Shoe Leather as a Renewable Resource: Penn Biologists Invent Power-Generating Backpack

From the Penn Office of University Communications
September 08, 2005

PHILADELPHIA -- If you already have a little spring in your
step, a team of biologists at the University of Pennsylvania
would like to put it 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."

Penn researchers involved in development and testing of the
Suspended-load Backpack at the Rome laboratory at Penn
include Louis Flynn, an engineer, and postdoctoral fellows
Evan M. Goldman and Taeseung D. Yoo. Funding for this
research comes from the Office of Naval Research and the
National Institute of Arthritis and Musculoskeletal and Skin
Diseases of the National Institutes of Health.