June 25, 2015 — With the present environment of wireless being the golden child, and it will resolve all the world’s problems. It seems the marketers are on an all-out binge to take anything that, even remotely, smells like money, no matter how impractical or minimally practical, and try to sell it.
I have written about Nikola Lab’s crowd-funding campaign to finance and bring to market a new smartphone case that, supposedly, turns energy from Wi-Fi, LTE and Bluetooth into direct current power that provides wireless charging to a smartphone.
This week’s “You Gotta Be Kidding Me!” comes in the form of harvesting the RF energy from Wi-Fi signals. Researchers at the University of Washington say they’ve developed the first Power over Wi-Fi (Powifi) system. You can find the original paper, “Powering the Next Billion Devices with Wi-Fi” here: http://arxiv.org/pdf/1505.06815v1.pdf.
RF energy harvesting isn’t new. There has been talk about capturing RF energy and harvesting it for quite a few years, and RF energy harvesting does work to some degree. I have even written some articles on it. However, the bottom line is that unless we are taking hundred-, kilo-, or megawatt-radiators, the practical applications are minimal at best.
One of the things the U of W experiment did was to recharge a fitness tracker to 41 percent of its battery power in 2.5 hours. Well, I guess that counts as an application, but a fitness tracker batter is very small and low capacity, and to only get it to 41 percent in two and a half hours isn’t all that practical.
The energy put out by a Wi-Fi signal is very low, even if it is optimized and linearized, as was done in the experiment. And, the law of RF energy decay says that the intensity of radiation passing through any unit area (directly facing the point source, so don’t put a coffee cup in the path) is inversely proportional to the square of the distance from the point source. Hence, doubling the distance from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.
Ok, so you can connect your cell phone to a RF energy harvesting device. Well, your Wi-Fi source will have to be very close to the harvester. And, practically, it will have to be on near full blast all the time to optimize the energy capture (which is counter intuitive to today’s energy conservation mindset). And, realistically, just how much of that one-watt output is going to be able to be harvested and used to charge the cell phone in a typical cluttered environment – not much, I suspect. And, don’t forget, some of that captured energy will have to go to circuit overhead, unless it’s plugged in, of course (being facetious).
Well, I guess if I’m on vacation I could let my fitness tracker get charged by my Wi-Fi router.
So my point is, like so many things we can do, just because we can, doesn’t always mean it will be all that practical. A few applications, maybe. I really can’t see low-power RF sources ever being all that practical as energy harvesting sources – your thoughts?