Can Energy Scavenging Save The World?

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Ambient heat, light, sound and radio can all be harnessed to power devices. It is well worth doing, says Peter Judge

I’m seeing a common thread in some recent stories, and it’s an interesting one. Through a bewildering array of techniques, scientists are finding ways to “harvest” waste energy, picking up and using some of the various kinds of energy that are by-products of other activities.  But could this have a real impact on our energy demands – and cut emissions?

There is a lot of wasted energy about. Running servers makes heat, and most activities produce noise. There are also a lot of radio signals surrounding us. Solar farms and wind turbines are giant instances of energy harvesting, but there is a lot of interest in using smaller systems to gather enough ambient energy to power a small device.

Doing away with batteries

Each time this is done, that is one more object that doesn’t have to include replaceable or rechargeable batteries (which use energy and chemicals to make) or regularly draw energy from the mains.

This summer, we’ve heard of an effort at UCLA to let mobile phone screens harvest sunlight as well wasted energy from the phone’s LCD screen backlight. It’s a lovely idea – the screen can use about 80 percent of the phone’s power, but a lot of that is absorbed back into the polarisers which make the dark parts of the  screen image.

Convert those polarisers into photocells, and you can recycle some of that energy. Add in the possibility of the screen multi-tasking as an actual solar panel, and you have a distant but conceivable prospect of a smartphone charging itself.

Solar powered phones are a possibility already, of course, but require a separate solar panel on the back of the phone, a hot country and the undemanding usage typical of a feature phone.

Other researchers have suggested harnessing the energy with which users hit the keys on a phone or laptop using a piezoelectric film, and still others (including a Nokia patent) have proposed scavenging power from the vibration created as devices are shaken.

Thermal energy can also create electricity, and it has been suggested that festival goers could charge their phones through thermal harvesting boots – although festival goers might also have piezo-electric T-shirts harvesting ambient sound. Those last two ideas are from Orange.

Scavenged energy is loose change

Radio signals can be harvested, using cheaply printed antennas (shown here), which could make sensors much easier to deploy and use. I like this because mobile devices don’t normally get deployed outside the areas where you can get a radio signal.

However, all these schemes are using the energy equivalent of loose change. When you put energy into a device, the waste is a few percent of the total. Getting useful energy back from that is the equivalent of raiding the penny jar.

The human race has a very large energy debt. It is using more energy than it can generate – in the long term – from the sources it currently uses. And unfortunately, you can’t pay off your mortgage in small change.

When considering energy harvesting, you have to remember that it is usually more efficient to reduce the waste in the first place, and don’t put harvesting measures in place that actually impact on usage (thermo-electric converting boots will make your feet warmer, for instance).

But despite all this, scavenging is still very much worth doing. Eliminating a power source for a mobile device can reduce its impact in many ways, including the manufacture of batteries or the cost of generating and transporting the electricity used to charge it.

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