US researchers have put a spring in their step with the development of an electricity-generating backpack that turns the normal movement of walking into electrical power.
The prototype Suspended-load Backpack, developed by a team of biologists at the University of Pennsylvania, converts mechanical energy from walking into up to 7.4 watts of electricity, more than enough to power a number of portable electronic devices.
"As efficient as batteries have become, they still tend to be heavy. Field researchers, for example, have to carry many replacement batteries to power their equipment, which take up a lot of weight and space in the pack," said Larry Rome, a professor in Pennsylvania's Department of Biology.
"The Suspended-load Backpack could help anyone with a need for power on the go, including researchers, soldiers, disaster relief workers or someone just looking to keep a mobile phone charged during a long trek."
The impetus for the research came during the recent war in Afghanistan when the Office of Naval Research approached Rome to develop a means to assist over-burdened soldiers.
The soldiers have to carry as much as 20lbs of spare batteries to power high-tech equipment such as global positioning systems, communications and night vision devices.
A typical soldier already marches into the battlefield carrying 80lbs of gear, so the researchers sought a way to capture the mechanical energy of marching to charge a lightweight battery that could replace all the spares.
The Suspended-load Backpack is based on a rigid frame pack, much like the type familiar to ramblers. However, rather than being rigidly attached to the frame, the sack carrying the load is suspended from the frame by vertically oriented springs.
The vertical movement of the backpack contents provides the mechanical energy to drive a small generator mounted on the frame.
Previous efforts to solve the dilemma of the over-burdened soldier incorporated devices placed in the heels of boots. According to Rome, however, little mechanical work is actually done at the point where the boots hit the ground.
"As humans walk they vault over their extended leg, causing the hip to rise 5-7 centimetres on each step. Since the backpack is connected to the hip, it too must be lifted 5-7 centimetres," said Rome.
"It is this vertical movement of the backpack that ultimately powers electricity generation."
The amount of power generated depends on how much weight is in the pack and how fast the wearer walks. The Penn researchers tested packs with loads of 40lbs to 80lbs and found that the wearer could generate as much as 7.4 watts while moving at a steady clip.
Cellphones, or even night vision goggles, typically require less than one watt of power.
Contrary to expectations, wearing the Suspended-load Backpack does not use up much more metabolic energy than walking with a conventional backpack of the same weight.
According to Rome and his colleagues, it is likely that wearers can change
their stride to compensate for movement of the load, which makes walking more
"Metabolically speaking, we've found this to be much cheaper than we anticipated," said Rome.
"The energy you exert could be offset by carrying an extra snack, which is nothing compared to weight of extra batteries. Pound for pound, food contains about 100-fold more energy than batteries."
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