The world is awash in RF energy, particularly in metropolitan areas. Intel recently demonstrated an experiment of ambient RF energy harvesting from a TV tower. Powercast did a similar demonstration several years ago from an AM radio station.
Intel Research Seattle harvested power at 4.1km from the KING-TV tower in Seattle which broadcasts 960KW at 674-680 MHz. The demonstration used a broadband log periodic antenna (5 dBi) designed for TV applications and
a 4-stage power harvesting circuit of the same design as the Intel WISP, with a front end tuned to the desired channel.
This ambient RF harvesting system was connected to the battery terminals of a commercially available indoor/outdoor temperature and humidity meter (“thermometer/hygrometer”) with an LCD display. The thermometer/hygrometer was measured to consume around 25uA at 1.5V from a laboratory power supply. About once per second, its current consumption briefly spiked up to around 50uA, presumably when sensor measurements were made.
The paper for this experiment is available from TechOnline.
We’ve been promoting the use of RF energy for battery-free wireless sensors. Although the antenna in this demonstration is a little large, it does show the concept of wireless power over distance and battery-free operation.
Powercast will be exhibiting and presenting at the 4th Annual Energy Harvesting Workshop at Virgina Tech on January 28-29, 2009. Powercast sends power through the air using radio waves and converts it back to DC power using RF energy harvesting.
Charlie Greene, PhD. will be presenting the topic “RF Energy Harvesting for Controllable Wireless Power Systems“. Mr Greene developed key patents related to RF energy harvesting and is Head, Technology Platforms for Powercast.
Powercast will also be demonstrating battery-free wireless sensors that are powered by RF energy harvesting. “Energy harvesting is an enabling technology that will drive the next wave of deployment for wireless sensor networks”, said Harry Ostaffe, Director of Marketing for Powercast. “Powercast’s RF energy harvesting technology will help to expand the market for wireless sensors and provide for premium product differentiation through the use of a controllable, wireless power solution.”
In the later part of 2008, Powercast began to demonstrate RF energy harvesting technology working with wireless sensors. An initial project was to wirelessly power a sensor platform from Crossbow Technology. Crossbow is a leader in the wireless sensor market and has a range of wireless sensor platforms, including the agricultural-specific eKo system.
Powercast integrated wireless power technology based on RF energy harvesting with Crossbow’s IRIS Mote platform (shown at right) and their MTS420 sensor board. The IRIS Mote was situated on top of a Powercast lab board as shown in the photo below and monitored with Crossbow’s MoteView user interface.
We could have just recharged the batteries with wireless power, but it’s hard to visualize that. So, we substituted a supercap for the batteries and connected that to the IRIS board. The power source was a 900 MHz transmitter (Powercaster™) designed by Powercast.
The IRIS Motes were loaded with the LP code which supports mesh networking and were programmed to wake up 8 times per second. Powercast will be continuing this effort with Crossbow’s ELP code (lower power, no mesh support) and have the motes sleep for a much longer period. This lower power configuration will provide for battery-free operation at a greater distance.
Crossbow has a related post on their blog about our visit to their office to make this demonstration.
Low power, low data rate wireless sensors such as Crossbow’s IRIS mote are great candidates for being powered by RF energy harvesting.
While this was just a concept of what can be done with RF energy harvesting, it’s an exciting proof point of the potential of wireless power.
Powercast anticipates that energy harvesting will be key to the next stage of growth in the wireless sensor market, and forsee our RF energy harvesting technology as a way to further expand the market and applications with a predictable wireless power platform.
A common question that we get asked is “How much power can I get?”. The answer is not straightforward because there are multiple factors involved, with the main controllable factors being:
- Transmitter power
- Transmitter antenna gain
- Transmission frequency
- Distance between transmitter and receiver
- Receiving antenna gain
There are other considerations too, like multi-path, antenna impedance matching, polarization etc.
Like most things, radio waves are subject to the laws of physics. That being the case, a formula known as Friis equation , named after Harald T. Friis, can be used to estimate the amount of power received at a certain distance.
P = Power, G = Antenna Gain, R = Distance
r = Receive, t = Transmit, λ = Wavelength (inverse of frequency)
Unless you are close to a transmitter (radio, TV, mobile, etc), or use a large antenna, pure ambient RF sources provide very little power. Powercast’s RF harvesting technology can also be coupled with a local transmitter to create a reliable wireless power system.
Since today’s wireless sensors have low power requirements (and are decreasing), harvesting power in the milliwatt and microwatt range can be sufficient to provide enough power for perpetual operation depending on the frequency of operation and duty cycle of the sensor system.
The Sensors Expo & Conference, the leading event on sensing technologies and solutions, recently announced some highlights for the 2009 event that will take place from June 8-10 in Rosemont, IL (USA).
A new area for the 2009 event is the Energy Harvesting and Power Management Pavilion. Powercast is a sponsor of this new area of the show floor that will provide energy harvesting and power management solutions capable of capturing, converting, storing, and delivering energy in a form that can be used to provide the power needed by the system it serves. Powercast will be showcasing RF energy harvesting and wireless power technology with specific application focus for wireless sensors.
RF-based wireless power can be used to recharge batteries in sensors, or to enable battery-free wireless sensors that can be deployed in commercial buildings and industrial facilities. Some unique applications of RF-based wireless power include the ability to power sensors over distance, behind walls, in low-light areas, or extreme cold or hot temperature environments.
This blog is designed to promote and provide information on the development of RF energy harvesting as a means of powering wireless sensors, Active RFID tags, and other sensing and tracking devices. RF energy harvesting and wireless power can be used to recharge batteries remotely or to create battery-free wireless sensors.
Powercast Corporation is a leader in RF energy harvesting technology and wireless power systems using RF energy. The content of this blog is authored by Harry Ostaffe, Director of Marketing for Powercast.
Energy Harvesting and Micro-Power