Cutting the cord – Energy harvesting in wearables
This past Christmas my girlfriend got me a Fitbit Flex. I didn't ask for one or even express interest in the devices because I saw them as little more than glorified pedometers, but I took the hidden message with a grain of salt and began dutifully using my new wearable.
Then a couple of weeks later at the Consumer Electronics Show I realized that I forgot to pack my Fitbit charger. On day 3 of the show, "The Little Fitbit That Could" finally couldn't, so I decided to make my way over to Fitbit's booth to see about getting some juice back into the band. But on my way there I started wondering, "Why can't a device that's so intimately involved with motion and the human body take advantage of piezoelectric, thermoelectric, or some other energy harvesting technology so I'm not always at the mercy of cords and power outlets?" So I asked.
Granted it was an impromptu stop by and none of Fitbit's technical representatives were available when I arrived, so I just asked a young lady working the booth. She responded that they were always open to new ideas, fished through a giant bag of spare USB chargers they had stashed under the counter, and sent me on my way.
Wearable energy harvesting – where are we now?
Given that wasn't much of an answer, I decided to ring up Robert Andosca, President and CEO of MicroGen Systems, a startup out of Rochester, NY that develops MEMS-based energy harvesting technology (www.microgensystems.co), for some insight. According to Andosca, there are currently three viable technologies for energy harvesting in wearable devices – piezoelectric, solar, and thermoelectric. However, none of them is without its faults:
Piezoelectric – Piezoelectric energy harvesting has become a popular method of gathering excess energy produced by motion, and when operating in resonance mode (when all parts of a system operate at the same frequency and from a fixed point in time) can generate about a milliwatt of free energy. But, because piezoelectric devices often operate in the 100s of hertz, whereas humans normally move at around 10, it's necessary to impulse them over time to prevent the output signal from decaying. In impulse mode, piezoelectric harvesters yield only about 20 percent of the energy produced in resonance mode (or a couple hundred microwatts), which is nearly an order of magnitude less than the 2.19 milliwatt output power of my Fitbit Flex.
Solar cells – A solar cell roughly 1 in.2, or about the size of a watch face, can create 3 milliwatts of energy in direct sunlight. Unfortunately for wearables (and many other solar-powered devices), when not in direct sunlight that power production drops off significantly. The average person gets about 5 minutes a day of straight sunshine, and indoors, for example, solar cells harvest less than 20 microwatts of power. All things considered, this amounts in a typical daily output of 50-100 microwatts for solar cells of that size, which is only a fraction of the 675 microwatts required to energize Nike+ SportBands.
Thermoelectric – Thermal energy harvesting is another intriguing technology for wearables, as heat generated by the human body can potentially provide milliwatts of power – given that a 30-degree temperature differential is maintained between the skin and its surroundings. It's possible to achieve this in thermoelectric systems, but maintaining this ΔT in dynamic environments necessitates heat sinks and cooling fins to insulate energy harvesters can quickly balloon to the size of a few golf balls. Although they can be scaled down, with the size, goes the power.
The problem, as you can see, is that we, as consumers (and therefore the companies that manufacture our consumer devices), want wearables that are infinitely small, infinitely cheap, and infinitely powerful. For instance, Andosca explained to me that the current Samsung Galaxy smart watches incorporate piezoelectric energy harvesting technology that is currently 10 mm (L) x 10 mm (W) x 3 mm (D). In their next-generation devices, Samsung is looking at cutting those dimensions basically in half, to 5 mm x 5 mm x 2 mm, necessitating a 2x improvement in harvesting capabilities just to maintain the status quo in that form factor.
Wearables and the energy harvesting fashion police
Keep in mind that throughout this article when referring to the power consumption of specific wearables, I have been referring to the power draw of the entire wearable system. The sensors on devices like Fitbit typically only require a few microwatts of power, which is a low enough draw to be accommodated by any of the previously mentioned technologies. Where the real snag in wearable devices (and IoT devices in general) comes in is connectivity. Every time a Bluetooth, Wi-Fi, ZigBee, or other SoC pings the network to transmit data, an exponential amount more power is used than when sensors themselves are simply taking readings.
All of this comes down then to a question of batteries and system design. Simply put, if wearables were designed from the ground up with the complete system in mind (including the resonance, sunlight capture, temperature differential, etc. of humans that make up part of a wearable system), you could minimize the challenges of trying to turn smartphones into armbands, and potentially lose the battery altogether. A good place to start would be calculating the power consumption of your wireless chip and your transmission frequency (especially the frequency of your transmissions), comparing it with the energy generated by your harvesting technology, and going from there. Aside from this, and barring the advent of cold fusion or an innovation in materials, energy harvesting technology will remain a way to extend, rather than eliminate, batteries for the foreseeable future.
And with that, right on cue, my Fitbit died. Again.