It all seems so simple, and at first glance, it makes a lot of sense. We live in a world run wild with electronic signals—radio, TV, Wi-Fi and cell phone signals, just to state a few of the obvious. Thanks to the advent of mobile devices, wearables and the Internet of Things (IoT), one of the major thrusts of R&D in electronics has been to develop applications that run on very little power.
Why not set up a little antenna to capture some of that Radio Frequency and rectify it? It’ll be easy to get a microwatt or so, and use it to slowly charge a battery or supercapacitor. That will be just enough to power an IoT device that only needs to turn on once in a while, transmit a reading, and then go back to sleep. Then, after our IoT device turns in again, the RF harvesting resumes, and plenty of stored energy awaits the next awakening.
Well at least, that’s the theory. And it sounds pretty good. After all, a TV station radiates an enormous amount of RF. Only a tiny fraction of it is dissipated in the detector stages of the combined totals of all the TV receivers that are tuned in. The rest of it is out there, waiting to be harvested.
RF harvesting techniques
RF power harvesting begins with an antenna. A given antenna can only efficiently harvest power radiated from a close bank of frequencies. A good place to start is with the example of UHF and VHF TV. Even at 500 MHz, a dipole would be 0.3 meters long. This already presents a red flag, because that’s a fairly large amount of real estate spent to harvest what will be a rather tiny amount of power. In addition, the antenna must be placed in a specific spatial orientation with respect to the TV station’s transmitting antenna. And both of these requirements make it impractical for a wearable device.
The harvester’s receiving antenna presents a 50-ohm impedance, which has to be matched to the input impedance of the rest of the device. The voltage harvested at the antenna then has to be increased to at least a volt so it can be rectified into DC. This can be done with an arrangement called a charge pump, which increases the voltage but, of course, can’t increase the total RF power.
RF energy harvesting research
An interesting series of experiments centers on harvesting RF power generated by a Tokyo, Japan TV broadcast station at a distance of 6.5 km. The block diagram for the project is as follows.
Figure 1: Representation of a system-level description of an RF energy harvesting device. (Source: “A Battery-Less, Energy Harvesting Device for Long Range Scavenging of Wireless Power from Terrestrial TV Broadcasts,” Georgia Institute of Technology)
The project was conducted at the Georgia Institute of Technology in conjunction with researchers from the University of Tokyo. In this implementation, the aforementioned charge pump is contained within the RF-DC block.
Important results of the project are summarized in the next diagram. The green blocks represent the amount of power—in microwatts—captured at the relevant 6.5 km distance by the antenna from emissions at UHF frequencies characteristic of Japanese TV. The blue and red bands represent the power needed to charge the supercapacitor mentioned in the block diagram to 1.8 volts and 3.0 volts, respectively.
Figure 2: The supercapacitor was charged to 2.9 volts in a reasonable amount of time. (Source: “A Battery-Less, Energy Harvesting Device for Long Range Scavenging of Wireless Power from Terrestrial TV Broadcasts,” Georgia Institute of Technology)
Limitations of RF energy harvesting
The backers of remote rf harvesting for IoT devices claim that this approach would be useful in powering a remote sensor in an urban area. But, as we have seen, a relatively long antenna is required, and it must be tightly oriented to a TV station or another power source. And, if the power source shifts or changes, all of the corresponding IoT devices must be realigned. This defeats the whole purpose of deploying power harvesting for the IoT, which is to avoid the task of physically accessing the device being powered. The antenna requirements alone make remote power harvesting for wearable devices impractical.
When one considers that the incidence of solar energy is so much greater than the amount of RF permitted in general population areas anywhere in the developed world, it’s hard to justify deployment. In addition, the situation is not likely to change, because there is a limit to how much RF power can be incident in any space open to the general public. If anything, the limits are likely to be scaled back, as RF exposure is being looked at with concern because of possible health risks to people.
RF energy harvesting solutions
Directed RF for power harvesting
There are situations where a sensor is deployed in an area that is hard to access, or perhaps the area itself is hazardous to humans. In these instances, a method has been developed whereby a sensor is powered not through the harvesting of random power, but from harvesting power specifically aimed at the sensor. Instead of depending on the vagaries of a tricky antenna or the presence or absence of a TV signal, a technician can shine an RF transmitter on the unit from a safe distance.
Powercast Corporation offers an evaluation kit to help organizations explore the possibilities of this technology. The company’s P2110-EVAL-02 evaluation kit is available from Arrow Electronics. The datasheet reveals that it includes an RF transmitter and receiver, an antenna, and a charging board to harness the transmitted power. And certainly, another important area to explore is RFID.
RFID - Remote Frequency Identification
Remote frequency identification, or RFID, uses radio wave signals to identify a tagged object. The device that reads the tag bathes it in an RF signal that serves two purposes. First, the tag—a tiny electronic device— “harvests” the incident RF power, which it uses to power itself on. Then the tag, which contains stored digital identification information, transmits that data back to the reader.
The reader now knows the identity of the item it scanned. The tags can be quite tiny compared to visual barcode tags. In addition, a human clerk can do the identification from a distance, and the approach easily lends itself to automation.
Practicality of RF power
So, unless you are designing an IoT or wearables regime for operation in the same building that houses a TV transmitter, the evidence strongly leads to the conclusion that it will be a quixotic and ultimately impractical effort. On the other hand, there are situations where RF power harvesting of specifically directed radio waves can be eminently practical.
