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Energy harvesting for wireless sensors

Many industrial end users will continue to resist adopting wireless sensing as long as these sensors require batteries, which must be replaced. While suppliers have made improvements in battery life, powering wireless sensors via energy harvesting is a potential game changer in both industrial and building automation. Harvesting is not one technology, but several, and the newest energy harvesting technology – thermoelectric generation – made significant progress toward commercialisation in 2010.

The use of batteries in wireless sensors has always been a barrier to wider end user acceptance of the technology in both building and process automation. Wireless process measurement products have achieved significant reductions in power consumption, and hence longer battery life. Industrial standards now dictate monitoring of battery health.

Suppliers have also made system-level improvements. But despite improvements, users have been slow to deploy battery-powered field sensors widely, seeing large numbers of field-mounted batteries as a source of maintenance cost and operational risk.

Can getting rid of batteries drive wider acceptance for wireless measurements? Suppliers and venture-stage firms alike now seek to develop devices with integral power supplies that have the same life expectancy as the devices themselves.

Rather than batteries, the ideal power supply would be capable of “energy harvesting” to provide energy over an indefinite period. The challenging aspect of this for suppliers is that, currently, no single energy harvesting technology dominates. Rather, suppliers must wrestle with several types of energy harvesting, most which are very new. Three energy-harvesting technologies now lead the industrial space: photo-voltaic, vibration, and thermoelectric.

The most established and widely deployed technology has been photo-voltaic (PV). However, PV solutions tend not to power individual devices but rather, recharge larger batteries that support small field systems in re-mote locations. In the earliest days of wireless field devices, suppliers showed (but did not commercialise) conceptual designs of field transmitters incorporating PV.

Embedding PV at the device level reduces device ruggedness, since the PV components can be destroyed or damaged during installation or operation. Another liability of device-integrated PV is that measurement locations vary widely in the amount of ambient light available and not all have enough to support a device indefinitely. So while widely deployed, PV is used as a separate sub-system rather than an integral part of field measurement devices.

A second source of ambient energy for harvesting is vibration. Vibrations can be converted into electric power by either piezoelectric or permanent magnet generators (PMG). Piezoelectric has been used in building automation applications, but under continuous vibration/deflection, long operating life has been a challenge. PMGs, on the other hand, have been commercialised (not surprisingly) in vibration-sensing applications.

PMGs offer the advantages of indefinite life and scalability to provide dedicated sensor power. However, the units are relatively heavy and bulky. This is not an issue in most equipment condition monitoring applications, especially when compared with the cost of new field sensor wiring. The sensors can operate with PMGs paired with “super-cap” capacitors for a measurement that is entirely free of batteries. This aspect proved attractive to Shell, an end user that had shown strong resistance to large-scale battery deployment.

The third and newest energy harvesting technology is thermoelectric generation. Thermoelectric technology uses essentially the same phenomenon as common thermocouples. However, instead of being optimised to create a voltage that is a measure of temperature difference, thermoelectric devices are optimised to use a temperature difference to create electric voltage.

A thermoelectric produces electrical power from heat flow across a temperature gradient. As the heat flows from hot to cold, free charge carriers in the material are also driven to the cold end. This causes a voltage proportional to the temperature difference. By connecting an electron conducting (n-type) and hole conducting (p-type) material in series, a net voltage is produced that can be applied to a load.

To achieve a useful volt-age, many thermoelectric couples must be connected in series within the device.
In theory, a thermoelectric could power a wireless temperature sensor indefinitely as long as there was a sufficient temperature difference between the measurement point and the ambient temperature. In practice, there are challenges. The device requires a significant heat sink to maintain the thermal gradient. The thermo generator must be packaged in an industrial device without compromising the ruggedness of the device.

Energy harvesters powering wireless field measurement is a compelling vision. But no magic bullet is in sight. No single energy harvesting technology can serve all industrial applications. Rather, each seems to work well within a certain application domain. It also seems that incorporating energy harvesting with sensing adds another dimension to the challenges of device design. However, the prospect of battery-free wireless sensing is so attractive that suppliers are investing and making progress in this area.

[Harry Forbes ( is Senior Analyst, ARC Advisory Group.]

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