From power walking to walking for power: Chapman harnesses human motion to power electronics

If your cell phone or iPod battery runs out of juice and a free electric outlet is nowhere to be found, you would not be alone in experiencing some deep frustration. Someday, though, such problems could be a thing of the past due to research begun by ECE Associate Professor Patrick Lyle Chapman .

Nearly five years ago, Chapman noticed that batteries on portable electronic devices, like cell phones and MP3 players, seemed to be the first components of the device to fail.

“The battery was becoming a bigger part of the phone than the phone itself, or whatever the portable item was,” Chapman said. “We also noticed overall that, despite all the increased functions, the average power needed to power those functions was getting smaller.”

Chapman set out to see if the human body wasted energy when moving, and if so, if that energy could be harvested to power electronics. He received a grant from the Office of Naval Research to investigate whether human motion could serve as a power source for soldiers. 

Chapman began assessing which bodily motions would be good candidates for harvesting wasted energy. He found that humans walk inefficiently, using a lot of energy to stay balanced. For example, walking to the top of the Grand Canyon would be very tiring. Gravity would help the walk to the bottom be less strenuous, but it would still be tiring. This suggests that humans unnecessarily spend muscle energy.

A couple motions were found to exert excess energy. One was bobbing up and down while walking. The head and shoulders move sideways, creating constant wasted motion to stay balanced. The other motion was a forward knee kick when walking. Muscle energy must be exerted to move the leg forward, and exerted again to slow the leg. 

“The idea was to do regenerative braking for your body,” Chapman said. “It’s the same thing a hybrid car does, actually. When you brake, energy is going back into your battery that you can use later. In a normal car, it’s just burned up as heat.”

After the motions were discovered, Chapman began building small electric machines that could use that motion. The machines could be carried in a backpack, on a belt clip, or in a knee brace.

“We mostly investigated the optimal design of the machine to take advantage of the erratic and unpredictable motion of the human body,” Chapman said.

An electronic circuit was then needed to convert the power generated by the body to power that the electronic device could use. They found that from walking, anywhere from a half watt to two watts of power could be generated. A cell phone at that time would need one watt to function.

“If you assume a battery has about 10 watt-hours of storage, and that the human walking could produce 1 watt of electricity while walking, it would take a minimum 10 hours of walking,” Chapman said. “Therefore, if you typically recharge once every few days now, the human energy harvesting method is borderline for being able to replace recharging.” 

“The idea would be that if you were continuously walking around throughout a day, could you produce enough energy to offset the amount of talking you did,” Chapman said. “It turns out that sometimes you can, and sometimes you can’t. Either way, it reduced the number of times you would have to plug in.”

Chapman worked on the project for three years, and there are still questions about converting human motion to usable energy. Carrying a machine means carrying more weight. Different studies have shown that the weight is offset when walking quickly, but walking around in an office would not be efficient. This might mean that it would be easier to carry more batteries.  

But this technology does have an advantage over batteries.

“You must always recharge a battery with electricity,” Chapman said. “For the soldier in the field application, you actually don’t have a place to plug in. You can always walk around more and recharge. As long as the soldier has food, he or she has a source of energy for their equipment.”