Engineers agree device power is the most challenging design problem.
Fitness trackers may have kicked off the wearables craze, but counting steps is just the tip of the iceberg when it comes to their functionality today.
As wearable devices grow in popularity, so does interest in energy harvesting technology, particularly to support secondary battery power or mitigate primary battery usage. In a recent survey, 69% of engineers say it will take some innovation, but will hit mainstream one day.
From wearables that detect glucose, ketones and lactate to monitoring blood pressure and even brain waves, the capabilities continue to expand. Some 303 million devices are expected to ship in 2024 (CCS Insight).
While engineers are overcoming many of the complexities associated with designing wearable devices—form-factor and materials to name just two—how to power them continues to vex. In fact, in a recent survey by Molex on diagnostic wearables, 38% of engineers reported that power is among the most challenging areas when designing wearables.
As they pack on features and functionality, designers still want to deliver acceptable runtimes without increasing battery or device size. An appeal of energy technology is that it’s highly complementary to batteries as a secondary power source and helps to mitigate primary battery usage.
Of course, the concept of energy harvesting isn’t new: The automatic winding mechanism of mechanical watches—which harvests kinetic energy from the human body in motion—was developed centuries ago. The first solar-powered watch debuted in 1972.
But once battery-operated quartz watches came on the scene, they quickly gained in popularity due to their lower cost and high reliability.
Now, energy harvesting is getting renewed interest. In fact, in the Molex survey, 69% of engineers say that while it will take some innovation, the technology will one day get here. When asked about which type of energy harvesting beyond solar will have practical application in diagnostic wearables, movement topped the list (49%) for engineers, followed by body heat (35%) and sweat (13%.)
“Every engineering team designing wearables that we’ve worked with recently wants to investigate energy harvesting as a potential power source,” says Walt Maclay. He is president of the design consultancy Voler Systems, which specializes in medical devices and wearables.
But while interest is high, he says it all comes down to weighing the trade-offs in energy harvesting versus batteries—namely size, power, complexity and cost. And, at least for his clients to date, batteries still reign supreme. “One of the issues is that energy harvesting today generates microwatts, while most of these devices need milliwatts,” Maclay pointed out. “Energy harvesting is a hot topic, but in the end a battery is the better solution for some design teams.”
From piezoelectrics to human motion
Research today in the use of energy harvesting for wearables abounds. At the University of Utah’s Laboratory of Integrated Self-Powered Sensing, for example, a team is investigating energy generation from human motion. They recently developed a prototype of a wrist-worn harvesting system using petal or star-shaped piezoelectric elements.
In another novel approach, scientists at the University of Surrey are working on wearable devices made entirely from recycled waste materials that run on energy harvested by the user’s movement.
And in work funded by the National Science Foundation, NASA, Rice University and others, researchers are working on a textile-based system based on pneumatic energy captured during walking to power wearable, assistive robotics.
Not an “all or nothing” proposition
Brian Zahnstecher, principal at PowerRox LLC, a design consultancy dedicated to solving power problems, concurs that interest in energy harvesting is on the rise. “Seven or eight years ago you might get laughed out of the room, but people are taking it a lot more serious today. We are at a point now where we have mainstream products based on the technology and a significant amount of research activity going on.”
He pointed out that energy harvesting technology should best be viewed in harmony with the minimization of power consumption and better batteries and storage devices. “With wearables, energy harvesting is not an all-or-nothing proposition. What we’re seeing is a focus on the balance of the increase in available energy and the decrease of the system’s overall power budget, and there are multiple scenarios to be explored.”
That could mean, for example, that energy harvesting is used to augment battery power—not replace it. Similarly, concerted efforts to reduce system power in a design may mean features can be added without needing supplemental power or a bigger battery at all.
As to whether wearables will be the killer app for energy harvesting, Zahnstecher thinks that the technology may take off faster in implantable medical applications, many of which are likely to require more modest power levels in the sub-milliwatt range.
And an even stronger motivation that just may kick energy harvesting into high gear, he stressed, might well turn out to be sustainability. “Within just a few years, we are looking at the potential for hundreds of billions or even a trillion wireless IoT devices in circulation. At that point, we’re likely to come to the conclusion that we cannot be tossing 100 million or more primary batteries a day into the landfill.”