Powering Wireless Efficiently

July 26, 2005
Energy harvesting powers independence for wireless devices

Following is an expanded version of this column under the same title in the August 2005 issue of Buildings magazine.

The wired vs. wireless dilemma has always been a choice between the lesser of two evils: One could either connect to line power and sacrifice the many benefits inherent to wireless - flexibility, mobility, and ease of installation - or use batteries and cope with the limited life, maintenance, and disposal issues that accompany them. As the number of wireless devices available for building applications such as HVAC, lighting, and security control grows, so does the challenge of how to power them efficiently and independently.

New energy-harvesting techniques, which scavenge minuscule amounts of ambient energy present in the environment, are quickly being developed to power a variety of wireless networked devices including sensors, switches, and the radio electronics necessary to transmit their signals.

Cost is also becoming less of a factor, not only because of the economies of scale associated with the growing commercialization of the technology, but through savings generated by eliminating the batteries and maintenance associated with battery-powered devices. While it is true that energy-harvesting solutions cost a few dollars more than battery-powered versions today, they should be comparable in cost in 2 to 3 years. The cost premium today is typically less than what it costs to swap the battery one time (battery cost, labor, etc.). So, over an expected lifetime of say 15 years, there would be significant savings from a self-powered sensor.

All the improvements in technology notwithstanding, one fact remains: Batteries are problematic for large-scale wireless applications. They have to be monitored for charge; useful lives vary depending on the operating environment; and there are labor, stocking, and replacement costs to consider. Furthermore, devices must be accessible for battery replacement, and batteries are fast becoming considered as a hazardous waste.

Energy harvesting provides continuous, renewable, and ample energy - meaning one can monitor sensors and transmit more frequently than what is typically permitted by battery-powered devices.

Better Techniques, Better Yield
Light, vibration, temperature gradients, or motion - present in the environment around us - all contain energy that can be harvested. Until fairly recently, the amount of energy generated via harvesting was not sufficient to send a wireless signal any practical distance. However, improvements to harvesting techniques, combined with very low-energy electronics, means independent “battery-less” technologies are a commercially viable reality.

Energy-harvesting radios output up to 10 mW compared to 1 mW (typical for battery-powered radios), so range is longer. With existing technologies, engineers can now expect transmission distances of up to 30 meters indoors, with signal strengths sufficient to reliably transmit through walls and other structural elements.

The key enabling features of these devices are their highly efficient power management technologies and extremely short signal duration. When the devices are not transmitting, they revert to an ultra-low-energy “sleep” mode. However, when signaled, the devices quickly wake up and transmit a burst of data, and then go back to sleep - all in just below 1/1,000 of a second.

Currently, solar-powered room temperature sensors are the most common application being specified today. A wall-mounted light switch that operates off the energy harvested when somebody pushes the switch is another device gaining acceptance among building system designers as well. Other applications include solar-powered magnetic contacts in security systems that monitor doors and windows, and solar-powered switches that remotely operate blinds, shutters, and awnings.

Can’t Do it All - Yet
By virtue of the necessity to keep the power demands of a given wireless device extremely low, data must be transmitted in short bursts. This is not an issue for applications such as a temperature sensor that intermittently transmits a small packet of data. But for high-bandwidth applications, such as streaming control or system monitoring data over extended periods of time, energy harvesting just can’t generate enough power to handle the job - yet.

Another concern is reliability, because a wireless signal can only be transmitted one way. With one-way communications, there is no acknowledgement that the signal was properly received by the intended device. This, of course, is not likely to be an issue for applications where the user can visually confirm transmission (e.g., the light turns on or the shade goes down), but for more sophisticated control systems, device status is often a critical aspect of overall system or network integrity.

As the technology evolves, fueled by increasing amounts of vendor-supplied development dollars and demand, we can expect to see more complex wireless devices capable of sending larger amounts of data go “battery-less.” It’s also equally likely to expect the number of applications to grow as companies figure out new ways to apply the technology in situations where the absence of wires can generate cost savings and other enterprise-wide facility management benefits.

Jeff Raimo is a product manager at Siemens Building Technologies located in Buffalo Grove, IL. Jim O’Callaghan is vice president of sales and marketing at EnOcean, Oberhaching Germany, which was founded by Siemens AG and spun-off in 2001. For more information on self-powered networking devices and other Siemens Building Technologies facility automation and management products, visit (www.enocean.com) or (www.us.sbt.siemens.com).

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