Shoe Leather As A Renewable Resource: Penn Biologists Invent Power-Generating Backpack

"As efficient as batteries have gotten, they still tend to be heavy.Field researchers, for example, have to carry many replacementbatteries to power their equipment, which take up a lot of weight andspace in the pack," said Larry Rome, a professor in Penn's Departmentof Biology. "The Suspended-load Backpack could help anyone with a needfor power on the go, including researchers, soldiers, disasterrelief-workers or someone just looking to keep a mobile phone chargedduring a long trek."

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

The Suspended-load Backpack is based on a rigid frame pack, muchlike the type familiar to hikers everywhere; however, rather than beingrigidly attached to the frame, the sack carrying the load is suspendedfrom the frame by vertically oriented springs. It is the verticalmovement of the backpack contents that provides the mechanical energyto drive a small generator mounted on the frame.

Previous efforts to solve dilemma of the over-burdened soldierincorporated devices placed in the heels of boots. According to Rome,however, little mechanical work is actually done at the point where theboots hit the ground.

"As humans walk, they vault over their extended leg, causing the hipto rise 5-7 centimeters on each step. Since the backpack is connectedto the hip, it to must be lifted 5-7 centimeters," Rome said. "It isthis vertical movement of the backpack that ultimately powerselectricity generation."

The amount of power generated depends on how much weight is in thepack and how fast the wearer walks. The Penn researchers tested packswith loads of 40 to 80 pounds and found that the wearer couldconstantly generate as much as 7.4 Watts while moving at a steady clip.Typically, cell phones or even night vision goggles require less thanone Watt to power.

Contrary to what might be expected, wearing the Suspended-loadBackpack does not use up much more metabolic energy than walking whilewearing a conventional backpack of the same weight. According to Romeand his colleagues, it is likely that wearers can change their strideto compensate for movement of the load, which makes walking moreefficient.

"Metabolically speaking, we've found this to be much cheaper than weanticipated. The energy you exert could be offset by carrying an extrasnack, which is nothing compared to weight of extra batteries," Romesaid. "Pound for pound, food contains about 100-fold more energy thanbatteries."

Penn researchers involved in development and testing of theSuspended-load Backpack at the Rome laboratory at Penn include LouisFlynn, an engineer, and postdoctoral fellows Evan M. Goldman andTaeseung D. Yoo. Funding for this research comes from the Office ofNaval Research and the National Institute of Arthritis andMusculoskeletal and Skin Diseases of the National Institutes of Health.