Power generation from radio waves shows way for smart devices without a battery

From Wi-Fi to Bluetooth and mobile communications, a large number of high-frequency radio waves can be detected in our immediate surroundings. These can be used to power chips with the help of a rectenna, a switching device that converts electromagnetic waves into direct current.

This is the only way to make an RFID chip work, for example, which does not require a battery. Even the small transponders in some winter clothing use this principle, so that people buried under an avalanche can be tracked down.

And in the range down to -10 dBm, which corresponds to a thousandth of the radio power of a WLAN router, this circuit is very reliable. Depending on the setup, 40 to 70 percent of the energy of the radio wave can be converted into direct current.

Energy: almost zero

The problem with this is that if a WLAN router is transmitting at full power, only one 30,000th of the power will reach the surface of such a chip at a distance of just one meter. This is 30 times weaker than would be necessary with current chips.

Typically, the energy of the electromagnetic waves found in our immediate surroundings is even several orders of magnitude lower.

In these areas there are also thermodynamic uncertainties. Tiny layers of material can completely block the radiation and even the smallest electrical resistances ensure that no current flows at all. This is the reason why only just under 1 percent of the energy can be converted below the power level of -20 dBm.

By superimposing and interconnecting a total of ten newly developed rectennas, the researchers at the University of Singapore nevertheless succeeded in operating a standard temperature sensor.

Its power was generated exclusively from unused electromagnetic radiation. This was only possible because even a power level of -62 dBm is still sufficient to generate direct current. If you multiply this power by 150 million, you get the radio of a homelike router. Thus, over the range from -20 to -62 dBm, almost 8 percent of the energy could be used to generate direct current.

It is then sufficient to be 10, 20 or even 30 meters away from such a device - or even better, from many such devices. And then it will be possible to power numerous sensors and, in the future, perhaps even more complex devices without the need for a battery.

However, this is unlikely to be possible in the countryside or even in a housing estate. In the heart of Singapore, on the other hand, quite easily.