The synergy between wireless, portable sensor products, and energy harvesting has created one of the newest hot topic areas in sensing. Today, the potential for "free energy" certainly gets engineers' attention. However, the broad range of market possibilities discussed for its near-term usage is also important since it includes interesting high-volume applications in home and building automation, industrial process/automated meter reading, medical, military, automotive/tire pressure sensors, radio frequency ID, and others.
Registered SAE sensor consultant William Fleming (SAE-ARI # 2083), who is well known in automotive circles as an instructor for SAE's "Sensors and Actuators" and currently is an editor for the IEEE Vehicular Technology Society's Magazine, is excited about the possibilities of energy harvesting for automotive tire pressure monitoring systems. "Batteries wear out. They also introduce weight imbalance on wheels and the sensors can get damaged when you replace a tire because currently the sensors are on the back of the air valve," he said. "They want to embed them right in the tread of the tire on the inside."
For a recent presentation on energy harvesting, Dr. Fleming summarized the power expectations from a variety of energy sources that are shown in Table 1. Getting these values is somewhat tricky. "The value for piezo vibration is for a resonant tuned device, tuned to the frequency at which the vibration is acting," said Fleming. "If you have broad band vibrations, you may not get nearly that much power."
Piezoelectric technology is not at the high end of potential power for energy harvesting. "The greatest sources of energy harvesting would be industrial temperature differences or outdoor solar sources," said Fleming. "But if they are unavailable, then the next greatest sources are vibration motion using piezoelectric or moving magnet generators."
Table 1. The range of available maximum power varies considerably depending on the energy source being harvested and other factors.
| Energy Harvesting Source | Maximum power (µW/cm2) |
| Vibration/Motion: Human activity Industrial—Piezoelectric generation —Electrostatic (electret moving plate) —Eletromechanical (moving magnet) | ~4 1000 ~100 1000 |
| Thermal: (Thermoelectric Seebeck effect) —Human activity —Industrial (engine cylinder heads, exhaust pipes) | 15/10°C ΔT ~25 10,000 |
| Light/Solar: —Indoor photovoltaic cell (typ.) —Outdoor photovoltaic cell (bright sun) | ~10 15,000 |
| RF Radiation: —Cell-phone tower radiation —WiFi radiation | ~0.1 ~1 |
Fleming is not the only industry observer monitoring the developments in energy harvesting. Market research firms closely following the potential for energy harvesting include Yole Developpement, Darnell Research, and others.
Yole Developpement recently announced a new report on "MEMS Energy Harvesting Devices, Technologies and Markets". They identify five companies in production with piezoelectric energy harvesters and three with electromagnetic technology and three with thin-film thermoelectric energy harvesting in limited production.
In the updated report, "Energy Harvesting, Micro Batteries and Power Management ICs" from Darnell Group, six "Critical Success Factors" for energy harvesting technologies are detailed. The report includes a cost-benefit model that identifies a specific path to commercial success within the next five years.
The companies presenting at this year's Energy Harvesting for Powering Sensor Applications Symposium at Sensors Expo promise to provide extensive insight into today's viable applications for energy harvesting based on the current level of technology, as well as future direction and capabilities that can be expected in the near future. Experts from the Who's Who of energy harvesting companies will update attendees with the latest in this exciting and intriguing area.