is possible.
Energy from the airwaves
Berland, whose company is working on the design with the US Defense Advanced Research Projects Agency (DARPA), says the antennas could be used to grab energy efficiently from ambient light and background radio waves to power mobile phones or soldiers鈥� radios. They could also be fitted around radio masts to absorb stray radio waves and recycle them as electricity.
As electronic devices becomes less and less power-hungry, ever more unlikely means could be used to power them. Starner has already worked out how much electrical energy can be captured from the beat of your pulse and the air pressure created as you breathe, not to mention the heat your body generates 鈥� which can produce up to 60 watts of power. Adam Heller, a biochemical engineer at the University of Texas, Austin, is even developing a tiny fuel cell that can generate electricity from the glucose and oxygen in your bloodstream. Attach one of these to your aorta and when you want to recharge your batteries, simply reach for a sandwich.
But scavenging energy from others raises a question: is the stuff really yours to take? If we are ever to power ourselves or the communities we live in with energy from passing pedestrians or vehicles, we鈥檒l all have to agree on one thing: no one will gain if we don鈥檛 all give a little.
