[Editor's Note: This was originally posted on the Embedded Master]
In this post I will explore RF energy harvesting – harvesting energy from radio waves. I spoke with Harry Ostaffe, Director of Marketing and Business Development at PowerCast to learn more about RF energy harvesting. Ostaffe informed me of another energy harvesting resource site. The Energy Harvesting Network focuses on disseminating the current and future capabilities of energy harvesting technologies to users in both industry and academia. The site currently lists contact information for 25 academic and 37 industrial members that are involved with energy harvesting.
The effectiveness of energy harvesting depends on the amount and predictable availability of an energy source; whether from radio waves, thermal differentials, solar or light sources, or even vibration sources. There are three categories for ambient energy availability: intentional, anticipated, and unknown. Building a device that powers itself in an environment with unknown and random sources of ambient energy is beyond the scope of this post. If you have experience with these types of designs, please contact me.
Building a device that relies on anticipated energy sources takes advantage of infrastructure that is already in place in the environment. For RF systems, this could include scavenging ambient transmissions from cell phones, mobile devices, as well as television and radio broadcasts located in the area. A challenge for systems that rely on anticipated energy sources is that available energy can fluctuate and there is no guarantee that there will be enough energy to scavenge from the environment.
Intentional energy harvesting designs rely on an active component in the system, such as an RF transmitter, that can explicitly provide the desired type of energy into the environment when the device needs it. PowerCast’s approach to support an intentional energy source is to offer a 4W 915 MHz RF transmitter. The intentional energy approach is also appropriate for other types of energy, such as placing an energy harvesting on a piece of industrial equipment that vibrates when it is operating. Another example could involve placing an energy harvesting near a light source that will emit light when the device will be operating and is no longer asleep. Using an intentional energy source allows designers to engineer a consistent energy solution.
An “obvious” frequency sweet spot for RF energy harvesters should be 2.4GHz because so many consumer devices work at that frequency. Ostaffe says that while they have made components that work in the 2.4GHz range, they are currently not publicly available. There is the potential for consumer frustration with a 2.4GHz harvester that currently makes offering harvesters in this frequency range a problematic idea. The first logical spot someone with one of these devices is likely to put them is near their 2.4GHz wireless access point. The problem is that these routers typically transmit in the 100mW range (versus 4W for the 915 MHz transmitter) and that does not provide enough energy for most harvester applications – especially because the energy drops off at 1/r2 from the source. The consumer is likely to attribute the poor performance of the device to a flaw in the device rather than an insufficient power source issue.
If you would like to be an information source for this series or provide a guest post, please contact me at Embedded Insights