Once the purview of upstart companies, energy harvesting—the use of free ambient energy to drive remote sensors, actuators, and low-power electronic devices—is now embraced by industrial heavy-hitters such as ABB, Analog Devices, Texas Instruments, and Schneider Electric. The harvested energy may come from ambient light, heat, motion, radio waves, or some combination of these. In general, we use energy harvesters to either operate a device without batteries at all or to prolong the life of batteries by using captured energy to supplement or recharge the batteries. The technology is appearing in home automation products, building management systems, industrial IoT devices, portable medical equipment, and other applications.
Energy harvesting has grown from a niche market a decade ago to a $500 million dollar industry, and is expected to increase to nearly a billion dollars by the year 2027. Let's take a look at the current state of the technology and see if your next design can be powered partly or entirely by free energy.
Energy Harvesting Switches
Nearly all grid electricity is based on the concept of electromagnetic induction: moving a magnetic field across a wire induces a current in the wire (the photovoltaic effect is one noteworthy exception, as it converts light directly into electricity). Utility-scale generators employ powerful magnets, massive coils of wire, and enormous mechanical forces to generate the juice that keeps our big machines running, but the same concept can be miniaturized to allow smaller forces to produce trickles of electricity.
For example, one of ZF's energy harvesting switches converts a force as low as 13 N (around 3 lbs) into one-third of a millijoule (0.33 mWs), enough energy to send a brief radio signal to a receiver up to 300 meters away at frequencies compatible with RF communication standards such as KNX-RF, ZigBee or Bluetooth Low Energy. That's with no obstructions; walls will cut the maximum range to 30 meters, still more than adequate for wireless doorbells, IoT devices, and building automation systems. Engineers looking to incorporate energy harvesting switches into their designs can experiment and build prototypes using an evaluation board from ON Semiconductor.
Piezoelectric
The piezoelectric effect stems from the properties of certain materials such as quartz. When a force squeezes a piezoelectric element, an electrical charge appears across the element. Stretching the element in the opposite direction generates a charge with the opposite polarity. Exposing the device to vibrations will produce a small AC voltage, which can be harvested, rectified, and used to power a circuit.
Piezoelectric devices with outputs in the milliwatt range are often used in vibration sensing, such as monitoring a machine for potential wear and tear, which facilitates predictive maintenance. Biomedical engineers are examining piezoelectricity to create self-powered implants, such as pacemakers, which are currently powered by batteries that need to be surgically replaced every few years.
For a thorough discussion of piezoelectric devices and their applications, see A review of energy harvesting using piezoelectric materials: state-of-the-art a decade later (2008–2018).
Thermoelectric
Photovoltaic panels provide power to the International Space Station, many orbital satellites, and some early Mars rovers. Due to its distance from the Sun, Mars is pretty much the cut-off point for solar power, but because of its dusty atmosphere, NASA opted for a different technology for the Mars Perseverance rover: thermoelectric generation (TEG), which is also the power source of choice for deep space missions. Taking advantage of the Seebeck effect, where a difference in temperature is converted to a voltage, these TEGs have no moving parts and are extremely reliable. How reliable? The Voyager missions, which are still transmitting data from beyond the solar system, are running on 45-year-old TEG technology.
TEGs require a temperature differential (i.e. a hot side and a cold side). Space provides the cold and the heat comes from a small amount of a radioisotope that emits heat as it naturally decays. The difference is enough to deliver more than 100 watts of power, which is used to power the rover and recharge the batteries that provide auxiliary power when the rover requires a little extra boost.
But you don't need a NASA budget or radioactive materials to take advantage of TEGs in Earth-bound applications. In fact, a high-school student created a battery-free flashlight using a hollow aluminum tube, a few LEDs, and a TEG. For many IoT applications that include low-power microcontrollers, TEG energy harvesters get the job done.
5G
Radio waves are a form of electromagnetic energy. Some antennas are directional, but many radio transmitters, like AM/FM, broadcast TV, Wi-Fi, and cell towers are omnidirectional. As a result, stray radio waves don't reach a receiving antenna; instead, they're absorbed by their surroundings. Freevolt developed an energy harvesting device that grabs this otherwise wasted energy and uses it to power electronic devices or to trickle-charge a device's batteries. Now, researchers are finding ways to turn 5G into a wireless power network. Electromagnetic energy from multiple directions is absorbed by antennas, and these waves are focused by a Rotman lens on the receiver. The resulting AC signal is rectified to produce several microwatts of DC power. Two Georgia Tech researchers describe some of the applications of 5G energy harvesting:
Video courtesy of Georgia Institute of Technology.
Optical
The first battery-free electronic device I owned was the solar-powered scientific calculator I bought when I was a graduate student in the mid-1980s (and it still works, by the way.) A mere four square centimeters of photovoltaic cells deliver enough power to drive the calculator's not-so-low-power microprocessor, support circuitry, and LCD, and it works just as well indoors under artificial lights as it does outside in full sun (this was far better than the TI-55 I used as an undergrad, which required me to carry around a spare 9V battery, especially on exam days!)
Epishine, a company that specializes in printable organic solar cells that are optimized for indoor, low-light conditions, has a cell that produces 2 µW/cm² under low light and 40 µW/cm² in a supermarket setting. The company has partnered with Evonik, a maker of printable rechargeable batteries, and Ynvisible, who makes low-power printable displays, to develop battery-free dynamic signage that can be used in stores, museums, and other public buildings. Evonik's batteries can produce up to 6.5 mWh of energy (1.3 V, 1 to 5 mAh) per cell. For higher voltages, currents, and capacities, the cells can be combined in series or parallel. Ynvisible's displays require less than 1 W/cm2; it only draws power when the display changes and during brief refresh cycles.
Are you using energy harvesting to power your products? Do you see something that might be useful in an upcoming design? Feel free to comment below and tell us about your project.