Microwave Oven = Smartphone Charger: Will RF Energy Harvesting Make It Possible?
By Jim Pomager, Executive Editor
Imagine setting your dead smartphone down on the kitchen counter as you grab a frozen dinner out of the freezer. You pop the meal in the microwave, hit the start button, and viola! — 5 minutes later you have both a hot, nutritious meal and a charged phone. Sounds too good to be true, right?
Well, to be honest, it is too good to be true, at least today. However, the dream might not be as outlandish as it sounds, based on research presented at the Conference on Ubiquitous Computing in Zurich last month.
Yoshihiro Kawahara, an associate professor in the department of information and communication engineering at the University of Tokyo, presented findings demonstrating that electromagnetic energy leaked from a common microwave oven could be harvested and stored, and used to power electrical devices.
Microwave ovens work by passing non-ionizing microwave radiation — usually at a frequency of 2.45 GHz — through food. Contrary to popular (paranoid?) belief, very little energy actually escapes a microwave oven, even at very short distances from the appliance. Kawahara and his team, collaborating with researchers from the Georgia Institute of Technology, measured leakage of about 500 μW/cm2 at a distance of 5 cm from the front of the oven door.
To capture this small amount of leaked power, Kawahara and his team designed a low-cost, energy-harvesting rectenna circuit tuned to the frequency of the microwave oven. The circuit consists of a dipole antenna and a custom-designed Dickson RF charge pump (about the size of a quarter — click on the thumbnail image above to view), which converts AC electrical power from a lower voltage to a higher DC voltage using a network of capacitors and diodes. During experiments, the antenna received instantaneous power levels of approximately 0.9 mW from the microwave oven when it was running. After 2 minutes of operation, the resistor consumed up to 8.87 mJ of maximum average power, and the capacitor stored up to 9.98 mJ.
According to the researchers, this amount of energy is sufficient to replace button batteries in powering low-power-consuming kitchen devices such as cooking thermometers, weight scales, and digital kitchen times. How long each could be operated by this power source varied anywhere from a few seconds to several minutes, depending on the device’s energy requirements.
Which brings us back to my introduction’s provocative and (overly) ambitious claim: Could this type of RF power harvesting technology eventually power your cellphone? Honestly, no time soon. Putting things in their proper context, if you fully discharge the battery on your iPhone, it will takes 5 minutes to establish even a minimum usable charge, and that’s using a 5W power adapter. With Kawahara’s rectenna, you would need to cook your TV dinner for a week or more to deliver a meaningful amount of power to your phone.
On the other hand, the researchers estimate that the amount of energy they were able to store was only about 15% of the ideal case, and they believe they can improve the circuit’s performance by using more sophisticated impedance matching and power management methods. Plus, the energy efficiency of electronic devices continues to improve, meaning their energy requirements will continue to drop. As a result, the technology could conceivably charge several low-power devices at once, power individual devices enough for longer operation, or even power more energy-hungry devices.
Though cell phone charging may still be a stretch, Kawahara has a grander vision for his energy harvester than powering up kitchen thermometers. He has published research on the potential development of autonomous wireless sensor networks that would scavenge the few dozen milliwatts needed for operation from ambient RF signals. His ultimate vision includes “thin and fluffy” wireless sensor nodes that could be carried by the wind and cover large areas like the “petals of a blossom.”
Such applications may sound fanciful, but RF energy harvesting is definitely an up-and-coming research area in the industry. At IMS2013 earlier this year, I sat in on a well-attended technical session on the topic, where researchers presented their novel techniques for harvesting and transmitting energy at different frequencies. (A paper by Kawahara and collaborators was presented during a separate technical session on RF energy scavenging at the conference, but I wasn't able to attend.) It will be interesting to see how this technology develops in the coming years to one day power satellites, sensors, appliances, and, just maybe, your smartphone.