The Gleaning Power of Piezo

New Approaches to Harvesting Energy from the Environment

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By Brian Fuller

The inventor and green-tech missionary Trevor Baylis walked 100 miles across an African desert nearly 10 years ago to prove piezoelectric technology could power a cell phone. It was an interesting story that quickly faded from memory, as the technology was deemed clever but impractical.

Enter Elizabeth Redmond and Andrew Katz, who have taken Baylis’ inspiration and piezo’s promise to the next level: The dance floor.

Redmond and Katz, only a few years removed from college, are an unlikely pair. She’s a hip artsy design type, graduated from the University of Michigan. He’s a mechanical engineer from Duke who took an early career detour to Wall Street. Together they’re breathing new life into a technology that, while promising, always seems to be the next big thing in low-power design.

She was working on her graduate thesis trying to figure out ways to generate electricity from the human body to power, among other things, an iPod music player.

Synchronicity?

Katz had published his thesis after participating in a Home Depot-sponsored project at Duke on the home of the future. Redmond came across the paper, “Residential Piezoelectric Energy Sources,” which was published in July 2004.

Redmond had been inspired in part by Baylis’ work on piezo ped power. Coming across Katz’s paper helped connect the dots.

“I was thinking about designing attachable instruments that would harvest more personalized energy to power things like the iPod,” she said. “Then my final solution was the flooring design. The reason was we settled on this it’s communal rather than personal use.”

Katz’s paper was influenced by other university work into piezoelectric-powered applications, including studies into smart flooring that tracks movement (Georgia Institute for Technology) and investigations into shoe-mounted piezoelectronics (MIT).

The promise and the challenge

The piezoelectric effect is well known, its first public demonstration being held in 1880 by Pierre and Jacques Curie. When compressed or tensile stress is induced in a material, an electric field is generated across it, creating a voltage gradient and a current. Crystals, plastic and ceramics are excellent materials for generating the piezoelectric effect.

MIT research, according to Katz, used two different materials in its test to generate power from foot strikes: polyvinylidene fluoride (PVDF) and lead zirconate titanate (PZT). The former material produced 1.3 mW per foot strike, and the latter 8.4mW.

Katz’s paper referenced experimental work on wireless sensors by MicroStrain Inc. that put piezoelectric transducers on support beams in a structure. Because the structure was constantly under strain, the piezoelectrics captured the voltage and storage it in a capacitor. After a certain level, the power was used to drive a transmitter that sent a wireless signal to a receiver. MicroStrain reported that the cycle time was about 20 to 80 seconds to store up a charge of 9.5 V on the capacitor given the size of the piezoelectric was 17 cm2.

Green-technophiles love piezo’s promise, but it comes with a price. It’s not cheap. A simple device involves an AC/DC rectifier, a filter capacitor and a DC-DC converter. The transducer used to transfer pressure into energy isn’t cheap, however. In some cases, costing more than $100. In Katz’s paper, which focused building a system to track movement, that made his flooring very expensive. A more cost-effective approach, he found, was piezoelectric cable, woven into a network that captures the footfalls. That cabling (in 2004 prices) would cost $30,000 to line a house at 4 inches apart.

Moving forward

Redmond said the two of them and their company, PowerLeap LLC, are working with flooring manufacturers and architectural designers to commercialize the product, and they’re researching ways to create the most efficient manufacturing process and supply chain to enable broad-market costs. They’ve opened discussions with several IC vendors whom they won’t name.

Currently their prototype design consists of a ceramic piezoelectric compound sandwiched between the top of the rigid flooring and the substrate. While neither Katz nor Redmond will talk publicly about their architecture in any detail during the patent-approval process, it’s designed to be modular so flooring can be fitted in pieces and if one portion goes down, it won’t take the entire flooring network with it.

Right now, they’re targeting $100 to $200 per square foot. By comparison, laminate flooring costs $1-$6 per square foot, while hardwood floors can run $5 to $15/sf. Of course, that flooring doesn’t generate electricity.

PowerLeap is currently working with an unnamed “green” dance club in San Francisco to install its piezo flooring there as a test bed.

“Some of our initial calculations that we’ve come up with in the lab is that standard pedestrian foot traffic will generate 1 watt-hour/sf, while a dance floor will generate 10 w-h/sf,” Redmond said.