Safety and Microwave Wireless Power
October 17th, 2022
Many times the first questions we are asked about wireless power transfer revolve around the safety of the beam. Our answer is that our Space Solar Power Satellites will comply with health and human safety regulations which limit the intensity to levels below that of sunlight - which we all acknowledge is a safe intensity level. Regulations are written to protect us and are separated into general population/uncontrolled and occupational/controlled exposure per FCC Part 47, and arise from IEEE standard C95.1 - 1992 (current version is 2019).
FCC Limit - Uncontrolled
The 10 W/m^2 limit is for uncontrolled exposure at the 10 GHz that our power beam operates at, and that is the averaged over 30 minutes. This limit is to protect the most vulnerable, in the least favorable conditions possible (hot, no wind, no shade) - and also if the persons exposed are unaware. Microwaves in particular are just photons, and their energy is converted to heat when incident on the skin just like visible sunlight. Healthy human thermoregulatory mechanisms are far more powerful (>100 times more) at rejecting this heat - in fact just the evaporation power of sweat is enough to reject 1,000 W or more, ignoring convection and perfusion (blood vessels expanding to carry more blood to the skin to raise it's temperature). Below is an image of a microwave emitter operating at the 10 W/m^2 intensity (a Starlink transceiver).
FCC Limit - Controlled
The limit for controlled exposure is 50 W/m^2 averaged over six minutes - this is for workers without personal protective equipment. Personal protective equipment (PPE) is anything that blocks microwaves but any conductor is enough to reflect away the energy, and eye PPE is recommended at 50 W/m^2 and above. There are clothes you can buy with conductive threads in them, effectively making a Faraday cage around you. Recent work has shown that cotton soaked in MXenes (a relative of graphene) is a very effective PPE solution: https://www.sciencedirect.com/science/article/abs/pii/S0008622320311982
Aircraft flying overhead will bounce the beam away as they are built from conductors - but active intelligent controls mean the beam will not intersect the path of commercial air traffic nor satellites in orbit, even though they would transit the beam in mere seconds. Proper use of RFI filtering on communication antennas means that a power beam wouldn't disrupt comms - as an added measure we chose a frequency that isn't used for telecom (and the harmonics won't interfere either).
The receiving antennas (rectennas) on the ground are elevated such that they can articulate and follow the satellite overhead, much like 2-axis tracking solar panels. The rectennas absorb 99.9% or more of the incident energy, so wild or plant life below will be unaffected. In fact, we are more concerned with wildlife hanging out on top of the rectennas to 'sun' themselves.
In a controlled environment without workers present - there is no intensity limit. Your microwave oven is far more than the 1,000 W/m^2 of sunlight (between 6,000-8,000 W/m^2 for typical ovens). So a fence and monitoring are all that are necessary to go to greater intensities. However, ionosphere heating is the ultimate limit for space solar power microwave power beaming - blocking energy at intensities above 250 W/m^2 for 2.5GHz.
Overall we mean for Space Solar Power to be a force for good in the world, and are actively planning to be a good neighbor to the people and infrastructure of this world. Our team has been working around microwave transmitters for nearly a year with no ill effects, and our CEO is happy to stick himself in a beam at any time to prove these low intensity microwaves are safe.
See https://www.law.cornell.edu/cfr/text/47/1.1310 for the FCC regulations.
A note on the IEEE standard - the limits arise from no human testing (just tissue simulant/ballistic gel) and quantifies the intensity that raises a 10mm cube simulant to 43deg C (above which biological harm is done) and then divides that intensity by ten for the limit. The simulant is generally in a small enclosure while illuminated. The limits by this test method are doubly conservative - by ignoring biological heat management mechanisms and dividing by ten. To be clear, it took six minutes at 500 W/m^2 for the simulant to reach 43deg C from 35 deg C. In more recent studies of patients in MRI systems (~64 to 127MHz EMF fields), core temperature increased by less than 1 deg C at ~300-400 W/m^2 (SAR of 4 W/kg) over 30 minutes. Those wishing to know more can read this overview: https://www.icnirp.org/cms/upload/publications/ICNIRPemfgdl.pdf
