Harvesting Human Power

People carry a lot of batteries around these days. The average person is lugging two to five batteries at all times in cell phones, iPods, laptops and other devices. Trades people and soldiers carry even more. But according to SFU assistant professor of Kinesiology Max Donelan, "Batteries can be a pain." They have very low energy density. "There’s a hundred times more energy in a granola bar than in an equivalent-sized battery," he says. That’s why he came up with a device that can take the energy from that granola bar and use it to charge your cell phone.
There is a catch: you have to eat the granola bar and then go for a walk. "Humans are efficient converters of chemical energy," says Donelan, whose amazing energy harvester fits on your knee and can charge your batteries as you walk.
Donelan is a physiologist who specializes in the biomechanics of walking. His PhD thesis (2001) was titled Mechanics and energetics of human walking.  He found that a person’s gate is largely defined by the energy expended transitioning between steps. Both walking too slowly and walking too quickly are not optimal.  "Everyone has a preferred walking speed which represents the minimal energy cost for that individual," says Donelan. He uses an inverted pendulum to model walking dynamics. Theoretically little or no energy is required to produce the swinging pendular motion of each step, but energy is required to make the transition from one leg to the other. The trailing leg expends energy acting as a motor, launching a person forward, while the leading leg simultaneously acts as a brake, requiring almost an equal amount of negative energy to complete the transition from one foot to the other.
"Unlike car brakes, muscles require energy to do negative work," says Donelan. To understand what he means, lift your hand up over your head. That act converts chemical energy in your muscles to potential energy in your hand. Now, let your hand fall back down. Unless you dropped your arm as dead weight, smashing your hand against whatever was directly below it, you more than likely moved your hand down in a controlled fashion. This required your muscles to do negative work, even though the force of gravity would have allowed you to flop your hand down "for free" like a rag doll. Animals never move like this. A controlled release of energy to counteract gravity is always applied.
It is precisely this energy that Donelan’s energy harvester collects. "It’s like regenerative braking," says Donelan, referring to the way electric cars slow down by making their rapidly rotating wheels recharge the batteries.
Human power is an attractive energy source. Muscles convert chemical energy with an efficiency of about 25%. For comparison, automobile engines only extract  about 18% of the energy of the gasoline they burn. The rest is lost as heat.
Donelan is not the first person to create electrical energy from human motion. Shoes which generate electricity by compressing piezoelectric (pressure sensitive) crystals in their soles produce about half a Watt of power. In 2005 an American physiologist named Larry Rome invented a backpack that converts the movement of a 38 Kg load into about 7 Watts of electrical power during fast walking.
A person does not have to carry much additional weight with Donelan’s energy harvester. It straps on across a knee joint just like an athletic knee brace. In fact the current prototype is a modified Generation II knee brace designed in Vancouver, BC. It weighs about 1.6Kg, but production units will be less than a kilogram. The functional heart of the device is a sensor system that detects an individual’s walking gait. Software works out the optimum point in the cycle to engage an electric generator when the muscles are doing negative work. "A special roller clutch engages only on extension of the leg," says Qingguo Li, lead research engineer on the project.
According to Donelan, the device does not interfere with walking. "When we engage it during treadmill experiments, subjects don’t even notice it, but when we disengage it they really notice because they have to start using their hamstrings to decelerate the leg at the end of the swing phase of a step." The unit produces 2.5 Watts from one leg in easy generative braking usage. To put it another way, you get 10 minutes of cell phone talk time for one minute of walking. If the wearer can tolerate a small increase in effort, a second mode of operation will generate up to 7 Watts per leg. Energy from the harvester will soon be used to charge batteries. Soldiers typically carry 13Kg of non-rechargeable batteries to power up to 30 electronic devices. Donelan sees his energy harvester coupled with a 1 Kg rechargeable lithium ion battery as an alternative for the military.
A civilian application might help amputees with power-assisted prostheses. An energy harvester on their good leg could charge the artificial leg’s batteries.
In March 2007, with the help of SFU’s University Industry Liaison Office, Donelan created a spinoff company called Bionic Power Inc. to bring his energy harvester to market. Many of the company’s six employees come from SFU. Qingguo Li is one of the founders of the company. He won the SFU Governor General’s Gold Medal for his PhD in 2006 and is now the control systems engineer. Kinesiology professor Andy Hoffer is another founder. Veronica Naing graduated in 2007 with a BA in Kinesiology from SFU. She is in charge of human subject testing at Bionic Power. CEO Yad Garcha helped raise the seed capital to launch the company. Donelan is the Chief Science Officer and he and his colleagues have applied for patents on the energy harvester technology.
As people continue to develop and use more portable technologies powered by batteries they will need increasingly reliable sources of energy. "Power from our bodies is efficient and portable," says Donelan. What’s more, people power is easy on the environment and there’s plenty of it around, but to get that power from a granola bar into a battery you have to understand how the human body works. That’s where Donelan and his company can show the way. Contact: http://fas.sfu.ca/locomotionlab/