devices which have long-term average power consumptions of micro-watts are perfectly reasonable. it's how some BLE sensors can run for years off a single coin cell battery.
you'd need some energy harvesting circuitry and something low-leakage to build it up in, but that's doable. more expensive than just stuffing a coin cell into the circuit, though.
> It looks more and more like one of those "free energy" type devices.
it's perfectly well understood that there's energy in RF. just very very little of it, as you note.
that's qualitatively different from the claims of most free energy devices.
> For the condition of synchronization of four oscillators at 2.4 GHz with Idc,sync = 3 mA (1.2 mA) for the parallel (series) configuration
That’s very low but I think a first step in order to power small biometric implants, for example some kind of pacemakers. It’s very frustrating have a surgical operation to change the battery every 10/15 years.
> The STOs are connected in parallel (Fig. 1a) or in series (Fig. 1b) configuration, stimulated by a single dc source. For the parallel connection, at the dc bias (Idc) value of 1.5 mA
Fig 2c and d shows the total output power which maxes out at 0.85 uW. Maybe this is enough to run a pacemaker or something small, I don't actually know. Fig 6 shows the rectification of the signal in to a voltage that can turn on an LED. The dc-dc converter puts it in the ~3V range. Assuming the dc-dc converter is near 100% efficiency (some are pretty damn good) this is a current of only 0.28 uA.
Its a novel way, but they talk about in their own paper that it is not a new more efficient way to convert RF energy to DC energy
> in the range of 1.65–2.8 GHz at Prf = 0 dBm and zero dc bias ac to dc conversion efficiency of ∼6% at Prf = −20 dBm. This is higher than the reported values for STOs53,55 but less than the recent reports of power conversion efficiency of 40% and 40–70% at an input rf power ~0 dBm for MoS2-based flexible rectenna and state-of-the-art Si and GaAs rectifiers56
Edit: Yes I understand that there is energy in RF signals, I am just questioning the actual practicality and claim that they can get usable energy from this. For a 200 mW (23 dBm) WiFi transmitter the free space loss at about 6 inches is 23 dB at which point you would have the same 0 dBm input that they used in the experiment. I have not included any antenna gain as they did not include the antenna gain from their microstrip patch antenna in their experiment, just the 0 dBm power input number.