Whilst the LEDs are not around the corner, I think the Kr-Cl excimer lamps might already be good enough.
When we wrote the original post on this, it was not clear how quickly covid was spreading through the air, but I think it is now clear that covid can hang around for a long time (on the order of minutes or hours rather than seconds) and still infect people.
It seems that a power density of 0.25W/m^2 would probably be enough to sterilize air in 1-2 minutes, meaning that a 5m x 8m room would need a 10W source. Assuming 2% efficiency that 10W source needs 500W electrical, which is certainly possible and in the days of incandescent lights you would have had a few 100W bulbs anyway.
EDIT: Having looked into this a bit more, it seems that right now the low efficiency of excimer lamps is not a binding constraint because the legally allowed far-UVC exposure is so low.
“TLV exposure limit for 222 nm (23 mJ cm^−2)”
23 mJ per cm^2 per day is just 0.002 W/m^2 , so you really don’t need much power until you hit legal limitations.
I think your math on the energy is wrong / incomplete. The energy hitting a person isn’t the same as the energy emitted from the source, and it falls off drastically with distance. So you can’t just divide wattage at the source by room area.
Also, the area being reached depends significantly on room geometry—meaning that it’s not easy to have a single lamp cover a room, especially the type of large, high occupancy room which is most likely to lead to large exposure events.
Whilst the LEDs are not around the corner, I think the Kr-Cl excimer lamps might already be good enough.
When we wrote the original post on this, it was not clear how quickly covid was spreading through the air, but I think it is now clear that covid can hang around for a long time (on the order of minutes or hours rather than seconds) and still infect people.
It seems that a power density of 0.25W/m^2 would probably be enough to sterilize air in 1-2 minutes, meaning that a 5m x 8m room would need a 10W source. Assuming 2% efficiency that 10W source needs 500W electrical, which is certainly possible and in the days of incandescent lights you would have had a few 100W bulbs anyway.
EDIT: Having looked into this a bit more, it seems that right now the low efficiency of excimer lamps is not a binding constraint because the legally allowed far-UVC exposure is so low.
“TLV exposure limit for 222 nm (23 mJ cm^−2)”
23 mJ per cm^2 per day is just 0.002 W/m^2 , so you really don’t need much power until you hit legal limitations.
Source
I think your math on the energy is wrong / incomplete. The energy hitting a person isn’t the same as the energy emitted from the source, and it falls off drastically with distance. So you can’t just divide wattage at the source by room area.
Also, the area being reached depends significantly on room geometry—meaning that it’s not easy to have a single lamp cover a room, especially the type of large, high occupancy room which is most likely to lead to large exposure events.