Extreme Processing: RF Energy Harvesting

By Robert Cravotta | May 28, 2010

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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.

RF Energy Harvesting

By Alf Resco June 2, 2010 - 23:35

I would not be surprised if this practice becomes illegal in some countries if not already. I believe there is at least one precedent case where a person residing close to a powerful radio transmitter was prosecuted for energy theft.

Efficiency...

By JohnStyles June 3, 2010 - 02:43

Using an intentional transmitter of 4W to power a small (low power) device seems a big waste of energy. The whole point of Energy Harvesting in my mind was that it scavenged energy from its environment. This is worth doing.

Power Harvesting or Power Sucking?

By Tarek Lule June 3, 2010 - 05:14

A little bit of nice history: when Radio Berlin was first set up in last century's 20's, gardeners in nearby garden plots found that their metallic clothesline (yes that was when plastics were not very widespread) were carrying measureable voltages. It sufficed to connect a bulb between the line and ground (literally) to light their evening beer and barbecue. Nice early example of energy harvesting?
Not nice for all: the downside was that the radio station had to exchange their power tubes much more often than anticipated. The power for the lighting was literally sucked out of the transmitter which got overloaded. That's why this early and practical way of energy harvesting got forbidden ever since. Wonder what happens if you place energy suckers near enough to other types of transmitters? Will these transmitters go bust as well? Will the drain of power be noticed and the transmitter cranks up the power to compensate - which completely disrupts the idea of only harvesting energy that is there "anyway" and "for free".

RF Energy Harvesting?

By Larry Sears June 3, 2010 - 08:18

When I was a youngster in the 50's, I had a Heathkit shortwave radio which used a long-wire antenna. I noticed that I got a shock from the antenna wire (when the radio was plugged in and the antenna attached). Excitedly, I assumed that I somehow had "free" power, so wired a 120V lamp from the antenna to ground. The lamp glowed - at least until the receiver's AC- line bypass capacitor exploded. Later in my career, I realized that the bypass cap must have been shorted and the chassis was raised to line potential. Since the antenna input was connect to the chassis through an RF coupling transformer, the antenna was also raised to line potential.

In this case, at least, no free lunch.

Power Sucking?

By Anonymous June 3, 2010 - 10:13

@Tarek

Was the radio station transmitter amplifier on some sort of feedback loop for power output? This seems counter intuitive that the clotheslines would "suck" the power out. Someone turned up the amp not knowing that there was a power sink. Measuring the power output of the transmission at some range using a theoretical efficiency number.

@ Alf

Given the dispersion of radio waves you aren't going to "attract them" with a harvester, only absorb the ones headed in your direction and potentially cause dead spots behind you as they are absorbed, so the closer you are to the transmitter, the greater the arc of your dead spot. Where is this illegal?

beam your RF through my property and I will harvest it ...

By KGround June 3, 2010 - 11:19

and the law be damned.

With enough (admittedly controversial) evidence to indicate that RF radiation at sufficiently high local energy levels is harmful to people, I say it is a fair trade. No body is paying me to use my airspace as a medium for their transmissions, so if I can tap off some of that trespassing energy to do useful (to me) work,. so much the better.
But note that I don't think this will ever be useful for any practical purpose beyond small gadgets (unless you happen to live across the street from Radio Berlin).
Also note that energy can also be harvested from high voltage transmission lines in the same way. My grandfather ran the electric fence chargers on his farm from a 300 foot long strand of wire runnning parallel to the high line for decades this way. No doubt at all that this was costing the electric company something since in this case what we had was a physically large single turn transformer subject to the usual transformer energy transform formulae.

RF Harvesting

By Anonymous June 8, 2010 - 17:44

My old SW radio antenna gave me a shock in the same way, except the real reason is that in the 50's there were no 3 way grounded AC plugs. That means if you plug the AC one way the chassis was gnd, flip it over and it was hot, and so was the antenna.

Harvesting available RF makes sense, but using transmitters does not. Besides the signal interference that a transmitter would cause, it is incredibly inefficient and wasteful.

Even so, RFIDs are an exception where the inefficiency is worth the savings in the receiver.

My grandfather ran the Zoloft

By Anonymous December 22, 2011 - 04:39

My grandfather ran the Zoloft dosage electric fence chargers on his farm from a 300 foot long strand of wire runnning parallel to the high line for decades this way.

Given the dispersion of radio

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I would not be surprised if

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