Shoe leather as a renewable resource: Penn biologists invent power-generating backpack

If you already have a little spring in your step, a team of biologists at the University of Pennsylvania would like to them to good use by adding a few more springs in the form of a power-generating backpack. Details of their prototype “Suspended-load Backpack” were announced today in the journal Science. The device converts mechanical energy from walking into electricity – up to 7.4 Watts – more than enough energy to power a number of portable electronic devices at once.

“As efficient as batteries have gotten, 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 Penn’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.”

Although “biologist” might seem like an unlikely job title for a mechanical inventor, Rome has found his study of muscular systems of locomotion to be directly applicable to the work. During the war in Afghanistan, the Office of Naval Research approached Rome to develop a means to assist over-burdened soldiers who must carry as much as 20 pounds of spare batteries required to power high-tech equipment such as global positioning systems, communications and night vision devices. A typical soldier already marches into the battlefield carrying 80 pounds of gear, so Rome sought a way to capture the mechanical energy of marching in order to charge a lightweight rechargeable battery that could replace all the spares.

The Suspended-load Backpack is based on a rigid frame pack, much like the type familiar to hikers everywhere; however, rather than being rigidly attached to the frame, the sack carrying the load is suspended from the frame by vertically oriented springs. It is the vertical movement of the backpack contents that provides the mechanical energy to drive a small generator mounted on the frame. Previous efforts to solve 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 centimeters on each step. Since the backpack is connected to the hip, it to must be lifted 5-7 centimeters,” Rome said. “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 40 to 80 pounds and found that the wearer could constantly generate as much as 7.4 Watts while moving at a steady clip. Typically, cell phones – or even night vision goggles – require less than one Watt to power.

Contrary to what might be expected, wearing the Suspended-load Backpack does not use up much more metabolic energy than walking while wearing 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 efficient.

“Metabolically speaking, we’ve found this to be much cheaper than we anticipated. The energy you exert could be offset by carrying an extra snack, which is nothing compared to weight of extra batteries,” Rome said. “Pound for pound, food contains about 100-fold more energy than batteries.”