Terahertz Power Beaming
The goal of power beaming is to use electromagnetic waves (eg. visible light, infrared light, terahertz waves, microwaves) to transmit energy over long distances. On the reciever end, the electromagnetic energy is converted into electrical power. Applications include continuous powering/ battery recharging of land-based and air-based drones without having to land the drones and switch out batteries!
Research of power beaming has typically focussed on using either visible/ infrared lasers or microwaves. The Federici/Gatley research group introduced the concept of terahertz power beaming. Why Terahertz? Since the wavelength of terahertz radiation is shorter than that of microwaves, it is less succeptable to beam diffraction, ie. spreading of the radiation beam as it propagates over long distance. As shown in the figure below, the microwave power beaming system is assumed to operate at a frequency of 2.45 GHz under clear weather conditions. For this calculation, it is assumed that the apertures of the transmitter and reciever are 1m in diameter. It is also assumed, for simplicity, that 10 W of power is required to be incident upon the receiver in order to provide sufficient power to the drone. This figure shows that a 2800 W transmitter would be required to receive 10.18 W at 100 meters. At this frequency, the rapid beam spreading (large angular divergence) with propagation distance dictates that only a portion of the transmitted beam will illuminate the targeted receiver. Microwave power beaming is not very efficient at large (~100 m) distances since the beam rapidly expands due to diffraction!
(left scale) Power received (dashed line) versus distance. (right scale) Beam diameter (solid line) as a function of distance from the transmitter for 2.45GHz. The horizontal line indicates the "hazard zone" or safety limit for the microwaves based on IEEE standards.
How does atmospheric weather effect power beaming? The figure below shows the calculated atmospheric attenuation in the microwave, terahertz, infrared, and visible bands. The dashed, dash-dot, solid, and dotted lines correspond to 4mm/hr of rain, 100m visibility of fog, US standard atmospheric conditions at sea level, and 15 g/m3 of water content, respectively. The long black dashed line corresponds to the expected attenuation due to ‘free-space damping’ (ie. spreading of the electromagnetic beam due to diffraction) for an assumed distance of 500 m with transmitter and receiver effective areas assumed to be 1m diameter disks. The corresponding Gaussian beam radius at the transmitter is 0.33 m. An infrared wavelength of 1.5 microns corresponds to a frequency of 2×105 GHz.
Note that for the microwave range ~10GHz, there is very little atmospheric attenuation. However, the free-space damping (ie.g spreading of the electromagnetic beam due to diffraction) contributes signifanctly to the effective damping of the microwave beam. As the electromagnetic frequency increases, free-space damping becomes small. In the THz band (~1 THz) the atmospheric attenuation due to water vapor becomes increasing more severe with increasing frquency. In the infrared and visible range, there are various 'atmospheric transmission windows' which enable power beaming. However, in the presence of fog, snow, dust, etc. infrared/ visible beams are strongly attenuated while Terahertz waves (as well as microwaves) are not effected as severely. Terahertz power beaming can be used in the presence of fog when infrared power beaming becomes impossible!
As an example, the figure below shows the calculated received power percentage from a Terahertz power beaming system through 100m of various atmospheric conditions. For all curves, the temperature is 17 C and the apertures are 1 m in diameter. The gray curve corresponds to clear weather and 5.78 g/m3 water vapor ( 40% relative humidity). The sold black line corresponds to clear weather conditions and 14.46 g/m3 water vapor (100% Relative Humity). The dashed, dotted and dash-dot lines correspond to 14.46 g/m3 water vapor with 1 g/m3, 5 g/m3, and 10 g/m3 fog droplet content, respectively.
In the presence of fog, the lower frequencies (100-300 GHz) show the most recieved power. Under conditions of rain (figure below) as well as sleet or snow, THz power beaming is most effective in the 100-300 GHz range. Details of the analysis may be found here.
For the figure above, the percent received power relative to transmitter power is calculated through 100m of rain. Atmosphere conditions correspond to an atmospheric pressure of 1013.25 hPa, temperature of T=288 K, and water vapor density of 7.5 g/m3. Curves top to bottom correspond to no rain (solid black) and equivalent rain rates of 2 mm/hr (dashed), 6 mm/hr (dotted), and 10 mm/hr (dash-dot). The effective radii of the transmitter and receiver are assumed to be 0.5m. The gray curve corresponds to the specific attenuation of rain for an equivalent rain rate of 10 mm/hr.